Rapid Increase in Log Populations in Drought-Stressed Mixed-Conifer and Ponderosa Pine Forests in Northern Arizona

Joseph L Ganey,Scott C Vojta

doi:10.4236/ojf.2012.22008

Abstract

Down logs provide important ecosystem services in forests and affect surface fuel loads and fire behavior. Amounts and kinds of logs are influenced by factors such as forest type, disturbance regime, forest management, and climate. To quantify potential short-term changes in log populations during a recent global- climate-change type drought, we sampled logs in mixed-conifer and ponderosa pine (Pinus ponderosa) forests in northern Arizona in 2004 and 2009 (n = 53 and 60 1-ha plots in mixed-conifer and ponderosa pine forests, respectively). Over this short time interval, density of logs, log volume, area covered by logs, and total length of logs increased significantly in both forest types. Increases in all log parameters were greater in mixed-conifer than in ponderosa pine forest, and spatial variability was pronounced in both forest types. These results document rapid increases in log populations in mixed-conifer forest, with smaller changes observed in ponderosa pine forest. These increases were driven by climate-mediated tree mortality which created a pulse in log input, rather than by active forest management. The observed increases will affect wildlife habitat, surface fuel loads, and other ecosystem processes. These changes are likely to continue if climate change results in increased warmth and aridity as predicted, and may require shifts in management emphasis.

Highlights

Coarse woody debris (CWD), defined as the large-size component of downed woody material (Harmon et al, 1986), provides important ecosystem services in forest systems (McComb & Lindenmayer, 1999; Butler et al, 2002; Woldendorp & Keenan, 2005), and amounts and types of CWD can affect surface fuel loads and fire behavior in these systems (Brown et al, 2003; Brewer, 2008)
Much of the volume of CWD in southwestern mixed-conifer and ponderosa pine forests consists of logs
Median log density increased by 36.4%, median log volume by 33.6%, median area covered by logs by 53.1%, and median total length of logs by 35.8% in mixed-conifer forest from 2004 to 2009 (Table 1)

Summary

Introduction

Coarse woody debris (CWD), defined as the large-size component of downed woody material (Harmon et al, 1986), provides important ecosystem services in forest systems (McComb & Lindenmayer, 1999; Butler et al, 2002; Woldendorp & Keenan, 2005), and amounts and types of CWD can affect surface fuel loads and fire behavior in these systems (Brown et al, 2003; Brewer, 2008). Much of the volume of CWD in southwestern mixed-conifer and ponderosa pine forests consists of logs (defined here as down woody material >20 cm in large-end diameter and ≥2 m in length). Logs perform various ecological roles, but are important in providing foraging sites and shelter for many species of wildlife (Bull et al, 1997). Because of their importance to native wildlife, specific guidelines for retention of logs were developed for a number of forest types in the Southwestern Region of the US Forest Service Information on abundance of logs in these forest types, as wells as trends in log populations, typically is sparse,

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