Graduate Thesis Or Dissertation
 

Height-related trends in structure and function of Douglas-fir foliage

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dr26z176b

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  • This dissertation investigated the impacts of tree height upon a range of physiological and structural characteristics of Douglas-fir foliage; relationships between structural and functional trends with height; and compensatory mechanisms that mitigate height-related growth constraints. Height-related trends in foliar physiological and anatomical characteristics were examined both within trees as well as between trees of different heights. Emphasis was placed on discovering the mechanistic nature of the observed growth constraints and adaptive strategies. This research was conducted in the interest of enhancing our knowledge of the causes of age- and size-related decline in forest productivity. The research conducted for this dissertation explored several previously uninvestigated mechanisms for size-related reductions in forest productivity that are associated with tree foliar structure and function. Height-related trends in foliar turgor associated with the gravitational hydrostatic pressure gradient were identified as well as subsequent impacts on leaf and shoot morphology. Osmotic potential declined with height providing evidence of osmotic adjustment to offset the height-related decline in leaf turgor. This osmotic adjustment occurred only seasonally, and not during the spring when turgor maintenance is most important for leaf expansion. Gradients in leaf turgor were reflected in height-related trends in leaf and shoot morphology. Previous work has documented height-related trends in foliar morphological characteristics and earlier studies have examined osmotic adjustment as an adaptation to drought and salinity stress. Height-related trends in foliar morphological characteristics have typically been attributed to light gradients along vertical profiles. The work in this dissertation is the first to evaluate the effects of the gravitational component of water potential on the osmotic, morphological and growth characteristics of foliage along a height gradient in tall trees. Trends in leaf hydraulic efficiency and safety along a height gradient were examined, and correlations between changes in leaf hydraulic conductance (Kleaf) and changes in stomatal conductance (gs) were identified that provided evidence of a control mechanism for gs. A consistent relationship was discovered between changes in Kleaf and gs providing evidence that leaf tracheid embolism is associated with stomatal closure during periods of declining water availability. Hydraulic vulnerability curves of foliage collected at different heights indicated that increased height is correlated with reduced hydraulic efficiency and increased hydraulic safety. These opposing trends suggest the existence of a compensatory mechanism in foliage that functions to reduce hydraulic vulnerability at the expense of transport capacity. The research in this dissertation is the first to examine the dynamic relationships between leaf hydraulics and stomatal behavior in temperate conifers, and the first to examine how these characteristics are affected by tree height. Height-related trends in leaf xylem anatomical characteristics were examined and strong correlations were identified between these anatomical characteristics and the patterns of leaf hydraulic efficiency and safety that were observed in previous work for this dissertation. Theoretical estimates of leaf hydraulic efficiency (Kleaf-theoretical) based upon the Hagen-Pouseille equation and measured leaf tracheid anatomical characteristics were strongly correlated with laboratory measurements of Kleaf, providing further evidence of a causal relationship between height-related trends in both leaf tracheid anatomical properties and leaf hydraulic function. Earlier studies have documented connections between leaf anatomical characteristics and leaf physiological characteristics such as correlations between leaf architecture and gas exchange and leaf architecture and transport efficiency. The research in this dissertation however, is the first to examine the impact of tree height on leaf anatomical characteristics and associated shoot physiological properties. Trends in gas-exchange along a height gradient were identified, independent of the immediate effects of path length and gravitational resistance. Photosynthesis under ambient [CO2] declined with height in a manner that was consistent with height-related trends in mesophyll anatomy and independent of leaf nitrogen content. Analyses of mesophyll resistance from assimilation versus internal CO2 concentration (A-Ci) curves indicated that height-related trends in mesophyll resistance were correlated with trends in both photosynthesis and mesophyll anatomy along a height gradient. Analyses of integrated and instantaneous water use efficiency indicated that height-related trends in integrated water use efficiency are attributable to the effects of path length and gravity on stomatal behavior as opposed to a height-related trend in intrinsic foliar properties. The research in this dissertation is the first to isolate the influence of height-related trends in foliar structure on different gas exchange properties such as assimilation and mesophyll conductance, independent of the direct effects of vertical gradients in xylem tension on stomatal and photosynthetic physiology.
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