Spruce-Fir Forest Decline in the Southwestern USA: Species Distribution Modeling Meets Ecological Realism

Spruce-fir forests in the southwestern USA are restricted to high elevation peaks and plateaus, severely limiting the upward migration of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa var. lasiocarpa), and corkbark fir (Abies lasiocarpa var. arizonica) driven by 21st Century climatic warming. My thesis revolves around using two fundamentally different types of species distribution models (SDMs), presence/absence vs. presence only, with two different applicable algorithms, logistic regression vs. Maximum Entropy Modeling (MaxEnt). These two methods are commonly used when building SDMs, but we have adapted unique ecological processes to each. The logistic regression method includes a migration component in post-development of bioclimatic envelopes for six high elevation tree species, and the MaxEnt method uses published tree-ring chronologies from the International Tree-Ring Data Bank (ITRDB) to determine the climate variables used to build a Climate-Growth Envelope (CGE) for Engelmann spruce. The bioclimatic envelopes are trained on spatial climate datasets from the years 1950–1999, and utilize statistically downscaled General Circulation Models (GCMs) coupled with species-specific migration rates to project shifts in species' ranges for three different time steps (2010–2039, 2040–2069, and 2070–2099). The CGE is built using significant climate variables influencing 31 ITRDB chronologies from 1895–1976, and displays an overall contraction of suitable climate space for Engelmann spruce to grow from 1980 to 2010.;A field site pertaining to each of the modeling methods were visited to verify trends in the shifting distribution of the focus species. Rectangular plots (0.1 ha) in the Spruce-fir/Mixed Conifer Transitional Forests on the North Rim of the Grand Canyon were first sampled in 1984 and resampled in 2010 to test for significant changes in density and basal area for mature trees (DBH ≥ 10 cm), as well as for densities of saplings and seedlings in subplots (0.01 ha) in relation to projected shifts in the bioclimatic envelopes. Circular plots (0.1 ha) measured the composition and density of three different forests (high-, mid-, and low-elevation forests) on north-northeastern aspects of Hawk Peak in the Pinaleño Mountains, and we associated the plot measurements with significant climate variables influencing tree-ring growth for Engelmann spruce and corkbark fir populations for each forests to verify general trends in the shift of the CGE. In both modeling and verification efforts, the decline of contemporary spruce-fir forests is evident within the recent past, and future range shifts project a dismal future for these high elevation tree species in their southernmost distributions.