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Accumulating evidence points to an anthropogenic ‘fingerprint’ on the global climate change that has occurred in the last century. Climate change has, and will continue to have, profound effects on the structure and function of terrestrial ecosystems. As such, there is a critical need to continue to develop a sound scientific basis for national and international policies regulating carbon sequestration and greenhouse gas emissions. This paper reflects on the nature of current global change experiments, and provides recommendations for a unified multidisciplinary approach to future research in this dynamic field. These recommendations include: (1) better integration between experiments and models, and amongst experimental, monitoring, and space-for-time studies; (2) stable and increased support for long-term studies and multi-factor experiments; (3) explicit inclusion of biodiversity, disturbance, and extreme events in experiments and models; (4) consideration of timing vs intensity of global change factors in experiments and models; (5) evaluation of potential thresholds or ecosystem ‘tipping points’; and (6) increased support for model–model and model–experiment comparisons. These recommendations, which reflect discussions within the TERACC international network of global change scientists, will facilitate the unraveling of the complex direct and indirect effects of global climate change on terrestrial ecosystems and their components. [Copyright &y& Elsevier]

Atmospheric nitrogen deposition to forested ecosystems is a concern because of both geochemical and biological consequences for ecosystem integrity. High levels of prolonged N deposition can lead to “N saturation” of the ecosystem. The Bear Brook Watershed in Maine is a long-term, paired forested watershed experiment with over a decade of experimental N additions (∼34 kg ha−1 per year=ambient+treatment) to investigate the biogeochemical consequences of N saturation. Both in situ and laboratory studies of N mineralization and nitrification were carried out to evaluate the changes in N cycling brought about by the long-term N additions. Consistent with hypotheses set forth in the literature (sensu [BioScience 39 (1989) 378]), the treated watershed had higher rates of N cycling compared to the reference watershed. In addition, we report important differences in N cycling rates as a function of forest cover type and soil horizon. Higher rates of net N mineralization occurred in hardwood O horizons compared to softwoods, but the opposite was true in the mineral soils suggesting an important link between litter type and N mineralization that varies with depth in the pedon. Nitrification showed the greatest response to N treatments, with the majority of mineralized N subsequently oxidized to nitrate in the mineral soils. By comparing the data herein with that previously reported for the Bear Brook experiment, it appears that the ecosystem response to N treatment continues to evolve on a decadal time scale and inherent differences in forest cover types and their underlying soils alter the fate of depositional N. [Copyright &y& Elsevier]

A field experiment was established in a high elevation red spruce ( Picea rubens Sarg.) – balsam fir ( Abies balsamea ) forest on Mount Ascutney Vermont, USA in 1988 to test the nitrogen (N) saturation hypothesis, and to better understand the mechanisms causing forest decline at the time. The study established replicate control, low and high dose nitrogen addition plots (i.e., 0, 15.7 and 31.4 kg NH 4 Cl-N ha − 1 yr − 1 ). The treatments began in 1988 and continued annually until 2010, but monitoring has continued to present. During the fertilization period, forest floor C:N, net in situ N mineralization, spruce foliar Ca%, and live spruce basal area decreased with increasing N addition, while foliar spruce N% and forest floor net nitrification increased with increasing N addition. The control plots aggraded forest floor N at a rate equal to the sum of the net in situ N mineralization plus average ambient deposition. Conversely, N addition plots lost forest floor N. Following the termination of N additions in 2010, the measured tree components returned to pre-treatment levels, but forest floor processes were slower to respond. During the 30 year study, site surface air temperature has increased by 0.5 °C per decade, and total N deposition has decreased 5.5 to 4.0 kg N ha − 1 yr − 1 . There have also been three significant drought years and at least one freeze injury year after which much of the forest mortality on the N addition plots occurred. Given that there was no control for the air temperature increase, discussion of the interactive impacts of climate and change and N addition is only subjective. Predicted changes in climate, N deposition and other stressors suggest that even in the absence of N saturation, regeneration of the spruce-fir ecosystem into the next century seems unlikely despite recent region-wide growth increases. [ABSTRACT FROM AUTHOR]