Considerable interest has developed in restoration of oak savanna which has almost disappeared in much of the United States resulting in endangered species.  In addition, oak savanna provides a fire buffer zone between forests and populated areas because grass fires are cooler than forest canopy fires and easier to control.  Oak trees are resistant to such fires and the wide spacing of the oak trees preclude difficult to control canopy fires.

Here we consider Western Oregon as an example although most of the work on oak savanna has been in the Midwest. A  working document summarizes some of the studies in the scientific literature. While the Calapooya in the Willamette Valley of Western Oregon managed oak savanna with fire,  it is likely that prairie and oak savanna would have existed without their intervention, as occurs from central California to SW Washington. For example, frequent fires from nearby forests in late summer would have likely spread into the Willamette Valley with 2-m high tinder dry prairie grass.  Though not frequent, some cloud to ground lightning strikes occur within the valley and surrounding foothills almost every summer.   Oak savanna supported a variety of forbs in addition to grasses.  Such forbs could compete partly because the grass “canopy” was periodically reduced by fire.

VEGETATION AND SOIL MOISTURE

Grasses growing without restrictions due to burning or grazing can grow in excess of 1 m, sometimes 2 m, with corresponding deep root systems.  Such grasses, whether senescent or growing, shade other native plants and are strong competition for soil moisture during their growing season.  The reduced solar radiation reaching shorter native plants probably delays the onset of native plants in the spring, leading to shorter period of growth before the soil moisture is depleted.  Presumably the optimal native vegetation and optimal grazing differs significantly between flatter ground surfaces and steeper south or southwest facing slopes, particularly those slopes with thin soil.

RESTORATION

Restoration normally begins with thinning of existing white oak forests and removing any other species, normally Doug Fir.  The white oak have normally grown in a relatively dense force and they are tall and spindly, at least with respect to the broad oak trees in a true oak savanna.  The thinning process falls well short of the tree spacing in a true oak savanna because isolated tall white oak are vulnerable to blow down.

GRAZING AND MAINTENANCE OF OAK SAVANNA

Helpful information for restoration of oak savanna in Western Oregon can be found here.   Burning maintains oak savanna by controlling grasses and brush but with a temperature sufficiently low to prevent serious damage to the oak trees.  Burning is generally not an option in the Northwest because of risk of becoming out of control and spreading to high-fuel forested areas and populated areas.  Mechanical removal of invasion of trees and brush is used but is labor intensive and thus expensive.

Light grazing can also be used to maintain oak savanna. The shorter grazed grass develops a less extensive root system and extracts less water. Shorter grass allows more sun to reach the soil surface which allows more rapid emergence of native plants in spring, before the soil dries out.  Cattle hooves can push new seed into the ground in which case the seed during the sprouting stage is more protected against daytime drying.

Advertisements