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Highlights • The peak of GPP lagged behind the peak in the colour indices. • GPP was strongly correlated with S green in the leaf expansion period in the rubber plantation. • Model using colour indices underestimated GPP. • FPAR was a better parameter than colour indices for modelling GPP. Abstract The widespread increase in the number of digital cameras mounted on flux towers provides an opportunity to better understand the relationship between the seasonality of canopy photosynthesis and canopy phenology. The challenge is due to fewer variations in rubber defoliation of rubber canopy. We examined the relationship between colour indices calculated from digital camera images and gross primary production (GPP) obtained from daily flux tower observations of carbon dioxide over two years in a rubber plantation and used these colour indices to model GPP. According to the results, (1) the strength of green (S green), Hue and GPP exhibited clear seasonal patterns in the rubber plantation, and the relationship between the camera-based indices and GPP appeared to have distinct characteristics at different phenological stages; (2) the peak GPP from the flux tower measurements lagged behind the peak in the colour indices calculated from digital camera imagery; (3) GPP was strongly correlated with S green derived from camera imagery in the rubber plantation, especially in the leaf expansion period; and (4) the GPP simulated by colour indices (S green and Hue) was underestimated, and the fraction of absorbed photosynthetically active radiation (FPAR) was the best parameter for modelling GPP in normal years. Our results indicate that colour indices calculated from digital camera images can be used to model GPP in rubber plantations and to monitor biotic and abiotic stress events. Future research should measure the pigment contents of canopy leaves to precisely quantify the relationship between colour indices and GPP. [ABSTRACT FROM AUTHOR]

Highlights • Precipitation exclusion induced a reduction of NPP in savanna. • Precipitation exclusion decreased NPP of perennials but elevated for the annuals. • NPP of savannas exhibited significant inter-annual variation. • Herb community had more resilience to precipitation exclusion than woody community in savanna. Abstract Declines in precipitation are expected to affect plant performance and ecosystem carbon uptake. The response of ecosystem productivity to declines in precipitation and potential underlying mechanisms have been well studied in many biomes; however, little is known about the role of declines in precipitation and the involved mechanisms in savanna ecosystems. In a 4-year field precipitation manipulation experiment, we simulated four levels of precipitation exclusion (control, 30%, 50% and 70%) to assess the effects of declines in precipitation on net primary productivity (NPP) in a savanna ecosystem in southwestern China. NPP was strongly correlated with soil water content during the experimental period. Precipitation exclusion significantly decreased the NPP of the entire vegetation including trees, shrubs, perennials and litterfall but significantly increased the NPP of annuals. Our results suggested that precipitation exclusion can reduce the productivity of savannas and that plant functional types differ in sensitivity to precipitation exclusion. These findings imply that future declines in precipitation in savanna regions may negatively impact carbon accumulation and may induce shifts in plant functional types to buffer the effects of declines in precipitation on productivity and stabilize ecosystem function in savannas. [ABSTRACT FROM AUTHOR]

At the beginning of 2015, a primary subtropical evergreen broadleaved forest in Southwest China experienced an extreme snow anomaly. We used a thermodynamic approach to evaluate the self-organization of the forest in response to snow disturbance. We found that the snow disturbance induced severe vegetation damage, as indicated by LAI significantly decreased by 33.19% and 40.85% in 2015 than the pre-disturbance years (2013–2014), respectively. The forest had the higher self-organization in 2015 with the higher ability of capture exergy (Rn/DR) and dissipation exergy (TRNc). The changes in vegetation patterns of the primary subtropical evergreen forest enhanced the ecosystem self-organization. Our finding was inconsistent with the general theory that the disturbance of natural systems reduces exergy capture ability and increases exergy dissipation. • We studied the self-organisation in snow disturbed subtropical forest. • The higher self-organisation occurred in 2015 than the pre-disturbance years. • The higher ability to capture and dissipation energy occurred during snow year. • The changes in vegetation patterns are the main reason for higher self-organization. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

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]

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]

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]

• 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]

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]