Enhancing Engelmann spruce productivity with careful seedling selection
Engelmann spruce trees from a variety of seed sources are surviving and growing at a provenance study established in 1970 on the White River National Forest.
Reforestation success and subsequent tree growth are predicted to decline in the future due to more severe droughts and warmer temperatures. However, trees that grow across a wide geographic range are often adapted to their local climate. Therefore, the use of seeds from areas that are warmer and drier for reforestation efforts might enhance success and tree growth.
In our study of Engelmann spruce trees sourced from a variety of areas, we found that productivity of future high elevation Engelmann spruce forests can be enhanced by planting seedlings from seeds that are genetically faster growers, faster reproducers, and/or better survivors. Therefore, forest managers can tailor their tree plantings to achieve specific goals at their site, whether it is reducing costs by having high post-planting survival or reducing the time to timber harvest by planting fast-growing trees. We planted Engelmann spruce seedlings from 20 different environments and took measurements on them over 45 years to compare survival and productivity of differently adapted trees at a high elevation site near Vail, Colorado. Our analysis shows that Engelmann spruce seeds from as north as Canada down to southern New Mexico are able to thrive as seedlings and young adults and can bounce back after drought at the site. There was only one seed source that had very low survival and growth over the 45 years of the study, potentially due to its adaptation to significant summer moisture, which does not typically occur in central Colorado.
The similarity of Engelmann spruce seed sources in their growth responses to climate show that this species reacts positively to warm temperatures throughout the year and dry conditions during the winter, but that it reacts negatively to dry growing seasons at our site. Given that our site has a relatively cool and dry climate, future warmer temperatures may promote growth, but only if they come with ample moisture. If spring conditions become drier, tree growth may be maintained by planting seeds from warmer and drier original locations, which we found to be less severely affected by hot and dry springs. The results of our study also bode well for assisted population migration efforts, demonstrating that even trees adapted to temperatures much warmer than the planting site can survive and grow there for the long-term. This means that we may be able to proactively plant trees now that are better adapted to future conditions, thereby giving the future forests a head start. These results are already being used to inform tree planting on high elevation National Forests and will promote productive future forests.