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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.

Atmospheric nitrogen deposition increases forest carbon sequestration across broad parts of the Northern Hemisphere. Slower organic matter decomposition and greater soil carbon accumulation could contribute to this increase in carbon sequestration. We investigated the effects of chronic simulated nitrogen deposition on leaf litter and fine root decomposition at four sugar maple (Acer saccharum)-dominated northern hardwood forests. At these sites, we previously observed that nitrogen additions increased soil organic carbon and altered litter chemistry. We conducted a 3-year decomposition study with litter bags. Litter production of leaves and fine roots were combined with decomposition dynamics to estimate how fine roots and leaf litter contribute to soil organic carbon. We found that nitrogen additions marginally stimulated early-stage decomposition of leaf litter, an effect associated with previously documented changes in litter chemistry. In contrast, nitrogen additions inhibited the later stages of fine root decomposition, which is consistent with observed decreases in lignin-degrading enzyme activities with nitrogen additions at these sites. At the ecosystem scale, slower fine root decomposition led to additional root mass retention (g m⁻²), and this greater retention of root residues was estimated to explain 5–51 % of previously documented carbon accumulation in the surface soil due to nitrogen additions. Our results demonstrated that simulated nitrogen deposition created contrasting effects on the decomposition of leaf litter and fine roots. Although previous nitrogen deposition studies have focused on leaf litter, this work suggests that slower fine root decomposition is a major driver of soil organic carbon accumulation under elevated nitrogen deposition.

Elevated CO2 (eCO2) experiments provide critical information to quantify the effects of rising CO2 on vegetation1–6. Many eCO2 experiments suggest that nutrient limitations modulate the local magnitude of the eCO2 effect on plant biomass1,3,5, but the global extent of these limitations has not been empirically quantified, complicating projections of the capacity of plants to take up CO27,8. Here, we present a data-driven global quantification of the eCO2 effect on biomass based on 138 eCO2 experiments. The strength of CO2 fertilization is primarily driven by nitrogen (N) in ~65% of global vegetation and by phosphorus (P) in ~25% of global vegetation, with N- or P-limitation modulated by mycorrhizal association. Our approach suggests that CO2 levels expected by 2100 can potentially enhance plant biomass by 12 ± 3% above current values, equivalent to 59 ± 13 PgC. The future effect of eCO2 we derive from experiments is geographically consistent with past changes in greenness9, but is considerably lower than the past effect derived from models10. If borne out, our results suggest that the stimulatory effect of CO2 on carbon storage could slow considerably this century. Our research provides an empirical estimate of the biomass sensitivity to eCO2 that may help to constrain climate projections.Elevated CO2 increases plant biomass, providing a negative feedback on global warming. Nutrient availability was found to drive the magnitude of this effect for the majority of vegetation globally, and analyses indicated that CO2 will continue to fertilize plant growth in the next century.

Fire activity, in terms of intensity, frequency, and total area burned, is expected to increase with a changing climate. A challenge for landscape-level assessment of fire effects, often termed burn severity, is that current remote sensing assessments provide very little information regarding tree/vegetation physiological performance and recovery, limiting our understanding of fire effects on ecosystem services such as carbon storage/cycling. In this paper, we evaluated whether spectral indices common in vegetation stress and burn severity assessments could accurately quantify post-fire physiological performance (indicated by net photosynthesis and crown scorch) of two seedling species, Larix occidentalis and Pinus contorta. Seedlings were subjected to increasing fire radiative energy density (FRED) doses through a series of controlled laboratory surface fires. Mortality, physiology, and spectral reflectance were assessed for a month following the fires, and then again at one year post-fire. The differenced Normalized Difference Vegetation Index (dNDVI) spectral index outperformed other spectral indices used for vegetation stress and burn severity characterization in regard to leaf net photosynthesis quantification, indicating that landscape-level quantification of tree physiology may be possible. Additionally, the survival of the majority of seedlings in the low and moderate FRED doses indicates that fire-induced mortality is more complex than the currently accepted binary scenario, where trees survive with no impacts below a certain temperature and duration threshold, and mortality occurs above the threshold.

The ability of plant litter to adsorb water is important for wildland fire, hydrological, and biogeochemical processes. Variation in water adsorption has largely been attributed to physical differences across species, with the role of litter chemistry in moisture dynamics receiving little attention. We hypothesized that lower specific leaf area ( SLA , cm 2 /g) and higher concentrations of hydrophobic lignin and lipid biomolecules would be associated with decreased litter water adsorption. Because plant biochemistry is tied to litter elemental and structural traits via biophysics and leaf economics, we expected to observe a suite of linked traits that were related to water adsorption, including element concentrations, carbon oxidation state, and energy concentration (Δ H c ). In litter from 22 species, we observed greater than fourfold variation in the maximum amount of liquid water adsorbed (adsorption capacity, g/g) and in the rate at which dry litter adsorbed water vapor (adsorption rate, mg g −1  min −1 ); there was a significant positive relationship between adsorption capacity and adsorption rate. Broadly, litter with low SLA had a low carbon oxidation state, low oxygen and ash concentrations, and high concentrations of carbon, hydrogen, lignin, and lipids. The two metrics of water adsorption had significant negative relationships with concentrations of energy, lignin, carbon, and hydrogen, and positive relationships with litter SLA and carbon oxidation state. However, water adsorption was better predicted by combinations of SLA with chemical and energy traits. Several traits associated with decreased litter water adsorption, such as concentrations of lignin and energy, also directly influence some of the same ecosystem processes affected by litter moisture (e.g., decomposition, wildland fires). In particular, because plants with the hydrophobic traits identified in this study are more abundant in dry environments, our observations suggest a mechanism that could accentuate the influence of litter traits on ecosystem processes and which merits further research. Understanding the role of these traits in water adsorption could be used to help predict shifts in ecosystem function as plant communities reassemble as result of climate change as well as provide quantitative information on how plant species influence wildland fire dynamics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Elevated concentrations of atmospheric CO₂ and tropospheric O₃ will profoundly influence future forest productivity, but our understanding of these influences over the long-term is poor. Leaves are key indicators of productivity and we measured the mass, area, and nitrogen concentration of leaves collected in litter traps from 2002 to 2008 in three young northern temperate forest communities exposed to elevated CO₂ and/or elevated O₃ since 1998. On average, the overall effect of elevated CO₂ (+CO₂ and +CO₂+O₃ versus ambient and +O₃) was to increase leaf mass by 36% whereas the overall effect of elevated O₃ was to decrease leaf mass by 13%, with similar effects on stand leaf area. However, there were important CO₂ x O₃ x year interactions wherein some treatment effects on leaf mass changed dramatically relative to ambient from 2002 to 2008. For example, stimulation by the +CO₂ treatment decreased (from +52 to +25%). whereas the deleterious effects of the +O₃ treatment increased (from -5 to -18%). In comparison, leaf mass in the +CO₂+O₃ treatment was similar to ambient throughout the study. Forest composition influenced these responses: effects of the +O₃ treatment on community-level leaf mass ranged from +2 to -19%. These findings are evidence that community composition, stand development processes, CO₂, and O₃ strongly interact. Changes in leaf nitrogen concentration were inconsistent, but leaf nitrogen mass (g m¯²) was increased by elevated CO₂(+30%) and reduced by elevated O₃(-16%), consistent with observations that nitrogen cycling is accelerated by elevated CO₃ but retarded by elevated O₃.

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.m-2.yr-1) and experimental NO3- deposition (ambient plus 3 g NO3--N.m-2.yr-1) for a decade; we also traced the flow of a 15NO3- pulse over a six-year period. Experimental 15NO3- 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.

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.

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.