Response of Overstory and Understory Conifers to Gap Size and Resource Levels

Andrew N Gray, Thomas A Spies, PNW Research Station, USDA Forest Service, Corvallis, OR, and Robert J Pabst, Oregon State University, Corvallis, OR

This study examined the long-term response of overstory and understory trees to creation of canopy gaps in mature Douglas-fir/western hemlock forests in the Cascade Range. Four circular gap sizes (diameters 0.2, 0.4, 0.6 and 1.0 times canopy height) and controls were established in four stands in 1990. Overstory trees in controls and within 10-20 m of gap edges were mapped and diameters measured prior to gap creation and 7 years later. A subsample of trees were cored to quantify growth before and after gap creation. Overstory tree mortality was evaluated annually. The tallest saplings of each species in each 4 x 4 m cell across the gaps were mapped and measured in 1998 and 2000. Results of the core analysis indicated that old-growth trees had a greater growth response to gap formation (137% of pre-gap growth rates) than mature trees (114%). However, diameter measurements of large trees were not accurate enough to detect a growth response. Growth of intermediate-stature, shade-tolerant trees tended to be greater on north sides than on south sides of small (0.2 and 0.4) gaps; the reverse was true for large (0.4 and 1.0) gaps. Overstory tree mortality did not vary with gap size. Sapling growth across gaps suggested trade-offs between optimal levels of solar radiation and soil moisture, with radiation being relatively more important than soil moisture for Pseudotsuga menziesii than for Tsuga heterophylla and Abies amabilis.