Boundary of savanna and forest in Africa.
Newswise — For many years, ecological studies have contended that climate's impact on the distribution of tropical forests and savannas is insignificant. Nevertheless, Prof. Dr. Steven Higgins and his international research team from the University of Bayreuth have recently managed to demonstrate that the presence of forests or savannas in African regions is primarily determined by climatic factors. The findings of their study, published in the journal "Science," thereby validate the pivotal influence of climate on the development of worldwide vegetation patterns.
The study emerged through a tight collaboration between plant ecologists from Bayreuth and research partners in Norway and South Africa. "Our examinations of forest and savanna distribution in Africa reveal the predominant impact of climate on vegetation. These findings further reinforce the notion that scientifically sound climate change predictions serve as a reliable foundation for accurately assessing forthcoming shifts in ecosystems and plant life, not only in Africa but also in other global regions," explains Prof. Dr. Steven Higgins, the first author and Chair of Plant Ecology at the University of Bayreuth. He further emphasizes, "By combining climate research and plant ecology, we can collectively contribute to the formulation of suitable strategies in nature conservation, environmental protection, agriculture, and forestry to adapt to the anticipated changes in the botanical realm."
Simultaneously, the study's authors caution against measures that underestimate the impact of climate on vegetation. For instance, ongoing efforts are being made to reforest African savanna regions to enhance the sequestration of CO₂ by forested areas. However, the new research contradicts the underlying assumption of these climate change mitigation plans – that forests and savannas can thrive under the same climatic conditions. "The belief that converting African savannas into forests on a large scale can augment the Earth's natural carbon storage is misguided and potentially perilous. Such afforestation endeavors would likely yield little success, as the climate in savanna regions is unsuitable for forests and would harm the biodiversity of savanna ecosystems," warns Dr. Timo Conradi, a co-author of the study from the Plant Ecology research group.
Climate dominance: A novel model refutes the paradigm of alternative ecosystem states
The perspective challenging the notion that forests and savannas in Africa are determined by specific climatic factors aligns with a paradigm that has gained increasing recognition in ecological research. This paradigm is based on the hypothesis of "alternative ecosystem states" (AES) existing in numerous regions worldwide. According to this hypothesis, the presence of AES is not primarily or solely attributable to climate but rather influenced by internal feedback processes within ecosystems. These processes override and diminish the impact of climatic factors. As supporting evidence for the AES hypothesis, researchers point to the observation that forests and savannas coexist in climatically similar regions, suggesting that vegetation is not solely determined by climate.
The study published in "Science" aimed to challenge and refute the aforementioned paradigm. The researchers employed a well-established model of plant growth and integrated it with extensive climate data and information regarding the regional distribution of plant species in Africa. They introduced a novel methodology called the "phytoclimatic transformation" that elucidates how various climatic factors either facilitate or impede the growth of different plant species. This method enables the inference of which plant species are likely to thrive under any combination of temperature, soil moisture, solar radiation, and atmospheric CO₂ concentration. By applying this approach, the researchers categorized a total of 678 sites in Africa based on whether they were more conducive to the development of forests or savannas. Remarkably, 89 percent of the predictions derived from this methodology aligned with the actual vegetation states observed in those locations.
The researchers were able to establish that discrepancies between predicted and observed vegetation states did not necessarily indicate the presence of alternative ecosystem states. Instead, they found that such discrepancies were largely attributable to abiotic environmental factors. These factors primarily encompassed topographic characteristics such as the configuration of the landscape through mountains and valleys, as well as soil composition and microclimatic influences stemming from features like waterfalls or river floodplains. These findings highlight the significant influence of these abiotic factors in shaping the distribution of forests and savannas, thereby challenging the notion of alternative ecosystem states as the primary driver.
Apologies for the confusion. The authors of the study do not fundamentally challenge the existence of alternative ecosystem states (AES). They acknowledge the theoretical validity of the AES concept and recognize its potential for stimulating further research inquiries. They also acknowledge that some compelling examples supporting the existence of AES can be found in the scientific literature. However, the authors note that empirically demonstrating the existence of AES in practice poses significant challenges. They assert that while they believe alternative ecosystem states exist, further investigation is needed to ascertain their prevalence and gain a better understanding of the specific conditions that facilitate their occurrence. Prof. Dr. Steven Higgins emphasizes the importance of ongoing research to explore the frequency and underlying factors that contribute to the existence of AES.
Research funding:
The collaboration between the Chair of Plant Ecology at the University of Bayreuth and the research partners in South Africa was funded by the German Federal Ministry of Education and Research (BMBF).
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