Climate change is expected to have a profound impact on forest growth and succession through a variety of mechanisms. Here we focus on temperature and precipitation and their effect on growth of trees. Due to the heat island effect, urban areas have elevated temperatures which can alter the water balance of trees which alters growth. We take advantage of this effect to ask two related questions: 1) What is the relationship between tree growth rate and water balance? 2) Is there an effect of urbanization on tree growth that cannot be explained by water balance?
To address this question we made use of tree growth data from >28000 trees from forest monitoring carried out by the National Park Service and the Smithsonian Institution at 20 protected areas. Monitoring took place in the US Mid-Atlantic region and spanned an urban to rural gradient from small forest fragments in Washington DC to locations in large protected woodlands ~100 km from the city, providing us with a temperature gradient. Climate data was incorporated as water balance, which integrates temperature and precipitation, as well as soil available water capacity, and topography to provide a biologically meaningful connection between climate and growth.
Results/Conclusions
As expected, larger trees had higher absolute growth rates. Climate impacts on tree growth varied by species. Trees species generally had a positive response to actual evapotranspiration (AET) and had negative response to water deficient. Furthermore within most tree species, impacts of climate on tree growth often depended on tree size, with larger trees typically more responsive to the climate gradient than smaller trees. Urbanization, outside of its impacts on climate, had a mixed effect on tree growth. Similar to climate, there was often an interaction between development and tree size.