Graduate Thesis Or Dissertation
 

The effects of stand density on the growth and microclimate of young Douglas-fir stands

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  • My objectives were to investigate the correlation of height and diameter growth in young Douglas-fir (Pseudotsuga menziesii) plantations with stand density, and to determine the effects of stand density on the canopy boundary layer conductance (g[subscript]ac) and microclimate characteristics of young Douglas-fir plantations. I measured annual height (h) and diameter (d) growth (retrospectively) of trees on three sites in southwestern Washington. Each site consisted of test plantations of eight- and twelve-year-old Douglas-fir in initial planting densities of 300, 1359, and 2960 trees hectare⁻¹. Both height and diameter growth increased with initial seedling density through the fifth year after planting. I used d²h as an index of seedling volume. Mean annual increase in d²h was greatest in the high-density treatment and lowest in the low-density treatment through the fifth year after planting. By the seventh year after planting, this trend was reversed (presumably due to the effects of competition from neighboring trees). The greatest observed difference in annual increase in d²h between high and low-density treatments occurred in the second year after planting. Mean d²h of the high-density treatment was 3 times that in the low-density treatment for that year. Measurements to determine the effects of stand density on the g[subscript]ac and microclimate characteristics were conducted in stands of 5-year-old Douglas-fir trees in densities of 300 and 1360 trees hectare ⁻¹. [CO₂], relative humidity, air temperature and wind speed were measured in a vertical profile at three heights in canopies (one, three and five meters) to gain information about how environmental conditions are affected by stand density. To quantify g[subscript]ac of different densities, evaporation rate of water from canopy foliage was measured. g[subscript]ac was calculated using a form of the Penman equation for evaporation. Differences in all parameters between density treatments decreased with height in the canopy. g[subscript]ac in the low-density treatment for individual trees was 1.5 times that in the high-density stand (p = 0.0038). Canopy boundary layers may reduce atmospheric mixing, resulting in different concentrations of gases within than above the canopy. The higher morning [CO₂] around plant foliage may function to increase morning photosynthetic rates. Stomata may respond to increased humidity by opening more and/or remaining open longer throughout the day, resulting in increased photosynthesis as well as increased photosynthetic water use efficiency.
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