Effects of drought and slope aspect on canopy facilitation in a mountainous rangeland

Aims Species composition and diversity of the mountainous rangelands are results of interactions between environmental severities, heterogeneous topography and facilitative effects by nurse plants. This research was aimed to compare relative effects of these three environmental variables on the natural vegetation of a mountainous rangeland. For a more detailed understanding, effects of four different nurse species were separately compared on the various plant growth forms and on two community plant responses (diversity and abundance). Methods A mountainous semiarid rangeland was selected in Baharkish, Quchan, Northeast of Iran. Density and canopy cover of all plant species were recorded under the canopy of four different shrubs and in open areas, in north and south-facing aspects, and in a normal and a drought year. Shannon diversity, total abundance (% cover) and the abundance of different growth forms were used as criteria for assessing effects of the environmental variables. Data were arranged in a factorial combination and analyzed by three-way analysis of variance using a GLM analysis. Important Findings (i) Drought, aspect and canopy created niche differentiation: annual forbs and shrubs were more affected by drought, whereas geophytes and grasses were more responsive to slope aspects. Effects of drought and slope aspect were more profound on species diversity, whereas that of canopy facilitation was stronger on plant abundance. (ii) Canopy facilitation was dependent on severity of the abiotic factors and life history of interacting species. Canopy facilitation allowed for the persistence of only annual forbs, but it was disadvantaged during the drought year. Plant community responses to abiotic factors (slope and drought) were more dependent on the plant growth form, while responses to canopy facilitation were more dependent on the morphology and/or ecology of nurse shrubs. (iii) Effect of shrubs was dependent on their morphology and ecology: shrubs with larger canopy area and nitrogen fixation capability increased, but those with allelopathic effects or a dense canopy structure decreased the diversity of the understory species.