Propagule Size Gradients in Arctostaphylos (Ericaceae): The Effects of Altitude and Latitude

ABSTRACT\ud PROPAGULE SIZE GRADIENTS IN ARCTOSTAPHYLOS\ud (ERICACEAE): THE EFFECTS OF\ud ALTITUDE AND LATITUDE\ud by\ud ?? Andrew G. Simpson 2009\ud Master of Science in Botany\ud California State University, Chico\ud Spring 2009\ud The latitudinal gradient in propagule size (fruits and seeds) is an unexplained\ud pattern in phytogeography and paleobotany. Proposed causes often focus on differences\ud in biome structure or mode of seed dispersal between tropical forests and temperate\ud grasslands or savannahs. Here we compare effects of latitude and altitude on fruit size in\ud Arctostaphylos (Ericaceae), an animal-dispersed and primarily Californian chaparral\ud shrub genus, using both a meta-analysis of published geographic and morphological\ud ranges, and field analysis of three species in the Sierra Nevada foothills. Stepwise linear\ud regression on multiple measures variables reveals a weak but statistically significant\ud negative correlation between fruit size and latitude and a similarly weak positive\ud correlation between fruit size and altitude. Species with burls typically have low\ud minimum latitude and high maximum fruit size. By contrast, intraspecific field data\ud vii\ud reveal a positive relationship between fruit size and both elevation and latitude. These\ud results reveal that Arctostaphylos does not exhibit the global propagule size gradients\ud exhibited by other angiosperms, even within California. Possible mechanisms responsible\ud for these anomalous results involve the reliance of Arctostaphylos on animal dispersal,\ud the disturbance ecology of chaparral shrublands, or the locally inverted latitudinal\ud gradient in precipitation characteristic of Mediterranean-type climates. Latitudinal\ud propagule-size gradients so far been predominantly investigated using floristic-scale\ud literature-based meta-analyses. More studies are needed at smaller taxonomic scales in\ud order to determine the complexity and mechanisms underlying the global pattern.CSU, Chic