Understanding turning points in dryland ecosystem functioning

Covering ca. 41% of Earth's surface, responsible for 40% of the global net primary productivity, and supporting more than one-third of human livelihoods, studying dryland ecosystems and changes in them is unquestionably important. Considering that these regions foster many livelihoods that are vulnerable due to poverty and food insecurity (issues that should be exacerbated by climate change), as well as highly biodiverse areas, understanding changes in dryland ecosystems and their drivers becomes even more important. Changes in dryland ecosystem functioning can take place gradually or abruptly (the so-called turning points, or TP). Although both can be prejudicial to the ecosystems and to the livelihoods that depend on them, abrupt ones can be more harmful, since their impact is drastic and can be felt promptly. Thus, better understanding where and how TPs occur, what causes them, and whether or not they can be predicted could support nature conservation and food security in drylands. Although abrupt changes were widely studied in several ecosystems, a global scale detection and characterization of TPs in dryland ecosystem functioning was never performed. Moreover, much is still unknown about the long-term drivers of abrupt change in drylands. Remote sensing data, acquired frequently and consistently worldwide, allow for such analyses. This dissertation aimed at expanding our knowledge on how dryland ecosystem functioning changes through time and the major drivers of these changes. More specifically, we here aimed at (i) detecting where and when TPs in ecosystem functioning occurred, and characterizing the detected TPs, in global dryland regions; (ii) studying the relative contribution of edaphic characteristics, anthropogenic pressure, and historic vegetation conditions in shaping an abrupt change (i.e., woody plants die-off) in the Senegalese drylands; (iii) disentangling climate and anthropogenic activity as drivers of vegetation productivity by quantifying the anthropogenic footprint over drylands and other water-limited ecosystems; and (iv) evaluating the performance of early warning indicators of abrupt changes in dryland ecosystems. A first logical step for better understanding TPs in dryland ecosystem functioning is detecting where and when they mostly occurred, and characterizing them based on the direction and rate of change before/after the TP. This was done here by using time series (1982-2015) of vegetation productivity and precipitation data in global drylands. A segmented trend analysis was used to detect and characterize the TPs. Hotspot regions with high TP occurrence were detected in North America, the Sahel, Central Asia, and Australia. Whereas in North America most of the TPs were characterized by a decreasing trend in ecosystem functioning, for the other three regions a positive reversal (i.e., a negative trend becomes a positive trend after the TP) was the prevailing TP type. High anthropogenic pressure coincided with TP occurrence in Asia and Australia, while most of the TPs in the Sahel occurred in areas with a high drought frequency. Next, we focused on better understanding the drivers of abrupt changes and ecosystem productivity in drylands. First, we used a drought-induced woody plants die-off case that occurred in Senegal in 2014-2015 to study the relative contribution of human pressure, edaphic characteristics, and biomass conditions prior to the drought in the severity of the die-off and in the post-disturbance recovery. Although anthropogenic pressure did not present a significant impact over the severity and recovery, soil physical characteristics showed to be important for both. Moreover, areas presenting a rapid accumulation of woody biomass before the drought were highly impacted, presenting higher mortality rates. This might be due to a change in species composition and to the depletion of water stored in the soils caused by increased evapotranspiration. Second, the anthropogenic footprint over global water-limited environments was assessed. By comparing vegetation productivity and precipitation trends, we detected areas presenting a decoupled response of vegetation to climate (i.e., over or under-productive given the rainfall conditions). Recent trends, between 1982 and 2015, suggest that there was a decrease in over-productive areas due to human activity, while the under-productive areas increased. This scenario is aggravated inside biodiversity hotspots. Finally, the predictability of TPs was evaluated in the Sahel. We demonstrated the usefulness of using changes in temporal autocorrelation, an indicator of an ecosystem's resilience, as an early warning indicator of abrupt changes. Moreover, we showed that human activity might hamper the use of such an early warning signal. With this dissertation, we contributed to the comprehensive understanding of abrupt changes in dryland ecosystems. We here provide information on changes and the current state of ecosystem functioning of global drylands. Moreover, information on the drivers of change and on how susceptibility to future TPs can be estimated were also provided. Such knowledge can support decision making and a correct allocation of resources, e.g., by supporting policymakers in the decision of whether or not intervention is needed, by helping in determining the necessary measures to be implemented, and by supporting in the delimitation of priority areas for conservation and/or management.

Jaar van publicatie:2021

Toegankelijkheid:Closed