Graduate Thesis Or Dissertation
 

Size-related variation in tree growth and physiology

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  • The goal of this dissertation was to improve our understanding of age-related constraints on aboveground production of forest trees. Previous research suggesting that carbon uptake of old trees is hydraulically constrained by tree size was used as the springboard for this research. Three specific working hypotheses were investigated: 1) compensation for height via decreasing the ratio of leaf area to sapwood area as trees become taller occurs for all tree species, 2) compensation via both physiological and structural mechanisms is insufficient to prevent age- or height-related reductions in stomatal conductance and photosynthesis of Douglas-fir, and 3) removal of competition via stand density reductions allows old ponderosa pine to increase growth via increased stomatal conductance. I found that, for most species in which data were available, the ratio of leaf area to sapwood area declines as trees become taller. This response should act to maintain leaf-specific hydraulic sufficiency as trees become taller. This decline was observed both within- and between-species growing in wet vs. dry climates, and for broadleaf and needleleaf species. Two species (Norway spruce and Balsam fir) increased the ratio with height, showing that compensation for hydraulic constraints via reducing leaf area per unit sapwood area is not a universal response. In Douglas-fir trees ranging in height from 15 m to 60 m, hydraulic limitation and compensation co-occur. Growth efficiency, stomatal conductance as indexed using stable carbon isotopes, and leaf-specific hydraulic conductance declined as trees became taller. A simple model based on Darcy's Law showed that the observed 44% decline in hydraulic conductance with increasing tree height would have been greater than 70% had the leaf area to sapwood area ratio and the soil-to-leaf water potential not changed in concert with height. I observed that growth and carbon isotope discrimination of 250-year-old ponderosa pine are very sensitive to increasing moisture availability after stand density reductions. The assumptions that old trees are unable to respond to increased growing space, or that they are genetically predisposed to grow slowly, were shown to be false. Together, these results support the theory that hydraulic constraints to tree height exist. Furthermore, these studies show that resource availability and hydraulic compensation can mitigate size-induced changes in hydraulic conductance and stomatal conductance. Future research on the mechanisms of age-related growth decline should focus on testing multiple hypotheses within the same set of forests. Incorporation of model predictions of each hypothesis will allow estimation of the relative role of each potential mechanism. Future research on the hydraulic limitation hypothesis should investigate the consequences of compensation for hydraulic driving forces-such as height-induced reductions in liquid phase conductance-on gas exchange and carbon allocation.
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