This project is focused on understanding the linkages between water acquisition by temperate forest trees (maples, oaks) and their strategies to acquire mineral nutrients, especially calcium and silica. Trees act as powerful "pumps" to bring up soil water and groundwater, and as the soil wets and dries through the season, trees shift their water sources as needed. The investigation will employ isotopes of hydrogen and oxygen to trace those shifts in a heavily instrumented biogeochemistry study site in central Pennsylvania, part of the NSF Critical Zone Observatory network. At the same time as they acquire water, trees acquire nutrients from the soil. Investigators will test ideas about how mineral nutrient sources may change as water sources change. Trees take up nutrients they need, and when leaf drop occurs, recycle them to the soil. One question is how important is the "mining" of nutrients from deep levels in the nutrient budget. If trees cannot mine nutrients, then they are limited to the store that is in shallow soil and in standing vegetation. Some evidence suggests that in some species tree roots leak water to shallow soils - one possible explanation is that they do this to make nutrients available from otherwise dry, shallow soils during times of water stress. Strontium isotopes and the ratios of germanium to silicon and strontium to calcium will be used to test these hypotheses more explicitly. This builds on previous work on mineral nutrient cycles the investigators have carried out in tropical systems.
The response of forests and of biogeochemical cycles to climate change is an important consideration because of the possible feedbacks on the carbon and water cycles. A prediction of most climate models is for more variable precipitation patterns, with increased periods of drought even under overall wetter conditions. Water and nutrient acquisition strategies for species that are better suited to these variable conditions may predict success patterns among species in adapting to climate change in temperate forests. Such species may also be able to increase their utilization of CO2 under increased CO2 levels if they are better able to acquire nutrients even under conditions of low soil water availability. It is critical to improve our understanding of the coupling between water and nutrient cycles in order to better predict the response of these important ecosystems to climate change, and their future impacts on the carbon and water cycles.
