Human impacts in African savannas are mediated by plant functional traits

Abstract

Summary Tropical savannas have a ground cover dominated by C4 grasses, with fire and herbivory constraining woody cover below a rainfall‐based potential. The savanna biome covers 50% of the African continent, encompassing diverse ecosystems that include densely wooded Miombo woodlands and Serengeti grasslands with scattered trees. African savannas provide water, grazing and browsing, food and fuel for tens of millions of people, and have a unique biodiversity that supports wildlife tourism. However, human impacts are causing widespread and accelerating degradation of savannas. The primary threats are land cover‐change and transformation, landscape fragmentation that disrupts herbivore communities and fire regimes, climate change and rising atmospheric CO2. The interactions among these threats are poorly understood, with unknown consequences for ecosystem health and human livelihoods. We argue that the unique combinations of plant functional traits characterizing the major floristic assemblages of African savannas make them differentially susceptible and resilient to anthropogenic drivers of ecosystem change. Research must address how this functional diversity among African savannas differentially influences their vulnerability to global change and elucidate the mechanisms responsible. This knowledge will permit appropriate management strategies to be developed to maintain ecosystem integrity, biodiversity and livelihoods.

Functional diversification enabled grassy biomes to fill global climate space

“…First, environmental change can alter feedbacks between grasses and woody plants via changes in the processes limiting the growth and mortality of woody plants. For example, rising CO 2 is hypothesised to increase tree recruitment in savannas and forest margins (54, 55), while extreme drought events and warming may cause forest dieback on large scales (56), where each process has feedbacks with fire leading to ongoing biome shifts (57). Second, environmental changes will shift the community composition of grass communities.…”

Woody plant encroachment intensifies under climate change across tundra and savanna biomes

“…Nitrogen-fixing taxa (e.g., Alnus in the tundra, Acacia in the savanna) can alter nitrogen cycling, reshaping nutrient availability across the landscape (Hu, Finney, & Brubaker, 2001), and in the case of the savanna leading to further woody encroachment and legume proliferation (Ritchie, Tilman, & Knops, 1998). Therefore, species traits can influence their responses to changing environmental conditions across these two biomes (Bjorkman et al, 2018;Osborne et al, 2018).…”

Droughts and the ecological future of tropical savanna vegetation

“…However, over the last couple of centuries, they have been drastically altered by human activities (Lambin, Geist, & Lepers, ; Miles et al, ; Murphy, Andersen, & Parr, ; Ramankutty & Foley, ; Ratnam et al, ; Strassburg et al, ). As human populations grow and expand, the remaining savanna habitats are increasingly threatened by a range of anthropogenic activities, including land‐use transformation, tree plantations for carbon–sequestration initiatives and alterations of fire and herbivory regimes (Abreu et al, ; Bond, ; Bond & Parr, ; Murphy et al, ; Osborne et al, ; Parr et al, ; Ratnam et al, ; Strassburg et al, ; Veldman et al, ). Further, global changes in the form of altered precipitation and temperature regimes, elevated atmospheric [CO 2 ] and atmospheric N and P deposition have emerged as major threats to the ecological integrity of these biomes (Bond & Midgley, ; Midgley & Bond, ; Parr et al, ), prompting Bond and Midgley () to ask if the rise to dominance of this biome in the Late Miocene is likely to be matched by an equally dramatic decline in the Anthropocene.…”

“…Changes in savanna structure, composition and function as a result of ongoing and future climate change can have major implications for human wellbeing and ecosystem processes (Midgley & Bond, ; Osborne et al, ). Here, I focus on the effects of altered rainfall regimes, specifically droughts, on savanna dynamics.…”

“…Importantly, inferring savanna responses based on studies carried out in other ecosystems is not straightforward. Because the two dominant life forms that characterize savanna ecosystems—C 4 grasses and C 3 trees and shrubs—respond differently to the same environmental controls, and because many vegetation traits of tropical savanna trees and grasses differ consistently from those of forests and temperate grasslands (Craine, Lee, Bond, Williams, & Johnson, ; Hoffmann, Franco, Moreira, & Haridasan, ; Ratnam et al, ; Charles‐Dominique, Beckett, Midgley, & Bond ), savannas are likely to respond to climate change in ways that forests and grasslands may not (Lehmann et al, ; Osborne et al, ).…”