Search

Annual gross primary productivity (AGPP) of terrestrial ecosystems is the largest carbon flux component in ecosystems; however, it's unclear whether photosynthetic capacity or phenology dominates interannual variation of AGPP, and a better understanding of this could contribute to estimation of carbon sinks and their interactions with climate change. In this study, observed GPP data of 494 site-years from 39 eddy covariance sites in Northern Hemisphere were used to investigate mechanisms of interannual variation of AGPP. This study first decomposed AGPP into three seasonal dynamic attribute parameters (growing season length (CUP), maximum daily GPP (GPP max ), and the ratio of mean daily GPP to GPP max (α GPP )), and then decomposed AGPP into mean leaf area index (LAI m ) and annual photosynthetic capacity per leaf area (AGPP lm ). Furthermore, GPP max was decomposed into leaf area index of DOY max (the day when GPP max appeared) (LAI max ) and photosynthesis per leaf area of DOY max (GPP lmax ). Relative contributions of parameters to AGPP and GPP max were then calculated. Finally, environmental variables of DOY max were extracted to analyze factors influencing interannual variation of GPP lmax . Trends of AGPP in 39 ecosystems varied from -65.23 to 53.05 g C m -2 yr -2 , with the mean value of 6.32 g C m -2 yr -2 . Photosynthetic capacity (GPP max and AGPP lm ), not CUP or LAI, was the main factor dominating interannual variation of AGPP. GPP lmax determined the interannual variation of GPP max , and temperature, water, and radiation conditions of DOY max affected the interannual variation of GPP lmax . This study used the cascade relationship of "environmental variables-GPP lmax -GPP max -AGPP" to explain the mechanism of interannual variation of AGPP, which can provide new ideas for the AGPP estimation based on seasonal dynamic of GPP., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

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 CO 2 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., (© 2021 John Wiley & Sons Ltd.)

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 (R eco ), 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., Competing Interests: The authors declare no competing financial interests.

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.

Nitrogen (N) cycle is very important for forest ecosystem, and soil N mineralization rate is often used as the index of soil N availability and its losses. Using Close-Top incubation method, we measured the N mineralization rate in soils (0-15 cm) of monsoon evergreen broadleaf forest, seasonal rain forest, rubber plantation, heavily disturbed seasonal rain forest, Millettia laptobotrya secondary forest and upland rice field. The results showed that the net N mineralization rate and N nitrification rate were decreased in the sequence of heavily disturbed seasonal rain forest > Millettia laptobotrya secondary forest > seasonal rain forest > monsoon evergreen broadleaf forest > rubber plantation > upland rice field. Rubber plantation and upland rice field were the most seriously disturbed land use patterns. Their soil N storage and mineralization rate were very low and exhibited significant variations, compared with other land use patterns. Lower net N mineralization rate correlated with lower N storage, and also, with lower fungi numbers. The N mineralization rate in disturbed forest soil which had recovered for several years was similar to that in primary forest soil, but decreased significantly when the forests were converted to agricultural land.