Assessing the dynamic effects of climate on individual tree growth throughout the development of naturally regenerated secondary forests
Tree growth increments are commonly used in dendroclimatology to infer past climatic conditions. To this end, trees growing in open light within extreme environments are preferentially selected to minimize the effects of competition and maximize sensitivity to climate. There is greater interest, from a forest dynamics perspective, in understanding the effects of climate on trees growing in naturally generated forests where competition for light and other resources exists. Here we develop and apply a dynamic hierarchical model to analyze the effects of climate on individual tree growth for over 2,000 trees from 35 naturally regenerated forest stands in northern Minnesota. PRISM output for each stand is used to derive past climatic growing conditions. The primary goal of the analysis is to understand the effects of temperature and water balance metrics (e.g., actual and potential evapotranspiration [AET, PET] and climatic deficit) on annual tree growth increment, and secondarily, to understand how the relationships between tree growth and climate variables evolve over the lifespan of a given tree. The model provides annual estimates of the effect of each climate variable on tree growth from 1897 to present.
There are strong annual effects on tree growth after controlling for tree age and diameter. The strength and direction of annual effects evolve through time capturing long-term growth trends and inter-annual growth variability. Results from the dynamic hierarchical model indicate that tree growth is most responsive to AET in the growing season (combined metric of energy and water availability) and minimum temperature during the spring (associated with the start of the growing season) with average annual growth increasing as a function of both AET and spring temperature. Climate variables, importantly, are significant only in years where there is a large deviation from average climatic conditions. Changes in the magnitude and direction of climate coefficients through time indicate that the effects of climate on tree growth are non-stationary likely due to complex interactions with forest health, competition level, and unmeasured micro-climatic conditions. These results differ from comparable non-dynamic models, which indicate that climate variables have significant unidirectional effects on tree growth. The study findings emphasize the importance of not oversimplifying the role of climate in models of forest ecosystem change. Future work will focus on isolating the effects of competition and climate on individual tree growth.