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

Abstract

Projected increases in the frequency and severity of drought events are expected to impose changes in the ecology of native forb communities in semi-arid ecosystems. We examined the state of forb communities during, and directly after an extreme drought event across two contrasting land-use types, which included a protected area (high diversity of free roaming wild herbivores) and communal rangeland (long history of intensive cattle grazing) in a semi-arid Lowveld savanna of the Gazankulu area, South Africa. Forb floristic data were collected towards the end of the drought and repeated after the drought release a few months later. Forb community composition was significantly different among land-use types. Community changes were not induced by annual forb emergence alone, but through species-specific dominance shifts, which differed among land-use types. Forb richness, diversity and biomass were equally low at both land-use types and increased significantly after the drought release, although the magnitude of response was much stronger in the protected area, whereas drought contributed to a directional change in the protected area with a complete post-drought turnover in both annual and perennial forb species, much less variability was observed in the communal rangeland, which may suggest long-term effects imposed by land-use history.

Extreme drought has limited effects on soil seed bank composition in desert grasslands

“…While perennial grasses are reduced during drought, lowering their seed production (Marone & Pol, 2021), annual species are able to grow during periods of soil moisture availability (Levine et al, 2008;Mayfield et al, 2014) derived from pulses of rain, like those that occur during the summer monsoon (Muldavin et al, 2008;Mulhouse et al, 2017). Under these conditions annual species can complete their life cycle during relatively short time periods when soil moisture is available (Ruthven III, 2007;Nenzhelele et al, 2018;Siebert et al, 2020), contributing seeds to the soil seed bank, even during relatively dry years.…”

“…Persistence is often species-specific, and viability will depend on environmental conditions (Ma et al, 2020). For example, some seeds germinate rapidly following a small moisture pulse (Nenzhelele et al, 2018;Siebert et al, 2020), but subsequent dry conditions may reduce their survival (Ooi et al, 2009). In contrast, those species with permanent seed banks and with bet-hedging strategies (i.e., with a high proportion of dormant seeds) have increased chances of survival during drought (Venable, 2007).…”

Effects of land-use on herbaceous vegetation in a semi-arid Mopaneveld savanna

“…in the degraded communal areas) (Shackleton 2000;Hillebrand, Bennet & Cadotte 2008). An improved understanding of the community ecology of herbaceous vegetation across land-use types will indicate how species and their respective functions are affected by land-use changes, and how they can be maintained under increasing land-use pressure and drought (Siebert & Dreber 2019;Siebert, Klem & Van Coller 2020;Kellner et al 2021).…”

A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…For instance, forbs often dominate savanna herbaceous communities after a drought event through drought-avoidance strategies (see regenerative traits), drought-tolerance traits (e.g. underground storage organs) and resprouting capacity (Siebert et al 2020). Similarly, when exposed to heavy grazing, herbaceous communities may be transformed in favour of grazing-resistant or -tolerant forbs (Siebert and Dreber 2019).…”