Thursday, August 6, 2009

PS 61-9: CANCELLED - Seed dispersal in Joshua trees: Dispersal by rodent mutualists

Benjamin A. Waitman, University of Nevada

Background/Question/Methods It has been suggested that Joshua trees (Yucca brevifolia) were dispersed by an extinct ground sloth, and that no disperser is extant. Preliminary research from my lab group has shown that Joshua tree seeds presented in small piles were dispersed to scattered caches in a manner typical of rodents. I examined the mechanisms of dispersal of the Joshua tree (Yucca brevifolia) in southern Nevada and California. Joshua tree fruits are indehiscent, and seeds must be removed from the fruit in order for dispersal to be effective. The removal of Joshua tree fruits and seeds from infructescences and on the soil surface was monitored using removal transects. Both merriam's kangaroo rats (Dipodomys merriami) and white-tailed antelope ground squirrels (Ammospermophilus leucurus) were given seeds in an arena. I also planted seeds in potential microsites to determine optimal sites for Joshua tree seedling emergence and survival through one season, both in the field and in a growth chamber. Results/Conclusions Seed half lives were 12.6 0.8 days and 2.9 0.3 days for seeds and fruits on the surface, respectively, and almost all propagules monitored were removed by animals. Both kangaroo rats and ground squirrels cached seeds in small numbers (5.6 0.4 seeds/cache for kangaroo rats and 6.6 1.4 seeds/cache for ground squirrels) from 0.5-2.5cm below the soil surface. Seedlings were most likely to emerge when planted between 1-3cm below the soil surface. The results show that scatter hoarding rodents cache Joshua tree seeds in small numbers in microsites from which they can emerge given suitable environmental conditions. Seeds planted on the soil surface did not survive in either the field or laboratory. This iconic Mojave Desert plant is dispersed by animal mutualists, and dispersal quality is effected rodent populations and the relative size of the seed crop.