Graduate Thesis Or Dissertation
 

The expression of functional traits during seedling establishment in two populations of Pinus ponderosa from contrasting climates

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  • Vegetation patterns and species distributions are strongly linked to soil moisture regimes, and populations within a species from contrasting climatic regimes could exhibit differences in the degree to which certain functional traits related to establishment are expressed. Tree seedling survival is crucial for forest regeneration, and thus may be a more important factor in determining species distributions than performance of adult trees. This study investigated the extent to which species populations from different climate zones exhibit differential expression of functional traits that facilitate their establishment. Seeds from two populations of ponderosa pine (Pinus ponderosa) from sites with contrasting climate regimes east (PIPO[subscript EAST]) and west (PIPO[subscript WEST]) of the Oregon Cascade Mountains were sown in raised soil beds in a common garden and grown under two water availability treatments (control and drought). Aboveground biomass accumulation, bud phenology, xylem anatomy, hydraulic architecture, foliar stable carbon isotope ratios (δ¹³C), gas exchange and water relations characteristics were measured in both populations and treatments. At the end of the first growing season, PIPO[subscript WEST] seedlings had grown more than PIPO[subscript EAST] seedlings in both the control (p<0.001) and drought (p=0.042) treatments, while PIPO[subscript EAST] seedlings formed dormant buds and ceased growing much earlier in the fall than PIPO[subscript WEST] seedlings, suggesting that phenological differences between the two populations could partly explain differences in their performance. Vapor pressure deficit (VPD) and low water availability likely resulted in reduced seedling water potential, turgor and rates of expansion of developing tracheids in both populations, resulting in smaller tracheid diameters at maturity, which potentially affected maximum hydraulic conductivity. Xylem-area specific hydraulic conductivity was higher in PIPO[subscript WEST] seedlings compared to PIPO[subscript EAST] seedlings in both the control (p=0.028) and drought (p=0.031) treatments. Intrinsic WUE based on δ¹³C values was higher in PIPO[subscript EAST] seedlings for both control and drought treatments (p<0.001 to p=0.027) across all sampling dates. There was a negative relationship (r²=0.95) between values of δ¹³C and leaf-specific hydraulic conductivity across populations and treatments, consistent with greater stomatal constraints on gas exchange with declining seedling hydraulic capacity. Integrated growing season assimilation and stomatal conductance estimated from foliar δ¹³C values and photosynthetic CO₂ response curves were 7% and 38% greater in PIPO[subscript WEST] seedlings. There were no significant differences found in predawn or midday water potentials between the two populations. PIPO[subscript EAST] seedlings had significantly more negative osmotic potential at full turgor and water potential at the turgor loss point compared to PIPO[subscript WEST] seedlings, suggesting that osmotic adjustment occurs in this population. Overall, PIPO[subscript EAST] seedlings exhibited more conservative behavior, suggesting that a reduction in growth is traded off for an increased resistance to drought. Results from this investigation will advance our understanding of the mechanisms involved in seedling establishment.
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file details KellyKerrL2014.pdf 2017-08-10 Public Télécharger