Drought effects on the herbaceous community structure of transformed Mopaneveld

Abstract

Semi-arid Mopaneveld vegetation structure is driven by herbivory, fire, nutrients and rainfall variability. The floristically homogenous herbaceous layer is highly dynamic and dominated by forbs. Forb species contribute significantly to the herbaceous species diversity of Mopaneveld. The responses of Mopaneveld herbaceous layer to increasing stressors such as climate- and land-use change may be heightened by its dynamic nature. Species- and functional diversity may provide crucial information on the responses of Mopaneveld to disturbances and the effect thereof on ecosystem function and stability. Rainfall variability is a characteristic feature of savannas such as the Mopaneveld and consequently some species have developed adaptive traits to cope with unpredictable precipitation and drought. Mopaneveld was previously shown to absorb anthropogenic disturbance effects through functional trait redundancy. However, studies investigating the effect of drought on anthropogenically transformed semi-arid savannas remain limited. This study therefore, aimed to determine the response of herbaceous community composition, in terms of both species- and functional diversity, on mine dumps in the semi-arid Mopaneveld savanna to a severe drought. The severe 2015-2016 drought period provided the opportunity to investigate the response of species- and functional attributes of herbaceous vegetation to a severe drought on the transformed Mopaneveld ecosystem at Palabora Copper Mine (PC) in Phalaborwa, Limpopo. Floristic- and environmental herbaceous vegetation surveys, following a random paired sampling design combined with the 1 m2 quadrate method, were conducted at two mine dumps at PC (copper tailings dam – TD; and waste rock dump -RD), and benchmarked against an adjacent protected area Cleveland Game Reserve (CGR), before drought and during drought release (In this study, defined as a period whereby an area received increased, but not above average, rainfall compared to a preceding drought period.). Herbaceous species were identified to species level and plant functional traits conferring drought- and stress tolerance were attributed to each species. Species- and functional trait data were used to analyse herbaceous community composition and diversity. Functional trait data were further analysed and compiled into plant functional types (PFT’s) to assess trait-based redundancy, and therefore resilience of the Mopaneveld herbaceous layer. Community-level analyses revealed that forb species diversity declined significantly in the protected area and on the TD, but not on the RD (intermediate land-use type). Forb functional trait diversity declined significantly on both mine dumps, but was more stable in the protected area. This may be due to the variety of forb lineages and their adaptation to natural disturbance in the protected Mopaneveld over evolutionary time. On the other hand, grass species diversity declined significantly in the protected area and remained stable on the mine dumps, which could be attributed to careful selection of stress-tolerant species for mine dump restoration practices. Furthermore, grass functional trait diversity remained stable across all land-use types. Functional trait evenness increased on the TD for both life forms and grass trait evenness also increased in the protected area. Productivity decreased significantly in the protected area, and grass biomass was significantly higher than forbs on the mine dumps. Both life forms were site specific, although more so for forbs. Indicator species analyses revealed forbs as the dominant indicator species in the protected area. Drought resulted in an increase in annual PFT’s across all land-use types. Forb PFT’s composed of allochorous dispersal modes and nitrogen fixing ability were particularly important during drought release, while grass PFT’s consisting of sun tolerant species with a low palatability and long-distance dispersal modes were favoured during drought release. Compared to mine dumps, the protected area was more associated with forb PFT’s made up of species with a specialized mode of dispersal and no nitrogen-fixing ability, and grass PFT’s consistent of species with shade tolerance, endozoochorous dispersal mode and high palatability. All PFT’s remained present during drought release, indicating that the Mopaneveld herbaceous layer retained its resilience, despite severe anthropogenic disturbance and drought. In the face of increasing drought frequency and land-use change, this study stresses the importance of promoting functional diversity and maintaining functional redundancy, by conserving more species with diverging responses to disturbance.