Decline in the health and vigor of Oregon white oak

Background/Question/Methods

Oregon white oak (OWO) is a dominant component of the savannah and woodland habitat that was once a large component of the landscape in western Oregon and Washington.  Land use changes over the past century have removed much of this habitat and have threatened or endangered species associated with this ecosystem, including Kincaid’s Lupine, Taylor’s Checker spot butterfly and Fender’s blue butterfly.  Remnants of this habitat can be found in forests in western Oregon and Washington but in many cases, the OWO appear to be declining in health and vigor primarily through competition with Douglas-fir. We’ve evaluated levels of competition from coexisting Douglas-fir and the effects of this competition on OWO vigor.

We selected 12 OWO on a hillside in Corvallis, OR.  Six trees were healthy and vigorous (canopy loss estimated at <30%) and six showed symptoms of declining health and vigor (canopy loss estimated at >60%). Height and diameter at breast height (DBH) of study trees were measured. Plots 16 m in radius were established around three study trees in each health category and the DBH, azimuth and distance from the study tree of each tree > 5 cm DBH in the plot was measured.

Results/Conclusions

OWO study trees did not differ significantly in height but trees with good health had larger mean DBH (0.50 m +/- 0.02 SE) than trees with declining health (0.43 m +/-0.01 SE). Consistent with competition being the cause of OWO decline. tree density was 2.5 times greater in plots surrounding trees with poor health compared to plots surrounding trees in good health (0.052 +/- 0.008 SE and 0.021 +/- 0.001 SE trees m^-2, respectively).  Additionally, dominance of Douglas-fir trees was high in declining OWO plots compared to healthy OWO plots (4.1 – 19.8 cm²m^-2 vs. 0 – 0.05 cm²m^-2, respectively). These results support the hypothesis that shade intolerant OWO are declining as forest density and Douglas-fir dominance increases. Additional data including changes in NSC and wood d¹³C are now being gathered on these study trees to assess the physiological mechanisms involved in the decline of OWO vigor.