Quantifying Ecosystem Trajectories: Tree Growth Response to Biophysical Gradients and Disturbance

Abstract

Disturbance and climate are important drivers of tree physiological functioning, community assemblages and trends in recruitment and species presence across time and space. Fire exclusion-driven changes to the disturbance regime of frequent fire-adapted forests of the southern Rocky Mountains, North America, followed by modern megafires has strongly influenced stand structure and led to density increases in many forest types. Recent decadal drought has led to widespread mortality of some tree species, exacerbated fire extent and effects, and contributed to insect outbreaks. With climate change ongoing, hotter and drier conditions and droughts are expected, leading to increased risk of widespread tree mortality and vegetation type change. Forest ecosystem changes result from compounding effects on individual tree establishment, growth, and survival, which leads to changes in stand structure and composition, and drives patterns appreciable at the broadest scale. Here I focus on tree-environment interactions in the context of disturbance and climate across three scales, from tree-stand, to stand-watershed, to forest-ecosystem. By working across scales we can observe how fine-scale tree responses to interact with the environment to create broad patterns. At the tree scale, I considered the influence of increased forest density on tradeoffs of water and nutrient limitation affecting growth and physiological functioning in old-growth ponderosa pine (Pinus ponderosa Douglas ex C. Lawson) (Appendix A). I identified a novel interaction in which lower leaf nitrogen in dense stands was associated with lower tree-ring growth yet higher carbon isotope discrimination, rather than the expected negative relationship between discrimination and density-driven water stress. Reduced leaf nitrogen likely limited photosynthetic capacity, resulting in discrimination values more decoupled from water stress than is expected in the Southern Rockies. At the stand level to watershed scale, I investigated climate-growth relationships and species distributions across a biophysical gradient in southwestern mixed-conifer forest (Appendix B). I used model selection to find how climate drivers of tree-ring growth varied by species, elevation, and aspect, and found differences were in accordance with relative species drought tolerance. I combined this with stand-level regeneration patterns to predict shifts in species dominance across the watershed. In the absence of fire, I found increased regeneration in pinyon pine (Pinus edulis Engelm.) at low elevations and white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.) at high elevations, while regeneration of ponderosa pine and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) decreased everywhere across the study area. At the ecosystem scale I used Forest Inventory and Analysis (FIA) data to derive the novel Community Mean Tolerance Index, based on relating species shade and drought tolerance to ecosystem changes and applied it to investigate demographic trends within and across forest types (Appendix C). With the index I mapped responses within and across forest ecosystems in the southern Rocky Mountains, and found areas at risk for vegetation type conversion to oak woodland following severe fire. Substantial shifts in mean drought and shade tolerance in tree regeneration was found in forest types that had exceeded their historic fire interval. Across forest types, drought tolerance in seedling groups increased at lower elevation sites, while shade tolerance increased at higher elevation sites. The difference in drought tolerance across demographic groups was significantly associated with PRISM-derived recent temperature and precipitation means, indicating the potential for climate-driven community shifts. Investigating the effects of disturbance and climate on trees, watersheds, and ecosystems allows for a holistic view across scales of the current state of southwestern forests, and the potential for future changes.