Latitudinal gradients of reproductive traits in Japanese woody plants

Latitudinal gradients in the variability of plant trait expression are insufficiently studied. Here, we infer the expression and variability of major reproductive traits in Japanese angiosperm woody plants using geographical distributions of 773 woody plant species and 4682 grid cells at the 10 × 10 km² spatial resolution. We focus on flower color, flower size, fruit size, fruit type, seed mass, and dispersal type and relate trait mean values and within cell variability to latitude, forest area and fragmentation, phylogenetic relatedness, and socioeconomic factors. To account for the correlation between variability and mean trait values, we use a recently developed model based on Taylor's power law (TPL). All studied reproductive traits exhibited strong although contrasting latitudinal gradients. Contrary to Gloger's rule, flowers tended to be darker at higher latitudes. Flower sizes increased and fruit and seed sizes decreased at higher latitudes. Color variability increased while variability in fruit size decreased toward higher latitudes. Higher human impact influenced fruit size positively and seed size negatively. We confirmed the usefulness of the TPL in the study of trait variability. We interpret the darker flower colors at higher latitudes as an adaptation to predominant bee pollination and thus as an effect of ecological filtering. Our study does not unequivocally corroborate the hypothesis that color variability within woody plant communities is governed by the intensity of ultraviolet radiation. We interpret the observed latitudinal trends toward higher variation in flower color and size in terms of increased numbers of generalist pollinators at higher latitudes.