Print PageClimate warming facilities (18-m2 chambers) were established at Harvard and Duke Forests in 2008-9, providing three open-top chamber (OTC) warming treatments: +0, +3 and +5oC soil and air temperatures. These two forests were selected for similar land-use histories and canopy oak-maple dominants. At each forest, we manually cleared gaps to provide a high-light treatment to compare with warming responses measured in adjacent intact forest understories. In each forest, each gap/understory habitat had nine OTC’s installed (N=3). Tree seeds were randomly assigned to each chamber and planted in 2009 and 2010. The same species originating from the same geographical genetic source(s) were planted at each forest.
We hypothesized light treatments would have bigger impacts on tree seedling growth rates than warming treatments. We also hypothesized that shade-tolerant trees would respond to warming more strongly in the shade by growing faster with warming, and shade-intolerant species would respond most strongly in gaps. We will present results from these new experiments and compare findings to longer-term soil warming research at Harvard Forest initiated in 2003.
Results/Conclusions
Most species grew faster in gaps than in forest understories, including shade-tolerant species (red maple, p=0.04; black gum, p=0.05) and shade-intolerant taxa (sweetgum, p=0.04; tulip poplar, p=0.07; loblolly p=0.05). However, shade-tolerant black cherry and southern magnolia grew larger in shaded than in gap habitats (p=0.02 and 0.04).
Responses to warming treatments depended on light availability. For example, red maple exhibited no growth enhancements with +3C warming in either gaps or shade. Yet understory maple trees grew 40% taller in response to +5C warming, while gap maples grew 102% taller, suggesting this shade-tolerant species will show the biggest warming impacts on growth in gap environments (p=0.02). In contrast to our hypothesis, shade-intolerant sweetgum and tulip poplar responded to the +5C warming by growing less quickly and reaching a smaller final height in both the gap and understory habitats. The response of intermediately-tolerant white oak to warming depended on light availability (p=0.05).
In our longer-term study at Harvard Forest, juvenile tree species that consistently grow faster, and often survive better, in response to +5C soil warming are those associating with arbuscular mycorrhizal fungi (sugar maple, red maple, black cherry, white ash), highlighting the importance of AM fungal symbioses for future forest tree composition. Impacts of climate warming on growth and competitiveness depend on tree species and abilities to interact with arbuscular mycorrhizae, as well as light environments.
