Intraspecific variability and reaction norms of forest understory plant species traits

Julia I. Burton, Steven S. Perakis, Sean C. McKenzie, Caitlin E. Lawrence and Klaus J. Puettmann

Plant traits are recognized as being increasingly important for understanding plant community assembly processes and predicting responses to changes in climate and disturbance regimes. Most approaches ignore trait variation within species, assuming individual species can be represented by a single average value. However, traits can vary broadly within species along gradients in resources and environmental conditions. Accounting for intraspecific variation may be particularly important when species rankings for different traits vary along those gradients. Trait data for understory species are lacking for many forest ecosystems, leading to uncertainty in the potential importance of trait variation within-species compared to variation among species. Moreover, most studies focus on leaf traits while few studies have considered whole-plant traits including water use efficiency and Δ15N (isotopic nitrogen composition – an integrated measure of nitrogen use strategy), stem traits or root traits. Our survey of 10 whole-plant, leaf, stem and root traits from 57 understory species from 7 forest sites in the Pacific Northwest shows that intraspecific variability is largely related to variation in resources and environmental conditions within sites, including gradients in light, understory competition and soil nitrogen, rather than variation between sites (i.e., climate). Consistent with a recent meta-analysis, intraspecific variability was generally more important for whole-plant and root traits than leaf and stem traits. However, our results show that species rankings are consistent across these gradients. Therefore, sampling species traits in a single set of conditions should be sufficient to establish species rankings, supporting the common practice of using a single trait value to represent a species.

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Image caption: Photo provided by authors.

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