Environmental variation in seed number, size, and dispersal of a fleshy‐fruited plant

Plant reproductive strategies may evolve from fitness trade‐offs between seed dispersal and seedling establishment. Small crops of large seeds generally are not dispersed as far as large crops of small seeds, but large seeds contain more reserves for seedling establishment. However, selection pressures imposed on these traits may vary among environments. This study examined the extent and consequences of environmental variation in the seed dispersal strategy of a fleshy‐fruited plant, the common chokecherry (Prunus virginiana). From 1996 to 1998, P. virginiana fruit crop and seed traits were measured for plants sampled in moist riparian or dry slope habitats in western Montana. The consequences of variation in P. virginiana location, seed size, and seed number for seed dispersal were estimated from 1996 to 1999 by collecting seeds in seed traps. In all years, P. virginiana produced larger crops of smaller seeds in riparian than in slope habitats. Reproductive biomass was lower for plants on moisture‐limited slopes than for similarly sized plants in riparian habitats. However, P. virginiana on slopes increased seed size rather than seed number in a year with relatively high levels of precipitation, suggesting that large offspring size may play a more important role than seed number in this environment. Environmental variation in P. virginiana reproductive traits may contribute to differences in seed dispersal patterns. Frugivores removed greater numbers of seeds from plants with large fruit crops and preferentially consumed relatively small seeds within sites (particularly in slope habitats). In riparian habitats, seed deposition by frugivores was greater away from rather than under P. virginiana canopies. In contrast, seed deposition on slopes was greatest under canopies. Thus, the production of many small seeds may be an adaptive response to increase seed dispersal in the riparian environment, where dispersal may be more important than seed size for successful offspring recruitment.