Aridity and intensive grazing have been confirmed to affect the
facilitative effects of dryland shrubs. However, their combined effects
on plant-plant interactions have rarely been tested. To test how these
two factors affect relations between plants, we analyzed 144 plots
(under shrub canopy vs. open areas) at 12 sampling areas established in
the conditions of two grazing regimes (high grazing vs. low grazing
intensity) and two different climatic regions (arid vs. semi-arid) in
northeastern Iran. A dominant shrub, Artemisia kopetdaghensis, was
selected as the model species. Further, we studied changes in plant life
strategies along the combined grazing and aridity stress gradients. We
used relative interaction indices to test the outcomes of plant-plant
interactions, calculated for species richness, Shannon diversity and
species abundances. Then we compared them using linear mixed-effect
models (LMM). The indicator species analysis was used to identify
species typical for the under-canopy of shrub and for the adjacent open
areas. The combination of stress factors affected the type and intensity
of plant-plant interactions and plant life strategies (CSR) of the
indicator species. Artemisia kopetdaghensis showed the highest
facilitation effect under the most intensive stress conditions (high
aridity/high grazing), which turned into competition under the low
stress conditions (low aridity/low grazing). In the arid region, the
canopy of shrub protected ruderal annual forbs and grasses with SR and
R-strategy, respectively, in both high (high aridity/high grazing) and
low grazing intensity (high aridity/low grazing). In the semi-arid
region and high grazing intensity (low aridity/high grazing), the shrubs
protected perennial forbs with C-strategy. Our FINDINGS highlight the
importance of context-dependent shrub management in the restoration of
vegetation damaged by intensive grazing.
