Mid-Scale Drivers of Variability in Dry Mixed-Conifer Forests of the Mogollon Rim, Arizona

Matthew Jaquette,Matthew A Bowker,Andrew J Sánchez Meador,David W Huffman

doi:10.3390/f12050622

Abstract

The structure and composition of southwestern dry mixed-conifer forests have changed significantly, decreasing forest resiliency to uncharacteristic disturbances which also threaten ecosystem services. Restoration of these forests can be informed by historical conditions; however, managers and researchers still lack a full understanding of how environmental factors influence forest conditions. We investigated historical and contemporary variability in dry mixed-conifer forests in northern Arizona and identified important environmental drivers. We utilized forest sample plots and dendrochronological reconstruction modelling to describe forest conditions in 1879 and 2014, respectively. We used correlogram analysis to compare spatial autocorrelation of average diameter, basal area and tree density, and structural equation modeling to partition the causal pathways between forest structure, forest composition, and a suite of environmental factors reflecting climate, topography, and soil. Historical (1879) reconstructed forests had significantly fewer trees, lower basal area, and higher average diameter than contemporarily (2014). Composition has shifted from ponderosa pine dominance towards a more mixed-species composition. Historically, forest structure did not exhibit strong spatial autocorrelation, but contemporary tree density and diameter were strongly autocorrelated. Environmental factors described little variation in historical forest conditions but are more important for contemporary conditions. Managers can utilize this increased understanding of variation to tailor silvicultural prescriptions to environmental templates.

Highlights

Mixed-conifer forests cover approximately 1 M hectares in the southwestern United States [1] and provide critical ecosystem services such as wildlife habitat [2], watershed protection [3], carbon sequestration, and nutrient cycling [4,5]
We focused on answering the following research questions: (1) What were the historical structural conditions in these ecosystems? (2) How did structural conditions vary spatially, and has spatial variation changed since fire exclusion? and (3) What were the drivers of variability in these forests, and how have drivers changed in relative importance since fire exclusion? Answers to these questions can help silviculturists design restoration treatments that have appropriate levels of spatial and structural variability and better reflect
Our study showed historically variable, heterogeneous, and open dry mixedconifer forests on the Mogollon Rim prior to anthropogenic fire exclusion, with ponderosa pine dominating stands on ridgetops, and more mixed composition persisting in drainages

Summary

Introduction

Mixed-conifer forests cover approximately 1 M hectares in the southwestern United States [1] and provide critical ecosystem services such as wildlife habitat [2], watershed protection [3], carbon sequestration, and nutrient cycling [4,5]. 20th-century landuse practices, unregulated logging, grazing, and active fire suppression, have disrupted natural fire regimes [6,7,8]. This has decreased these forests’ resilience [9,10]—the “ability of an ecosystem to regain structural and functional attributes that have suffered harm from stress or disturbance” [11]. While there has been criticism that historical conditions will become irrelevant under future climatic conditions [16], restoring a more characteristic composition and structure would increase ecosystem resiliency [3,9] and allow these ecosystems to resist type conversion due to wildfire and climate warming [9,10,17]

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