Contributions of Epicormic Branches and Delayed Adaptive Foliage in Coastal Douglas-fir under Variable Density Management Regimes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jq085r01s

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  • The contribution of delayed adaptive reiteration to crown maintenance was explored across a wide range of adjacent open space conditions in early-mature (approximately 60 year old) Douglas-fir located on the eastern slope of Oregon’s Coast Range. The stands had experienced uniform thinning in 1964-65 and 1980-81 to release dominant and codominant trees, and again in 1993 to create a wide range of stand density and spatial uniformity conditions. A subset of plots had been re-thinned in 2001 to return them to target densities. A combination of ground-based, in-crown, and needle cohort measurements were employed to characterize branch and foliage characteristics. Epicormic branches were present throughout tree crowns but contributed less than 10 percent of total branch length and only 2.4 percent of foliage mass. Very few epicormic branches occurred below the base of the regular crown in the sample trees, and those present were too small to impact log or lumber quality. However, reiteration of foliage from dormant buds (delayed adaptive reiteration) was ubiquitous, occupying 60 percent of total branch length and accounting for more than 40 percent of total foliage mass. The extent of adjacent open space did not influence patterns of branch length or location for either original or epicormic branches, nor did it affect the proportion of branch length occupied by delayed foliage. Increasing adjacent open space may have had a modest negative impact on the proportion of sequential (regular) foliage occurring on original branches (p=0.0548).Paired samples of delayed and sequential (regular) foliage were compared to determine if they differed in structure or physiological performance. Regardless of crown position, delayed foliage had higher average specific leaf area (SLA) and exhibited higher levels of discrimination against 13C (Δ13C), lower intrinsic water use efficiency (iWUE), and higher δ18O, than sequential foliage. The results indicated that delayed foliage was, on average, more shade adapted than sequential foliage. In addition, cohorts of both types of foliage demonstrated distinct reductions in average SLA with increasing age of their leaf cohort, a result attributed to more rapid shedding of high-SLA needles. Year-to-year variation in Δ13C, iWUE, and δ18O was correlated with weather conditions, but trends were complicated by the integration of isotope signals across multiple growth seasons. Delayed foliage provides Douglas-fir with an ongoing source of new leaf area and the capacity to adapt to changing growth conditions. It provides a significant proportion of the species’ photosynthetic capacity, and very likely increases its ability to recover from crown damage, foliage disease, and herbivory. It may allow Douglas-fir to more readily utilize the increased levels of CO2 available in the earth’s atmosphere, and to respond positively to other environmental changes at both local (site-level) and global (climatic) scales.
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