Topography and vegetation structure mediate drought impacts on the understory of the South American Atlantic Forest

Response of vegetation dynamic change to multi-scale drought stress in the high-latitude Nenjiang River basin in China

Vegetation is an integral part of terrestrial ecosystem and plays an important role in responding to climate change, with its dynamic characteristics reflecting the ecological environmental quality. Recently, the continually increasing frequency and intensity of droughts has greatly changed how vegetation growth and development respond to drought. In this study, using normalized difference vegetation index and standardized precipitation evapotranspiration index (SPEI), we studied the response characteristics of vegetation dynamics to multi-scale drought stress (SPEI-1, SPEI-3, and SPEI-12) in the Nenjiang River basin (NRB) via Pearson correlation analysis, along with further exploration of the vegetation stability under drought. The results showed that the same period effect of drought on vegetation growth in NRB mainly occurs during the early and middle stages of vegetation growth. Furthermore, the proportion of significant positive correlation between them is 15.3%–43.3%, mainly in the central and southern parts of the basin. The lagged period effect of drought on vegetation growth mainly occurred during autumn in the southeast and middle of the basin, with a significant positive correlation of 20.8%. Under drought stress, the forest vegetation stability in NRB was the highest, with the resilience of wetland and grassland vegetation being the best and worst, respectively. Our study results will not only deepen our understanding of the dynamic vegetation changes in the high-latitude semi-arid basin under global climate change, but also provide a scientific basis for the management and water resources allocation of “agriculture-wetland-forest” complex ecosystem in the future.

Regional and local determinants of drought resilience in tropical forests

The increase in severity of droughts associated with greater mortality and reduced vegetation growth is one of the main threats to tropical forests. Drought resilience of tropical forests is affected by multiple biotic and abiotic factors varying at different scales. Identifying those factors can help understanding the resilience to ongoing and future climate change. Altitude leads to high climate variation and to different forest formations, principally moist or dry tropical forests with contrasted vegetation structure. Each tropical forest can show distinct responses to droughts. Locally, topography is also a key factor controlling biotic and abiotic factors related to drought resilience in each forest type. Here, we show that topography has key roles controlling biotic and abiotic factors in each forest type. The most important abiotic factors are soil nutrients, water availability, and microclimate. The most important biotic factors are leaf economic and hydraulic plant traits, and vegetation structure. Both dry tropical forests and ridges (steeper and drier habitats) are more sensitive to droughts than moist tropical forest and valleys (flatter and wetter habitats). The higher mortality in ridges suggests that conservative traits are not sufficient to protect plants from drought in drier steeper habitats. Our synthesis highlights that altitude and topography gradients are essential to understand mechanisms of tropical forest's resilience to future drought events. We described important factors related to drought resilience, however, many important knowledge gaps remain. Filling those gaps will help improve future practices and studies about mitigation capacity, conservation, and restoration of tropical ecosystems.

Spatio-temporal pattern of ecological droughts and their impacts on health of vegetation in Northwestern China

The ecological implications of drought have been widely discussed in recent years. Ecological drought was thus proposed as a new drought type to describe the impact of drought on ecosystems. The current study used an innovative drought index, called the standardized ecological water deficit index (SEWDI), to monitor terrestrial ecological drought in Northwestern China, which is an ecologically fragile region. Droughts and their characteristics, including drought affected area, drought severity, drought duration, drought frequency, and drought orientation, were extracted using a spatial and temporal identification method based on SEWDI at a three-month timescale. To investigate the variation in dominant factors determining vegetation health, the contributions of moisture and thermal conditions during different ecological drought events were determined using a gradient boosting regression model. The main results indicated that (1) the spatial and temporal identification method successfully identified the spatio-temporal patterns of ecological drought; (2) a total of 184 ecological drought events during 1982-2020 were identified, of which 56.6% occurred prior to the 21st century. Drought events in the 21st century always exhibit larger affected areas, longer durations, a higher frequency, and greater severity, and migrated westward; and (3) in all ecological drought events, vegetation health dominated by thermal conditions accounted for 42.7% and 48.2% before and during the 21st century, respectively. This illustrated that vegetation has experienced more severe thermal stress during the 21st century.