Abstract
First-year tree seedlings represent a particularly vulnerable life stage and successful seedling establishment is crucial for forest regeneration. We investigated the extent to which Pinus ponderosa P. & C. Lawson populations from different climate zones exhibit differential expression of functional traits that may facilitate their establishment. Seeds from two populations from sites with contrasting precipitation and temperature regimes east (PIPO
dry) and west (PIPO
mesic) of the Oregon Cascade mountains were sown in a common garden experiment and grown under two water availability treatments (control and drought). Aboveground biomass accumulation, vegetative phenology, xylem anatomy, plant hydraulic architecture, foliar stable carbon isotope ratios (δ
13C), gas exchange and leaf water relations characteristics were measured. No treatment or population-related differences in leaf water potential were detected. At the end of the first growing season, aboveground biomass was 74 and 44% greater in PIPO
mesic in the control and drought treatments, respectively. By early October, 73% of PIPO
dry seedlings had formed dormant buds compared with only 15% of PIPO
mesic seedlings. Stem theoretical specific conductivity, calculated from tracheid dimensions and packing density, declined from June through September and was nearly twice as high in PIPO
mesic during most of the growing season, consistent with measured values of specific conductivity. Intrinsic water-use efficiency based on δ
13C values was higher in PIPO
dry seedlings for both treatments across all sampling dates. There was a negative relationship between values of δ
13C 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 δ
13C values and photosynthetic CO
2-response curves were 6 and 28% lower, respectively, in PIPO
dry seedlings. Leaf water potential at the turgor loss point was 0.33 MPa more negative in PIPO
dry, independent of treatment. Overall, PIPO
dry seedlings exhibited more conservative behavior, suggesting reduced growth is traded off for increased resistance to drought and extreme temperatures.
Keywords
carbon isotope discrimination,
gas exchange,
genetic variation,
hydraulic architecture,
osmotic potential,
phenology.
Citation
Kerr, K. L.; Meinzer, F. C.; McCulloh, K. A.; Woodruff, D. R.; Marias, D. E. 2015. Expression of functional traits during seedling establishment in two populations of Pinus ponderosa from contrasting climates. Tree Physiology. 35(5): 535-548.