Not All Plant Taxa Display Typical Latitude-Propagule Size Gradients: A Case Study In Arctostaphylos L. (Ericaceae)

Abstract

Proposed causes of the latitudinal-propagule size gradient invoke differences among biome structures or seed dispersal syndromes. Latitudinal seed-size gradients so far have been predominantly investigated using entire floras, prompting the question of whether such trends exist at smaller scales. Here we consider effects of latitude and elevation on fruit size between and within species in Arctostaphylos L. (Ericaceae), a zoochorous, primarily Californian chaparral shrub genus. We measured fruit size, strongly correlated with seed size in Arctostaphylos, in three species of this genus from multiple localities in the Sierra Nevada foothills, and conducted a character analysis of fruit size across the genus using the standard California flora plant manual (Baldwin et al. 2012). Across the genus we found a weak negative correlation between fruit size and latitude (0.0026 mm diameter/km) and a weak positive correlation between fruit size and elevation (1.3 mm diameter/km). AIC indicates that these trends are not explained by autocorrelation between fruit size and other variables such as maximum plant size. By contrast, intraspecific field data revealed a positive relationship between fruit size and both elevation and latitude. Propagule size gradients within Arctostaphylos oppose those reported for angiosperms globally. This contrast may result from uniformity of fruit structure and animal dispersal, disturbance ecology of chaparral, or local precipitation gradients characteristic of Mediterranean-type climates. Studies of propagule size gradients within taxa can uncover ecological mechanisms behind this trend that remain obscure at global scales.