Forb ecology research in dry African savannas: Knowledge, gaps, and future perspectives

Abstract

Savannas are commonly described as a vegetation type with a grass layer interspersed with a discontinuous tree or shrub layer. On the contrary, forbs, a plant life form that can include any nongraminoid herbaceous vascular plant, are poorly represented in definitions of savannas worldwide. While forbs have been acknowledged as a diverse component of the herbaceous layer in savanna ecosystems and valued for the ecosystem services and functions they provide, they have not been the primary focus in most savanna vegetation studies. We performed a systematic review of scientific literature to establish the extent to which forbs are implicitly or explicitly considered as a discrete vegetation component in savanna research. The overall aims were to summarize knowledge on forb ecology, identify knowledge gaps, and derive new perspectives for savanna research and management with a special focus on arid and semiarid ecosystems in Africa. We synthesize and discuss our findings in the context of different overarching research themes: (a) functional organization and spatial patterning, (b) land degradation and range management, (c) conservation and reserve management, (d) resource use and forage patterning, and (e) germination and recruitment. Our results revealed biases in published research with respect to study origin (country coverage in Africa), climate (more semiarid than arid systems), spatial scale (more local than landscape scale), the level at which responses or resource potential was analyzed (primarily plant functional groups rather than species), and the focus on interactions between life forms (rather seldom between forbs and grasses and/or trees). We conclude that the understanding of African savanna community responses to drivers of global environmental change requires knowledge beyond interactions between trees and grasses only and beyond the plant functional group level. Despite multifaceted evidence of our current understanding of forbs in dry savannas, there appear to be knowledge gaps, specifically in linking drivers of environmental change to forb community responses. We therefore propose that more attention be given to forbs as an additional ecologically important plant life form in the conventional tree–grass paradigm of savannas.

Forb community responses to an extensive drought in two contrasting land-use types of a semi-arid Lowveld savanna

“…Nevertheless, some notes on forb dynamics in rangeland studies are useful contributions to our current understanding. For instance, similar to what is known for grasses, herbivory, climate (particularly rainfall variability), Introduction fire and microhabitat properties are considered as important drivers of forb community dynamics in semi-arid and arid savannas (Siebert and Dreber 2019).…”

“…Studies on herbaceous vegetation dynamics are strongly focused on grass productivity related to grazing and climate variability (Trollope et al 2014) with limited focus on forbs (non-graminoid herbaceous vascular plants) (Siebert and Dreber 2019). Forbs are an important source of biodiversity in grasslands and savannas (Uys 2006;Buitenwerf et al 2011;Trollope et al 2014; Scott-Shaw and Morris 2015; Siebert and Scogings 2015) that are functionally diverse, which suggest stronger resilience to environmental disturbances (Shackleton 2000;Jacobs and Naiman 2008;Bond and Parr 2010;Scott-Shaw and Morris 2015;Van Coller et al 2018).…”

“…Cattle may even suppress forb cover through forb forage preferences towards the end of a dry season (Young et al 2005;Odadi et al 2007;Kimuyu et al 2017). Despite significant contributions to ecosystem goods and services, both annual and perennial forbs are often perceived negatively by land managers, because of their association with savanna land degradation (Siebert and Dreber 2019) and dominance over grasses under particular environmental conditions, such as on nutrient-rich soils (Reich et al 2003;Van Coller and Siebert 2015), when grazing pressure is moderate to high (Scholes 1987;Hejcmanová et al 2010;Cowley et al 2014) or after drought events (Fynn and O'Connor 2000;Buitenwerf et al 2011).…”

“…The majority of studies that are aimed at describing grazing and/or drought impacts in savanna ecosystems are not particularly designed for an improved understanding of forb community ecology, neither the drivers responsible for community dynamics (Siebert and Dreber 2019). Nevertheless, some notes on forb dynamics in rangeland studies are useful contributions to our current understanding.…”

“…After drought events, forbs emerge before grasses and often dominate the herbaceous layer while the grasses recover (Fynn and O'Connor 2000;Kallah et al 2000;Buitenwerf et al 2011). Drought events in grassland ecosystems may increase native annual forbs in the seed bank (LaForgia et al 2018), although emergence following disturbances may be species-specific for forbs (Siebert and Dreber 2019).…”