Vegetation change in semiarid communities

Abstract

In arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event-driven dynamic behavior, unpredictable and low rainfall and complicated interactions between species make it difficult to assess probabilities and time scales of vegetation change.To gain a better understanding of the main processes and mechanisms involved in vegetation change, we have developed a spatially explicit individual based model that simulates changes in plant communities over long time spans. The model, based on life-history attributes of the five dominant component plant species of a typical Karoo shrub community, follows the fate of each individual plant within the community, the sum of which is community dynamics. The model explores the differential effects of a realistic range of rainfall pattern on the abilities of these species to compete, survive, grow and reproduce.The specific aim of the model is to identify key processes of vegetation change and to calculate probabilities and timespans for transitions between different vegetation states. Such knowledge is needed for species conservation and sustained animal production.We show that the time-scale for changes of the dynamic state of the system are long compared with human lifespans. Employing the full range of possible rainfall scenarios showed that short-term community dynamics (years to decades) and species composition depend strongly on the short-term (years) sequence of rainfall events. In all simulation experiments the final vegetation state varied by more than 37% after a 60 year simulation period. Simulating resting of an overgrazed part of the shrub community indicated that little improvement in rangland condition was likely during a period of 60 years. Even such active management, as (simulated) clearing of unpalatable shrubs, resulted in only a 66% probability that degraded shrubland would be in good condition after 60 years resting. Simulated overgrazing of a rangeland in good initial condition only became obvious 40 or 50 years after the initiation of heavy grazing, and after 70 years the mean vegetation state eventually reached that of an overgrazed rangeland.

Long-term vegetation change in the Succulent Karoo, South Africa following 67 years of rest from grazing

“…This model argues that even the availability of long-term records of the dynamics of a semi-arid plant community would have limited predictive value mainly due to the high stochasticity of this system. Wiegand and Milton (1996) suggest that a dynamic inertia exists in overgrazed rangeland. Once a long-lived species dominates, it will persist for a long-time and occupy sites that otherwise could serve as establishment sites for competing species.…”

“…An alternative understanding of Karoo vegetation dynamics suggests that changes in plant composition and cover are unpredictable on short time scales (years) but episodic on long-time scales in response to a combination of factors such as climate, grazing pressure, rare events and changed disturbance regimes (Ellis and Swift, 1988;Wiegand and Milton, 1996). This model argues that even the availability of long-term records of the dynamics of a semi-arid plant community would have limited predictive value mainly due to the high stochasticity of this system.…”

“…Those that do exist (Bossdorf et al, 2000;Roux and Vorster, 1983;Wiegand and Milton, 1996) have relied largely on models to explain the long-term dynamics of vegetation composition and change in the Succulent Karoo. Earlier ideas of vegetation change in the Karoo adopted the Clementsian succession paradigm developed for more mesic areas in North America (Acocks, 1953(Acocks, , 1988.…”

Species, functional groups and community structure in seed banks of the arid Nama Karoo: Grazing impacts and implications for rangeland restoration

“…In arid and semiarid ecosystems, recovery processes in degraded rangelands are slow and stochastic (Call and Roundy, 1991;Visser et al, 2004), and the time span for improvement might last several decades (Allington and Valone, 2010;Wiegand and Milton, 1996), which is beyond what is relevant for management. Both the time needed and the lack of a promising natural restoration potential at DEG to support species which are common to sustainably managed rangelands, point towards the need for active interventions.…”

Factors affecting establishment of a gypsophyte: the case of Lepidium subulatum (Brassicaceae)

“…The knowledge of the factors controlling seedling abundance and survival at spatial and temporal scales provide a basic tool for the management and conservation of this type of arid systems (Milton, 1995;Wiegand and Milton, 1996). In this sense, gypsum systems are considered one of the most threatened systems in the Mediterranean basin (Gomez-Campo, 1987) and have been included in the European Community Habitats and Species Directive (Comunidad Europea, 1992), which promotes the conservation of the most vulnerable ecosystems in Europe.…”