Graduate Thesis Or Dissertation
 

Production and turnover of fine roots of mature Douglas-fir in relation to site

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  • Fine roots are a dynamic component of Douglas-fir ecosystems. Changes in standing crops indicate that fine roots require a major portion of the stand's energy resources to fulfill their role in the functioning of these systems. We observed standing crops of live and dead fine (< 1 mm diameter) and small (1-5 mm diameter) roots over a 30-month period based on intact soil core samples taken monthly from grids established in dry, moderate, and wet stands of mature Douglas-fir. Environmental measurements included temperature of soil and air, water potential of soil and xylem at predawn, and potential evaporation. Changes in standing crops of live and dead fine roots in the top 75 cm of soil indicate that seasonal patterns vary from one year to next and can vary by site, but may not. Differences from one year to the next may be greater than differences between sites. Large changes in standing crop can occur within a few months. Fine roots are most abundant in the uppermost layer of soil; 75% are in the top 25 cm of soil. Changes within various layers by depth are statistically significant during seasonal and "long-term" time frames and are most pronounced in upper layers. Counts of "new" root-tips taken from these samples permit an assessment of root activity independent of changes in standing crop. Periods of root-tip activity do not necessarily correspond to changes in standing crop, indicating that fine-root growth and mortality can occur simultaneously in the same area. Based on a simple conceptual model of fine-root dynamics we estimated fine-root production and turnover by inferring transfers needed to account for observed standing crops of live and dead fine roots. When averaged over the entire study period, we estimate fine-root growth as 6.5, 6.3, and 4.8 Mg/ha/yr, fine-root mortality as 7.2, 7.2, and 5.5 Mg/ha/yr, and decomposition of fine roots as 8.2, 8.0, and 6.9 Mg/ha/yr for the dry, moderate, and wet sites, respectively. These values are 3-5 times greater than foliage litterfall. Thus fine roots constitute a major sink of carbohydrates and source of detritus for the belowground ecosystem.
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