Offspring size is a key functional trait that can affect subsequent life history

stages; in many species, it exhibits both local adaptation and phenotypic plasticity.

Variation among populations in offspring size may be explained by various factors,

including local climatic conditions. However, geographic variation in climate

may be partitioned into long-term and interannual sources of variation, which

may differ in their effects on population mean offspring size. To assess environmental

correlates of offspring size, we evaluated geographic variation in seed mass

among 88 populations representing 6 species of Streptanthus (Brassicaceae) distributed

across a broad climatic gradient in California. We examined the effects of

temperature-mediated growing season length and precipitation on population

mean seed mass to determine whether it is best explained by (1) long-term mean

climatic conditions; (2) interannual climate anomalies (i.e., deviations in climate

from long-term means) during the year of seed development, or (3) interactions

between climate variables. Both long-term mean climate and climate anomalies

in the year of collection were associated with population mean seed mass, but

their effects differed in direction and magnitude. Relatively large seeds were

produced at chronically wet sites but also during drier-than-average years. This

contrast indicates that these associations may be generated by different mechanisms

(i.e., adaptive evolution vs. phenotypic plasticity) and may be evidence of

countergradient plasticity in seed mass. In addition, populations occurring in locations

characterized by relatively long growing seasons produced comparatively

large seeds, particularly among chronically dry sites. This study highlights the

need to consider that the responses of seed mass to long-term versus recent

climatic conditions may differ and that climate variables may interact to predict

seed mass. Such considerations are especially important when using these patterns

to forecast the long- and short-term responses of seed mass to climate change. The

results presented here also contribute to our broader understanding of how climate

drives long-term (e.g., local adaptation) and short-term (e.g., phenotypic plasticity)

variation in functional traits, such as offspring size across landscapes.