Tree biomass allocation to leaves, roots, and wood affects the residence time of carbon in forests, with potentially dramatic implications for ecosystem carbon storage. However, drivers of tree ...biomass allocation remain poorly quantified. Using a combination of global data sets, we tested the relative importance of climate, leaf habit, and tree mycorrhizal associations on biomass allocation. We show that trees that associate with arbuscular mycorrhizal (AM) fungi allocate roughly 4% more of their biomass to root tissue than trees that associate with ectomycorrhizal (ECM) fungi. Further, the effect of mycorrhizal association on root biomass allocation was greater than that of climate and similar in magnitude to that of leaf habit (evergreen vs. deciduous). These patterns in whole‐plant biomass allocation are likely due to differences in carbon investment toward root versus fungal tissues, where trees with AM fungi favor root production while trees with ECM fungi favor fungal tissue production. These results suggest that considering tree mycorrhizal associations could improve our understanding of ecosystem carbon storage in terrestrial biosphere models: specifically, that greater within‐tree allocation to root biomass in AM‐associated tree species may contribute to stable soil carbon pools in forests dominated by AM fungi.
Soil organic matter (SOM) stocks, decomposition and persistence are largely the product of controls that act locally. Yet the controls are shaped and interact at multiple spatiotemporal scales, from ...which macrosystem patterns in SOM emerge. Theory on SOM turnover recognizes the resulting spatial and temporal conditionality in the effect sizes of controls that play out across macrosystems, and couples them through evolutionary and community assembly processes. For example, climate history shapes plant functional traits, which in turn interact with contemporary climate to influence SOM dynamics. Selection and assembly also shape the functional traits of soil decomposer communities, but it is less clear how in turn these traits influence temporal macrosystem patterns in SOM turnover. Here, we review evidence that establishes the expectation that selection and assembly should generate decomposer communities across macrosystems that have distinct functional effects on SOM dynamics. Representation of this knowledge in soil biogeochemical models affects the magnitude and direction of projected SOM responses under global change. Yet there is high uncertainty and low confidence in these projections. To address these issues, we make the case that a coordinated set of empirical practices are required which necessitate (1) greater use of statistical approaches in biogeochemistry that are suited to causative inference; (2) long-term, macrosystem-scale, observational and experimental networks to reveal conditionality in effect sizes, and embedded correlation, in controls on SOM turnover; and (3) use of multiple measurement grains to capture local- and macroscale variation in controls and outcomes, to avoid obscuring causative understanding through data aggregation. When employed together, along with process-based models to synthesize knowledge and guide further empirical work, we believe these practices will rapidly advance understanding of microbial controls on SOM and improve carbon cycle projections that guide policies on climate adaptation and mitigation.
Summary
●Fine roots and mycorrhizal fungi may either stimulate leaf litter decomposition by providing free‐living decomposers with root‐derived carbon, or may slow decomposition through nutrient ...competition between mycorrhizal and saprotrophic fungi.
●We reduced the presence of fine roots and their associated mycorrhizal fungi in a northern hardwood forest in New Hampshire, USA by soil trenching. Plots spanned a mycorrhizal gradient from 96% arbuscular mycorrhizal (AM) associations to 100% ectomycorrhizal (ECM)‐associated tree basal area. We incubated four species of leaf litter within these plots in areas with reduced access to roots and mycorrhizal fungi and in adjacent areas with intact roots and mycorrhizal fungi.
●Over a period of 608 d, we found that litter decayed more rapidly in the presence of fine roots and mycorrhizal hyphae regardless of the dominant tree mycorrhizal association. Root and mycorrhizal exclusion reduced the activity of acid phosphatase on decomposing litter.
●Our results indicate that both AM‐ and ECM‐associated fine roots stimulate litter decomposition in this system. These findings suggest that the effect of fine roots and mycorrhizal fungi on litter decay in a particular ecosystem likely depends on whether interactions between mycorrhizal roots and saprotrophic fungi are antagonistic or facilitative.
Conspecific negative density dependence (CNDD) promotes tree species diversity by reducing recruitment near conspecific adults due to biotic feedbacks from herbivores, pathogens, or competitors. ...While this process is well‐described in tropical forests, tests of temperate tree species range from strong positive to strong negative density dependence. To explain this, several studies have suggested that tree species traits may help predict the strength and direction of density dependence: for example, ectomycorrhizal‐associated tree species typically exhibit either positive or weaker negative conspecific density dependence. More generally, the strength of density dependence may be predictably related to other species‐specific ecological attributes such as shade tolerance, or the relative local abundance of a species. To test the strength of density dependence and whether it affects seedling community diversity in a temperate forest, we tracked the survival of seedlings of three ectomycorrhizal‐associated species experimentally planted beneath conspecific and heterospecific adults on the Prospect Hill tract of the Harvard Forest, in Massachusetts, USA. Experimental seedling survival was always lower under conspecific adults, which increased seedling community diversity in one of six treatments. We compared these results to evidence of CNDD from observed sapling survival patterns of 28 species over approximately 8 years in an adjacent 35‐ha forest plot. We tested whether species‐specific estimates of CNDD were associated with mycorrhizal association, shade tolerance, and local abundance. We found evidence of significant, negative conspecific density dependence (CNDD) in 23 of 28 species, and positive conspecific density dependence in two species. Contrary to our expectations, ectomycorrhizal‐associated species generally exhibited stronger (e.g., more negative) CNDD than arbuscular mycorrhizal‐associated species. CNDD was also stronger in more shade‐tolerant species but was not associated with local abundance. Conspecific adult trees often have a negative influence on seedling survival in temperate forests, particularly for tree species with certain traits. Here we found strong experimental and observational evidence that ectomycorrhizal‐associating species consistently exhibit CNDD. Moreover, similarities in the relative strength of density dependence from experiments and observations of sapling mortality suggest a mechanistic link between negative effects of conspecific adults on seedling and sapling survival and local tree species distributions.
Temperate forest soils are net sources of carbon dioxide (CO
2
) and net sinks for methane (CH
4
), the two greenhouse gases most responsible for contemporary global climate change. Both soil carbon ...fluxes are sensitive to their local tree communities due to the direct effects of tree traits as well as indirect effects of associated soil properties. We asked how tree species identity and diversity predicts the flux of CO
2
and CH
4
from soils, how the two net fluxes are related, and what tree and soil characteristics predict their magnitudes. In a mixed temperate forest in central Massachusetts, we established 49 plots containing either a single tree species or a combination of those species and measured growing season soil CO
2
and CH
4
fluxes for two years. We found generally greater soil CO
2
and CH
4
fluxes associated with deciduous tree species. CH
4
uptake rates were more sensitive to tree species than were CO
2
fluxes. Tree species mixtures lead to predictable intermediate fluxes of CO
2
, but mixtures resulted in lower than predicted CH
4
uptake. Soil CO
2
emission and CH
4
uptake were both positively related to total litter inputs. Soil CO
2
emission was additionally associated with warmer temperatures and a lower ratio of soil carbon to nitrogen; in contrast, CH
4
uptake was associated with lower soil moisture and a shallower organic horizon. Thus, tree species community composition may prove useful for predicting soil carbon fluxes, but much remains to be discovered about the mechanisms linking tree species to associated microbial and biogeochemical processes.
Soil respiration is the dominant pathway by which terrestrial carbon enters the atmosphere. Many abiotic and biotic processes can influence soil respiration, including soil microbial community ...composition. Mycorrhizal fungi are a particularly important microbial group because they are known to influence soil chemistry and nutrient cycling, and, because the type of mycorrhizal fungi in an ecosystem can be assessed based on the plant species present, they may be easier than other soil microbes to incorporate into ecosystem models. We tested how the type of mycorrhizal fungi—arbuscular (AM) or ectomycorrhizal (ECM) fungi—associated with the dominant tree species in a mixed hardwood forest was related to soil respiration rate. We measured soil respiration, root biomass, and surface area, and soil chemical and physical characteristics during the growing season in plots dominated by ECM-associated trees, AM-associated trees, and mixtures with both. We found rates of soil respiration that were 29% and 32% higher in AM plots than in ECM and mixed plots, respectively. These differences are likely explained by the slightly higher nitrogen concentrations and deeper organic horizons in soil within AM plots compared with soil in ECM and mixed plots. Our results highlight the importance of considering mycorrhizal associations of dominant vegetation as predictors of carbon cycling processes.
Aboveground litter production is an important biogeochemical pathway in forests whereby carbon and nutrients enter soil detrital pools. However, patterns and controls of aboveground litter production ...are often based on an understanding of how autumnal, foliar inputs are related to aboveground tree production. Here we use three separate data sources of aboveground litter production in temperate forests to ask how aboveground woody productivity affects foliar litter production in light of other factors, such as the climate sensitivity of litter production and the seasonality of not only foliar but also fine woody debris and reproductive litter inputs. We find that foliar litter production increases with aboveground woody production, and this relationship is modified both by plant functional group and climate. Basal area also provides a crucial control on litter production. Conifer forests produce approximately half as much foliar litter as broadleaf deciduous forests. Litter production is sensitive to both among-site and among-year variation in climate, such that more litter is produced in warmer, wetter locations and years. On average 72% of aboveground litter is foliar material, with the remaining split about evenly between fine woody debris and reproductive material, and although about 88% of broadleaf litter falls during autumn, only about 61% of needles, 37% of fine woody debris and 43% of reproductive material falls during the same period. Together these results illustrate key differences in the controls of litter production in coniferous and deciduous forests, and highlight the importance of often overlooked litter fluxes, including non-autumn and non-foliar litterfall.
Wood decomposition is regulated by multiple controls, including climate and wood traits, that vary at local to regional scales. Yet decomposition rates differ dramatically when these controls do not. ...Fungal community dynamics are often invoked to explain these differences, suggesting that knowledge of ecosystem properties that influence fungal communities will improve understanding and projection of wood decomposition. We hypothesize that deadwood inputs decompose faster in forests with higher stocks of downed coarse woody material (CWM) because CWM is a resource from which lignocellulolytic fungi rapidly colonize new inputs. To test this hypothesis, we measure decomposition of 1,116 pieces of fine woody material (FWM) of five species, incubated for 13 to 49 months at five locations spanning 10°-latitude in eastern U.S. forest. We place FWM pieces near and far from CWM across observational transects and experimental common gardens. Soil temperature positively affects location-level mean decomposition rates, but these among-location differences are smaller than within-location variation in decomposition. Some of this variability is caused by CWM, where FWM pieces next to CWM decompose more rapidly. These effects are greater with time of incubation and lower initial wood density of FWM. The effect size of CWM is of the same relative magnitude as for the known controls of temperature, deadwood density and diameter. Abundance data for CWM is available for many forests and hence may be an ecosystem variable amenable for inclusion in decomposition models. Our findings suggest that conservation efforts to rebuild depleted CWM stocks in temperate forests may accelerate decomposition of fresh deadwood inputs.
Invasive insect pests are a common disturbance in temperate forests, but their effects on belowground processes in these ecosystems are poorly understood. This study examined how aboveground ...disturbance might impact short-term soil carbon flux in a forest impacted by emerald ash borer (Agrilus planipennis Fairmaire) in central New Hampshire, USA. We anticipated changes to soil moisture and temperature resulting from tree mortality caused by emerald ash borer, with subsequent effects on rates of soil respiration and methane oxidation. We measured carbon dioxide emissions and methane uptake beneath trees before, during, and after infestation by emerald ash borer. In our study, emerald ash borer damage to nearby trees did not alter soil microclimate nor soil carbon fluxes. While surprising, the lack of change in soil microclimate conditions may have been a result of the sandy, well-drained soil in our study area and the diffuse spatial distribution of canopy ash trees and subsequent canopy light gaps after tree mortality. Overall, our results indicate that short-term changes in soil carbon flux following insect disturbances may be minimal, particularly in forests with well-drained soils and a mixed-species canopy.
Feedbacks between plants and their soil microbial communities often drive negative density dependence in rare, tropical tree species, but their importance to common, temperate trees remains unclear. ...Additionally, whether negative density dependence is driven by natural enemies (e.g., soil pathogens) or by high densities of seedlings has rarely been assessed. Density dependence may also depend on seedling size, as smaller and/or younger seedlings may be more susceptible to mortality agents. We monitored seedlings of Quercus rubra, a common, canopy‐dominant temperate tree, to investigate how the density of neighboring adults and seedlings influenced their survival over two years. We assessed how the soil microbial community influenced seedling survival by growing seedlings in a glasshouse inoculated with soil collected from beneath conspecific and heterospecific mature trees. In the field, seedling survival was lower in areas with high densities of mature conspecifics but was unrelated to either conspecific or heterospecific seedling density. Smaller seedlings were also more sensitive than larger seedlings to neighboring adult conspecifics. In the glasshouse, seedlings grown with soil from beneath a conspecific adult had a higher mortality rate than seedlings grown with soil from beneath heterospecific adults or sterilized soil, suggesting that soil microbial communities drive the patterns of mortality in the field. These results illustrate the importance of negative density‐dependent feedbacks resulting from the soil microbial community in a common and ecologically important temperate tree species.
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