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Soil microbes mediate major biogeochemical processes in forest ecosystems. Soil pH is considered a “master variable” with a strong positive effect on many biogeochemical processes. To better understand how soil pH influences microbial activity and nitrogen (N) dynamics in forests, we utilized a set of long-term measurements of surface soil pH, N availability, and microbial biomass and respiration from the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA. We compared the strengths of these relationships in an unmanipulated watershed, where naturally acidic soils have been further acidified by anthropogenic acid deposition, to those in a nearby watershed, where soils were treated with calcium silicate to ameliorate the effects of acid deposition. While we expected to observe strong positive relationships between soil pH and microbial biomass and activity, we instead found weak and/or curvilinear relationships. In many cases, microbial biomass and activity peaked at unexpectedly low pH values (~ 4.5), and decreased at higher pH values, especially in the calcium-treated soils. It is likely that complexities in plant-microbial interactions inhibit and/or mask microbial response to changes in pH in these acidic soils. These results raise questions about pH as a controller of microbial processes and how ecosystems recover in response to decreases in acid deposition.

Resource allocation theory posits that increased soil nutrient availability results in decreased plant investment in nutrient acquisition. We evaluated this theory by quantifying fine root biomass and growth in a long term, nitrogen (N) × phosphorus (P) fertilization study in three mature northern hardwood forest stands where aboveground growth increased primarily in response to P addition. We did not detect a decline in fine root biomass or growth in response to either N or P. Instead, fine root growth increased in response to N, by 40% for length (P = 0.04 for the main effect of N in ANOVA), and by 36% for mass, relative to controls. Fine root mass growth was lower in response to N + P addition than predicted from the main effects of N and P (P = 0.01 for the interaction of N × P). The response of root growth to N availability did not result in detectable responses in fine root biomass (P = 0.61), which is consistent with increased root turnover with N addition. We propose that the differential growth response to fertilization between above- and belowground components is a mechanism by which trees enhance P acquisition in response to increasing N availability, illustrating how both elements may co-limit northern hardwood forest production.

Functional balance theory predicts that plants will allocate less carbon belowground when the availability of nutrients is elevated. We tested this prediction in two successional northern hardwood forest stands by quantifying fine root biomass and growth after 5–7 years of treatment in a nitrogen (N) x phosphorus (P) factorial addition experiment. We quantified root responses at two different levels of treatment: the whole-plot scale fertilization and small-patch scale fertilization of ingrowth cores. Fine root biomass was higher in plots receiving P, and fine root growth was highest in plots receiving both N and P. Thus, belowground productivity did not decrease in response to long-term addition of nutrients. We did not find conclusive evidence that elevated availability of one nutrient at the plot scale induced foraging for the other nutrient at the core scale, or that foraging for nutrients at the core scale responded to addition of limiting nutrients. Our observations suggest NP co-limitation of fine root growth and indicate complex interactions of N and P affecting aboveground and belowground production in early successional northern hardwood forest ecosystems.

Increasingly, land managers seek ways to manage forests for multiple ecosystem services and functions, yet considerable challenges exist in comparing disparate services and balancing trade-offs among them. We applied multi-criteria decision analysis (MCDA) and forest simulation models to simultaneously consider three objectives: (1) storing carbon, (2) producing timber and wood products, and (3) sustaining biodiversity. We used the Forest Vegetation Simulator (FVS) applied to 42 northern hardwood sites to simulate forest development over 100 years and to estimate carbon storage and timber production. We estimated biodiversity implications with occupancy models for 51 terrestrial bird species that were linked to FVS outputs. We simulated four alternative management prescriptions that spanned a range of harvesting intensities and forest structure retention. We found that silvicultural approaches emphasizing less frequent harvesting and greater structural retention could be expected to achieve the greatest net carbon storage but also produce less timber. More intensive prescriptions would enhance biodiversity because positive responses of early successional species exceeded negative responses of late successional species within the heavily forested study area. The combinations of weights assigned to objectives had a large influence on which prescriptions were scored as optimal. Overall, we found that a diversity of silvicultural approaches is likely to be preferable to any single approach, emphasizing the need for landscape-scale management to provide a full range of ecosystem goods and services. Our analytical framework that combined MCDA with forest simulation modeling was a powerful tool in understanding trade-offs among management objectives and how they can be simultaneously accommodated.

Physiological tolerance of environmental conditions can influence species-level responses to climate change. Here, we used species-specific thermal tolerances to predict the community responses of ant species to experimental forest-floor warming at the northern and southern boundaries of temperate hardwood forests in eastern North America. We then compared the predictive ability of thermal tolerance vs. correlative species distribution models (SDMs) which are popular forecasting tools for modeling the effects of climate change. Thermal tolerances predicted the responses of 19 ant species to experimental climate warming at the southern site, where environmental conditions are relatively close to the ants' upper thermal limits. In contrast, thermal tolerances did not predict the responses of the six species in the northern site, where environmental conditions are relatively far from the ants' upper thermal limits. Correlative SDMs were not predictive at either site. Our results suggest that, in environments close to a species' physiological limits, physiological trait-based measurements can successfully forecast the responses of species to future conditions. Although correlative SDMs may predict large-scale responses, such models may not be accurate for predicting site-level responses.

Climate warming could disrupt species interactions if organisms’ phenologies respond to climate change at different rates. Phenologies of plants and insects can be sensitive to temperature and timing of snowmelt; however, many important pollinators including ground-nesting bees have been little studied in this context. Without knowledge of the environmental cues affecting phenologies of co-occurring species, we have little ability to predict how species assemblages, and species interactions, will be affected by climate change. Here, we studied a hardwood forest understory over six years, to determine how spring temperatures, snowmelt timing, and photoperiod influence the phenology of two spring wildflowers (Anemone spp. and Trillium grandiflorum), activity of ground-nesting bees, and their temporal overlap. Surface degree-day accumulation was a better predictor of phenology for Anemone spp. (plant) and Nomada (bees) than were day of year (a proxy for photoperiod) or snowmelt date, whereas Trillium flowering appeared most sensitive to photoperiodic cues. Activity periods of Andrena and Lasioglossum bees were equally well described by degree-day accumulation and day of year. No taxon’s phenology was best predicted by snowmelt date. Despite these differences among taxa in their phenological responses, timing of bee activity and flowering responded similarly to variation in snowmelt date and early spring temperatures. Furthermore, temporal overlap between flowering and bee activity was similar over the years of this study and was unaffected by variability in snowmelt date or temperature. Nevertheless, the differences among some taxa in their phenological responses suggests that diverging temporal shifts are a possibility for the future.

Over the next century, many mid and high latitude temperate ecosystems are projected to experience rising growing season temperatures and increased frequency of soil freeze/thaw cycles (FTCs) due to a reduction in the depth and duration of the winter snowpack. We conducted a manipulative field experiment in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire to determine the interactive effects of climate change across seasons on rates of net N mineralization, foliar N, and natural abundance foliar ¹⁵N (δ¹⁵N) in red maple (Acer rubrum) trees. We warmed soils 5 °C above ambient temperatures and induced winter FTCs to simulate projected changes over the next century. Net N mineralization was dominated by ammonification and increased with warmer soil temperatures, but was not affected by soil FTCs in the previous winter. Similarly, warming led to increased foliar N concentrations and δ¹⁵N, with no effect of soil FTCs. Together, our results show that growing season soil warming increases soil N availability and N uptake by trees, which may offset the previously observed negative effects of a smaller snowpack and more frequent soil freezing on N cycling. We conclude that soil warming in the growing season may counteract the trend of reduced soil N availability relative to plant N demand (i.e. N oligotrophication) observed in northern hardwood forests. This research demonstrates that climate change across seasons affects N cycling in northern hardwood forests in ways that would have not been apparent from examining one season alone.

Plant and microbial processes exert control on the stoichiometry of available nutrients, potentially influencing forest ecosystem responses to nitrogen enrichment and other perturbations that alter resource availability. We tested whether an excess of one nutrient influenced the available pool of another, to learn the net outcome of various feedbacks on mineralization and uptake processes. We examined nitrogen and phosphorus availability (assayed with buried ion-exchange resin strips) in the first year of fertilizing northern hardwood forests with 30 kg/ha N, 10 kg/ha P, or N and P together. Fertilizing with a single nutrient raised the availability of the added nutrient and had no detectable effect on availability of the other nutrient. However, resin-available N was raised substantially more by adding N+P than it was by adding N alone. This effect of N+P must be the result of either reduced biotic uptake of N or increased mineralization of N, and suggests that N loss following forest disturbances will be enhanced in cases where the availability of both N and P are increased. That P interacts with N to enhance N availability, by whatever mechanism, could help explain observations of N and P co-limitation in ecosystems and calls attention to the need to carefully elucidate mechanisms underlying co-limitation of forest productivity.

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.

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.