Progress 04/17/04 to 12/31/06
Outputs
Pinyon-juniper woodlands in the Great Basin have undergone a distinct increase in density and extent over the past century. Restoration practices of land management agencies use natural and mechanical treatments as a means to reverse this change. More quantitative information on woodland structure over large landscapes is needed for restoration purposes. This study of woodland characteristics over a central Nevada landscape examines the environmental factors that influence woodland structure, and the influences of environmental factors and woodland structure on woodland mortality. Field measurements of woodland structure and mortality for 217 plots were collected across 11 mountain ranges in 2005. Object-oriented classification was used to classify canopy cover from air photos at 1-m resolution over the entire study area. This semi-automated classification method is a multi-scale approach that can easily be aggregated to the desired resolution. GIS-derived
environmental variables were used along with canopy cover estimates from air photos to scale up field plot measurements to a 3,250-km2 area. This study found that mesic sites with greater soil development promote higher density and cover of P-J, while less productive sites host fewer large trees and have greater basal area. High stocking levels on xeric sites with reduced soil development had greater mortality and occurrence of pinyon Ips and mistletoe infestation. Individual trees with larger diameters were prone to a higher percent dead crown. Basal area and stand density index of singleleaf pinyon were positively correlated with the magnitude of singleleaf pinyon mortality. Results also indicated that the relationship between woodland mortality and stand structure is scale dependent, with stronger correlations at coarser spatial scales. This suggests that density dependence in woodland mortality, particularly during drought years, may arise primarily because of limited availability
of favorable microsites, and less because of direct effects of intraspecific competition. Denser sites are more likely to host trees on less suitable microsites, which presumably become established only during especially favorable climatic periods. Thus, pinyon mortality on sub-optimal microsites can be considered part of normal, long-term woodland dynamics. Previous Nevada land use history might have obscured stronger relationships between environmental factors and woodland structure. Woodland mortality by single mortality agents may be difficult to estimate because disturbances interact in tandem and consecutively in time and space. The statistical models developed will help guide P-J woodland treatments and restoration efforts by determining which woodland structure attributes and environmental factors are important in influencing tree mortality in the Great Basin.
Impacts
Our research has significant impacts for both land managers and scientists. On the management side, we will provide the Battle Mountain Field Office with ground data and spatial predictive models describing structure, distribution and vigor of pinyon-juniper woodlands over 3250 sq. km. encompassing 11 mountain ranges. Our method of classifying woodland patches and tree canopy cover from widely available, black and white aerial photography should prove useful for other applications that seek to quantify historical woodland change or inventory current condition. Our main scientific contribution lies in an improved understanding of the density-dependent nature of tree mortality in these woodlands. Our work suggests that mortality in denser stands may not result primarily from increased competition among trees, as is commonly believed, but may be caused by trees occupying sub-optimal microsites in such stands, resulting in mortality during drought periods. Further study is
needed to test this hypothesis.
Publications
- Greenwood, D.G. 2006. Landscape analysis of tree mortality and pinyon-juniper woodland structure in the Great Basin. M.S. Thesis, University of Nevada, Reno.
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Progress 01/01/05 to 12/31/05
Outputs
Field sampling was completed in summer of 2005, with 217 stand structure plots sampled in pinyon-juniper woodlands over 11 mountain ranges. Variables sampled included: tree diameter class structure, tree height class structure, tree canopy cover, crown vigor, presence of tree mortality agents (pinyon Ips, dwarf mistletoe, etc.), ages of oldest trees, tree seedling inventory, shrub species composition and cover, and understory cover by plant functional type. Tree locations were mapped in the field, so that relationships between density and mortality can be assessed over multiple scales within the 0.1-ha plots. All field data have been entered into a database and error-checked. An object-oriented classification algorithm (in the eCognition environment) has been modified from that developed within project NEV052NGA, to better account for effects of tree crown shadows. Woodland canopy cover has been completely mapped for two mountain ranges at 1-m resolution from
panchromatic digital orthophotos, and canopy cover classification is in process for nine others. A comparison between field canopy cover and remotely sensed canopy cover across two mountain ranges yields a coefficient of determination of 41%, and work is underway to refine our understanding of how the two measures are related. Tree mortality throughout the study area was widespread but generally diffuse in impact, with an average percent dead crown of 17%, percent of dead trees per plot of 8%, and approximately 41% of plots having at least one Ips-killed tree. Preliminary analyses show weakly significant relationships between mortality measures and stand structural measures such as percent dead crown and stand density index. Statistical analysis is ongoing to model: (1) woodland structural attributes according to environmental factors and canopy cover as mapped from panchromatic aerial photography; (2) measures of tree mortality according to woodland structure and environmental
variables. Future efforts will investigate analyze the relationship between understory and overstory structure and composition over the broad spatial extent of this study, with a focus on whether threshold effects observed by others for site-specific studies will still hold over a broader range of environmental conditions.
Impacts
We expect to develop methods for cost-effectively and accurately estimating pinyon-juniper woodland structure and mortality over large areas. Our methods will include development of statistical models relating what can be measured on the ground, to commonly available GIS data layers and crown cover as can be estimated using panchromatic aerial photography (scales from 1:15,000 - 1:50,000). Our data will establish a useful baseline for assessing future changes in pinyon-juniper vegetation, including both old-growth and expansion woodlands. We will develop maps identifying and describing what are likely to be critical priority areas for restoration management.
Publications
- No publications reported this period
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Progress 04/17/04 to 12/31/04
Outputs
A graduate student was hired, who began work on the project in late July, 2004. An initial, brief field season in the Simpson Park Range lasted approximately four weeks, and will serve as a valuable pilot study. In coordination with BLM personnel involved with the project, a field sampling protocol was devised and successively refined through preliminary sampling of 16 pinyon-juniper (P-J) woodland sites. The sampling protocol involves sampling of understory and overstory components of P-J stands, including diameter structure, tree height, vegetation cover, crown mortality, stem mortality, and identification of mortality agents. Our sampling scheme uses a clustered design, where the central plot in each cluster is marked as a permanent plot for future monitoring efforts. In addition to the 16 field plots, crown cover was sampled from 56 other sites. All 72 plots are being combined with additional crown cover plots, from another study, to serve as ground-truthing for
an aerial photography-based classification of tree crown cover over much of this central Nevada study area. This effort, to be undertaken in the early months of 2005, will facilitate spatial modeling of field-sampled woodland structure and mortality over large portions of the 11 mountain ranges included in this study. We have insufficient data at this time to formulate even preliminary results concerning the relationship between woodland structure, tree mortality, and environmental factors. However, our brief field season in 2004 led to successful development of a field protocol, such that we will be able to efficiently collect needed data during spring and summer of 2005. Classification of aerial photography is underway, and spatial modeling protocols will be developed during Fall and Winter of 2005-6.
Impacts
We expect to develop methods for cost-effectively and accurately estimating pinyon-juniper woodland structure and mortality over large areas. Our methods will include development of statistical models relating what can be measured on the ground, to commonly available GIS data layers and crown cover as can be estimated using panchromatic aerial photography (scales from 1:15,000 - 1:50,000). Our data will establish a useful baseline for assessing future changes in pinyon-juniper vegetation, including both old-growth and expansion woodlands. We will develop maps identifying and describing what are likely to be critical priority areas for restoration management.
Publications
- No publications reported this period
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