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Snag and log populations in northern Arizona mixed-conifer and ponderosa pine forests

Date: August 24, 2016

Little is known about the dynamics of snags in southwestern forests, despite their intimate connection to fire behavior, native wildlife, decay dynamics and biological legacies


Background

Snags and logs provide important resources and biological legacies in mixed-conifer forests. Photo: U.S. Forest Service.
Snags and logs provide important resources and biological legacies in mixed-conifer forests.
Snags (standing dead trees) and logs are a critical component of ecosystems. They contribute to decay dynamics and other ecological processes in forested ecosystems, provide important resources for native wildlife, and influence fuel loads and fire behavior. As a result of this rich biological legacy, land managers and researchers have focused special attention on snags and logs in recent decades, and many public land agencies have established specific guidelines for retention of snags and logs. Even with this increased focus, little is known about the dynamics of snag and log populations in southwestern forests.Since 1997, RMRS scientists have monitored populations of snags (standing dead trees) and downed logs in northern Arizona mixed-conifer and ponderosa pine forests, as well as patterns of climate-mediated tree mortality influencing inputs to snag and log populations.

RMRS scientists established a series of permanent plots in mixed-conifer and ponderosa pine forest in northern Arizona in the 1970s. Snag and log populations are inventoried within these plots at five year intervals, providing information on dynamics of snag and log populations as well as on patterns of tree mortality.

These data have been used to describe structure and composition of snag and log populations. In addition, this set of five decades of data is used to evaluate changes in snag and log populates over time, to model the longevity of individually marked snags and logs, and to critically evaluate the feasibility and performance of current guidelines for retention of large snags and logs in these forest types.

Because the study area has been subject to an ongoing drought, data on patterns of tree mortality provide detailed insight into the response of these forests to the warmer and drier conditions predicted for this area due to climate change.

This information is critical to managers charged with maintaining populations of snags and logs, as well as of the species dependent on these structures, in a rapidly changing landscape.

Key Findings 

  • Densities of both snags and logs are increasing in these forest types due to climate-mediated tree mortality. This mortality is changing forest structure and species composition.

  • Densities of large logs generally meet minimum standards for retention of these structures. In contrast, large snags are deficient in many areas, especially in ponderosa pine forests.

  • Many snags used by cavity nesting birds are smaller than the minimum snag sizes in current guidelines for snag retention, suggesting that some relaxation of size standards may be possible.

  • Gaps in research include: studies explicitly linking snag size and density to demography of cavity nesting birds and studies documenting ecologically sustainable snag densities. 

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Principal Investigators: 
External Partners: 
Jeffrey S. Jenness, Jenness Enterprises
Gary C. White, Colorado State University (Emeritus)