Effects of Wood Cutting on Wildlife Habitat in Blue Oak Woodlands in the Northern Sacramento Valley

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Wood cutting in California's hard-wood rangelands to modify rangelandand provide wood products has been a persistent land use since the late1700s, when domestic livestock were introduced to California. Between 1945and 1985, almost 1.2 million acres of California's estimated 7.4 millionacres of hardwood woodlands had been converted to other habitats throughtree thinning and clearing. Wood cutting continues throughout the state,particularly in the northern Sacramento Valley where more than 50 percentof California's firewood is harvested in hardwood rangelands dominated byblue oak. Between 1988 and 1992, wood was cut on almost 25,000 acres ofCalifornian hardwood rangelands. Little information exists on effects towildlife from habitat loss or modification due to wood cutting.

This study was conducted to quantify baseline wildlife habitat characteristicsof lands where wood had been cut, as well as estimate effects on wildlifehabitat using existing models of habitat relationships. Data on tree andshrub habitat parameters were collected from vegetation plots located inharvested areas on 12 ranches in Tehama County and seven ranches in ShastaCounty. Blue oak was the dominant tree; foothill pine and interior liveoak were present in fewer numbers.

Pre-cut tree conditions of uncut stands were estimated from relationshipsderived from uncut trees using a statewide growth and yield model for blueoaks. This same model was used to estimate stand growth with and withoutwood cutting over a 50-year period. Effects on 21 wildlife species wereassessed using habitat models from Version 5.2 of the California WildlifeHabitat Relationships System (CWHR) database. These 21 species were selectedbecause they are strongly associated with blue oak habitats and indicatea wide range of habitat conditions and potential effects. Effects were assessedby comparing habitat changes between cut and uncut woodlands. A change inhabitat values of plus or minus 50 percent was considered a significanteffect.

Differences in habitat conditions between cut and uncut conditions wereplainly evident. Wood cutting resulted in substantially lower amounts ofcanopy cover, while stem densities were variously affected depending onthe tree diameters selected during the harvest. Among the 19 ranches studied,uncut conditions had the greatest amount of canopy cover over the 50-yearperiod, ranging from 26-83 percent during the first decade and growing to38-94 percent during the fifth decade. Average stem diameters for the uncutcondition averaged 7.4-12.3 inches during the first decade, growing to 10.1-13.3inches during the fifth decade. In contrast, when cut, canopy cover andstem diameters averaged 0-53 percent and 6.5-14.1 inches, respectively,during the first decade. By the fifth decade after cutting, canopy coverand stem diameters averaged 7-83 percent and 9.6-16.7 inches, respectively.

Two of the five ranch scenarios used for assessing effects to wildlifehad relatively similar stem diameters and canopy cover between the cut anduncut conditions. This may indicate that harvesting was proportionatelydirected at trees of all sizes. The other three scenarios had substantialdecreases in stem diameter and canopy cover, indicating that larger treeswere 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 grasslandfor the first decade. One ranch remained annual grassland over the 50-yearperiod because tree canopy never exceeded 10 percent, while two ranchesgrew into blue oak woodland habitat by the second decade.

Wildlife were variously affected depending on the CWHR-predicted habitatsuitabilities for each species. Generally, species favoring closed-canopyconditions with larger diameter trees, such as the Cooper's hawk and Pacific-slopeflycatcher, were negatively affected when cutting resulted in grasslandsor oak woodlands with small diameter trees and open canopies. Conversely,species favoring grasslands or very open woodlands, such as the westernmeadowlark, California ground squirrel, and gopher snake, were positivelyaffected. Some species favoring tree habitats with open canopies, such asthe mourning dove, ash-throated flycatcher, western bluebird, gray fox,and mule deer, were positively affected or unaffected depending on the magnitudeof the harvest. The western screech owl, acorn woodpecker, scrub jay, bushtit,white-breasted nuthatch, and western gray squirrel were significantly negativelyaffected only when substantial differences occurred between the cut anduncut 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 magnitudeof tree cutting influenced effects to wildlife. Scenarios with relativelysmall amounts of tree cutting had predicted impacts that were either positive(two ranches with nine species and seven species) or not significant (tworanches with 12 species and 13 species). Harvesting in these cases actuallyresulted in woodlands with larger diameter trees and more open canopy conditionsthan the uncut condition. Ranches with heavy harvests had effects that weremostly negative (two ranches with eight species and 12 species). One ranchhad relatively similar canopy cover levels for the cut and uncut woodlandconditions, and only one and two species were negatively and positivelyaffected, respectively.

To minimize negative effects and have some positive effects, wood cuttingshould be proportionately directed at trees of all sizes or disproportionatelymore large trees should be left so that average tree diameter changes relativelylittle or increases, respectively. In keeping with the tree retention standardsemployed by the California Department of Fish and Game, immediate post-harvesttree canopy cover should be between 25-40 percent, to minimize adverse effectsto wildlife. Land management practices should allow for tree recruitmentthrough stump sprouting, acorn germination, and retention of smaller, presumablyyounger, trees. Furthermore, snags, shrubs, downed woody debris, acorn-producingtrees, brushpiles, and other habitat elements should remain or be enhanced.


prepared and edited by Justin Vreeland, Bill Tietje, and Pam Tinnin