ALBERT

Description: Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant–plant interaction may promote vegetation recovery in ecosystems affected by human activities. Determining the relevance of this process, however, requires assessing how nurse plants influence the establishment of other species, as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas. Further, since vegetation recovery is a time-dependent process, the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history. Thus, an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region. This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts, the creosotebush ( Larrea tridentata ). Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries. For this study, we selected 10 floodplain valleys differing in their age (i.e. the time elapsed after human activities were ceased). At each landscape, we measured the cover of creosotebushes and the proportion of plant species positively associated with them, as well as the density of seeds in the soil beneath creosotebush canopies. All these data were regressed against the age of the landscapes. Further, to assess whether positive association patterns were due to facilitation or other processes, we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies. Important Findings Most plant species from the target region were positively associated to creosotebushes, and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions. In most landscapes, the density of seeds was higher beneath creosotebushes than in the surrounding habitats, suggesting that these shrubs may also act as seed traps. The community-level effects of creosotebushes increased with landscape age and creosotebush cover, indicating that magnitude of these effects depends on the disturbance history of each site. These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time. We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.