Category: Theses and Dissertations
Title: Reference conditions and spatial dynamics in a southwestern dry mixed-conifer forest.
Author: Rodman, K.
Subject: Reference conditions, Mixed Conifer
Abstract: Structural conditions in southwestern “frequent-fire” forests, or forests that have a history of frequent, low-severity fire, have changed considerably since Euro-American settlement (ca. late 1800's); these changes are primarily due to excessive livestock grazing, extractive logging, road construction, and organized fire suppression. As a result of these land-use changes, forest densities in many locations have increased, contributing to increases in fire size and severity and alterations to ecosystem function. A recognition of these impacts has prompted interest in forest restoration treatments across broad swaths of the landscape. Forest restoration attempts to mimic the natural range of variability - or the structure and function inherent to intact natural ecosystems - but many aspects of these ecosystems are poorly understood. In combination with studies of the natural disturbance regime, reference conditions and studies of the natural range of variability also provide an ecological basis for silvicultural treatments. Spatial pattern, or the horizontal distribution of trees, is an important component of the natural range of variability and influences a wide array of ecological processes. With this study, we reconstructed historical structure (density and spatial pattern) and species composition for four dry mixed-conifer stands on the Mogollon Rim, iii Arizona, USA. We reconstructed conditions from the year 1879, the time of the widespread cessation of fire in the area. We found that these forests showed evidence of increased tree density between 1879 and 2014 with major increases in density in the smaller size classes. During this time period, the number of live trees ha-1 increased from an average of 129.5 to 744.3, basal area increased from 10.1 to 41.9 m2 ha-1, and mean canopy cover increased from 14.8 to 54.7 percent. Our results also showed shifts in species composition to a higher relative proportion of Abies concolor, and Pseudotsuga menziesii, and a lower relative proportion of Pinus ponderosa and Quercus gambelii. Mean group size, or the average number of trees in explicit tree groups, also increased as tree canopy cover became more continuous. Finally, we investigated the roles of different species groups within fine-scale spatial pattern. Species groups showed distinctly different patterns, suggesting that the patterns of individual species are an important consideration during the implementation of silvicultural treatments that are meant to mimic natural patterns and processes. Mechanical treatments and thinning operations in dry mixed-conifer forests on the Mogollon Rim should target an aggregated spatial structure of leave trees with a range of group sizes within a given site. Treatments based on the natural range of variability are likely to increase resistance and resilience of these ecosystems in the face of uncertain future conditions.
Source: Northern Arizona University
Identifier: 173 p.
Publisher: NAU School of Forestry, http://www.for.nau.edu