The relationship between drought resistance traits and seedling establishment in Ponderosa pine populations from contrasting climates

Background/Question/Methods

Vegetation patterns and species distributions are strongly linked to soil moisture regimes, and populations within a species from contrasting climatic regimes could exhibit different establishment mechanisms. Conifer 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 aims to investigate the extent to which species populations from different climate zones exhibit different drought resistance traits and strategies that facilitate their establishment. Seeds from two populations of ponderosa pine (Pinus ponderosa) from sites with contrasting climate regimes east (dry population) and west (wet population) of the Oregon Cascade Mountains were sown in raised soil beds in a common garden and grown under two water availability treatments (high and low). Seedling biomass, leaf-specific conductivity (k_l), rates of desiccation of excised shoots, and foliar stable carbon isotope ratios (δ¹³C) were measured in both populations. We hypothesized that seedlings grown under the low water availability treatment would exhibit greater control over desiccation, reduced k_l, and greater intrinsic water-use efficiency (WUE) based on δ¹³C values. We further hypothesized that seedlings from the dry site population would exhibit more pronounced drought resistance traits.

Results/Conclusions

At the end of the season, seedlings from the wet population had grown more than seedlings from the dry population under both high (p=0.004) and low (p=0.048) water availability treatments. Leaf-specific conductivity was more than twice as high in the wet population for both water availability treatments (p < 0.04) and was consistently, but not significantly lower in the low water treatment.  Intrinsic WUE based on δ¹³C values was higher in the dry site population for both high and low water availability treatments (p = 0.001 to 0.044), but there was no significant treatment effect on δ¹³C values.  There was a negative relationship (p < 0.0001) between values of δ¹³C and k_l across populations and treatments, consistent with greater stomatal constraints on gas exchange with declining seedling hydraulic capacity. Within the treatment, desiccation rates of excised shoots were greater in the dry population (p < 0.05). By mid-November, 92% of seedlings from the dry population had formed dormant buds compared to 33% of seedlings from the wet population, suggesting that phenological differences between the two populations could partly explain differences in their performance. Results from this investigation will advance our understanding of the mechanisms involved in seedling establishment.