Quercus Garryana - Natural History

Natural History

Garry oak is the only native oak species in British Columbia, Washington State, and northern Oregon. In these areas, Garry oak woodlands are seral, or early-successional - they depend on disturbance to prevent their being overtaken by Douglas-fir. The disturbance that allowed Garry oak to persist in an area that would otherwise succeed to coniferous forest was primarily fire. Natural wildfires are relatively common in the drier portions of the Pacific Northwest where Garry oak is found, but fire suppression has made such events much less common. In addition, early settler's records, soil surveys, and tribal histories indicate that deliberate burning was widely practiced by the First Nations people of these areas. Fire perpetuated the grasslands that produced food plants such as camas, chocolate lily, bracken fern, and oak; and that provided grazing and easy hunting for deer and elk. Mature Garry oaks are fire resistant, and so would not be severely harmed by grass fires of low intensity. Such fires prevented Douglas-fir, and most other conifer seedlings from becoming established, allowing bunch grass prairie and Garry oak woodland to persist. Fire also kept oak woodlands on drier soils free of a shrub understory. Wetter oak woodlands historically had a substantial shrub understory, primarily snowberry. (Perdue IN Dunn and Ewing).

Garry oak woodlands in British Columbia and Washington state are critical habitats for a number of species that are rare or extirpated in these areas, plant, animal, and bryophyte:

  • Propertius duskywing butterfly Erynnis propertius, sole larval food plant is oak
  • Bucculatrix zophopasta leaf-mining moth, sole larval food plant is oak
  • Lewis woodpecker Melanerpes lewis
  • Slender billed nuthatch Sitta carolinensis aculeata
  • Sharp tailed snake Contia tenuis
  • Western gray squirrel Sciurus griseus
  • Western tanager Piranga ludoviciara
  • Western wood peewee Contopus sordidulus
  • Western bluebird Sialia mexicana
  • Sessile trillium Trillium parviflorum
  • Banded cord-moss Entosthodon fascicularis
  • Apple moss Bartramia stricta
  • (liverwort) Riccia ciliata

(Hanna and Dunn IN Dunn and Ewing; T. Lea, Miles and McIntosh, GOERT Colloquium 2006).

Garry oak woodlands create a landscape mosaic of grassland, savanna, woodland, and closed-canopy forest. This mosaic of varied habitats, in turn, allows many more species to live in this area than would be possible in coniferous forest alone. Parks Canada states that Garry oak woodlands support more species of plants than any other terrestrial ecosystem in British Columbia. It grows in a variety of soil types, for instance, rocky outcrops, glacial gravelly outwash, deep grassland soils, and seasonally flooded riparian areas. (Hanna and Dunn IN Dunn and Ewing; T. Lea, GOERT Colloquium 2006).

The Donation Land Claim Act of 1850 encouraged Anglo settlement of Washington and Oregon, and marked the beginning of the end of regular burning by Indians of the area (Perdue IN Dunn and Ewing). The arrival of Europeans also reduced the number of natural fires that took place in Garry oak habitat. With fire suppression and conversion to agriculture, Garry oak woodlands and bunch grass prairies were invaded by Douglas-fir, Oregon ash (Fraxinus latifolia}, and imported pasture grasses. Oaks were logged to clear land for pasture, and for firewood and fence posts. Livestock grazing trampled and consumed oak seedlings. By the 1990s, more than half the Garry oak woodland habitat in the South Puget Sound area of Washington State was gone. (Hanna and Dunn IN Dunn and Ewing. On Vancouver Island, more than 90% was gone. (T. Lea, GOERT Colloquium 2006). Remaining Garry oak woodlands are threatened by urbanization, conversion to Douglas-fir woodland, and invasion by shrubs, both native and nonnative (Scotch broom Cytisus scoparius, sweetbriar rose Rosa eglanteria, snowberry Symphoricarpos albus, Indian plum Oemleria cerasiformis, poison-oak Toxicodendron diversilobum, English holly Ilex aquifolium, bird cherry Prunus avens). Conversely, oak groves in wetter areas that historically had closed canopies of large trees are becoming crowded with young oaks that grow thin and spindly, due to lack of fires that would clear out seedlings.(Hanna and Dunn IN Dunn and Ewing;

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