Multiscale soil and vegetation patchiness along a gradient of herbivore impact in a semi-arid grazing system in West Africa

Author： Rietkerk M.

Publisher： Springer Publishing Company

ISSN： 1385-0237

Source： Vegetatio, Vol.148, Iss.2, 2000-06, pp. : 207-224

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Abstract

We studied the degree and scale of patchiness of vegetation and selected soil variables along a gradient of herbivore impact. The gradient consisted of a radial pattern of `high', `intermediate' and `low' herbivore impact around a watering point in a semi-arid environment in Burkina Faso (West Africa). We hypothesised that, at a certain range of herbivore impact, vegetated patches alternating with patches of bare soil would occur as a consequence of plant-soil feedbacks and run-off-run-on patterns. Indeed, our transect data collected along the gradient showed that vegetated patches with a scale of about 5–10 m, alternating with bare soil, occurred at intermediate herbivore impact. When analysing the data from the experimental sites along the gradient, however, we also found a high degree of patchiness of vegetation and soil variables in case of low and high herbivore impact. For low herbivore impact, most variation was spatially explained, up to 100% for vegetation biomass and soil temperature, with a patch scale of about 0.50 m. This was due to the presence of perennial grass tufts of Cymbopogon schoenanthus. Patterns of soil organic matter and NH_4-N were highly correlated with these patterns of biomass and soil temperature, up to r=0.7 (P<0.05) for="" the="" in="" situ="" correlation="" between="" biomass="" and="" nh_4-n.="" for="" high="" herbivore="" impact,="" we="" also="" found="" that="" most="" variation="" was="" spatially="" explained,="" up="" to="" 100%="" for="" biomass="" and="" soil="" temperature,="" and="" 84%="" for="" soil="" moisture,="" with="" three="" distinct="" scales="" of="" patchiness="" (about="" 0.50 m,="" 1.80 m="" and="" 2.80 m).="" here,="" microrelief="" had="" a="" corresponding="" patchy="" structure.="" for="" intermediate="" herbivore="" impact,="" again="" most="" variation="" was="" spatially="" explained,="" up="" to="" 100%="" for="" biomass="" and="" soil="" temperature,="" and="" 84%="" for="" soil="" moisture,="" with="" a="" patch="" scale="" of="" about="" 0.95 m.="" here,="" we="" found="" evidence="" that="" vegetated="" patches="" positively="" affected="" soil="" moisture="" through="" less="" run-off="" and="" higher="" infiltration="" of="" rainwater="" that="" could="" not="" infiltrate="" into="" the="" bare="" soil="" elsewhere,="" which="" was="" not="" due="" to="" microrelief.="" thus,="" we="" conclude="" that="" our="" findings="" are="" in="" line="" with="" our="" initial="" hypothesis="" that,="" at="" intermediate="" herbivore="" impact,="" vegetated="" patches="" alternating="" with="" patches="" of="" bare="" soil="" persist="" in="" time="" due="" to="" positive="" plant-soil="" feedbacks.="">