Mean annual precipitation predicts primary production resistance and resilience to extreme drought

Rapid recovery of ecosystem function following extreme drought in a South African savanna grassland

Climatic extremes, such as severe drought, are expected to increase in frequency and magnitude with climate change. Thus, identifying mechanisms of resilience is critical to predicting the vulnerability of ecosystems. An exceptional drought (<first percentile) impacted much of southern Africa during the 2015 and 2016 growing seasons, including the site of a long-term fire experiment in Kruger National Park, South Africa. Prior to the drought, experimental fire frequencies (annual, triennial, and unburned) created savanna grassland plant communities that differed in composition and function, providing a unique opportunity to assess ecosystem resilience mechanisms under different fire regimes. Surprisingly, aboveground net primary productivity (ANPP) recovered fully in all fire frequencies the year after this exceptional drought. In burned sites, resilience was due mostly to annual forb ANPP compensating for reduced grass ANPP. In unburned sites, resilience of total and grass ANPP was due to subdominant annual and perennial grass species facilitating recovery in ANPP after mortality of other common grasses. This was possible because of high evenness among grass species in unburned sites predrought. These findings highlight the importance of both functional diversity and within-functional group evenness as mechanisms of ecosystem resilience to extreme drought.

Low growth resilience to drought is related to future mortality risk in trees

Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality.

Quantifying security and resilience of Chinese coastal urban ecosystems

The emerging threat to the coastal urban ecosystems from increased intensity and frequency of weather events is a compelling reason for improving our understanding of the integrity of the existing ecosystem. Resilience of an ecosystem is a critical property that aids recovery and adaptation when subject to intense stress. Quantifying the resilience of an ecological system requires a detailed understanding of the vulnerabilities and interactions within a complex web of interconnected social, technological and economic networks. Through an ecological network analysis of ascendency and redundancy of the flux of energy and material flows, the causal relationships are established through structural equations modeling (SEM) techniques. A model based-on the five factors of driving force (D), pressure (P), state (S), impact (I), and response (R) (DPSIR), recognizes the different roles these factors play in the coastal urban ecological security system of China. Energy and material flows transmission equations of the ecological security network are developed to evaluate the resilience of the ecological security network. The results show that the ecological security network of Chinese coastal cities has a relatively high network occupation rate (A/C = 0.6898), indicating a relatively mature state of the ecological security network of coastal cities with sufficient metabolic capacity and steady status. The low vacancy rate (R/C = 0.3102) shows that the coastal ecological security network lacks flexibility of surplus space. The energy and material flows conversion and dissipation ability in the network are strong: the five factors of DPSIR are highly interdependent, and the ecological security network framework is both steady and mature. However, the resilience of the coastal urban ecosystem against external impacts is weak. It is critical for coastal cities to broaden their planning protocols to introduce more flexible space to increase resilience and guarantee a robust pathway for sustainable development. This study contributes to a rational method for testing the internal causal relationships among DPSIR linkages toward quantifying our understanding of the resilience of a security ecosystem.

Community Response to Extreme Drought (CRED): a framework for drought-induced shifts in plant-plant interactions

Contents Summary 52 I. Introduction 52 II. The Community Response to Extreme Drought (CRED) framework 55 III. Post-drought rewetting rates: system and community recovery 61 IV. Site-specific characteristics influencing community resistance and resilience 63 V. Conclusions 64 Acknowledgements 65 References 66 SUMMARY: As climate changes, many regions of the world are projected to experience more intense droughts, which can drive changes in plant community composition through a variety of mechanisms. During drought, community composition can respond directly to resource limitation, but biotic interactions modify the availability of these resources. Here, we develop the Community Response to Extreme Drought framework (CRED), which organizes the temporal progression of mechanisms and plant-plant interactions that may lead to community changes during and after a drought. The CRED framework applies some principles of the stress gradient hypothesis (SGH), which proposes that the balance between competition and facilitation changes with increasing stress. The CRED framework suggests that net biotic interactions (NBI), the relative frequency and intensity of facilitative (+) and competitive (-) interactions between plants, will change temporally, becoming more positive under increasing drought stress and more negative as drought stress decreases. Furthermore, we suggest that rewetting rates affect the rate of resource amelioration, specifically water and nitrogen, altering productivity responses and the intensity and importance of NBI, all of which will influence drought-induced compositional changes. System-specific variables and the intensity of drought influence the strength of these interactions, and ultimately the system's resistance and resilience to drought.