Colonization by annual plants of an experimentally altered desert landscape: source-sink relationships

We studied how annual plant species colonize a set of patches (natural and human-made) in a shrubland landscape in the Negev desert. We asked what patch properties and species' attributes affect colonization during 4 years after experimental formation of eight sets of pits and mounds. The 'sink function' of patches was measured as the numbers of colonizing species, the proportion of patches colonized during each year, and the abundance of the colonizers. The effects of species' dispersal mode and propagule size were examined. In the first growing season, pits and mounds were colonized by large numbers of species. The total number of colonizing species was similar for the two patch types, but incidence was higher in pits than in mounds. The early colonizers were mainly wind-dispersed and showed both high abundance and incidence. Dispersal mode did not affect colonization in the second year and later, nor did propagule size throughout the experiment. Most early colonizers became resident in the patches, but pits and mounds showed some further colonization during the second year. In the third and fourth year both the numbers of colonizing species and their incidence decreased. Pits were mainly colonized from outside the experimental units to which they belonged, whereas mounds received species mostly from the adjacent pits within the same units. From the second year on, patches of undisturbed crusted soil were also colonized, mainly from the adjacent pits and mounds. The number of colonizing species in undisturbed patches increased during the last, high-rainfall, year both per patch and for the whole set of patches. We conclude that the sink function of patches for colonization increases with removal of vegetation, soil crust and seed bank, with patch capacity to capture resource and plant propagules, and with amount of rainfall. Sink function decreased with time due to decreasing availability of new species, but increased with the presence of stronger sink patches in the vicinity. We suggest that the concept of sink function, in terms of the numbers, incidence and abundance of species colonizing sets of patches, can be a powerful tool for linking community and landscape processes.