Fraser, Valerie D. 2001. Biomass and productivity in an old-growth Douglas-fir/western hemlock stand in the western Cascades of Oregon. New Haven, CT: Yale University; project report for a non-thesis M.S. degree. 15 p.
Annual bole productivity was determined for a 0.25-ha old-growth Douglas-fir (Pseudotsuga menziesii)/western hemlock (Tsuga heterophylla) stand in western Oregon for a 29-year period using increment cores, repetitive stem diameter remeasurement and mortality monitoring. Annual values for fine and coarse litterfall were estimated for an18-year period. These data were analyzed to investigate changes in standing bole biomass with time, annual variation in bole productivity, monthly and annual patterns of litterfall, and the correlation between bole productivity and litterfall. The effects of climatic variables (precipitation, maximum and minimum temperature), treefall, and atmospheric carbon dioxide on bole productivity were also examined.
Standing bole biomass increased from 543 Mg/ha in 1971 to 587 Mg/ha in 1999, with mortality accounting for 1.1 Mg/ha/yr. Annual bole productivity averaged 2.7 Mg/ha/yr and followed an upward trend of 0.0125 Mg/ha/yr. Fine litterfall ranged from 2.2Mg/ha/yr to 3.3 Mg/ha/yr. No relationship was apparent between bole productivity and litterfall.
The critical time period for cumulative precipitation in predicting annual bole productivity was from mid-May to mid-July. Annual bole productivity was best predicted by mean maximum temperature during this same time period, with higher temperatures projecting lower productivity for the year. When using mean minimum temperature as a predictor for bole productivity, the critical period extended from mid-April to mid-August.
Because it has been shown in earlier studies that productivity in old-growth forests either reaches a "steady state" or declines over time (Grier & Logan 1977, DeBell & Franklin 1987), it could be argued that the upward trend in productivity in this stand corresponds to the similar trend in atmospheric carbon dioxide. However, a closer analysis of the available long-term data indicates that it is likely that some trees within the stand experienced accelerated growth following tree falls on the edge of the stand, and that this growth response accounts for the upward trend in productivity for the stand as a whole.
These results illustrate 1) the value of long-term data sets in this line of research and 2) the importance of examining local effects before applying results on an expanded scale. Because this long-term reference stand is only one of many in an extensive network, an excellent opportunity exists to examine productivity trends on larger spatial and temporal scales.