Summer drought as a filter to seedling establishment in the restoration of Mediterranean-climate ecosystems

Background/Question/Methods The establishment of seedlings is a critical but often difficult step in restoration projects. Summer drought is a primary filter to seedling establishment in Mediterranean-climate ecosystems. Understanding how drought, additional environmental factors, and species traits interact to determine summer survival allows restoration practitioners to make better educated guesses about what to plant. We first investigated how soil type and weed cover interact to influence the effect of summer drought on seedling survival. Second we asked if certain plant traits, in particular seed mass, influence a species’ likelihood of surviving summer drought. In spring 2008 we established permanent plots at an ex-pine plantation north of Perth, Western Australia and at an abandoned farm near Albany, Western Australia. Both sites had been direct seeded in late winter 2008. We recorded the species and location of all recruits within the plots, and in autumn 2009 we recorded their survival. For each plot we measured soil type and nutrient content, and weed cover. We related survival to these variables and to plant traits (e.g. seed mass).

Results/Conclusions Summer survival was lowest in plots with sandy soils. High water infiltration rates in these soils likely exacerbated the effect of summer drought. Weed cover was also lowest at these sites, suggesting that abiotic conditions filtered out weed species as well as native species. In plots with silty soil, weed cover was an important determinant of both initial recruitment and summer survival. Recruitment and survival were both greater in low-weed microsites. The ex-pine plantation was characterized by uniformly sandy soils, and we predicted that traits influencing drought survival would therefore be evident at that site. Large seeds can facilitate deeper roots and consequently drought avoidance, and summer drought survival at the ex-pine plantation increased with seed mass consistently across all three planted families. Our study illustrates the importance of considering interactions between abiotic and biotic filters at a given site when selecting species to plant.