Functional traits underlie specialist-generalist strategies in whitebark pine and limber pine

Abstract

Plant species life history strategies are described by functional variation spanning an acquisitive and conservative resource use continuum. Specialist species can exhibit traits promoting one end of the continuum, while generalist species can display traits promoting both acquisitive and conservative resource use. Whitebark pine (Pinus albicaulis, PIAL) and limber pine (Pinus flexilis, PIFL) are two high-elevation pines that have similar growth and morphology, yet contrasting elevational distributions with PIAL viewed as a specialist inhabiting a narrower elevation range, and PIFL as a generalist inhabiting a broader elevation range. We compared the physiological and morphological traits of greenhouse-grown 5-year-old PIAL and PIFL. Our results suggest that PIFL’s acquisitive and conservative resource use traits contribute to its generalist strategy and ability to inhabit a greater range of elevations than PIAL. PIFL had greater acquisitive resource use traits including: high-light tolerance (greater Qsat, greater fascicle density), increased biomass allocation to photosynthetic tissue (higher needle biomass, aboveground:belowground biomass, needle:branch + stem biomass), and higher C and water uptake (greater stomatal density and size, higher C assimilation rate), as well as greater conservative resource use traits including: greater physical stress resistance (shorter height, higher stem and branch diameters, greater branch and stem diameter:length), drought tolerance (higher SWC, leaf starch proportion), and drought avoidance (earlier budburst phenology, smaller hydroscape area) than PIAL. Our results suggest that PIFL may make more efficient use of high-light loads and maximize C and water uptake when moisture is abundant during spring snowmelt before the onset of dry summer conditions. Other conservative resource use traits describing cold tolerance, heat tolerance, and drought tolerance did not differ between species, suggesting that both species exhibit traits that promote similar conservative resource use enabling their overlapping persistence at higher elevations. Comparing the physiology of PIAL and PIFL within the same environment enables us to identify physiological mechanisms that underlie species establishment and survival, and how juvenile physiology contributes to their contrasting distributions and their generalist-specialist strategies.