Alan F. Talhelm

SIMULATED ATMOSPHERIC NO3− DEPOSITION INCREASES SOIL ORGANIC MATTER BY SLOWING DECOMPOSITION

Presently, there is uncertainty regarding the degree to which anthropogenic N deposition will foster C storage in the N-limited forests of the Northern Hemisphere, ecosystems which are globally important sinks for anthropogenic CO2. We constructed organic matter and N budgets for replicate northern hardwood stands (n = 4) that have received ambient (0.7-1.2 g N x m(-2) x yr(-1) and experimental NO3- deposition (ambient plus 3 g NO3(-)-N x m(-2) x yr(-1)) for a decade; we also traced the flow of a 15NO3- pulse over a six-year period. Experimental NO3- deposition had no effect on organic matter or N stored in the standing forest overstory, but it did significantly increase the N concentration (+19%) and N content (+24%) of canopy leaves. In contrast, a decade of experimental NO3- deposition significantly increased amounts of organic matter (+12%) and N (+9%) in forest floor and mineral soil, despite no increase in detritus production. A greater forest floor (Oe/a) mass under experimental NO3- deposition resulted from slower decomposition, which is consistent with previously reported declines in lignolytic activity by microbial communities exposed to experimental NO3- deposition. Tracing 15NO3- revealed that N accumulated in soil organic matter by first flowing through soil microorganisms and plants, and that the shedding of 15N-labeled leaf litter enriched soil organic matter over a six-year duration. Our results demonstrate that atmospheric NO3- deposition exerts a direct and negative effect on microbial activity in this forest ecosystem, slowing the decomposition of aboveground litter and leading to the accumulation of forest floor and soil organic matter. To the best of our knowledge, this mechanism is not represented in the majority of simulation models predicting the influence of anthropogenic N deposition on ecosystem C storage in northern forests.

Species-specific responses to atmospheric carbon dioxide and tropospheric ozone mediate changes in soil carbon

We repeatedly sampled the surface mineral soil (0-20 cm depth) in three northern temperate forest communities over an 11-year experimental fumigation to understand the effects of elevated carbon dioxide (CO(2)) and/or elevated phyto-toxic ozone (O(3)) on soil carbon (C). After 11 years, there was no significant main effect of CO(2) or O(3) on soil C. However, within the community containing only aspen (Populus tremuloides Michx.), elevated CO(2) caused a significant decrease in soil C content. Together with the observations of increased litter inputs, this result strongly suggests accelerated decomposition under elevated CO(2.) In addition, an initial reduction in the formation of new (fumigation-derived) soil C by O(3) under elevated CO(2) proved to be only a temporary effect, mirroring trends in fine root biomass. Our results contradict predictions of increased soil C under elevated CO(2) and decreased soil C under elevated O(3) and should be considered in models simulating the effects of Earths altered atmosphere.

Fine roots are the dominant source of recalcitrant plant litter in sugar maple‐dominated northern hardwood forests

Summary Most studies of forest litter dynamics examine the biochemical characteristics and decomposition of leaf litter, but fine roots are also a large source of litter in forests. We quantified the concentrations of eight biochemical fractions and nitrogen (N) in leaf litter and fine roots at four sugar maple (Acer saccharum)‐dominated hardwood forests in the north‐central United States. We combined these results with litter production data to estimate ecosystem biochemical fluxes to soil. We also compared how leaf litter and fine root biochemistry responded to long‐term simulated N deposition. Compared with leaf litter, fine roots contained 2.9‐fold higher acid‐insoluble fraction (AIF) and 2.3‐fold more condensed tannins; both are relatively difficult to decompose. Comparatively, leaf litter had greater quantities of more labile components: nonstructural carbohydrates, cellulose and soluble phenolics. At an ecosystem scale, fine roots contributed over two‐thirds of the fluxes of AIF and condensed tannins to soil. Fine root biochemistry was also less responsive than leaf litter to long‐term simulated N deposition. Fine roots were the dominant source of difficult‐to‐decompose plant carbon fractions entering the soil at our four study sites. Based on our synthesis of the literature, this pattern appears to be widespread in boreal and temperate forests.

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests.

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2) and tropospheric ozone (O3) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3. Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r2 = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m−2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (ΔNPP/ΔN) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2. Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content.

No evidence that chronic nitrogen additions increase photosynthesis in mature sugar maple forests

Atmospheric nitrogen (N) deposition can increase forest growth. Because N deposition commonly increases foliar N concentrations, it is thought that this increase in forest growth is a consequence of enhanced leaf-level photosynthesis. However, tests of this mechanism have been infrequent, and increases in photosynthesis have not been consistently observed in mature forests subject to chronic N deposition. In four mature northern hardwood forests in the north-central United States, chronic N additions (30 kg N ha(-1) yr(-1) as NaNO3 for 14 years) have increased aboveground growth but have not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increases in growth, we hypothesized that the NO3(-) additions increased foliar N concentrations and leaf-level photosynthesis in the dominant species in these forests (sugar maple, Acer saccharum). The NO3(-) additions significantly increased foliar N. However, there was no significant difference between the ambient and +NO3(-) treatments in two seasons (2006-2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (micromol CO2 s(-1) g(-1) N) was significantly decreased (-13%) by NO3(-) additions. Furthermore, we found no consistent NO3(-) effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for delta 13C and delta 18O, isotopes that can be used together to integrate changes in photosynthesis over time. We observed a small but significant NO3(-) effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number. These photosynthesis and leaf area data together suggest that NO3(-) additions have not stimulated photosynthesis. There is no evidence that nutrient deficiencies have developed at these sites, so unlike other studies of photosynthesis in N-saturated forests, we cannot attribute the lack of a stimulation of photosynthesis to nutrient limitations. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation.

Towards a new paradigm in fire severity research using dose–response experiments

Most landscape-scale fire severity research relies on correlations between field measures of fire effects and relatively simple spectral reflectance indices that are not direct measures of heat output or changes in plant physiology. Although many authors have highlighted limitations of this approach and called for improved assessments of severity, others have suggested that the operational utility of such a simple approach makes it acceptable. An alternative pathway to evaluate fire severity that bridges fire combustion dynamics and ecophysiology via dose–response experiments is presented. We provide an illustrative example from a controlled nursery combustion laboratory experiment. In this example, severity is defined through changes in the ability of the plant to assimilate carbon at the leaf level. We also explore changes in the Differenced Normalised Differenced Vegetation Index (dNDVI) and the Differenced Normalised Burn Ratio (dNBR) as intermediate spectral indices. We demonstrate the potential of this methodology and propose dose–response metrics for quantifying severity in terms of carbon cycle processes.

Air pollution and the changing biogeochemistry of northern forests

Industrialization has greatly affected the biogeochemistry of northern forests by increasing the atmospheric deposition of acid and nitrogen (N). In 1990, the US Congress amended the Clean Air Act to include tighter emissions regulations; this reduced acid deposition (by >50% in this study), but did not effectively lower N deposition. Here, we demonstrate that since this legislation was enacted, there have been marked decreases in sulfur (−16%), calcium (−17%), and aluminum (−42%) concentrations in sugar maple (Acer saccharum) foliage across the Upper Great Lakes region of the US, signaling a declining influence of acid deposition. In contrast, N deposition has persistently been over 75% greater than the amount of N needed to offset annual plant N sequestration, creating increases in N availability and soil N leaching. Recent emissions regulations will reduce N deposition somewhat, but further increases in soil N availability and leaching are likely. Policy decisions regarding N deposition will have to weigh increased carbon storage against negative impacts on water quality and species diversity.

Chapter 10 – Belowground Carbon Cycling at Aspen FACE: Dynamic Responses to CO2 and O3 in Developing Forests

The Aspen free-air carbon dioxide (CO2) enrichment (FACE) experiment tested how three developing forest communities responded to elevated concentrations of CO2 and/or tropospheric ozone (O3). Throughout the 11-year experiment, elevated CO2 increased aboveground productivity, whereas the initial negative effects of elevated O3 on aboveground productivity became insignificant over time. During the first 2 years, fine root biomass and soil respiration responded positively to elevated CO2 and negatively to elevated O3. However, after 5 years, O3 effects on fine root biomass were weakly negative or positive and effects on soil respiration were positive. Despite altering litter inputs, neither elevated O3 nor elevated CO2 affected overall soil C storage at the end of the experiment, consistent with observations that elevated CO2 increased and elevated O3 tended to decrease the activity of litter-degrading extracellular enzymes. Overall, our understanding of belowground processes is still insufficient to predict how ecosystems will respond to global change.Abstract The Aspen free-air carbon dioxide (CO2) enrichment (FACE) experiment tested how three developing forest communities responded to elevated concentrations of CO2 and/or tropospheric ozone (O3). Throughout the 11-year experiment, elevated CO2 increased aboveground productivity, whereas the initial negative effects of elevated O3 on aboveground productivity became insignificant over time. During the first 2 years, fine root biomass and soil respiration responded positively to elevated CO2 and negatively to elevated O3. However, after 5 years, O3 effects on fine root biomass were weakly negative or positive and effects on soil respiration were positive. Despite altering litter inputs, neither elevated O3 nor elevated CO2 affected overall soil C storage at the end of the experiment, consistent with observations that elevated CO2 increased and elevated O3 tended to decrease the activity of litter-degrading extracellular enzymes. Overall, our understanding of belowground processes is still insufficient to predict how ecosystems will respond to global change.

Effects of fire radiative energy density dose on Pinus contorta and Larix occidentalis seedling physiology and mortality

Climate change is projected to exacerbate the intensity of heat waves and drought, leading to a greater incidence of large and high-intensity wildfires in forested ecosystems. Predicting responses of seedlings to such fires requires a process-based understanding of how the energy released during fires affects plant physiology and mortality. Understanding what fire ‘doses’ cause seedling mortality is important for maintaining grasslands or promoting establishment of desirable plant species. We conducted controlled laboratory combustion experiments on replicates of well-watered nursery-grown seedlings. We evaluated the growth, mortality and physiological response of Larix occidentalis and Pinus contorta seedlings to increasing fire radiative energy density (FRED) doses created using natural fuels with known combustion properties. We observed a general decline in the size and physiological performance of both species that scaled with increasing FRED dose, including decreases in leaf-level photosynthesis, seedling leaf area and diameter at root collar. Greater FRED dose increased the recovery time of chlorophyll fluorescence in the remaining needles. This study provides preliminary data on what level of FRED causes mortality in these two species, which can aid land managers in identifying strategies to maintain (or eliminate) woody seedlings of interest.

Impacts of fire radiative flux on mature Pinus ponderosa growth and vulnerability to secondary mortality agents

Recent studies have highlighted the potential of linking fire behaviour to plant ecophysiology as an improved route to characterising severity, but research to date has been limited to laboratory-scale investigations. Fine-scale fire behaviour during prescribed fires has been identified as a strong predictor of post-fire tree recovery and growth, but most studies report these metrics averaged over the entire fire. Previous research has found inconsistent effects of low-intensity fire on mature Pinus ponderosa growth. In this study, fire behaviour was quantified at the tree scale and compared with post-fire radial growth and axial resin duct defences. Results show a clear dose–response relationship between peak fire radiative power per unit area (W m–2) and post-fire Pinus ponderosa radial growth. Unlike in previous laboratory research on seedlings, there was no dose–response relationship observed between fire radiative energy per unit area (J m–2) and post-fire mature tree growth in the surviving trees. These results may suggest that post-fire impacts on growth of surviving seedlings and mature trees require other modes of heat transfer to impact plant canopies. This study demonstrates that increased resin duct defence is induced regardless of fire intensity, which may decrease Pinus ponderosa vulnerability to secondary mortality agents.