Abstract
Positive interspecific plant–plant interactions in (semi-)arid ecosystems are crucial for supporting ecosystem diversity and stability, but how interactions respond to grazing combined with temporal variation in drought is poorly understood. In a semi-arid area in south-eastern Spain (Murcia region), we planted 1280 saplings of the palatable shrub Anthyllis cytisoides (beneficiary) under the canopy of the unpalatable shrub Artemisia herba-alba (nurse) or in open microsites between shrub patches. We applied four grazing treatments (no grazing, low goat grazing pressure, high goat grazing pressure and rabbit grazing) and two watering treatments. Sapling height and survival were followed for two consecutive years, during which one extreme drought event occurred. We analysed how grazing, watering and their combination affected nurse effects throughout the course of the study. Grazing and the drought event, but not watering, significantly altered the nurse effects. Under ungrazed conditions prior to the extreme drought event, nurse effects on sapling survival were neutral, whereas they were positive at rabbit-grazed plots. At low goat grazing, sapling growth was higher under nurse shrubs than in open microsites. However, after the extreme drought event, sapling survival was higher in open microsites at ungrazed plots, whereas at rabbit-grazed plots, nurse effects shifted from positive to neutral. Our findings highlight the importance of rabbit grazing in determining the direction of plant–plant interactions in arid ecosystems. Moreover, our findings support the idea that positive plant–plant interactions may wane under the combination of high grazing and drought stress.
Highlights
Over the last two decades an increasing number of studies showed the importance of interspecific facilitation in structuring plant communities, and in recent years, the importance of interspecific facilitation for structuring ecosystems (Bruno and others 2003) and for supporting biodiversity (Michalet and others 2006, McIntire and Fajardo 2014) has been widely recognized
Before the drought period we found equal survival rates for the ungrazed, low goat grazing and high goat grazing plots, but lower survival rates for the rabbit-grazed plots (Figure 1)
During the drought event sapling survival dropped in all four plots, but less on the low goat grazing plots compared to the other three grazing treatments
Summary
Over the last two decades an increasing number of studies showed the importance of interspecific facilitation (that is, net positive interactions between plants) in structuring plant communities (for reviews see: Callaway 2007; Brooker and others 2008; Pugnaire and others 2011; He and others 2013; Michalet and Pugnaire 2016), and in recent years, the importance of interspecific facilitation for structuring ecosystems (Bruno and others 2003) and for supporting biodiversity (Michalet and others 2006, McIntire and Fajardo 2014) has been widely recognized. In arid ecosystems, observational studies showed that plant–plant interactions may shift back from facilitation to competition at sites or during periods with very low rainfall (Tielborger and Kadmon 2000; Maestre and Cortina 2004; Armas and Pugnaire 2005; Saccone and others 2009; Butterfield and others 2016). The predictions from the original SGH were falsified by studies from grazed ecosystems showing that facilitation intensity increased from low to high grazing pressure, but decreased again with very high grazing pressure (Smit and others 2007; Graff and Aguiar 2011; Saiz and Alados 2012) This was because at very high grazing pressure nurse plants themselves got damaged by grazing or trampling (Michalet and others 2014), or because herbivores started searching more intensively for resources, effectively removing the protective effects of the nurse plant (Soliveres and others 2011). Adding consumer pressure to drought stress may possibly act as an accelerator in the waning of positive interactions at the extreme end of an aridity gradient (Verwijmeren and others 2013)
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