Long-term global change effects on forest biogeochemistry in the north-central United States

Human activities have substantially altered the composition of the atmosphere and many of these changes directly affect the biogeochemistry of forest ecosystems. Because of the geography of industrialization, these impacts are particularly acute in northern temperate forests. Unfortunately, most studies examining the effects of altered atmospheric composition on forest ecosystems may not be accurate predictors of the long-term impacts on mature forests because these studies used immature trees and were short in duration. Here, I use measurements from two large long-term collaborative experiments to examine the impacts of altered atmospheric composition on forest biogeochemistry in the north-central United States.At the Rhinelander free-air carbon dioxide (CO2) enrichment experiment in Wisconsin, I examined the independent and interactive effects of increased concentrations of atmospheric CO2 and tropospheric ozone (O3) on leaf production and soil carbon (C) storage in three forest communities. To estimate leaf production, litter traps were used to collect fallen leaves from 2002 to 2008 (years five through eleven of the experiment). In addition to leaf production (g m-2), these collections were used to assess leaf area (m2 m-2), leaf litter nitrogen (N) concentration (mg g-1), and the leaf N content (g N m-2). To estimate changes in soil C pools, the top 20 cm of the mineral soil was sampled seven times between 1998 and 2008. Initially, soils from the +CO2+O3 treatment had less new C than soils from the +CO2 treatment, but this difference gradually disappeared. This gradual disappearance matched trends in fine root production. Combining the leaf production study with the soil C study, these results suggest that the rate of soil C cycling accelerated under elevated CO2 and declined under elevated O3 because changes in soil C accumulation did not match changes in litter production.The other long-term experiment tests the influence of atmospheric deposition on four mature northern hardwood forests spread across 500 km in northern Michigan. Using data only from the plots receiving ambient deposition, I found that there have been decreases in leaf sulfur, calcium, and aluminum concentrations over the past two decades. Acid deposition usually increases concentrations of these elements in soil solution, so the observed changes in leaf chemistry signal a waning influence of this pollutant. In comparison, leaf delta 15N and soil lysimeter data show that persistent ambient N deposition has caused widespread increases in both the availability of inorganic nitrogen and soil nitrate leaching. The declining influence of acid deposition shows that environmental policy can quickly and broadly influence forest biogeochemistry. Although there are large amounts of nitrate being leached from these forests as a result of ambient N deposition, the parallel N addition experiment at these same sites resulted in increased aboveground growth. We tested the effects of 14 years of N additions on photosynthesis in two ways: by making instantaneous measurement from both canopy towers and excised branches, and by analyzing leaf tissue for delta13C and delta18O, isotopes integrate changes in photosynthesis through time. Trees receiving N additions had higher foliar N concentrations, but there were no differences in instantaneous measurements of photosynthesis from canopy towers or excised branches. Further, there were no significant changes in delta13C and delta 18O in either current foliage or leaf litter collected annually throughout the N addition experiment (1994-2007). Together, these data suggest that increases in photosynthesis are not responsible for the higher rates of aboveground growth.Together, these experiments show that changes in atmospheric composition expected to occur in the next century will alter the functioning of forest ecosystems in the northcentral United States. However, predictions from short-term experiments did not often match the results observed in these long-term projects. Alternately, the recovery of forests in the north-central United States from acid deposition suggests that forest biogeochemistry can respond positively if pollution reductions are prioritized by policy makers. (Abstract shortened by UMI.)...