Climatic and competitive constraints drive clinal variation in Populus tremuloides leaf morphology

Background/Question/Methods

A clear understanding of the relationship between leaf morphology and climate is essential to predict plant responses to anthropogenic climate change. I collected 90 Populus tremuloides (quaking aspen) saplings (juvenile trees < 2m in height) in summer of 2014 across the species’ elevational range, which covers significant precipitation and temperature gradients, in the La Plata Mountains, Colorado. For each sapling, I measured the specific leaf area (SLA), the ratio of area of one side of a leaf (cm²) to whole leaf dry mass (g). “Shade leaves” with high relative SLA are relatively thin with greater surface area for light capture and “sun leaves” with low relative SLA are thicker with decreased area to minimize water loss. Sapling growth rate was measured by measuring height growth (cm) per year (age) with age calculated from annual ring analysis. We then tested the effect of elevation (climate) and competition for sunlight on SLA and growth rate. Competition for sunlight was quantified as continuous leaf area index (LAI) analyses via fisheye photos. Further, I analyzed leaf venation networks (density, variation in diameter) to assess the role of leaf vascular tissue anatomy in an increasingly warmer and drier climate. 

Results/Conclusions

There is a strong positive relationship between SLA and LAI, with individuals under open canopies producing leaves with less surface area but greater thickness, while individuals under closed canopies produce leaves with greater surface area but less thickness. There was a slight significant difference in SLA across elevations, with low elevation individuals producing leaves of lower SLA while individuals of middle and higher elevations produced leaves of greater SLA values. Further, we found a positive interaction between elevation and LAI, such that shaded individuals at high elevations produced higher SLA leaves than shaded individuals at low elevation. Annual growth showed no significant relationships as a function of SLA, LAI, and the interaction between the two and elevation. Preliminary vein density analysis suggests fewer venation adjustments across elevation than hypothesized which suggests alternate mechanisms of survival possibly independant of vein density. These results indicate that light availability is the strongest driver of intra-specific variation in leaf morphology and that juvenile aspen growth is not strongly constrained by either light availability or geographic variation in drought stress. These data suggest that aspen understories might be more microclimatically homogenous than macroclimatic gradients, or that clonality or variation in traits other than leaf morphology sustain growth rates even under water and light limitation.