Acorn Size Research Articles

Phenotypic variation in seedlings of a "keystone" tree species (Quercus douglasii): the interactive effects of acorn source and competitive environment.

Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination (Δ) as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.

Latitudinal trends in acorn size in eastern North American species of Quercus

Latitudinal variation in acorn size was examined in 32 species of the genus Quercus geographically restricted to eastern North America. Within-species comparisons showed the existence of a prevalent trend: individuals at high latitude produce, on the average, smaller acorns than individuals of the same species at low latitude. This observed trend, clines of decreasing acorn size with increasing latitude, could represent (i) the selected or plastic response to a shorter and (or) cooler growing season constraining the size of acorns that can mature on a tree, (ii) a consequence of past size-selective dispersal that accompanied postglacial range expansion, or (iii) a combination of these effects. The within-species trend contrasted with an observed among-species trend: species with large acorns have ranged that extend into higher latitudes than species with small acorns. Current evidence strongly suggests that small acorn size is maladaptive for successful seedling establishment at higher latitudes. Our results caution against inferring selective forces operating on seed size from among-species comparisons. Key words: acorn size, Quercus L., latitudinal clines, eastern North America.

Acorn Size and Geographical Range in the North American Oaks (Quercus L.)

In this work we show that among North American oak species, acorn size and geographical range are correlated. Two geographically isolated sets of oak species are evaluated. The ecological significance of a positive relationship between acorn size and geographical range is explored