Effects of Wood Cutting on Wildlife Habitat in Blue Oak Woodlands in the Northern Sacramento Valley
Barret A. Garrison and Richard B. Standiford. Oaks ‘n’ Folks – Volume 12, Issue 1 – February, 1997
This study was conducted to quantify baseline wildlife habitat characteristics of lands where wood had been cut, as well as estimate effects on wildlife habitat using existing models of habitat relationships. Data on tree and shrub habitat parameters were collected from vegetation plots located in harvested areas on 12 ranches in Tehama County and seven ranches in Shasta County. Blue oak was the dominant tree; foothill pine and interior live oak were present in fewer numbers.
Pre-cut tree conditions of uncut stands were estimated from relationships derived from uncut trees using a statewide growth and yield model for blue oaks. This same model was used to estimate stand growth with and without wood cutting over a 50-year period. Effects on 21 wildlife species were assessed using habitat models from Version 5.2 of the California Wildlife Habitat Relationships System (CWHR) database. These 21 species were selected because they are strongly associated with blue oak habitats and indicate a wide range of habitat conditions and potential effects. Effects were assessed by comparing habitat changes between cut and uncut woodlands. A change in habitat values of plus or minus 50 percent was considered a significant effect.
Differences in habitat conditions between cut and uncut conditions were plainly evident. Wood cutting resulted in substantially lower amounts of canopy cover, while stem densities were variously affected depending on the tree diameters selected during the harvest. Among the 19 ranches studied,uncut conditions had the greatest amount of canopy cover over the 50-year period, ranging from 26-83 percent during the first decade and growing to 38-94 percent during the fifth decade. Average stem diameters for the uncut condition averaged 7.4-12.3 inches during the first decade, growing to 10.1-13.3 inches during the fifth decade. In contrast, when cut, canopy cover and stem diameters averaged 0-53 percent and 6.5-14.1 inches, respectively,during the first decade. By the fifth decade after cutting, canopy cover and stem diameters averaged 7-83 percent and 9.6-16.7 inches, respectively.
Two of the five ranch scenarios used for assessing effects to wildlife had relatively similar stem diameters and canopy cover between the cut and uncut conditions. This may indicate that harvesting was proportionately directed at trees of all sizes. The other three scenarios had substantial decreases in stem diameter and canopy cover, indicating that larger trees were harvested.
Three ranches had immediate post-cut canopy coverages less than 10 percent,and harvested habitats on these ranches had to be classified as annual grassland for the first decade. One ranch remained annual grassland over the 50-year period because tree canopy never exceeded 10 percent, while two ranches grew into blue oak woodland habitat by the second decade.
Wildlife were variously affected depending on the CWHR-predicted habitat suitabilities for each species. Generally, species favoring closed-canopy conditions with larger diameter trees, such as the Cooper’s hawk and Pacific-slopeflycatcher, were negatively affected when cutting resulted in grasslands or oak woodlands with small diameter trees and open canopies. Conversely,species favoring grasslands or very open woodlands, such as the western meadowlark, California ground squirrel, and gopher snake, were positively affected. Some species favoring tree habitats with open canopies, such as the mourning dove, ash-throated flycatcher, western bluebird, gray fox,and mule deer, were positively affected or unaffected depending on the magnitude of the harvest. The western screech owl, acorn woodpecker, scrub jay, bushtit,white-breasted nuthatch, and western gray squirrel were significantly negatively affected only when substantial differences occurred between the cut and uncut conditions. The ensatina salamander, red-tailed hawk, wild turkey,mule deer, and western fence lizard were relatively unaffected by the cutting,despite varying degrees of canopy cover preference. Clearly, the magnitude of tree cutting influenced effects to wildlife. Scenarios with relatively small amounts of tree cutting had predicted impacts that were either positive(two ranches with nine species and seven species) or not significant (two ranches with 12 species and 13 species). Harvesting in these cases actually resulted in woodlands with larger diameter trees and more open canopy conditions than the uncut condition. Ranches with heavy harvests had effects that were mostly negative (two ranches with eight species and 12 species). One ranch had relatively similar canopy cover levels for the cut and uncut woodland conditions, and only one and two species were negatively and positively affected, respectively.
To minimize negative effects and have some positive effects, wood cutting should be proportionately directed at trees of all sizes or disproportionately more large trees should be left so that average tree diameter changes relatively little or increases, respectively. In keeping with the tree retention standards employed by the California Department of Fish and Game, immediate post-harvest tree canopy cover should be between 25-40 percent, to minimize adverse effects to wildlife. Land management practices should allow for tree recruitment through stump sprouting, acorn germination, and retention of smaller, presumably younger, trees. Furthermore, snags, shrubs, downed woody debris, acorn-producing trees, brushpiles, and other habitat elements should remain or be enhanced.
prepared and edited by Justin Vreeland, Bill Tietje, and Pam Tinnin