Moisture matters: Linking soil water availability to forest productivity

Background/Question/Methods

The availability of soil water is critical in determining the growth and productivity of both trees and forests, yet measurements of available soil water are often overlooked in ecological studies. As such, few studies investigating the linkage between in situ available soil water and annual tree growth exist. We explored this relationship between available soil water and forest productivity by utilizing a unique 45-year record of soil moisture from the Marcell Experimental Forest (MEF) in northern Minnesota. Nine 1/20^th ha plots were established in each of three dominant upland cover types in Minnesota  (aspen, red pine, and northern hardwoods). Plots were established along topographic gradients in the ridge, sidelsope, and toeslope positions. We installed a total of three transects per cover type and had 27 total plots. Two increment cores were collected at diameter-at-breast-height (DBH) from each tree (DBH > 10 cm) within the plots. A total of 2265 increment cores were collected for analysis. Increment cores were mounted, measured, and cross-dated according to standard dendrochronological procedures and growth-climate relationships were calculated using correlation analysis with seasonal soil moisture, mean monthly precipitation, and mean monthly temperature, which were all measured at the MEF.

Results/Conclusions

Annual forest productivity varied between cover types and topographic position. As anticipated, red pine exhibited the higher annual growth rates than either aspen or northern hardwood stands. Plots located on sideslopes had lower available soil water and lower annual growth than plots located on ridges or toeslopes. Variables that contributed significantly to growth varied amongst species but, in the majority of species, growth was strongly correlated with seasonal available soil water (p < 0.05). Unexpectedly, temperature was significantly correlated with annual growth. These results suggest that available soil water may be a limiting factor of tree growth for a variety of species living in northern Minnesota. Given that available soil water has been declining at the MEF over the past-45 years, species ranges in the region may begin to shift as water availability declines due to climatic changes.