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Abstract: CH4 and N2O fluxes from soil under a tropical seasonal rain forest in Xishuangbanna, Southwest China were measured for one year using closed static chamber technique and gas chromatography method. Three treatments were set in the studied field: (A) litter-free, (B) with litter, and (C) with litter and seedling. The results showed that the soil in our study was a sink of atmospheric CH4 and source of atmospheric N2O. The observed mean CH4 fluxes from treatments A, B, and C were –50.0 ± 4.0, –35.9 ± 2.8, –31.6 ± 2.8 μgC/(m2·h), respectively, and calculated annual fluxes in 2003 were –4.1, –3.1, and –2.9 kgC/hm2, respectively. The observed mean N2O fluxes from treatments A, B, and C were 30.9 ± 3.1, 28.2 ± 3.5, 50.2±3.7 μgN/(m2·h), respectively, and calculated annual fluxes in 2003 were –4.1, –3.1, and –2.9 kgC/hm2, respectively. The observed mean N2O fluxes from treatments A, B, and C were 30.9 ± 3.1, 28.2 ± 3.5, 50.2±3.7 μgN/(m2·h), respectively, and calculated annual fluxes in 2003 were 2.8, 2.6, and 3.7 kgN/hm 2, respectively. Seasonal variations in CH4 and N2O fluxes were significant among all the three treatments. The presence of litter decreased CH4 uptake during wet season (P < 0.05), but not during dry season. There was a similar increase in seedlings-mediated N2O emissions during wet and dry seasons, indicating that seedlings increased N2O emission in both seasons. A strong positive relationship existed between CH4 fluxes and soil moisture for all the three treatments, and weak relationship between CH4 fluxes and soil temperature for treatment B and treatment C. The N2O fluxes correlated with soil temperature for all the three treatments. [Copyright &y& Elsevier]

Root distribution and mycorrhizal associations were compared in primary, secondary and limestone forests in Xishuangbanna, southwest China. Soil cores to a depth of 20 cm were collected at random points from four 50 m2 quadrats in each forest type. Arbuscular mycorrhizal (AM) associations were the only form of mycorrhiza found in all forest types. The primary forest was characterized by high root mass, root lengths and AM colonization levels higher than other forest types. In contrast, secondary forests had greater AM fungal spore numbers and specific root length, indicating that plant species in secondary forests achieved a greater degree of soil exploration with less biomass allocation to roots. Root density, AM colonization and AM fungal spore numbers decreased with soil depth in all forest types. Although the correlation between AM colonization levels and spore numbers was insignificant when all forest types were considered together, significant relationships emerged when each forest type was considered individually. AM colonization and spore numbers were correlated with several root variables. [Copyright &y& Elsevier]

Background and aims: Drought is predicted to have a profound impact on soil respiration. This study aimed to assess the effects of long-term precipitation decrease on soil respiration in a tropical rainforest. Methods: A precipitation reduction experiment was conducted in a tropical forest in southwest China at the beginning of 2011. Soil respiration and environmental parameters were measured monthly for three years. Results: The continuous precipitation reduction treatment did not affect the seasonal patterns of soil respiration, but it significantly increased soil respiration in the study plot during the rainy season, and the relationship between soil respiration and soil moisture differed in the control and reduction treatment in the rainy season. Compared with the net ecosystem exchange of carbon in this system, the increment of annual soil carbon emissions in the reduction treatment was considerable and should not be ignored. Conclusions: Our results indicate that the responses of soil respiration to precipitation decrease may vary seasonally and the variation of volumetric water content in different seasons may be an important factor leading to the seasonal variation. The variation of soil moisture among different ecosystems as well as in different seasons should be taken into consideration when predicting the future response of soil respiration to drought globally. [ABSTRACT FROM AUTHOR]

Subtropical forests are important ecosystems globally due to their extensive role in carbon sequestration. Extreme climate events are known to introduce disturbances in the ecosystem that cause long‐term changes in carbon balance and radiation reflectance. However, how these ecosystem function changes contribute to global warming in terms of radiative forcing (RF), especially in the years following a disturbance, still needs to be investigated. We studied an extreme snow event that occurred in a subtropical evergreen broadleaved forest in south‐western China in 2015 and used 9 years (2011–2019) of net ecosystem CO2 exchange (NEE) and surface albedo (α) data to investigate the effect of the event on the ecosystem RF changes. In the year of the disturbance, leaf area index (LAI) declined by 40% and α by 32%. The annual NEE was −718 ± 128 g C m−2 as a sink in the pre‐disturbance years (2011–2014), but after the event, the sink strength dropped significantly by 76% (2015). Both the vegetation, indicated by LAI, and α recovered to pre‐disturbance levels in the fourth post‐disturbance year (2018). However, the NEE recovery lagged and occurred a year later in 2019, suggesting a more severe and lasting impact on the ecosystem carbon balance. Overall, the extreme event caused a positive (warming effect) net RF which was predominantly caused by changes in α (90%–93%) rather than those in NEE. This result suggests that, compared to the climate effect caused by forest carbon sequestration changes, the climate effect of α alterations can be more sensitive to vegetation damage induced by natural disturbances. Moreover, this study demonstrates the important role of vegetation recovery in driving canopy reflectance and ecosystem carbon balance during the post‐disturbance period, which determines the ecosystem feedbacks to the climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] Terrestrial species are predicted to migrate northward under global warming conditions, yet little is known about the direction and magnitude of change in microbial distribution patterns. In this continental-scale study with more than 1600 forest soil samples, we verify the existence of core microbiota and lump them into a manageable number of eco-clusters based on microbial habitat preferences. By projecting the abundance differences of eco-clusters between future and current climatic conditions, we observed the potential warming-driven migration of the core microbiota under warming, partially verified by a field warming experiment at Southwest China. Specifically, the species that favor low pH are potentially expanding and moving northward to medium-latitudes (25°–45°N), potentially implying that warm temperate forest would be under threat of soil acidification with warming. The eco-cluster of high-pH with high-annual mean temperature (AMT) experienced significant abundance increases at middle- (35°–45°N) to high-latitudes (> 45°N), especially under Representative Concentration Pathway (RCP) 8.5, likely resulting in northward expansion. Furthermore, the eco-cluster that favors low-soil organic carbon (SOC) was projected to increase under warming scenarios at low-latitudes (< 25°N), potentially an indicator of SOC storage accumulation in warmer areas. Meanwhile, at high-latitudes (> 45°N) the changes in relative abundance of this eco-cluster is inversely related with the temperature variation trends, suggesting microbes-mediated soil organic carbon changes are more responsive to temperature variation in colder areas. These results have vital implications for the migration direction of microbial communities and its potential ecological consequences in future warming scenarios. [ABSTRACT FROM AUTHOR]

The land use change from tropical forests to rubber plantations has had a great influence on ecosystem‐level carbon (C) exchange and soil C dynamics. This study aimed to assess the variation of soil respiration and its components in rubber plantations of different stand age. A trenching method was used to partition soil respiration (RS) into autotrophic respiration (RA) and heterotrophic respiration (RH) for 3 years in 12, 24, 32 and 49‐year‐old rubber plantations. Our results showed that RS changed significantly among rubber plantations, with the highest RS in the old‐growth rubber plantation (49‐year plantation, 147.30 ± 6.91 mg CO2‐C m−2 hr−1) followed by the mature (137.16 ± 7.68 and 108.10 ± 4.83 in 24‐year and 32‐year plantations, respectively) and the young (12‐year plantation, 78.43 ± 3.84 mg CO2‐C m−2 hr−1) rubber plantations. RS was significantly and exponentially related to soil temperature (p < .0001) rather than soil water content, which led to higher RS in the rainy season than in the dry season. The contribution of RA to RS was higher in the mature rubber plantation than in the young (27%) and the old‐growth (20%) rubber plantations. The sensitivity of RS to increasing temperature (Q10) was higher in 32‐ and 49‐year plantations than in 12‐ and 24‐year rubber plantations. The largest Q10 value of RH and RA was found in the old‐growth (49‐year plantation, 2.95) and the mature (32‐year plantation, 8.01) rubber plantations, respectively. Our results highlight the importance of environmental factors in determining the variation in RS in rubber plantations with different stand age. However, further studies are still required to elucidate the mechanisms impacting stand age‐related effects on soil respiration, such as the differences in photosynthesis in rubber plantations. Highlights: Soil respiration (RS) in four rubber plantations of different stand age was examined.RS tended to increase with the increasing stand age.The contribution of autotrophic respiration (RA) to RS was higher in the mature stands.Q10 values of heterotrophic respiration (RH) and RA were higher in old‐growth and mature stands, respectively. [ABSTRACT FROM AUTHOR]

Questions: In the changing climate scenario, the decline in precipitation is expected to alter water availability for plants, which in turn affects plant community structure and composition. The responses of community composition and structure to declines in precipitation are well documented in other biomes but remain understudied in water‐limited savannas. Location: A savanna ecosystem in southwest China. Methods: We used a four‐year (2014–2017) precipitation manipulation experiment to examine changes in herbaceous community composition and structure across the species, functional group and community levels under precipitation reduction. Results: Precipitation reduction significantly decreased the average height and percentage cover of the herbaceous community, while increasing species richness and the Pielou evenness index. Precipitation reduction significantly decreased average height, percentage cover and relative abundance of graminoids and perennials, but increased those of forbs and annuals. Precipitation reduction prompted a shift in the dominant species of the herbaceous community towards Fimbristylis monostachya. Conclusions: The results show that precipitation reduction changed the composition and structure of the herbaceous community of this savanna. Furthermore, they provide strong evidence that changes in herbaceous community structure and composition in response to the intensity and duration of precipitation reduction in this savanna exhibited relatively low thresholds, which suggests that the herbaceous community's response to a decline in precipitation was essentially nonlinear. These findings imply that even relatively small declines in precipitation may stimulate shifts in plant community structure and composition and affect the function and stability of savanna ecosystems. [ABSTRACT FROM AUTHOR]

The pattern of intense defoliation of rubber trees ( Hevea brasiliensis ) in Southwest China is assumed to mainly be attributable to drought stress. However, this has never been proven, and the pattern remains unclear. Our objectives were to reveal the driving factor and pattern of intense defoliation, and to explore the characteristics of sap flow density ( J s ) and relate them to climatic factors before, during and after the intense defoliation period. We continuously measured J s and related climatic factors in a rubber plantation in Southwest China from 2013 to 2016 and found that (1) the cold stress (<10 °C) was more relevant to intense defoliation than drought stress, and after encountering cold stress the rubber trees might enter dormancy for approximately 50 days during the defoliation period; (2) J s showed a self-regulating ability to respond to high air temperature ( T a ) and water vapor pressure deficit ( VPD ) with a time lag of approximately one hour during all periods except during the defoliation period; (3) the sensitivity of J s to T a weaken ( slope  = 0.14, r  = 0.17, P  > 0.05) during the defoliation period, but it rebounded back to a relative high level ( slope  = 1.44, r  = 0.55, P  < 0.01) during the refoliation period which was similar to that during the remaining periods; and (4) the relationship between J s and T a was generally positive, while both SWC 5 and SWC 100 exerted inhibitory effects on J s , and the interactive effects among T a , SWC 5 and SWC 100 on J s were significant during the refoliation period ( P  < 0.01). Based on these findings, we evaluated J s as a crucial physiological indicator of the processes underlying important phenological transition periods, and the cold stress rather than drought stress was likely the driving factor of the pattern of intense defoliation occurred in Southwest China rubber plantations. [ABSTRACT FROM AUTHOR]

The potential impact of drought on the carbon balance in plants has gained great attention. Non-structural carbohydrate (NSC) dynamics have been suggested as an important trait reflecting carbon balance under drought conditions. However, NSC dynamics under drought and the response mechanisms of NSC to drought remain unclear, especially in water-limited savanna ecosystems. A precipitation exclusion experiment was performed to simulate different drought intensities in a savanna ecosystem in Yuanjiang valley in southwestern China. Growth, total NSC concentration and diurnal change of NSC were determined for the leaves and non-photosynthetic organs of three dominant species (Lannea coromandelica, Polyalthia cerasoides and Heteropogon contortus) throughout the growing season. Drought significantly reduced the growth of all the three species. Total NSC concentration averaged ~8.1%, varying with species, organ and sampling period, and did not significantly decrease under drought stress. By contrast, the diurnal change of NSC in these three species increased under drought stress. These results indicate that these three dominant species did not undergo carbon limitation. Thus, relative change in NSC is a more sensitive and effective indicator than carbon reserves in evaluation of plant carbon balance. These findings provide new insights for the understanding of carbon balance and the mechanisms of carbon starvation. [ABSTRACT FROM AUTHOR]

Savanna ecosystems play a crucial role in global N 2 O emissions. However, our understanding of N 2 O emissions under limiting precipitation conditions is lacking. This study evaluates the effects of precipitation reduction on soil N 2 O fluxes from a woody savanna ecosystem in Yunnan Province, Southwest China. Precipitation exclusion shelters were installed above the tree canopy, and four total treatments were established as follows: a control (CK) and precipitation exclusions of 30% (PE3), 50% (PE5), and 70% (PE7). Two years (2015–2016) of N 2 O fluxes, soil temperature and soil water content data were collected. The N 2 O fluxes were generally low, ranging from 0.039 to 0.245 mg N m −2 day −1 , and they were strongly linked to precipitation events. Additionally, the N 2 O fluxes during the rainy season were significantly greater than those during the dry season. The maximum N 2 O flux was observed in August, and the minimum flux occurred in December. Precipitation exclusion had a significant negative influence on the N 2 O fluxes. The N 2 O emissions of CK, PE3, PE5, and PE7 were 0.20, 0.17, 0.13, and 0.12 kg N ha −1 yr −1 , respectively. With the exacerbation of precipitation exclusion, the decrease rate of precipitation exclusion on the N 2 O emissions increased over the entire year (eventually reaching 41.8% in PE7), but the decrease rate of precipitation exclusion on the soil N 2 O emission during the dry season was stronger than that during the rainy season. Additionally, the proportion of dry season N 2 O emissions to total annual emissions decreased (from 45% to 41%), and that of rainy season N 2 O emissions to total annual emissions increased (from 55% to 59%) over the year, whereas they exhibited a stable trend from PE5. The data show that the Yuanjiang savanna is a net source of N 2 O; precipitation reduction decreases the N 2 O emissions in the savanna regions, indicating that precipitation reduction can only slow the increase in the N 2 O concentration in the atmosphere and can therefore slow global warming. In addition, the N 2 O emissions during the dry season may play a significant role in total N 2 O emissions and be more sensitive to precipitation reduction than those during rainy season. These possibilities should be considered in future studies, especially in those ecosystems that experience substantial inter-annual climatic fluctuations. [ABSTRACT FROM AUTHOR]

Litter inputs are expected to have a strong impact on soil N2O efflux. This study aimed to assess the effects of the litter decomposition process and nutrient efflux from litter to soil on soil N2O efflux in a tropical rainforest. A paired study with a control (L) treatment and a litter-removed (NL) treatment was followed for 2 years, continuously monitoring the effects of these treatments on soil N2O efflux, fresh litter input, decomposed litter carbon (LCI) and nitrogen (LNI), soil nitrate (NO3−-N), ammonium (NH4+-N), dissolved organic carbon (DOC), and dissolved nitrogen (DN). Soil N2O flux was 0.48 and 0.32 kg N2O-N ha−1 year−1 for the L and NL treatments, respectively. Removing the litter caused a decrease in the annual soil N2O emission by 33%. The flux values from the litter layer were higher in the rainy season as compared to the dry season (2.10 ± 0.28 vs. 1.44 ± 0.35 μg N m−2 h−1). The N2O fluxes were significantly correlated with the soil NO3−-N contents (P < 0.05), indicating that the N2O emission was derived mainly from denitrification as well as other NO3− reduction processes. Suitable soil temperature and moisture sustained by rainfall were jointly attributed to the higher soil N2O fluxes of both treatments in the rainy season. The N2O fluxes from the L were mainly regulated by LCI, whereas those from the NL were dominated jointly by soil NO3− content and temperature. The effects of LCI and LNI on the soil N2O fluxes were the greatest in the 2 months after litter decomposition. Our results show that litter may affect not only the variability in the quantity of N2O emitted, but also the mechanisms that govern N2O production. However, further studies are still required to elucidate the impacting mechanisms of litter decomposition on N2O emission from tropical forests. [ABSTRACT FROM AUTHOR]

Water-use efficiency ( WUE ) at the whole-plant level plays as an important role as an indicator of both the water-use strategy of the plant, and the ecophysiological processes linking the carbon and water cycles. However, the variation in the annual pattern of WUE , the interannual trends, and the regulating factors of WUE in rubber plantations remain poorly understood. Here, we used a four-year (2013–2016) continuous biometric survey along with sap flow data recorded at a monthly resolution to examine the annual and interannual dynamics of WUE and the relationships of WUE with environmental and biological factors in rubber ( Hevea brasiliensis ) plantations in Southwest China. The results showed that (1) there was no definitive pattern in WUE between April and December, but the peak WUE values generally occurred around August; (2) the multi-year mean WUE was 2.34 ± 0.19 g C·kg −1 H 2 O, and a negative correlation of WUE to photosynthetically active radiation was found at the interannual scale ( r  = −0.962, P  < 0.05); and (3) a moderate, significant and parabolic relationship of WUE with air temperature ( R 2  = 0.40), net radiation ( R 2  = 0.37) and growth rate of the diameter at breast height ( R 2  = 0.35) was observed at the monthly scale during the growing season ( P  < 0.001). These findings indicate that WUE might be regulated by factors affecting the carbon sequestration rather than water consumption, and it will provide a deeper understanding of the ecohydrological and coupled processes between the carbon and water cycles of rubber plants. [ABSTRACT FROM AUTHOR]

Background and aims: Drought is expected to be more frequent and more intense with global warming. Our aim was to investigate how soil respiration would respond to different levels of precipitation exclusion (‘drought strength’).Methods: We conducted a two-year drought experiment in a woody savanna ecosystem in south west of China, which consisted of four treatments: a control treatment (CK); 30% precipitation exclusion (PE3), 50% precipitation exclusion (PE5) and 70% precipitation exclusion (PE7).Results: The cumulative soil respiration rates were significantly decreased in both rainy and dry seasons as drought became more intense. The sensitivity of soil respiration to soil moisture decreased as drought severity increased. There were bursts of CO2 emission when dry soils were rewetted by rainfall after the dry season. Unlike most other exponential relationships between soil respiration and soil temperature, a parabolic function was observed in all treatments (P < 0.05), which was due to higher soil temperature (>28 °C) coinciding with insufficient soil water content (<11% Vol). Respiration rate is best represented by a model which combines soil moisture and temperature.Conclusion: Soil respiration rates were significantly decreased as drought enhanced. The response of soil respiration to drought in the dry season should not be ignored especially when evaluating the effect of drought on soil respiration in a whole year in savanna ecosystems. The quadratic relationship between soil respiration and soil temperature may be important for modeling the response of soil respiration to climate change (drought) in savanna ecosystems. [ABSTRACT FROM AUTHOR]

Forest ecosystems play an increasingly important role in the global carbon cycle. However, knowledge on carbon exchanges, their spatio-temporal patterns, and the extent of the key controls that affect carbon fluxes is lacking. In this study, we employed 29-site-years of eddy covariance data to observe the state, spatio-temporal variations and climate sensitivity of carbon fluxes (gross primary productivity (GPP), ecosystem respiration (R eco ), and net ecosystem carbon exchange (NEE)) in four representative forest ecosystems in Yunnan. We found that 1) all four forest ecosystems were carbon sinks (the average NEE was − 3.40 tC ha − 1 yr − 1 ); 2) contrasting seasonality of the NEE among the ecosystems with a carbon sink mainly during the wet season in the Yuanjiang savanna ecosystem (YJ) but during the dry season in the Xishuangbanna tropical rainforest ecosystem (XSBN), besides an equivalent NEE uptake was observed during the wet/dry season in the Ailaoshan subtropical evergreen broad-leaved forest ecosystem (ALS) and Lijiang subalpine coniferous forest ecosystem (LJ); 3) as the GPP increased, the net ecosystem production (NEP) first increased and then decreased when the GPP > 17.5 tC ha − 1 yr − 1 ; 4) the precipitation determines the carbon sinks in the savanna ecosystem (e.g., YJ), while temperature did so in the tropical forest ecosystem (e.g., XSBN); 5) overall, under the circumstances of warming and decreased precipitation, the carbon sink might decrease in the YJ but maybe increase in the ALS and LJ, while future strength of the sink in the XSBN is somewhat uncertain. However, based on the redundancy analysis, the temperature and precipitation combined together explained 39.7%, 32.2%, 25.3%, and 29.6% of the variations in the NEE in the YJ, XSBN, ALS and LJ, respectively, which indicates that considerable changes in the NEE could not be explained by variations in the temperature and precipitation. Therefore, the effects of other factors (e.g., CO 2 concentration, N/P deposition, aerosol and other variables) on the NEE still require extensive research and need to be considered seriously in carbon-cycle-models. [ABSTRACT FROM AUTHOR]

The lasting imprint of phylogenetic history on current day ecological patterns has long intrigued biologists. Over the past decade ecologists have increasingly sought to quantify phylogenetic signals in environmental niche preferences and, especially, traits to help uncover the mechanisms driving plant community assembly. However, relatively little is known about how phylogenetic patterns in environmental niches and traits compare, leaving significant uncertainty about the ecological implications of trait-based analyses. We examined phylogenetic signals within known environmental niches of 64 species, at seedling and adult life stages, in a Chinese tropical forest, to test whether local environmental niches had consistent relationships with phylogenies. Our analyses show that local environmental niches are highly phylogenetically labile for both seedlings and adult trees, with closely related species occupying niches that are no more similar than expected by random chance. These findings contrast with previous trait-based studies in the same forest, suggesting that phylogenetic signals in traits might not a reliable guide to niche preferences or, therefore, to community assembly processes in some ecosystems, like the tropical seasonal rainforest in this study. [ABSTRACT FROM AUTHOR]

Soil respiration, the second largest CO 2 flux in terrestrial ecosystems, involves the microbial respiration of litter from aboveground sources, belowground litter and root respiration, from rhizodeposition. In the subtropical forests of the Ailao Mountain, a chamber with automated CO 2 efflux was set up with two treatments: a control treatment with litterfall to measure the total soil respiration (R T) and a litter removal treatment to measure aboveground litterfall respiration (R L). This study aimed to examine the responses of R L to unexpected heavy snowfall events and soil temperature (ST), soil moisture (SM), rainfall (RF), total litterfall (TL), litter water content (LWC), nitrate nitrogen (NO 3 −-N), and ammonium nitrogen (NH 4 +-N). The period of the study was divided into two: before the heavy snowfall event (BS) and during and after the heavy snowfall event (AS). The rate of R L was slightly decreased in AS (1.18 ± 0.03 μmol CO 2 m−2 s−1) compared with that in BS (1.19 ± 0.02 μmol CO 2 m−2 s−1). The relationships between R L and ST, SM, RF, and LWC, respectively, were all statistically significant (p < 0.05) in both periods. However, the relationships between R L and TL and NH 4 +-N, respectively, were not statistically significant for either period. The relationship between R L and NO 3 −-N was statistically significant for AS but not for BS. The relationship between R L and RF was statistically significant from 2011 to 2018. The temperature dependence of soil respiration was higher in BS than in AS, and the effect of litter removal was 2.55 % and 2.32 % for AS and BS respectively. The results indicate that current global terrestrial models underestimate R L trends for the feedback of global climate change in subtropical forests. Aboveground litterfall respiration (R L) had strong connection with soil temperature, soil moisture, rainfall and litter water content for two periods but not for ammonium nitrogen (NH 4 +-N). Unexpected snowfall events took effects the connection between R L and total litterfall. Heavy snowfall events declined the relationship between R L and nitrate nitrogen (NO 3 −-N). [Display omitted] [ABSTRACT FROM AUTHOR]

Forests play an important role in global mercury (Hg) cycling. To explain the high Hg accumulation in subtropical forest ecosystems, we studied temporal dynamics of Hg, carbon (C), nitrogen (N), and sulfur (S) in forest soil profiles, as well as litterfall flux and precipitation, in an old-growth moist evergreen broadleaf (EB) forest and a mossy coppice (MC) forest from South China over seven years. The mean soil Hg concentration was 257 ± 14 ng g −1 in the O-horizon and 248 ± 15 ng g −1 in the A-horizon for the EB forest, and 94 ± 27 ng g −1 in the O-horizon and 70 ± 11 ng g −1 in the A-horizon for the MC forest. Annual variations in Hg concentration were suggested to be associated with variations in precipitation and litterfall biomass. Significant vertical Hg transport was only observed in the MC forest, which was attributed to its lower organic matter content. Correlation and stoichiometry analyses further suggested that the dynamics in Hg concentration in the forest floor was also closely linked to the variation in S concentration. Additionally, the difference in the soil Hg pool between these two forests was attributed to different litterfall biomass fluxes. [ABSTRACT FROM AUTHOR]

Conspecific negative density dependence ( CNDD) has been recognized as a key mechanism underlying species coexistence, especially in tropical forests. Recently, some studies have reported that seedling survival is also negatively correlated with the phylogenetic relatedness between neighbors and focal individuals, termed phylogenetic negative density dependence ( PNDD). In contrast to CNDD or PNDD, shared habitat requirements between closely related individuals are thought to be a cause of observed positive effects of closely related neighbors, which may affect the strength and detectability of CNDD or PNDD. In order to investigate the relative importance of these mechanisms for tropical tree seedling survival, we used generalized linear mixed models to analyze how the survival of more than 10 000 seedlings of woody plant species related to neighborhood and habitat variables in a tropical rainforest in southwest China. By comparing models with and without habitat variables, we tested how habitat filtering affected the detection of CNDD and PNDD. The best-fitting model suggested that CNDD and habitat filtering played key roles in seedling survival, but that, contrary to our expectations, phylogenetic positive density dependence ( PPDD) had a distinct and important effect. While habitat filtering affected the detection of CNDD by decreasing its apparent strength, it did not explain the positive effects of closely related neighbors. Our results demonstrate that a failure to control for habitat variables and phylogenetic relationships may obscure the importance of conspecific and heterospecific neighbor densities for seedling survival. [ABSTRACT FROM AUTHOR]

• The effect of fog on rubber trees (Hevea brasiliensis) was quantified for 3 years. • Visibility, sap flux density and crop water productivity (WP c) were studied. • Under the fog days, rubber trees were physiologically less active. • Carbon assimilation and evapotranspiration were suppressed but WP c enhanced by fog. • Dry seasonal fog can reduce water demand and alleviate drought stress. The rapid conversion of tropical rainforests into monoculture plantations of rubber (Hevea brasiliensis) in Southeast Asia (SEA) necessitates understanding of rubber tree physiology under local climatic conditions. Frequent fog immersion in the montane regions of SEA may affect the water and carbon budgets of the rubber trees and the plantation ecosystems. We studied the effect of fog on various plant physiological parameters in a mature rubber plantation in southwest China over 3 years. During the study period, an average of 141 fog events occurred every year, and the majority occurred during the dry season, when the temperature was relatively low. In addition to the low temperature, fog events were also associated with low vapor pressure deficit, atmospheric water potential, relative humidity and frequent wet-canopy conditions. We divided the dry season into cool dry (November-February) and hot dry (March-April) seasons and classified days into foggy (FG) and non-foggy (non-FG) days. During the FG days of the cool dry season, the physiological activities of the rubber trees were suppressed where carbon assimilation and evapotranspiration showed reductions of 4% and 15%, respectively, compared to the cool dry non-FG days. Importantly, the unequal declines in carbon assimilation and evapotranspiration led to enhanced crop water productivity (WP c) on cool dry FG days but insignificant WP c values were found between FG and non-FG days of the hot dry season. Our results suggest that, by regulating plant physiology, fog events during the cool dry season significantly reduce water demand and alleviate water stress for the trees through improved WP c. [ABSTRACT FROM AUTHOR]

The survival strategy and ecophysiological process of rubber trees (Hevea brasiliensis), in which water-use efficiency (WUE) represents a vital indicator linking the carbon and water cycles, are mediated by phenology and can be severely affected by a low fruit-set rate and cold stress. However, the phenology-related WUE and its temporal variation as well as its responses to site heterogeneity in rubber plantations remain poorly understood. In this study, we used a four-year (2013–2016) continuous dataset of biometric survey, sap flow and phenology of rubber trees to examine the WUE in response to phenological changes and cold stress. The planting density and stand age, factors exhibiting a considerable difference and contributing to site heterogeneity, were explored to reveal their effects on the WUE of rubber plantations in Yunnan Province of Southwest China. Unexpectedly, the response of the WUE to phenological changes was characterized by the peak value (2.73±0.59 g C kg−1 H 2 O) that occurred simultaneously with the fruiting period (P f), and the WUE values during the P f were significantly higher than that of other phenological periods (P <0.01), indicating that the survival strategy adopted by the rubber trees to cope with the low fruit-set rate as well as cold stress during the defoliation period (P d) and adapt to the local climate might be very effective. However, this effectiveness might be reduced to some extent, since the WUE was significantly affected by the planting density (df =2, F -value=11.51) and stand age (df =3, F -value=22.84) indicated by one-way analysis of variance (P <0.01). These findings can provide a deeper understanding for the adaptation strategies to adverse climate and the cultivation and management of rubber plantations. [Display omitted] • The annual life cycle of rubber trees was divided into four phenological periods. • Water-use efficiency (WUE) revealed the survival strategy of rubber trees. • Maximum WUE values occurred simultaneously with the fruiting period (P f). • The planting density and stand age significantly affected the WUE. • Increases in the planting density tended to decrease the WUE of rubber trees. [ABSTRACT FROM AUTHOR]

Aims: The aims were to identify the effects of interactions between litter decomposition and rhizosphere activity on soil respiration and on the temperature sensitivity of soil respiration in a subtropical forest in SW China. Methods: Four treatments were established: control (CK), litter removal (NL), trenching (NR) and trenching together with litter removal (NRNL). Soil CO efflux, soil temperature, and soil water content were measured once a month over two years. Soil respiration was divided into four components: the decomposition of basic soil organic matter (SOM), litter respiration, root respiration, and the interaction effect between litter decomposition and rhizosphere activity. A two-factor regression equation was used to correct the value of soil CO efflux. Results: We found a significant effect of the interaction between litter decomposition and rhizosphere activity ( R) on total soil respiration, and R exhibited significant seasonal variation, accounting for 26 and 31 % of total soil respiration in the dry and rainy seasons, respectively. However, we found no significant interaction effect on the temperature sensitivity of soil respiration. The temperature sensitivity was significantly increased by trenching compared with the control, but was unchanged by litter removal. Conclusions: Though the interaction between litter decomposition and rhizosphere activity had no effects on temperature sensitivity, it had a significant positive effect on soil respiration. Our results not only showed strong influence of rhizosphere activity on temperature sensitivity, but provided a viable way to identify the contribution of SOM to soil respiration, which could help researchers gain insights on the carbon cycle. [ABSTRACT FROM AUTHOR]

Increasingly, ecologists are using functional and phylogenetic approaches to quantify the relative importance of stochastic, abiotic filtering and biotic filtering processes shaping the pattern of species co-occurrence. A remaining challenge in functional and phylogenetic analyses of tropical tree communities is to successfully integrate the functional and phylogenetic structure of tree communities across spatial and size scales and habitats in a single analysis., We analysed the functional and phylogenetic structure of tree assemblages in a 20-ha tropical forest dynamics plot in south-west China. Because the influence of biotic interactions may become more apparent as cohorts age, on local scales, and in resource-rich environments, we perform our analyses across three size classes, six spatial scales and six distinct habitat types, using 10 plant functional traits and a molecular phylogeny for the >400 tree taxa found in the plot., All traits, except leaf area and stem-specific resistance, had significant, albeit weak phylogenetic signal. For canopy species, phylogenetic clustering in small and medium size classes turned to phylogenetic overdispersion in the largest size class and this change in dispersion with size was found in each habitat type and across all spatial scales. On fine spatial scales, functional dispersion changed from clustering to overdispersion with increasing size classes. However, on larger spatial scales assemblages were functionally clustered for all size classes and habitats., Phylogenetic and functional structure across spatial and size scales and habitats gave strong support for a deterministic model of species co-occurrence rather than for a neutral model. The results also support the hypothesis that abiotic determinism is more important at larger scales, while biotic determinism is more important on smaller scales within habitats. [ABSTRACT FROM AUTHOR]

Highlights: [•] Subtropical and tropical forest showed different temporal variations in carbon fluxes. [•] Seasonal variation in NEE of tropical forest was driven by ER. [•] Seasonal variation in NEE of subtropical forest was driven by both GPP and ER. [•] Annual rainfall drives interannual variation in subtropical or tropical forest. [ABSTRACT FROM AUTHOR]

The annual pattern of the carbon-water coupling of rubber trees (Hevea brasiliensis) has never been explored on the plant and ecosystem scales, and the responses of the carbon-water coupling to cold stress and hail events remain unknown. We continuously measured the sap flow density (J s), diameter at breast height (DBH), and the carbon and water flux of rubber plantations in Southwest China from 2012 to 2016, to compare the water-use efficiency (WUE) of rubber plantations by considering different scales (plant and ecosystem) and methods (eddy covariance and isotope techniques) and to reveal the adaptation strategy of rubber trees to cold stress and hail events from the perspective of WUE. We found the followings: (1) high correlation in the annual WUE pattern were noted on the plant and ecosystem scales (r =0.74, P <0.05), with higher WUE values obtained during the rainy seasons than during the dry seasons; (2) The rubber trees responded to cold stress with the reduction of the strength of the carbon-water coupling, which was characterized by a lower ratio of growth rate of DBH (R dbh) to J s during the cold stress period (slope =0.007, P >0.05) compared with after the cold stress (slope =0.03, P <0.05); and (3) The hail event severely disrupted the WUE pattern of rubber trees, which was characterized by a weak relationship between R dbh and J s after experiencing the hail event (slope =0.01, r =0.35, P >0.05) than without experiencing the hail event (slope =0.07, r =0.63, P <0.05). We concluded that the response of rubber trees to the frequent cold stress might involve the long-term adaptation by short-term decoupling of WUE ; however, the effects of the occasional hail event on the growth of rubber trees were more profound. [Display omitted] • Water-use efficiency (WUE) revealed the adaption of rubber trees to cold stress. • The WUE of rubber trees was highly correlated on plant and ecosystem scales. • Rubber trees adapted to cold stress by decoupling the WUE for a short period. • Hail event severely reduced the WUE of rubber trees on the plant scale. • Hail event had greater effects on photosynthesis than transpiration of rubber trees. [ABSTRACT FROM AUTHOR]

This study seeks a further understanding on climate trends in a subtropical mountain forest, SW China. Air ( Ta) and soil temperature ( Ts), both in open land (1983-2010) and under a forest canopy (1986-2010), were investigated. Short-term radiation components were also measured simultaneously both in open land and understory to explore the relationships of microclimatic variables. Correlations of Ta and Ts with sunshine hours ( St) and wind speed ( Ws) were also analysed as driving factors of the temperature trends. The results showed that (1) Understory radiation components were greatly reduced by the forest canopy, showing a strong effect of forest canopy on microclimatic variables. Ts_0 in open land was significantly correlated with solar radiation. Wind speed had significant influences on differences between Ta and Ts_0, between open land Ts_0 and understory Ts_0. The long-term data showed that Ts_0 under forest canopy were closely coupled with Ta in open land. (2) Ta had a larger increase than Ts_0 in open land, and temperature increases in winter were greater than in other seasons. Soil temperature at depths under forest canopy had nearly twice the increases of those on open land; we attributed this to the higher relative increase of Ws over St. (3) A slope change in 1998 was detected in the Ts_0 and Ta difference ( Ts_0 − Ta) series, suggesting different response of Ts_0 and Ta since that year. Deceleration of St and stability of Ws may have been factors. This study improves our understanding of warming in a nature reserve where anthropogenic influences are absent. Further studies are needed for the biological and biochemical implications on subtropical mountain forest. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Epiphytic plants play an important role in the nutrient cycle of forest ecosystems. There had been fewer studies in subtropical regions than in other climate zones. Prior research showed that the canopy epiphyte could fix nitrogen combined with microorganism in tropical forest. The epiphytic plants enwrapping trees in canopy layer are very abundant in the subtropical mountainous cloud forest of Ailao Mountain (central and southern Yunnan Province, SW China). This forest lacks widespread nitrogen-fixing plants, and the nitrogen origin is elusive. Maybe there also exist such nitrogen-fixing systems in epiphyte community. Nitrogen-fixing potentials of canopy epiphytes increased greatly from dry season to wet season. There occurred an obvious difference on the epiphytic nitrogen fixation abilities between upper canopy layer and sub-canopy layer in alternant period between wet season and dry season. Epiphytic nitrogen-fixing potentials for the subtropical moist forest in Ailao Mountains ranged between 0.027 and 2.24 kg ha year. Our results indicate that the canopy epiphytes in the subtropical moist forest of Ailao Mountains can fix a significant amount of atmospheric nitrogen. This finding suggests a new nitrogen source for the subtropical forest ecosystem, thus can have profound impact on the studies of nitrogen cycling. [ABSTRACT FROM AUTHOR]

• Comparing tropical rainforest (TRF) to rubber plantation (RP) emissions, CH 4 sink decreased and CO 2 emission was similar. • Leaf area index and soil water content mainly influences soil CO 2 and CH 4 emissions in RP respectively. • Soil water content, soil temperature, and dead fine roots were the most important factors of soil CO 2 and CH 4 emissions in TRF. • Soil temperature and fine root growth explained the difference in soil CO 2 and CH 4 between TRF and RP. Soil CO 2 (a) and CH 4 (b) fluxes in tropical rainforest (black circles) and rubber plantation (open circles) at Xishuangbanna, Southwest China. [Display omitted] [ABSTRACT FROM AUTHOR]

• In situ 15NO 3 −-N labeling experiment for microbial processes of N 2 O production. • Only 0.027% of 15 NO 3 −-N was lost as N 2 O. • Of the total N 2 O production, 3.1% was from 15N-labeled NO 3 −. • 48.6% of the total N 2 O was from the added NH 4 + and the simulated native N. • H + AN, D and Co-D contributed 97.6%, 1.6% and 0.9% of total N 2 O, respectively. Tropical forests are important contributors for N 2 O emissions from soil to the atmosphere; however, the microbial processes responsible for soil N 2 O production in tropical forest soils remain unclear. In this study, we employed a 40-day-long in situ 15NO 3 − labeling experiment to determine the microbial processes that are responsible for N 2 O production in an undisturbed tropical rainforest in Xishuangbanna, southwest China. We observed that during the experiment, most of the 15N-labeled NO 3 − were immobilized and only 0.027% was emitted as N 2 O. Of the total N 2 O production, 3.1% was from the added 15N-labeled NO 3 −, 48.6% was from the added NH 4 + and native N. Within the first 27 h, nitrification (heterotrophic and autotrophic nitrification) was the predominant microbial process of N 2 O production, accounting for 97.6% of total soil N 2 O emissions, whereas denitrification and co-denitrification resulted in only 1.6% and 0.9% emissions, respectively. To the best of our knowledge, this is the first study to demonstrate that nitrification, rather than denitrification, is the predominant microbial process of N 2 O production in situ in the study area in the tropical forest. [ABSTRACT FROM AUTHOR]

As one of the important factors affecting plant productivity and plant distribution, temperature also affects the physiological and ecological characteristics of plants to a large extent. We report canopy leaf temperature distribution over a 36 m tall primary tropical rain forest and samplings of 28 tree species in SW China by means of two high resolution thermal cameras (P25, Flir systems, Wilsonville, OR, USA). The leaf temperature of dominant tree Species Pometia tomentosa was the highest (31.8 °C), 10.2 °C higher than that of tree species Mezzettipsis creaghii (21.6 °C). The mean leaf to air temperature difference (Tc–Ta) of Pometia tomentosa was the highest (6.4 K), the second highest was Barringtonia pendula (6.1 K), and Mezzettipsis creaghii had the lowest (Tc–Ta) (1.9K). (Tc–Ta) of tree species with smaller leaves and larger stomatal conductance was lowly sensitive to climate factors. Leaf size and stomatal conductance together decided the effect of climate change to (Tc–Ta) of the different tree species. We have shown that the composition of tree species in tropical rain forest areas is important to the climate through our research. [ABSTRACT FROM AUTHOR]

• α GPP , integrated index of GPP seasonality, was ratio of mean to maximum daily GPP. • α GPP in Northern Hemisphere ranged from 0.47 to 0.85 with an average of 0.62. • α GPP decreased as latitude increased, and was stable between years. • Spatial pattern of astronomical radiation seasonality affected that of α GPP. • Spatiotemporal variation of α GPP provided a new approach to assess annual GPP. The seasonal dynamic of gross primary productivity (GPP) has influences on the annual GPP (AGPP) of the terrestrial ecosystem. However, the spatiotemporal variation of the seasonal dynamic of GPP and its effects on spatial and temporal variations of AGPP are still poorly addressed. In this study, we developed a parameter, α GPP , defined as the ratio of mean daily GPP (GPP mean) to the maximum daily GPP (GPP max) during the growing season, to analyze the seasonal dynamic of GPP based on Weibull function. The α GPP was a comprehensive parameter characterizing the shape, scale, and location of the seasonal dynamic curve of GPP. We calculated α GPP based on the data of GPP for 942 site-years from 115 flux sites in the Northern Hemisphere, and analyzed the spatiotemporal variation and influencing factors of the α GPP. We found that the α GPP of terrestrial ecosystems in the Northern Hemisphere ranged from 0.47 to 0.85, with an average of 0.62 ± 0.06. The α GPP varied significantly both among different climatic zones and different ecosystem types. The α GPP was stable on the interannual scale, while decreased as latitude increased, which was consistent across different ecosystem types. The spatial pattern of the seasonal dynamic of astronomical radiation was the dominating factor of the spatial pattern of α GPP , that was, the spatial pattern of the seasonal dynamic of astronomical radiation determined that of the seasonal dynamic of GPP by controlling that of seasonal dynamics of total radiation and temperature. In addition, we assessed the spatial variation of AGPP preliminarily based on α GPP and other seasonal dynamic parameters of GPP, indicating that the understanding of the spatiotemporal variation of α GPP could provide a new approach for studying the spatial and temporal variations of AGPP and estimating AGPP based on the seasonal dynamic of GPP. [ABSTRACT FROM AUTHOR]

Climate models predict that droughts will increase in Southeast Asia, yet little is known about how soil respiration (Rs) and its components heterotrophic respiration (Rh) and autotrophic respiration (Ra) will change following drought years. To clarify this issue and to detect underlying mechanisms, we conducted a 2-year field experiment in the seventh and eighth year of long-term artificially droughted plots within a tropical rainforest in Xishuangbanna, southwest China. We separated Rh and Ra by trenching and we measured dissolved organic carbon in the soil and microbial biomass. In average, the drought stress, reduced through-fall by 50%, reduced fine root biomass by 36%. Although Ra declined by 35%, active inorganic N and Rh increased by 31% and 29%. Further, the coefficient of determination (R2) between soil microbial community composition, mainly, group-specific phospholipid fatty acid and the variation of Rh was among 17%–59% during the dry and rainy season of 2018. However, changes in dissolved organic carbon, microbial biomass carbon and nitrogen, ammonium nitrogen, were inconsistent with the increase in Rh. There was an inconsistent significant positive correlation between seasonal change of CO 2 flux and these processes dynamic across 2017 and 2018. Soil temperature, soil moisture, and litterfall jointly determined annual variation in Rs and Rh across two years. These findings have improved understanding of how long-term drought stress influences soil CO2 effluxes via change of soil-based biochemical indicators (like fine root, active inorganic nitrogen, and soil microorganisms). • Through-fall reduction decreased fine root and autotrophic respiration. • Nitrate-N and heterotrophic respiration increased under through-fall reduction. • Effect of through-fall reduction on soil respiration decreased after 6-year. • PLFAs correlated with heterotrophic respiration. [ABSTRACT FROM AUTHOR]

Savanna ecosystems play a crucial role in the global carbon cycle. However, there is a gap in our understanding of carbon fluxes in the savanna ecosystems of Southeast Asia. In this study, the eddy covariance technique (EC) and the biometric-based method (BM) were used to determine carbon exchange in a savanna ecosystem in Southwest China. The BM-based net ecosystem production (NEP) was 0.96 tC ha−1 yr−1. The EC-based estimates of the average annual gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem carbon exchange (NEE) were 6.84, 5.54, and −1.30 tC ha−1 yr−1, respectively, from May 2013 to December 2015, indicating that this savanna ecosystem acted as an appreciable carbon sink. The ecosystem was more efficient during the wet season than the dry season, so that it represented a small carbon sink of 0.16 tC ha−1 yr−1 in the dry season and a considerable carbon sink of 1.14 tC ha−1 yr−1 in the wet season. However, it is noteworthy that the carbon sink capacity may decline in the future under rising temperatures and decreasing rainfall. Consequently, further studies should assess how environmental factors and climate change will influence carbon-water fluxes. [ABSTRACT FROM AUTHOR]

Conspecific negative density‐dependence (CNDD) has been recognized as a key mechanism underlying species coexistence, especially in tropical forests. Recently, some studies have reported that seedling survival is also negatively correlated with the phylogenetic relatedness between neighbors and focal individuals ‐ termed phylogenetic negative density‐dependence (PNDD). In contrast to CNDD or PNDD, shared habitat requirements between closely related individuals are thought to be a cause of observed positive effects of closely related neighbors, which may affect the strength and detectability of CNDD or PNDD. In order to investigate the relative importance of these mechanisms for tropical tree seedling survival, we used generalized linear mixed models to analyze how the survival of more than 10,000 seedlings of woody plant species related to neighborhood and habitat variables in a tropical rainforest in southwest China. By comparing models with and without habitat variables, we tested how habitat filtering affected the detection of CNDD and PNDD. The best‐fitting model suggested that CNDD and habitat filtering played key roles in seedling survival, but that, contrary to our expectations, phylogenetic positive density‐dependence (PPDD) had a distinct and important effect. While habitat filtering affected the detection of CNDD by decreasing its apparent strength, it did not explain the positive effects of closely‐related neighbors. Our results demonstrate that a failure to control for habitat variables and phylogenetic relationships may obscure the importance of conspecific and heterospecific neighbor densities for seedling survival.This article is protected by copyright. All rights reserved. [ABSTRACT FROM AUTHOR]

The relative degree to which stochastic and deterministic processes underpin community assembly is a central problem in ecology. Quantifying local-scale phylogenetic and functional beta diversity may shed new light on this problem. We used species distribution, soil, trait and phylogenetic data to quantify whether environmental distance, geographic distance or their combination are the strongest predictors of phylogenetic and functional beta diversity on local scales in a 20-ha tropical seasonal rainforest dynamics plot in southwest China. The patterns of phylogenetic and functional beta diversity were generally consistent. The phylogenetic and functional dissimilarity between subplots (10 × 10 m, 20 × 20 m, 50 × 50 m and 100 × 100 m) was often higher than that expected by chance. The turnover of lineages and species function within habitats was generally slower than that across habitats. Partitioning the variation in phylogenetic and functional beta diversity showed that environmental distance was generally a better predictor of beta diversity than geographic distance thereby lending relatively more support for deterministic environmental filtering over stochastic processes. Overall, our results highlight that deterministic processes play a stronger role than stochastic processes in structuring community composition in this diverse assemblage of tropical trees. [ABSTRACT FROM AUTHOR]