Forests of the future: Demographic responses of juvenile trees to soil warming at Harvard Forest

Background/Question/Methods   How will the abundance and distribution of eastern temperate forests and species change in response to climate warming? Projected increases in surface temperatures of 2-6 C and associated changes in soil moisture will affect temperate forest tree recruitment. It has recently become apparent that seedling recruitment most often determines tree species' abundances and range limits. In contrast to mature trees, which have extensive root systems and carbohydrate and water reserves that buffer against environmental variation, seedlings can be highly sensitive to short-term changes in climate. Juvenile tree demographic responses and establishment success are starting to be used to track species' range shifts in response to changing climate, but surprisingly few studies examine recruitment impacts of climate variation and change in relevant settings. At Harvard Forest we began monitoring the growth and survivorship of over 800 naturally-recruited juvenile trees in 2003 and have annually re-censured them through 2009. Half of these trees were in a 900-m2 plot with soil temperature increased by 5 degree C above ambient, and the remaining occurred in an adjacent control plot.

Results/Conclusions   Species that grew faster with soil warming include sugar maple (Acer saccharum, p<0.05 in all years), black cherry (Prunus serotina, p<0.05 in all years), red maple (A. rubrum, p<0.05 in most years), and white ash (Fraxinus americana, p<0.10 in most years). Some of these species showed earlier spring bud-break and a longer growing season (sugar maple, cherry), whereas others did not (red maple, ash). July maximum photosynthetic rates were also increased for sugar maple and black cherry. We believe these responses are at least partially mediated by warming-induced increases in soil nitrogen mineralization rates. As juveniles, these species are all shade-tolerant and thus better able to take advantage of increased nitrogen availability in shady forest understories (generally ranging from 3-8% full sunlight). In addition, all of these species associate with arbuscular mycorrhizal fungi which are especially adept at garnering soil phosphorus resources. We believe that an important response to warmer soils in the eastern U.S. will be an enhanced successional shift towards shade-tolerant, usually slower-growing, tree species which on an annual basis sequester less carbon in woody tissues, thus providing a positive feedback to climate warming