RESUMO
Leaf litter inputs can influence the structure and function of both terrestrial and adjacent aquatic ecosystems. Dioecy and herbivory are two factors that together have received little attention, yet have the potential to affect the quantity, quality, and timing of riparian litterfall, litter chemistry, and litter decomposition processes. Here, we explore litter chemistry differences for the dioecious Sitka willow (Salix sitchensis Sanson ex. Bong), which is establishing on primary successional habitats at Mount St. Helens (WA, USA) and is heavily infested with a stem-boring weevil (Cryptorhynchus lapathi). Weevil-attacked branches produced summer senesced litter that had significantly higher %N, lower C:N ratios, and lower condensed tannins than litter from branches that were unattacked by the weevil and senesced naturally in the autumn. Weevils more often attack female willows; however, these common litter chemicals did not significantly differ between males and females within the weevil-attacked and -unattacked groups. High-resolution mass spectrometry was used to isolate compounds in litter from 10 Sitka willow individuals with approximately 1500-1600 individual compounds isolated from each sample. There were differences between weevil-attacked litter and green leaf samples, but at this level, there was no clustering of male and female samples. However, further exploration of the isolated compounds determined a suite of compounds present only in either males or females. These findings suggest some variation in more complex litter chemistry between the sexes, and that significant differences in weevil-attacked litter chemistry, coupled with the shift in seasonality of litter inputs to streams, could significantly affect in-stream ecological processes, such as decomposition and detritivore activity.
RESUMO
Selection on quantitative traits by heterogeneous climatic conditions can lead to substantial trait variation across a species range. In the context of rapidly changing environments, however, it is equally important to understand selection on trait plasticity. To evaluate the role of selection in driving divergences in traits and their associated plasticities within a widespread species, we compared molecular and quantitative trait variation in Populus fremontii (Fremont cottonwood), a foundation riparian distributed throughout Arizona. Using SNP data and genotypes from 16 populations reciprocally planted in three common gardens, we first performed QST -FST analyses to detect selection on traits and trait plasticity. We then explored the environmental drivers of selection using trait-climate and plasticity-climate regressions. Three major findings emerged: (1) There was significant genetic variation in traits expressed in each of the common gardens and in the phenotypic plasticity of traits across gardens, both of which were heritable. (2) Based on QST -FST comparisons, there was evidence of selection in all traits measured; however, this result varied from no effect in one garden to highly significant in another, indicating that detection of past selection is environmentally dependent. We also found strong evidence of divergent selection on plasticity across environments for two traits. (3) Traits and/or their plasticity were often correlated with population source climate (R2 up to .77 and .66, respectively). These results suggest that steep climate gradients across the Southwest have played a major role in shaping the evolution of divergent phenotypic responses in populations and genotypes now experiencing climate change.
