Can seed caching enhance seedling survival of Indian ricegrass (Achnatherum hymenoides) through intraspecific facilitation?

Abstract

Positive interactions among individual plants (facilitation) may often enhance seedling survival in stressful environments. Many granivorous small mammal species cache groups of seeds for future consumption in shallowly buried scatterhoards, and seeds of many plant species germinate and establish aggregated clusters of seedlings from such caches. Scatterhoards made by desert heteromyid rodents provide a major source of seedling recruitment for Indian ricegrass (Achnatherum hymenoides), a perennial bunchgrass species occurring widely across arid southwestern deserts of North America. However, effects of the resulting clumping of seedlings on subsequent survival have not been quantified under field conditions, which include extended periods of excessive summer heat and little precipitation. We monitored Indian ricegrass seedlings transplanted into field exclosures at two western Nevada study sites as either single seedlings or clumps of 25 or 35 seedlings and compared their survival. Survival was positively correlated with the number of seedlings growing together, and survival of whole clumps of 25 or 35 seedlings was significantly greater than that of seedlings growing singly. Moreover, individual seedlings within clumps of 35 seedlings had significantly higher survival than seedlings growing singly. No single seedlings survived through the hot and very dry summer following their planting, but a small proportion (1.8–2.8 %) of individual seedlings within clumps survived. Results of this field experiment suggest that facilitative benefits accruing to Indian ricegrass due to seed-caching desert rodents can extend past seedling establishment and into the long-term survival of the plant.