Limited climatic space for alternative ecosystem states in Africa

Abstract

One of the foundational premises of ecology is that climate determines ecosystems. This has been challenged by alternative ecosystem state models, which illustrate that internal ecosystem dynamics acting on the initial ecosystem state can overwhelm the influence of climate, and by observations suggesting that climate cannot reliably discriminate forest and savanna ecosystem types. Using a novel phytoclimatic transform, which estimates the ability of climate to support different types of plants, we show that climatic suitability for evergreen trees and C4 grasses are sufficient to discriminate between forest and savanna in Africa. Our findings reassert the dominant influence of climate on ecosystems and suggest that the role of feedbacks causing alternative ecosystem states is less prevalent than has been suggested.

Climate, fire, and anthropogenic disturbance determine the current global distribution of tropical forest and savanna

“…In interpreting model output of forest probability, a probability range of 40%-60% was interpreted as indicative of a zone where forest presence was indeterminant, and it should be noted that this interpretation is somewhat arbitrary, and wider bounds would result in a conclusion of greater area of indeterminant vegetation. However, our zone of indeterminacy does concur with recent work identifying a limited zone of ASS in Africa based on climatic suitability for various plant growth forms (Higgins et al 2023). Furthermore, a wide cover range definition of bistability would undermine the concept by artificially emphasizing the continuous cover gradient between the distinct forest and savanna biomes.…”

Reassessment of the risks of climate change for terrestrial ecosystems

“…While in principle any suitability or species distribution model could be used in the workflow, in practice, many existing alternative models may be inappropriate for this task. First, correlative suitability models have limited transferability 33 , 44 , producing implausible growth-form suitability surfaces 24 , whereas the process-based TTR-SDM used here was successful in model transferability tests 33 and produces plausible suitability surfaces 24 , 30 . Second, alternative models do not consider how elevated atmospheric CO 2 concentrations may influence photosynthesis and thereby species ranges.…”

“…The premise of this analysis is that we can characterize changes in the climatic forcing of ecosystems by analysing shifts in climatic suitability for the growth forms that define ecosystems. Previous work showed that the growth-form suitability values are useful predictors of ecosystem states 30 , suggesting that shifts in growth-form suitability may be useful indicators of the changing climatic forcing. It should be emphasized that the growth-form suitability may be quite different from growth-form abundances because other ecological processes such as biotic interactions, dispersal or disturbances, determine whether physiological suitability can predict abundance.…”

The need for mechanistic explanations in (seed) ecology

“…Species distributions are also typically explained at the macroclimate scale only, despite the existence of alternative drivers (Schwilk & Keeley, 2012;Pausas & Bond, 2021). A recent example is the prediction of African vegetation using a plant growth model applied to species distribution and variables (Higgins et al, 2023). While the model may correctly predict some current vegetation, it fails to incorporate the most important ecological mechanisms that shaped African vegetation, such as large herbivores and frequent fires (Bond, 2019;Owen-Smith, 2021).…”