Christopher W. Swanston

Carbon dynamics during a long-term incubation of separate and recombined density fractions from seven forest soils

Abstract Density fractions in soils differ in their turnover rates, but direct measurement of the C dynamics in the individual density fractions is limited. In 300-day incubations of mineral soils from forests in Washington and Oregon, USA, light fractions (LF), heavy fractions (HF), whole soils (WS), and physically recombined light and heavy fractions (RF), were measured for respiration and shifts in microbial biomass. A combined fraction was calculated from the incubation results of the light and heavy fractions, and called the summed fraction (SF). Carbon concentration followed the pattern: LF>RF>HF. In accordance with this pattern, when cumulative respiration was considered per gram of substrate, the physical fractions exhibited a predictable response: LF>RF>HF. However, when expressed per gram of initial C, the respiration of the LF was not significantly different from that of the HF. These findings suggest the recalcitrance of HF is similar to that of LF and, consequently, differences in their turnover rates in WS may be due to microbial accessibility or physical protection. Whether expressed per gram of substrate or per gram of initial C, the respiration of the SF was not different from that of the WS. Within the SF, the HF was responsible for 35% of the total respiration. Lower respiration in the RF compared with WS and SF might be explained by an antagonistic interaction between the varied microbial communities that degrade LF and HF; in the heterogeneous WS, these communities may be spatially separated to a greater extent than in the laboratory substrate. Unfortunately, the microbial data were highly variable and provided little evidence to either support or refute this idea. The density separation technique appears to be a viable method for isolating different soil organic matter fractions. However, the function of these fractions should be considered more carefully in the context of accessibility and C content.

Fire effects on temperate forest soil C and N storage

Temperate forest soils store globally significant amounts of carbon (C) and nitrogen (N). Understanding how soil pools of these two elements change in response to disturbance and management is critical to maintaining ecosystem services such as forest productivity, greenhouse gas mitigation, and water resource protection. Fire is one of the principal disturbances acting on forest soil C and N storage and is also the subject of enormous management efforts. In the present article, we use meta-analysis to quantify fire effects on temperate forest soil C and N storage. Across a combined total of 468 soil C and N response ratios from 57 publications (concentrations and pool sizes), fire had significant overall effects on soil C (-26%) and soil N (-22%). The impacts of fire on forest floors were significantly different from its effects on mineral soils. Fires reduced forest floor C and N storage (pool sizes only) by an average of 59% and 50%, respectively, but the concentrations of these two elements did not change. Prescribed fires caused smaller reductions in forest floor C and N storage (-46% and -35%) than wildfires (-67% and -69%), and the presence of hardwoods also mitigated fire impacts. Burned forest floors recovered their C and N pools in an average of 128 and 103 years, respectively. Among mineral soils, there were no significant changes in C or N storage, but C and N concentrations declined significantly (-11% and -12%, respectively). Mineral soil C and N concentrations were significantly affected by fire type, with no change following prescribed burns, but significant reductions in response to wildfires. Geographic variation in fire effects on mineral soil C and N storage underscores the need for region-specific fire management plans, and the role of fire type in mediating C and N shifts (especially in the forest floor) indicates that averting wildfires through prescribed burning is desirable from a soils perspective.

Ecosystem vulnerability assessment and synthesis: a report from the Climate Change Response Framework Project in northern Wisconsin

The forests of northern Wisconsin will likely experience dramatic changes over the next 100 years as a result of climate change. This assessment evaluates key forest ecosystem vulnerabilities to climate change across northern Wisconsin under a range of future climate scenarios. Warmer temperatures and shifting precipitation patterns are expected to influence ecosystem drivers and increase stressors, including more frequent disturbances and increased amount or severity of pests and diseases. Forest ecosystems will continue to adapt to changing conditions. Identifying vulnerable species and forests can help landowners, managers, regulators, and policymakers establish priorities for management and monitoring.

Michigan forest ecosystem vulnerability assessment and synthesis: a report from the Northwoods Climate Change Response Framework project

Forests in northern Michigan will be affected directly and indirectly by a changing climate during the next 100 years. This assessment evaluates the vulnerability of forest ecosystems in Michigans eastern Upper Peninsula and northern Lower Peninsula to a range of future climates. Information on current forest conditions, observed climate trends, projected climate changes, and impacts to forest ecosystems was considered in order to draw conclusions on climate change vulnerability. Upland spruce-fir forests were determined to be the most vulnerable, whereas oak associations and barrens were determined to be less vulnerable to projected changes in climate. Projected changes in climate and the associated ecosystem impacts and vulnerabilities will have important implications for economically valuable timber species, forest-dependent wildlife and plants, recreation, and long-range planning.

Central Appalachians forest ecosystem vulnerability assessment and synthesis: a report from the Central Appalachians Climate Change Response Framework project

Forest ecosystems in the Central Appalachians will be affected directly and indirectly by a changing climate over the 21st century. This assessment evaluates the vulnerability of forest ecosystems in the Central Appalachian Broadleaf Forest-Coniferous Forest-Meadow and Eastern Broadleaf Forest Provinces of Ohio, West Virginia, and Maryland for a range of future climates. Information on current forest conditions, observed climate trends, projected climate changes, and impacts on forest ecosystems was considered by a multidisciplinary panel of scientists, land managers, and academics in order to assess ecosystem vulnerability to climate change. Appalachian (hemlock)/northern hardwood forests, large stream floodplain and riparian forests, small stream riparian forests, and spruce/fir forests were determined to be the most vulnerable. Dry/mesic oak forests and dry oak and oak/pine forests and woodlands were determined to be least vulnerable. Projected changes in climate and the associated impacts and vulnerabilities will have important implications for economically valuable timber species, forest-dependent wildlife and plants, recreation, and long-term natural resource planning.

Forest ecosystem vulnerability assessment and synthesis for northern Wisconsin and western Upper Michigan: a report from the Northwoods Climate Change Response Framework project

Forest ecosystems across the Northwoods will face direct and indirect impacts from a changing climate over the 21st century. This assessment evaluates the vulnerability of forest ecosystems in the Laurentian Mixed Forest Province of northern Wisconsin and western Upper Michigan under a range of future climates. Information on current forest conditions, observed climate trends, projected climate changes, and impacts to forest ecosystems was considered in order to assess vulnerability to climate change. Upland spruce-fir, lowland conifers, aspen-birch, lowland-riparian hardwoods, and red pine forests were determined to be the most vulnerable ecosystems. White pine and oak forests were perceived as less vulnerable to projected changes in climate. These projected changes in climate and the associated impacts and vulnerabilities will have important implications for economically valuable timber species, forest-dependent wildlife and plants, recreation, and long-term natural resource planning.

Two-year dynamics of foliage labelling in 8-year-old Pinus pinaster trees with 15N, 26Mg and 42Ca—simulation of Ca transport in xylem using an upscaling approach

Abstract• IntroductionAtmospheric deposition is an important input of major nutrients into forest ecosystems. The long-term goal of this work was to apply stable isotope methodology to assess atmospheric nutrient deposition in forest systems.• Materials and methodsA labelling experiment of foliage with stable isotopes of primary and secondary macro nutrients (15N, 26Mg and 42Ca injected into the stem sapwood) was carried on standing trees to monitor interactions between canopy and precipitations. 15N rapidly reached the foliage; however, Mg and Ca were not detected in foliage until more than a year after injection.• Results and discussionThe delay in mobilization of Mg and Ca prevented us from accurately modelling deposition contributions of these two elements. Nonetheless, an upscaling approach based on published results on Ca transport in shoots xylem was used to simulate our results. These simulations of Ca transport at the tree scale were consistent with our experimental data.• ConclusionThis consistency suggested that mechanisms of nutrient transport are the same at the different scales. Nitrogen was rapidly transported in the xylem to foliage, probably mainly by mass flow. Conversely, transport of Mg and particularly Ca was considerably delayed, probably due to successive cation exchanges along the xylem vessels.

Radiocarbon dating of American pika fecal pellets provides insights into population extirpations and climate refugia

The American pika (Ochotona princeps) has become a species of concern for its sensitivity to warm temperatures and potential vulnerability to global warming. We explored the value of radiocarbon dating of fecal pellets to address questions of population persistence and timing of site extirpation. Carbon was extracted from pellets collected at 43 locations in the western Great Basin, USA, including three known occupied sites and 40 sites of uncertain status at range margins or where previous studies indicated the species is vulnerable. We resolved calibrated dates with high precision (within several years), most of which fell in the period of the mid-late 20th century bomb curve. The two-sided nature of the bomb curve renders far- and near-side dates of equal probability, which are separated by one to four decades. We document methods for narrowing resolution to one age range, including stratigraphic analysis of vegetation collected from pika haypiles. No evidence was found for biases in atmospheric 14C levels due to fossil-derived or industrial CO2 contamination. Radiocarbon dating indicated that pellets can persist for >59 years; known occupied sites resolved contemporary dates. Using combined evidence from field observations and radiocarbon dating, and the Bodie Mountains as an example, we propose a historical biogeographic scenario for pikas in minor Great Basin mountain ranges adjacent to major cordillera, wherein historical climate variability led to cycles of extirpation and recolonization during alternating cool and warm centuries. Using this model to inform future dynamics for small ranges in biogeographic settings similar to the Bodie Mountains in California, extirpation of pikas appears highly likely under directional warming trends projected for the next century, even while populations in extensive cordillera (e.g., Sierra Nevada, Rocky Mountains, Cascade Range) are likely to remain viable due to extensive, diverse habitat and high connectivity.

Regional Highlights of Climate Change

Climatic extremes, ecological disturbance, and their interactions are expected to have major effects on ecosystems and social systems in most regions of the United States in the coming decades. In Alaska, where the largest temperature increases have occurred, permafrost is melting, carbon is being released, and fire regimes are changing, leading to a transition from conifers to hardwoods in some forests. In Hawaii and the U.S.-affiliated Pacific islands, an altered climate and sea level rise are changing hydrology and fire regimes, affecting both forest ecosystems and human communities. In the Northwest, insect outbreaks (already prominent) and increased area burned, in combination with declining snowpack, are expected to have a major effect on dry, interior forests. In the Southwest, recent large wildfires and forest dieback in pinyon pine exemplify the kinds of changes that may occur in arid and semi-arid forests if droughts become more common in the future. In the Great Plains, where trees currently occupy only a small portion of the landscape, warmer temperature and non-native insects could reduce the amount of forested area and alter species distribution. In the Midwest, warmer temperature is expected to affect the distribution and abundance of many tree species, associated habitat, and human use of forests in a region where private lands are mixed with public lands. In the Northeast, warmer temperature is expected to affect the distribution and abundance of many tree species, although the productivity of hardwood species may increase significantly. In the Southeast, biodiversity and productivity may be affected by a combination of warmer climate, altered fire regimes, and invasive plants and insects.

Adapting to climate change

Federal agencies have led the development of adaptation principles and tools in forest ecosystems over the past decade. Successful adaptation efforts generally require organizations to: (1) develop science-management partnerships, (2) provide education on climate change science, (3) provide a toolkit of methods and processes for vulnerability assessment and adaptation, (4) use multiple models to generate projections of climate change effects, (5) incorporate risk and uncertainty, (6) integrate with multiple management objectives, (7) prioritize no-regrets decision making, (8) support flexibility and adaptive learning, and (9) incorporate adaptation in planning and projects. Resistance, resilience, response, and realignment strategies help to identify the scope of appropriate adaptation options at broad spatial scales in forest ecosystems. At the local scale, it is necessary to: (1) define management objectives, spatial extent, and timeframes, (2) analyze vulnerabilities, (3) determine priorities, (4) develop local tactics associated with strategies, (5) implement plans and projects, and (6) monitor, review, and adjust. The best examples of vulnerability assessment and adaptation planning in forests have occurred in national forests, where science-management partnerships have been established across multiple institutions. Although strategic planning for adaptation has been increasing, implementation of on-the-ground adaptation projects has been rare, primarily because of a lack of budget, personnel, and mandate for action. No one agency or organization can fully meet the challenge of adaptation, but this task is within reach if willing partners work collaboratively toward sustainable management grounded in knowledge of climate science and dynamic ecosystems.