Graduate Thesis Or Dissertation
 

Relative competitive abilities of several common forest species and planted Douglas-fir in western Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x059cb425

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  • In terms of production forestry, more often than not any species that is not the crop species is considered a competitor as they are using finite growing resources that would otherwise be available to the crop species. With specific regard to Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) production in the Pacific Northwest, this study evaluated an array of morphological and physiological plant attributes to discern the relative competitive abilities of several common forest species and planted Douglas-fir in western Oregon during the first year of plantation establishment in the presence and absence of vegetation management treatments. The competitive ability of a species refers to the morphological and physiological characteristics associated with resource acquisition and internal allocation; a concept lacking a specific metric for evaluation. A conceptual model of plant resource utilization including proxy metrics for key aboveground plant-environment interactions was used a framework for synthetic assessment of species relative competitive ability. The relative competitive abilities of species were evaluated over a summer growing season with assessments of saturated specific leaf area (SLA[subscript SAT]) and saturated leaf dry matter content (LDMC[subscript SAT]), diurnal and seasonal leaf-level gas exchange (net photosynthesis (P[subscript n]), stomatal conductance (G[subscript s]), and derived instantaneous water-use efficiency (WUE)), midday leaf xylem pressure potential (Ψ[subscript md]), aboveground proportional allocation of biomass into stem, leaf, and reproductive body components, leaf area index (LAI), and morphological development and growth (height, crown radius, and diameter and bud density for Douglas-fir). The study employed a complete randomized block design (RCBD) with four replicates (blocks) and three vegetation management treatment regimes: untreated control (C), site preparation only (SP), and site preparation with a spring and summer release (SP+R). The relative competitive ability of all species was evaluated in the C, whereas only dominant competitor species remained for evaluation in the SP treatment. Vegetation treatment effects were evaluated among remaining competitor species and Douglas-fir in the C and SP treatments, whereas the response of Douglas-fir was assessed across all three vegetation management treatments (C, SP, SP+R). Selected forest competitor species included two woody perennial shrubs, two ferns, one herbaceous dicot, and two herbaceous graminoid species: trailing blackberry (Rubus ursinus Cham. & Schlecht), snowberry (Symphoricarpos albus (L.) Blake), swordfern (Polystichum munitum (Kaulfuss) K. Presl), bracken fern (Pteridium aquilinum (L.) Kuhn), woodland groundsel (Senecio sylvaticus L.), California brome (Bromus carinatus Hook. & Arn.), and false brome (Brachypodium sylvaticum (Huds.) Beauv). Forest competitor species evaluated in the SP treatment were limited to trailing blackberry, swordfern, woodland groundsel, and California brome. Species relative competitive ability varied dramatically. However, similarities related to herbaceous and woody life forms were observed. Based on both univariate and multivariate response variable analyses, the relative competitive abilities of species examined in the study were ranked as follows: woodland groundsel > false brome > California brome > trailing blackberry = bracken fern > snowberry > swordfern > Douglas-fir. Although vegetation treatments effectively reduced total cover below 20%, a threshold of putative importance, with observed effects on soil moisture content and species performance, species relative competitive ability remained unchanged. Physiological responses were more variable than morphological responses for species performance and expressed greater sensitivity to vegetation treatment. Vegetation treatment effects were most pronounced for Douglas-fir in the SP+R treatment where mean total cover was 6.5%. In the SP+R treatment Douglas-fir exhibited decreased moisture stress coupled with significant increases in both diurnal and seasonal P[subscript n] and G[subscript s] rates and patterns.
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