On the variation of traits and tree range constraints
Love-Anderegg, Leander Dean
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Ecology in the 21st century faces the considerable challenge of predicting how ecosystem structure and function will respond to rapid global environmental change. In order to meet this challenge, ecology must transcend description through the development of broad ecological theory and ecological tools that can explain and predict ecological phenomena across multiple scales of spatial, temporal and taxonomic organization. This dissertation leverages within-species geographic variation in plant performance and functional traits to test the biogeographic predictive power of long-standing ecological theory, illuminate how tree drought resistance strategies will mediate geographic range shifts in a warming world, and explore the strengths and weaknesses of leaf functional traits as ecological tools. Species geographic ranges are, in essence, the spatial manifestation of their ecological niche, yet the exact mechanisms that constrain species ranges remain elusive, limiting our ability to predict range shifts. In the first chapter of this dissertation, I collected tree cores from over 700 trees across the western U.S. to determine how climate and competition jointly constrain elevation tree ranges. This work is based on the longstanding but rarely tested hypothesis that biotic and abiotic stress trade off, with species interactions (competition) being the main fitness constraint in benign environments and abiotic/climatic stress proving the main constraint in harsh environments. I found broad-scale evidence for this tradeoff in the tree core record. Across multiple species on multiple mountains, populations with the fastest tree growth (the most ‘benign’ sites) were most sensitive to competition while the slowest growing populations were the most sensitive to climate. However, this trade-off did not map cleanly onto range position. Of the nine species ranges examined, only two showed strong evidence for a trade-off between climatic and competitive growth constraints, although evidence for climatic constraints in harsh environments was more consistent. These findings highlight multiple processes that complicate local range dynamics, but suggest that the constraints on large-scale (e.g. latitudinal) tree distributions may still be predicted from ecological theory. Thus, existing correlational tools such as Climate Envelope Models may be appropriate for predicting shifts of large-scale plant range boundaries in climatically harsh environments. Second, I used within-species variation in drought tolerance traits to elucidate the physiological mechanisms by which drought controls two specific tree range boundaries. I quantified elevational variation in the drought tolerance and drought avoidance traits of a widespread gymnosperm (ponderosa pine –Pinus ponderosa) and angiosperm (trembling aspen – Populus tremuloides) tree species in the southwestern USA. Although water stress increased and growth declined strongly at the lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth. By contrast, ponderosa pine showed little elevational trait variation but avoided drought stress at low elevations through stomatal closure, such that its dry range boundary experienced limited carbon assimilation even in good years. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms – a result that has important implications for process-based modeling approaches to tree biogeography. Further, I show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range-limit context, helps elucidate a mechanistic understanding of range constraints. Finally, I collected and compiled an extensive dataset on leaf functional trait variation within and between species in order to test some of the foundational assumptions of trait-based ecology. Functional traits have great potential to stimulate a predictive ecology, providing scale-free tools for understanding ecological interactions, community dynamics and ecosystem function. Yet their utility relies in part on four key assumptions: 1) that most trait variation lies between rather than within species, 2) that global patterns of trait covariation are the result of universal evolutionary or physiological trade-offs that are independent of taxonomic scale, and 3) that traits respond predictably to environmental gradients. I examined three traits central to the leaf economics spectrum, leaf mass per area (LMA), leaf lifespan, and leaf nitrogen content, and quantified patterns of leaf trait variation, particularly within-species. Although I found that some foliar traits do vary primarily between species (as predicted), others – particularly area-based leaf nitrogen content – vary enormously within-species. I also found that some of the global trait relationships central to the leaf economics spectrum hold true across taxonomic scales. However, other patterns of trait covariation show surprisingly different patterns within- versus between-species, calling into question some of the putative evolutionary and physiological mechanisms linking these leaf traits. Finally, in a subset of well sampled conifers in the northwestern U.S.A., I found that leaf lifespan was reasonably responsive to environmental gradients but other foliar traits had very weak links to environmental variation. Taken together, my results challenge the ‘scale-free’ nature of the currently proposed mechanisms driving leaf trait covariation. However, my results demonstrate the potential power of intra-specific trait variation to deepen our understanding of the causes and consequences of functional trait variation.
- Biology