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Key Results and Products from LTER4 Vegetation Studies

Key Results and Products from LTER4 Vegetation Studies

Prepared by Kari Bisbee O'Connell, February, 2002

During LTER4, we continued measurements of the permanent plots in our regional network as well as measurements of plots in three HJA watersheds that were established 40 years ago after experimental logging. Data from the permanent study areas have provided important insights into natural processes of succession, tree mortality, biomass accumulation, and timber growth, and have helped to validate growth-and-yield and mechanistic succession models. For example, recent management plans have proposed to extend the time between timber harvests from the conventional 40 to 80 years to closer to 150 years for federal forests in western Oregon and Washington. Acker et al. (1998) analyzed data from 20 permanent plots established between 1910 and 1940 in natural Douglas-fir stands to investigate whether extended rotations in Douglas-fir forests will promote biological diversity associated with old-growth or will result in large quantities of forfeited timber production. For this set of plots, timber yield declined quite gradually up to the maximum observed age (about 150 years). Plots that started with lower densities of trees were more similar in stand structure to typical old-growth at 150 years of age than those that began with higher densities. Recruitment of medium-sized, shade tolerant trees was negligible over the 4 to 8 decades of record, suggesting that promotion of this component of late successional structure may require management intervention.

From the analysis of 60 years of long-term plot records from the Cascade Head Experimental Forest, Acker et al. (2000) concluded that between the age of 85 and 145 years, bolewood net primary production (NPP) declined by a factor of two. Only 6% of the decline could be accounted for by increases in autotrophic respiration. The most likely explanation is that gross primary production declined significantly during this period.

Acker et al. (2002) used permanent plot observations in young (10-35 yr), mature (100-120 yr), and old (450+ yr) Douglas-fir and western hemlock forests to investigate the relative contributions of reduced NPP and mortality to the decline in stand biomass accumulation with age. Bole biomass accumulation rate increased over time in the young watershed, remained constant over time in the mature watershed, and varied between positive and negative in the old watershed. They concluded that both decreasing bole NPP and increasing tree mortality contribute about equally to the decline in biomass accumulation with stand age. Valerie Frazer, a graduate student from Yale University, cored all the trees in the permanent plot in HJA reference stand 07 (RS07) and analyzed litterfall for an 18-year period up to 1995 to investigate annual variation in bole productivity, the relationship between litterfall and bole productivity, and the correlation between precipitation and temperature and bole productivity. She found that annual bole productivity followed an upward trend for the period between 1971 to 1998, and that cumulative precipitation from mid-May to mid-July was best in predicting annual bole productivity. There was no relationship between litterfall and bole productivity for this stand.

Bible (2001) used plots from our permanent forest plot network to examine temporal patterns of mortality rates, tree population structure, biomass accumulation, input of coarse woody debris, and causes of mortality of Douglas-fir and western hemlock in the Cascade Mountains of western Washington and Oregon. Temporal patterns of stand structure and biomass differed between Douglas-fir and western hemlock (Figure 1 and Figure 2). High mortality rates for Douglas-fir occurred in young to mature stands due largely to suppression, whereas in old growth, mortality rates were substantially lower and causes were from density-independent agents. As growth of old-growth Douglas-fir declined, death of even a few trees removed more biomass than was produced, resulting in a sharp decline in biomass stocks in old-growth forests (Figure 1). Rates of western hemlock mortality were low in young and mature stands, but in old-growth, rates were greater due to a combination of suppression, snow loading and physical damage from falling trees.

Smithwick et al. (in press) approximated an upper limit on C storage in the PNW by estimating total ecosystem carbon (TEC) stores of 43 old-growth forest stands from the PSP network in 5 distinct biogeoclimatic provinces. They suggested that the upper bound of C storage in forests of the PNW is higher than estimates of current C stores, presumably due to a combination of natural and anthropogenic disturbances. This result indicates a potentially substantial and economically significant role of C sequestration in the region. Large-footprint laser altimetry is an emerging remote sensing technology with great promise for describing vertical characteristics of forests, as well as providing more accurate estimates of biomass and leaf area. Collaborative work by the HJA vegetation group and colleagues in the Forest Service and NASA is demonstrating that this technology provides new perspectives on the structure of temperate coniferous forests. In the first publication on this work, Means et al. (1999) have shown that data from an airborne version of large-footprint laser altimetry can be used to predict height, basal area, total biomass, and leaf biomass with r2 values ranging from 0.84 to 0.96 on sites ranging from those with only herbaceous vegetation to old-growth forest.

In 1996, a large wildfire occurred at Torrey-Charlton Research Natural Area where we had existing permanent plots in high-elevation mountain hemlock forest. Since then, the HJA vegetation group has collaborated with the USFS to monitor the progress of early succession after wildfire at Torrey-Charlton. The first publication from this project is currently in preparation. We have improved access to information on our long-term datasets. Portions of the PSP datasets are accessible on-line, and web reports for four sets of permanent plots are currently available (http://www.fsl.orst.edu/lter/research/compplfr.htm). Most significantly, 46 data requests have occurred since 1996 for the PSP dataset.

References

Acker, S.A., T.E. Sabin, L.M. Ganio, and W.A. McKee. 1998. Development of old-growth structure and timber volume growth trends in maturing Douglas-fir stands. Forest Ecology and Management 104:265-280.

Acker, S.A., P.A. Harcombe, S.E. Greene, and M.E. Harmon. 2000. Biomass accumulation over the first 150 years in coastal Oregon spruce?hemlock forest. Journal of Vegetation Science 11: 725-738.

Acker, S. A., C. B. Halpern, M. E. Harmon, and C. T. Dyrness. 2002. Trends in bole biomass accumulation, net primary production, and tree mortality in Pseudotsuga menziesii forests of contrasting age. Tree Physiology, 22: 213-217.

Bible, K. 2001. Long-term patterns of Douglas-fir and western hemlock mortality in the western Cascade Mountains of Washington and Oregon. Ph.D. dissertation. University of Washington.

Means, J.E. S.A. Acker, D.A. Harding, J.B. Blair, M.A. Lefsky, W.B. Cohen, M.E. Harmon, and W.A. McKee. 1999. Use of large-footprint scanning airborne lidar to estimate forest stand characteristics in the western Cascades of Oregon. Remote Sensing of Environment 67:298-308.

Smithwick, E.A.H., M.E. Harmon, S.M. Remillard, and S.A. Acker. 2002. Potential upper bounds of carbon stores in forests of the Pacific Northwest. Ecological Applications 12(5): 1303-1317.