PS 3-31
Effects of experimental climate warming on seedling growth for three subalpine conifers along a forest to alpine tundra gradient
Global warming is anticipated to shift tree species’ current ranges. These projections rely on climate envelope models developed from observations of the adult life-stage. However, because seedling recruitment is a critical bottleneck to tree and forest range expansion, we need to understand environmental controls on seedling emergence, growth, and establishment. Our objective was to examine variation in seedling growth of three subalpine tree species (Picea Engelmannii, Pinus contorta, Pinus flexillis) from two seed source elevations sown in common gardens at forest, treeline, and alpine sites at Niwot Ridge, Colorado, and subjected to heating and watering treatments. Seedlings that emerged from seed sown the previous fall were harvested at approximately three and six weeks old. Above- and belowground biomass was obtained on oven-dried samples. Above- and belowground length and silhouette area were obtained from photographs of fresh samples using image-processing software. We hypothesized that seedling growth differs among species and seed provenances, and is influenced by environmental conditions such as soil temperature and moisture. After snowmelt when soil moisture is high, we expect root growth to be controlled by temperature. As soils dry, overall growth (and survival) is likely limited by the root’s ability to access receding soil moisture.
Results/Conclusions
Our preliminary analyses indicate that aboveground seedling biomass increased with warming, while root biomass decreased with heating. Above- and belowground biomass was greatest for P. flexilis and for the low-elevation provenances. Aboveground growth was greatest in the treeline ecotone, while root growth was greatest at the lowest forest site. Root:total biomass decreased with heating and with watering, was greatest at the lower forest site and for P. Engelmannii, and remained constant over time. Greater first year survival in P. flexilis and in the low elevation provenances may be explained by their faster overall growth rate, which we observed previously. In addition, without increased soil moisture, warming may negatively impact seedling growth (and survival). To better link growth responses to survival, we are evaluating whether warming increases root-length growth, with the expectation that specific root length (cm/g) is not conserved. Because variation in microclimate among plots within and across treatments may be a more direct control on seedling growth, we will further evaluate our hypotheses by quantifying sensitivity of growth rates to continuous variation in soil temperature and moisture.