Climate change will cause non-analog vegetation states in Africa and commit vegetation to long-term change

Abstract

Abstract. Vegetation responses to changes in environmental drivers can be subject to temporal lags. This implies that vegetation is committed to future changes once environmental drivers stabilize; e.g., changes in physiological processes, structural changes, and changes in vegetation composition and disturbance regimes may happen with substantial delay after a change in forcing has occurred. Understanding the trajectories of such committed changes is important as they affect future carbon storage, vegetation structure, and community composition and therefore need consideration in conservation management. In this study, we investigate whether transient vegetation states can be represented by a time-shifted trajectory of equilibrium vegetation states or whether they are vegetation states without analog in conceivable equilibrium states. We use a dynamic vegetation model, the aDGVM (adaptive Dynamic Global Vegetation Model), to assess deviations between simulated transient and equilibrium vegetation states in Africa between 1970 and 2099 for the RCP4.5 and 8.5 scenarios using regionally downscaled climatology based on the MPI-ESM output for CMIP5. We determined lag times and dissimilarity between simulated equilibrium and transient vegetation states based on the combined difference of nine selected state variables using Euclidean distance as a measure for that difference. We found that transient vegetation states over time increasingly deviated from equilibrium states in both RCP scenarios but that the deviation was more pronounced in RCP8.5 during the second half of the 21st century. Trajectories of transient vegetation change did not follow a “virtual trajectory” of equilibrium states but represented non-analog composite states resulting from multiple lags with respect to vegetation processes and composition. Lag times between transient and most similar equilibrium vegetation states increased over time and were most pronounced in savanna and woodland areas, where disequilibrium in savanna tree cover frequently acted as the main driver of dissimilarities. Fire additionally enhanced lag times and dissimilarity between transient and equilibrium vegetation states due to its restraining effect on vegetation succession. Long lag times can be indicative of high rates of change in environmental drivers, of meta-stability and non-analog vegetation states, and of augmented risk for future tipping points. For long-term planning, conservation managers should therefore strongly focus on areas where such long lag times and high residual dissimilarity between most similar transient and equilibrium vegetation states have been simulated. Particularly in such areas, conservation efforts need to consider that observed vegetation may continue to change substantially after stabilization of external environmental drivers.

Representation of global change drivers across biodiversity datasets

“…For example, we are currently lacking biodiversity data from places with high magnitudes of climate change including Arctic and boreal forest regions, as well as tropical regions that are currently entering non-analog climate spac 90 . These data are important not just for understanding current effects of climate change, but also as sentinels of future change around the world [90][91][92] . Another underrepresented part of the global change spectrum is relatively intact sites with low human impact (Figures 1-2), which provide a necessary comparator for testing the impacts of human use, pollution and other global change drivers.…”

Woody plant encroachment drives population declines in 20% of common open ecosystem bird species

“…woody plant encroachment) impacts species' populations. This insight is urgently needed as high rates of encroachment are occurring in these systems and are projected to continue (Kumar et al, 2021;Pfeiffer et al, 2020).…”

“…woody plant encroachment) impacts species' populations. This insight is urgently needed as high rates of encroachment are occurring in these systems and are projected to continue (Kumar et al., 2021; Pfeiffer et al., 2020). Additionally, many open grassy ecosystems are mistakenly targeted for afforestation and tree ‘restoration’ suggesting that these ecosystems have the potential to become significantly woodier at large extents (Bond et al., 2019).…”