Rapid recovery of ecosystem function following extreme drought in a South African savanna grassland

Abstract

Climatic extremes, such as severe drought, are expected to increase in frequency and magnitude with climate change. Thus, identifying mechanisms of resilience is critical to predicting the vulnerability of ecosystems. An exceptional drought (<first percentile) impacted much of southern Africa during the 2015 and 2016 growing seasons, including the site of a long‐term fire experiment in Kruger National Park, South Africa. Prior to the drought, experimental fire frequencies (annual, triennial, and unburned) created savanna grassland plant communities that differed in composition and function, providing a unique opportunity to assess ecosystem resilience mechanisms under different fire regimes. Surprisingly, aboveground net primary productivity (ANPP) recovered fully in all fire frequencies the year after this exceptional drought. In burned sites, resilience was due mostly to annual forb ANPP compensating for reduced grass ANPP. In unburned sites, resilience of total and grass ANPP was due to subdominant annual and perennial grass species facilitating recovery in ANPP after mortality of other common grasses. This was possible because of high evenness among grass species in unburned sites predrought. These findings highlight the importance of both functional diversity and within‐functional group evenness as mechanisms of ecosystem resilience to extreme drought.

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

“…Alternatively, this effect may not be in terms of vegetation composition in systems where the dominant species rely less on a persistent seed bank (Gibson, 2009). But, the high abundance of annual species in the seed bank might play an important role in ecosystem functioning after the dry period is over (e.g., Wilcox et al, 2020). These species develop rapidly and can cover and protect the soil, and create favorable niches facilitating the recruitment of perennial species.…”

“…Changes in rainfall event size and lengthening the time between rain events may have several impacts on community composition and diversity (Knapp et al, 2002;Miranda et al, 2009;Talmon et al, 2011). In grassland ecosystems, droughts can rapidly alter plant community composition (Fynn & O'Connor, 2000) and ecosystem functioning (Wilcox et al, 2020). Drought can decrease the abundance of dominant perennial grasses (Báez et al, 2013), and increase the abundance of annual and perennial forbs during the drought and after it ends (Milton & Dean, 2000;Hodgkinson & Müller, 2005;Porensky et al, 2013;O'Connor, 2015;Munson et al, 2016;Swemmer et al, 2018).…”

Nitrogen addition amplifies the nonlinear drought response of grassland productivity to extended growing‐season droughts

“…However, other studies have reported nonsignificant effects of N addition on post-drought recovery (Tilman 1996 because N addition was associated with stronger responses by plant shoots, such as mortality or wilting, during drought periods (Southon et al 2012) offsetting the positive effects of N addition on post-drought recovery. Wilcox et al (2020) suggested that compensation plays a critical role in ecosystem resilience to extreme drought in savanna grasslands, and thus long-term N addition will negatively impact recovery if diversity or compensation are reduced. Hence, we still do not have a clear understanding of the effects of N FIG.…”

“…Wilcox et al. (2020) suggested that compensation plays a critical role in ecosystem resilience to extreme drought in savanna grasslands, and thus long‐term N addition will negatively impact recovery if diversity or compensation are reduced. Hence, we still do not have a clear understanding of the effects of N addition on grassland recovery immediately after drought events or at longer time scales.…”

Legacy Effects of Extreme Drought on the Belowground Bud Bank of Bunchgrass and Rhizomatous Steppe Communities in Inner Mongolia

“…In grasslands, drought can alter community composition and structure, decrease aboveground net primary productivity (ANPP), and shift biomass allocation patterns (Frank 2007;Evans et al 2011;Hoover et al 2014;Meng et al 2019;Zhang et al 2019;Zhao et al 2020); however, grasslands are also highly resilient ecosystems. Previous studies suggested that ANPP can completely recover to pre-drought levels within one year after drought (Hoover et al 2014;Wilcox et al 2020). While legacy effects of drought are often negative (Sala et al 2012), some grasslands are more productive than expected one year after drought (Gri n- Nolan et al 2018).…”