Northern hardwood forests have long been assumed to be primarily nitrogen limited, but may often be co‐limited by multiple elements. Nutrient limitation can be inferred through responses of foliar and litter chemistry to nutrient addition over time. We compared community‐level foliar and litter chemistry and resorption efficiency in a long‐term, factorial nitrogen (N) and phosphorus (P) fertilization study across 10 forest stands at three sites in New Hampshire, where N and P were added annually. We measured N, P, calcium (Ca), magnesium (Mg), and potassium (K) in foliage from codominant trees and in fresh litter in 2008–2010 (pretreatment) and again in 2014–2016 and 2021–2022. Foliar N and P concentrations indicated co‐limitation in 2014–2016 based on reduced concentrations of one nutrient following addition of the other, suggesting a dilution effect. In 2021–2022, an interactive effect of N and P addition was observed: foliar P concentrations were lower under N+P addition, consistent with dilution following a greater growth response to N + P than to P addition, which was observed by 2015–2019. Changes in litter N and P concentrations with N and P addition mirrored those in foliar N and P. Resorption efficiency of N and P decreased with addition of these respective nutrients and P resorption efficiency was higher in the N+P treatment than the P treatment. Foliar Ca and litter Ca and K decreased with N addition but increased with P addition. Results indicated N and P co‐limitation and revealed biogeochemical interactions among N, P and base cations.

Soil respiration (Rsoil) is the second largest terrestrial carbon (C) flux, and therefore, it is imperative to understand and quantify its responses to global environmental change. Rsoil consists of two component CO2 fluxes: autotrophic respiration from the metabolic activity of roots (Ra-root) and heterotrophic respiration (Rh) derived from the metabolic activity of mycorrhizal fungi and microbial decomposition of detritus, soil organic matter, and rhizodeposits. Increased nitrogen (N) availability often reduces Rsoil in forest ecosystems, but it remains unclear which contributing fluxes govern Rsoil responses and if suppression of Rsoil results from increased N availability itself or from the tendency of added N to acidify soil. Here, we address these uncertainties in a long-term, large-scale factorial N × pH experiment in six temperate forest stands in central New York, USA. We anticipated that increasing soil N availability would decrease plant belowground C allocation and related root-associated respiration and that soil acidification would suppress microbial decomposition, thereby reducing Rh. We found that both acidifying and deacidifying N additions suppressed annual Rsoil by 19% and 13%, respectively (-1.8 Mg C ha-1 year-1 overall), but acidification (from pH 4.67 to 4.22) alone did not detectably affect this flux. Annual Rsoil decreased steeply (R2 = 0.66, p < 0.001) as soil N availability increased. Nitrogen additions generally suppressed Rh, especially in the forest floor (-34%), whereas the effects of acidification alone varied by soil depth, with substantial suppression in the forest floor (-33%) partially offset by stimulation at depth. A novel partitioning of Rsoil component responses suggests that N additions suppressed root-associated respiration by ~1.1 Mg C ha-1 year-1 (62% of the Rsoil suppression), while acidification alone had no effect. Our findings demonstrate that soil N availability, not soil pH, is the predominant biogeochemical control over Rsoil in these temperate forests, with larger responses of plant-driven C fluxes than microbial-driven C fluxes.

Soil carbon dioxide (CO2) flux, or soil respiration, is a critical control on net ecosystem carbon (C) balance. Using long-term (2002-2020) measurements at the Hubbard Brook Experimental Forest (New Hampshire, U.S.), we show that soil respiration rates have notably increased since similar to 2015. In 2020, cumulative summer respiration flux was approximately 90% higher than the average summer flux over the 2002-2015 period. The increase in soil respiration cannot be explained directly by temperature or pH change alone. We also found that heterotrophic microbial C mineralization and microbial biomass C have also increased rapidly since similar to 2015, pointing towards an increase in the bioavailability of organic C substrates. We suggest that these observations are consistent with a hypothetical increase in plant allocation of C belowground in response to changing climatic and soil conditions. Quantification of interactions among co-occurring global change factors (e.g., warming temperatures, increasing atmospheric CO2, and nutrient limitation) is needed to predict how the soil C reservoir will continue to respond to global environmental changes.

Foliar traits can reflect fitness responses to environmental changes, such as changes in nutrient availability. Species may respond differently to these changes due to differences in traits and their plasticity. Traits and community composition together can influence forest nutrient cycling. We compared five traits-foliar N, foliar P, specific leaf area (SLA), leaf dry matter content (LDMC), and leaf carbon isotope ratio (δ13C)-in six northern hardwood tree species (Acer rubrum, Acer saccharum, Betula alleghaniensis, Betula papyrifera, Fagus grandifolia, and Prunus pensylvanica) in a nitrogen (N) and phosphorus (P) fertilization study across 10 mid- and late-successional forest stands in New Hampshire, USA. We also analyzed the response of tree growth to N and P addition. Nutrient addition shifted trait values towards the "acquisitive" side of the spectrum for all traits except δ13C, reflecting a tradeoff between water-use efficiency and nutrient-use efficiency. Treatment responses in relative basal area increment revealed that the Betula species were N-limited, but traits of all species responded to either or both N and P addition in ways that suggest N and P co-limitation. Two species displayed lower foliar P under N addition, and three species displayed lower foliar N under P addition, which also suggests co-limitation. These indications of co-limitation were reflected at the community level. Specific leaf area, LDMC, and δ13C differed with stand age within several species. Examining trait responses of tree species and communities to nutrient availability increases our understanding of biological mechanisms underlying the complex effects of nutrient availability on forests.

Declining nitrogen (N) availability relative to plant demand, known as N oligotrophication, is a widespread phenomenon that has been particularly well documented in northern hardwood forests of the northeast U.S. It is hypothesized that later fall senescence contributes to this trend by increasing tree resorption of N, resulting in higher carbon:nitrogen ratios (C:N) in litterfall and reduced N availability in soil. To examine the effects of litterfall C:N on soil N cycling, we conducted a litter quality manipulation experiment comparing low C:N and high C:N litter with native litter along an elevation and aspect gradient at Hubbard Brook Experimental Forest, NH, USA. We found that potential net ammonification and mineralization rates were positively correlated with litter N and negatively correlated with litter C:N under high C:N litter, but these relationships were not present under native or low C:N litter. Differences in nitrate pools and net mineralization rates between high- and low-quality litter treatments were greater at colder sites where native litterfall tends to have lower C:N than at low elevation sites. Together, these results demonstrate that higher C:N litter and a warming climate may contribute to N oligotrophication through effects on microbially driven N cycling rates in organic soils.

Co-limitation is defined as the coincident limitation of biological activity by multiple resources. According to theories of resource optimization, co-limitation should be common as organisms adjust to changes in the availability of resources in the environment. We review the multi-faceted nature of the co-limitation concept and provide a synthesis of recent experimental studies of co-limitation in northern hardwood forests to illustrate the complexities of nitrogen (N) and phosphorus (P) co-limitation and possible responses to environmental stressors such as acid rain, N deposition, elevated CO2, land-use, and climate change. In a factorial nutrient addition experiment, cycling of one nutrient changed in response to addition of the other through synergistic interactions and feedbacks between N and P, including microbial recycling, soil enzyme activity, and foliar nutrient resorption; these responses were suggestive of some degree of N–P co-limitation in these forests. After 8 years of treatment, aboveground growth increased in response to either N or P added individually and even more in response to N + P addition, indicating N–P co-limitation. Surprisingly, fine root growth increased in response to nutrient addition, with significantly greater root growth in N + P plots in five successional stands and in N plots in three mature stands. In contrast, fine litterfall did not respond significantly to nutrient addition. Collectively, these results demonstrate the complexity of the interactions between macronutrients in regulating production processes in forest ecosystems.

Foliar resorption is a principal nutrient conservation mechanism in terrestrial vegetation that could be sensitive to ongoing changes in climate and atmospheric nitrogen (N) deposition. We quantified N resorption in northern hardwood forests along an elevation gradient of decreasing temperature and increasing soil N availability to evaluate how this critical nutrient cycling process can be expected to respond to global and regional environmental changes. Foliar N resorption proficiency (NRP) increased significantly at lower elevations for both sugar maple and American beech, the dominant species in these forests. Foliar N resorption efficiency (NRE) also decreased with increasing elevation, but only in one year. Both species exhibited strong negative relationships between NRP and soil N availability. Thus, we anticipate that with climate warming and decreasing N inputs, northern hardwood forests can be expected to exhibit stronger N conservation via foliar resorption. Both species also exhibited strong correlations between resorption efficiency of N and C, but resorption of both elements was much greater for beech than sugar maple, suggesting contrasting mechanisms of nutrient conservation between these two widespread species.

Many tree species form dual mycorrhizal symbioses, arbuscular mycorrhizae (AM) and ectomycorrhizae (EM), especially at the seedling stage, but the effect of soil nutrient availability in shaping these mycorrhizal relationships is not well understood. We quantified the effects of N and P fertilization on survival, growth, and mycorrhizal associations of white pine (Pinus strobus) seedlings planted into a long-term nutrient addition experiment (MELNHE) in four northern hardwood forests in New Hampshire, USA. Seedling survival was significantly reduced by N addition, potentially due to changes in soil pathogen communities. In contrast, seedling growth indicated N limitation but was unaffected by mycorrhizal colonization. Both microscopic and metagenomic evidence indicated the formation of dual mycorrhizae across all the stands and treatments. The two types of mycorrhizal colonization were equally abundant and were neither positively nor negatively correlated. Addition of P alone significantly reduced AM colonization relative to controls and increased EM colonization. The EM fungal community composition was strongly affected by N and P additions, and P addition caused increases in abundance of common EM fungi with widely differing functional traits (e.g., exploration type, nitrogen affinity).

Nutrient limitation of forest growth has been difficult to predict, and in temperate forests, long-term tests of single-nutrient versus multiple-element limitation are few. Nutrient co-limitation is the expected outcome of the ability of plants to adjust allocation to minimize limitation by any single resource. Nutrient limitation of productivity in northern hardwood forests was predicted by the Multiple Element Limitation (MEL) model to shift over time since harvest from single limitation by N to P at ~30 years and then, in mature forests, to co-limitation by N and P. Our work tested those predictions for tree growth in a fully factorial N and P addition experiment in 13 forest stands that we grouped in young (20-30 years), mid-age (40-50 years), and mature (>100 years old) age classes in New Hampshire, USA. Over 8 years of treatment, we found evidence of additive co-limitation of tree growth by N and P. We did not find evidence that limitation varied with time since disturbance. Our results suggest that processes contributing to co-limitation in these northern hardwood forests are effective across stands that vary widely in N status and are not sensitive to disturbance by forest harvest over time periods of several decades.

Fine roots and root-associated carbon (C) inputs contribute disproportionally to soil C stocks. Here, we quantified fine root dynamics in a mixed northern hardwood forest at the Woods Lake Watershed in the Adirondack Park, NY, USA, where an experimental lime application in 1989 led to the near-doubling of forest floor organic matter stocks two decades later. Prior work linked this organic matter accumulation with lower heterotrophic respiration and decreased abundances of major fungal saprotrophs and ectomycorrhizal fungi. We investigated whether liming-driven shifts in fine root dynamics and depth distribution also contributed to forest floor accumulation. Forest floor mass in limed plots again roughly doubled that in unlimed plots 32 years after liming, with persistently large accumulations in the Oa horizon. Liming decreased fine root production across all horizons measured (Oe, Oa, 0–10 cm mineral soil) and by 40% overall, with largest effects in the Oa horizon (−45%) where root turnover was also reduced (−59%). Thus, liming decreased, rather than increased, root detrital C inputs to the forest floor. These results suggest that liming must have suppressed decomposition even more than shown previously or increased some other C input to the forest floor to explain its substantial C accumulation.

Soil respiration is the largest single efflux in the global carbon cycle and varies in complex ways with climate, vegetation, and soils. The suppressive effect of nitrogen (N) addition on soil respiration is well documented, but the extent to which it may be moderated by stand age or the availability of soil phosphorus (P) is not well understood. We quantified the response of soil respiration to manipulation of soil N and P availability in a full-factorial N x P fertilization experiment spanning 10 years in 13 northern hardwood forests in the White Mountains of New Hampshire, USA. We analyzed data for 2011 alone, to account for potential treatment effects unique to the first year of fertilization, and for three 3-year periods; data from each 3-year period was divided into spring, summer, and fall. Nitrogen addition consistently suppressed soil respiration by up to 14

Accurate measurement of decomposition of fine roots in forests is important for understanding nutrient cycling, carbon dynamics and biotic responses; however, various sources of error or bias interfere with commonly used approaches. Most studies of fine root decomposition have used the buried root litter bag method, but this method may underestimate decay rates based on comparisons with the less disruptive, intact soil core approach. We compared these two methods for one year for seven temperate forest trees growing in twelve monospecific plantations in central New York. The experimental design included four species that form arbuscular mycorrhizae (sugar maple, red maple, cherry, tulip poplar) and three with ectomycorrhizae (pine, spruce, oak). We also examined macronutrients (N, P) release and fungal community composition of decaying fine roots using Illumina sequencing. Across the seven species and 12 plots, fine root decay rate was significantly higher for intact cores than litter bags, with the difference being approximately 12 %, a smaller disparity than that observed in previous comparisons. However, one species (Norway spruce) showed the opposite trend. Similarly, N and P release from decaying roots was significantly greater for the intact core method, but again with an exception (red oak). No consistent effects of mycorrhizal type on the method effect or decay rate and nutrient release were detected. Fungal communities in decaying roots differed between the two methods; nevertheless, Mycena spp. dominated most plots for both methods. These results further emphasize the notion that decay rates and nutrient mobilization of fine roots in forests are usually underestimated by the litter bag approach.

The importance of woody detritus as a source of soil organic matter is not well constrained. We quantified the recovery of 13C derived from isotopic-enriched sugar maple wood in various C fractions of two temperate forest soils in central New York, USA. Decay rates of small woody debris were quite rapid (k = 0.362 to 0.477 per year) and after 10 years less than 1

Coarse roots represent a globally important belowground carbon pool, but the factors controlling coarse root decomposition rates remain poorly understood relative to other plant biomass components. We compiled the most comprehensive dataset of coarse root decomposition data including 148 observations from 60 woody species, and linked coarse root decomposition rates to plant traits, phylogeny and climate to address questions of the dominant controls on coarse root decomposition. We found that decomposition rates increased with mean annual temperature, root nitrogen and phosphorus concentrations. Coarse root decomposition was slower for ectomycorrhizal than arbuscular mycorrhizal associated species, and angiosperm species decomposed faster than gymnosperms. Coarse root decomposition rates and calcium concentrations showed a strong phylogenetic signal. Our findings suggest that categorical traits like mycorrhizal association and phylogenetic group, in conjunction with root quality and climate, collectively serve as the optimal predictors of coarse root decomposition rates. Our findings propose a paradigm of the dominant controls on coarse decomposition, with mycorrhizal association and phylogeny acting as critical roles on coarse root decomposition, necessitating their explicit consideration in Earth-system models and ultimately improving confidence in projected carbon cycle-climate feedbacks.

Forests in northeastern North America have undergone dramatic transformations due to losses and gains of species, changes in land use and pollution. Historic stressors combined with new threats of white-tailed deer and non-native earthworms are threatening native plant diversity. We developed a transplant approach to gauge the importance of deer and earthworms in allowing understorey species recovery. This approach, instead of censusing existing individuals, avoids problems imposed by past land use, dispersal limitation or climate change. We selected 20 native species in different taxonomic and functional groups, with different palatability to herbivores, different life-history strategies and a range of plant traits (%N, specific leaf area [SLA]) and our selection included species indicative of primary or secondary forests. Using a 2 x 2 full factorial design, we planted species into fenced and unfenced plots with and without existing earthworm invasions in five secondary hardwood forests. We measured survival, plant growth and reproduction over 4-6 years. Earthworm biomass was associated with increased survival of 13 and decreased survival of five species. Surviving transplants grew taller (eight of 12 species measured) and wider (seven of nine species measured) in earthworm plots but were more likely to be attacked by insects. Excluding deer benefitted most species' survival and growth. Taxonomic class, SLA or foliar N did not affect species sensitivity to deer or earthworms. Synthesis. Secondary forests in our region continue to provide suitable habitat for native species. Earthworms and deer are major structuring forces affecting survival and growth of our transplant species, but their impacts are species-specific and change over time. We observed poorly recognized indirect negative impacts of deer on many unpalatable species. Taxonomic class and differences in plant traits offered no important insights regarding vulnerability to deer or earthworms. Transplanting native species into secondary forests can succeed in restoring understorey communities, but will require fencing or substantial deer population reductions to be successful long term. Secondary forests in our region continue to provide suitable habitat for native species. Earthworms and deer are major structuring forces affecting survival and growth of our transplant species, but their impacts are species-specific and change over time. The authors observed poorly recognized indirect negative impacts of deer on many unpalatable species. Taxonomic class and differences in plant traits offered no important insights regarding vulnerability to deer or earthworms. Transplanting native species into secondary forests can succeed in restoring understorey communities, but will require fencing or substantial deer population reductions to be successful long term.image

One of the principal inputs of organic matter to forest soils is turnover of tree fine roots, but the process of decomposition of fine root litter and its conversion into stable soil organic matter (SOM) has received limited study. We labeled fine roots of sugar maple ( Acer saccharum Marsh.) with 13 C and traced the label for 7 years into four contrasting soils to improve understanding of this process. After 7 years we recovered an average of 8.9% of the 13 C label, with about two-thirds recovered as coarse particulate organic matter and one-third in microaggregates and on silt and clay particles. No differences in 13 C recovery were detected between 1–2 and 3–4 order fine roots. Most of the 13 C in microaggregates (53–250 µm, 58%) was occluded within macroaggregates, and the recovery in this fraction increased significantly from year 2 to 7, illustrating the role of fine root detritus in the formation of microaggregates. This process was most pronounced in the A horizon of a higher pH soil (pH = 5.5) with high iron oxide content. Conversely, the lowest 13 C recovery in this fraction was observed in the A horizon of an acidic, fine-textured Inceptisol (Cambisol—World Reference Base). We estimate that annual input into relatively stable fractions of SOM represents about 14% of the total annual accumulation in these fractions; thus, our results support recent evidence that fine root litter is only a moderate contributor to stable SOM in acid temperate forest soils.

Nitrogen and phosphorus, alone or in combination, are the nutrients most often limiting to plants. Resorption is one way plants conserve nutrients, thereby reducing dependence on nutrient uptake from soil. We investigated foliar nutrient concentrations, ratios, and resorption in three northern hardwood species growing in eight stands across three sites and in two age classes as part of a long-term N x P factorial fertilization experiment. We found that neither P nor N addition affected N resorption, but trees in plots receiving P addition exhibited lower P resorption. Foliar N:P ratios often indicated P limitation in the control and N plots, but co -limitation by N and P in plots where P was added, alone or with N. Green leaf N and P concentrations and P resorption were highest at the site with the highest N availability and intermediate P availability. Though these stands are in a region where trees are commonly assumed to be N limited, we found numerous indications of P limitation in these stands, as well as site and species differences in resorption proficiency and efficiency.

Nitrogen (N) is a critical element in many ecological and biogeochemical processes in forest ecosystems. Cycling of N is sensitive to changes in climate, atmospheric carbon dioxide (CO 2 ) concentrations, and air pollution. Streamwater nitrate draining a forested ecosystem can indicate how an ecosystem is responding to these changes. We observed a pulse in streamwater nitrate concentration and export at a long-term forest research site in eastern North America that resulted in a 10-fold increase in nitrate export compared to observations over the prior decade. The pulse in streamwater nitrate occurred in a reference catchment in the 2013 water year, but was not associated with a distinct disturbance event. We analyzed a suite of environmental variables to explore possible causes. The correlation between each environmental variable and streamwater nitrate concentration was consistently higher when we accounted for the antecedent conditions of the variable prior to a given streamwater observation. In most cases, the optimal antecedent period exceeded two years. We assessed the most important variables for predicting streamwater nitrate concentration by training a machine learning model to predict streamwater nitrate concentration in the years preceding and during the streamwater nitrate pulse. The results of the correlation and machine learning analyses suggest that the pulsed increase in streamwater nitrate resulted from both (1) decreased plant uptake due to lower terrestrial gross primary production, possibly due to increased soil frost or reduced solar radiation or both; and (2) increased net N mineralization and nitrification due to warm temperatures from 2010 to 2013. Additionally, variables associated with hydrological transport of nitrate, such as maximum stream discharge, emerged as important, suggesting that hydrology played a role in the pulse. Overall, our analyses indicate that the streamwater nitrate pulse was caused by a combination of factors that occurred in the years prior to the pulse, not a single disturbance event.

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands. A new method to quantify three masting components from individual tree-years has revealed that globally, masting is uncommon in tree species that depend on mutualist dispersers, with its distribution further mediated by climate and nutrient availability.

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.