Rabbit Grazing as the Major Source of Intercanopy Heterogeneity in a Juniper Shrubland

Abstract

Semi-arid shrublands are prone to wildfires especially if there are no firebreaks separating shrubs to reduce the spread of fire. The accumulated grass vegetation is a potential combustion source which also contributes to the spread of wildfires. We present a field study conducted in a semi-arid community, where the fire risk is high and the grass density surrounding the Juniperus communis shrubs is highly varied. We tested simultaneously the relative significance of the main factors contributing to plant cover around shrubs: allelopathy, shading, and herbivory. Grass cover was compared around junipers which were either currently occupied by rabbits or were unoccupied. We visually estimated plant cover and measured the Normalized Difference Vegetation Index. Both sides of the shrubs were sampled at various distances to see if allelopathy and/or shading reduced grass density. The allelopathy effect was also tested by lettuce bioassay. We found that the vegetation was open only around occupied junipers and that the effect of occupation decreased by distance from the shrub. The spatial distribution of plant cover showed that, although we could detect significant allelopathic effect of juniper by bioassay, neither the shading nor the allelopathy were as important as herbivory. We also compared the rabbit diet composition with plant availability for each occupied patch and found that the consumed species were under represented due to selective grazing. Our findings indicate that local herbivore activity could affect the spatial heterogeneity of combustible material, therefore, moderate rabbit grazing can be considered as a management tool to reduce the spread of fire.