Understanding how an organism might respond to changing climate requires observing the organism across different environments. Ecologically dominant tree species serve as a major carbon sink to buffer the effects of climate change. However, trees have long generation times that limit the number of individuals that can be assessed through traditional agronomic approaches and our ability to study responses to climate change. Luckily, temperate trees record their history of growth in their trunk as tree rings, which are formed when growth pauses over the cold winter months. Annual growth measurements in conjunction with historical environmental data from weather stations, satellites, and historical records allow the Swarts Lab to observe how a given tree responds to the diverse environments it experiences over its lifespan.
Integrating environmental responses with genetic variation measured from the same individuals, the Swarts lab will be able to identify the genetics underlying adaptive traits. In addition, this approach will allow them to identify individuals better adapted to different environments. Ultimately, the insights obtained by the Swarts lab will be able to inform reforestation decisions, so that following fire or disease, more adapted individuals can be planted to ensure healthy future forests.
Tree rings record differences in growth across years and between individuals. The Swarts lab samples annual growth with hand drills that generate a 5-millimeter increment core. This approach is very similar to a biopsy in humans. Like people, tree growth is influenced by their “home life” as well as their local neighborhood and place of origin. Sampling for the project “Tree Ring Genomics” is carried out in natural forests, where very local influences on growth can be assessed by mapping and recording information for all trees within a 21-meter radius, in addition to the ~80 trees sampled. To capture variation at the “neighborhood” scale, such as differences in elevation and slope, multiple small plots are sampled across a broader environment, such as a valley. Finally, by sampling populations across Europe, the group aims to understand genetic responses to continental-scale differences in climate.
Norway Spruce, a model organism of conifers, is native to central Europe and Scandinavia but has been an important commercial tree for millennia. As a result, it has large, established populations outside of the native range giving spruce a distribution across much of western and central Europe. Including closely related species, it is possible to observe how genetic variation responds to environments across much of Eurasia. Conifer pollen is wide-ranging and can spread up to 400 kilometers. Hence, much of the genetic variation is shared across this range, making it an ideal system for the Swarts lab to understand the genetics underlying adaptation to changing environments.
Boreal conifers are expected to be an important component of climate mitigation as global temperatures continue to rise. However, annual growth can be estimated from any species that forms annual rings, even in the tropics where some species form rings associated with reliable dry seasons. Hence, the Swarts lab’s “Tree Ring Genomics” approach could be easily adapted to examine the health of various forests. Beyond understanding the genetics underlying adaptation to the environment, the Swarts lab aims to use “Tree Ring Genomics” to predict genetic responses to novel environments. “Predicting which trees would be best suited for planting in specific areas would ultimately contribute to maintaining healthy and resilient forests that can withstand the changing climate,” says Kelly Swarts.