The biological controls of soil carbon accumulation following wildfire and harvest in boreal forests: A review.

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha-1 year-1, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

Fire severity as a key determinant of aboveground and belowground biological community recovery in managed even-aged boreal forests.

Changes in fire regime of boreal forests in response to climate warming are expected to impact postfire recovery. However, quantitative data on how managed forests sustain and recover from recent fire disturbance are limited.Two years after a large wildfire in managed even-aged boreal forests in Sweden, we investigated how recovery of aboveground and belowground communities, that is, understory vegetation and soil microbial and faunal communities, responded to variation in the severity of soil (i.e., consumption of soil organic matter) and canopy fires (i.e., tree mortality).While fire overall enhanced diversity of understory vegetation through colonization of fire adapted plant species, it reduced the abundance and diversity of soil biota. We observed contrasting effects of tree- and soil-related fire severity on survival and recovery of understory vegetation and soil biological communities. Severe fires that killed overstory Pinus sylvestris promoted a successional stage dominated by the mosses Ceratodon purpureus and Polytrichum juniperinum, but reduced regeneration of tree seedlings and disfavored the ericaceous dwarf-shrub Vaccinium vitis-idaea and the grass Deschampsia flexuosa. Moreover, high tree mortality from fire reduced fungal biomass and changed fungal community composition, in particular that of ectomycorrhizal fungi, and reduced the fungivorous soil Oribatida. In contrast, soil-related fire severity had little impact on vegetation composition, fungal communities, and soil animals. Bacterial communities responded to both tree- and soil-related fire severity. Synthesis: Our results 2 years postfire suggest that a change in fire regime from a historically low-severity ground fire regime, with fires that mainly burns into the soil organic layer, to a stand-replacing fire regime with a high degree of tree mortality, as may be expected with climate change, is likely to impact the short-term recovery of stand structure and above- and belowground species composition of even-aged P. sylvestris boreal forests.

Ericoid shrubs shape fungal communities and suppress organic matter decomposition in boreal forests.

Mycorrhizal fungi associated with boreal trees and ericaceous shrubs are central actors in organic matter (OM) accumulation through their belowground carbon allocation, their potential capacity to mine organic matter for nitrogen (N) and their ability to suppress saprotrophs. Yet, interactions between co-occurring ectomycorrhizal fungi (EMF), ericoid mycorrhizal fungi (ERI), and saprotrophs are poorly understood. We used a long-term (19 yr) plant functional group manipulation experiment with removals of tree roots, ericaceous shrubs and mosses and analysed the responses of different fungal guilds (assessed by metabarcoding) and their interactions in relation to OM quality (assessed by mid-infrared spectroscopy and nuclear magnetic resonance) and decomposition (litter mesh-bags) across a 5000-yr post-fire boreal forest chronosequence. We found that the removal of ericaceous shrubs and associated ERI changed the composition of EMF communities, with larger effects occurring at earlier stages of the chronosequence. Removal of shrubs was associated with enhanced N availability, litter decomposition and enrichment of the recalcitrant OM fraction. We conclude that increasing abundance of slow-growing ericaceous shrubs and the associated fungi contributes to increasing nutrient limitation, impaired decomposition and progressive OM accumulation in boreal forests, particularly towards later successional stages. These results are indicative of the contrasting roles of EMF and ERI in regulating belowground OM storage.

Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation.

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N2 -fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N2 -fixation with the functioning of vascular plants. Neglecting nonvascular N2 -fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N2 -fixing plants are more common. Adequately accounting for nonvascular N2 -fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N2 -fixation responses to global change factors (elevated CO2 , N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N2 -fixation using nine Earth system models. The majority of empirical studies found positive effects of CO2 , warming, precipitation, or light on nonvascular N2 -fixation, but N deposition strongly downregulated N2 -fixation in most empirical studies. Furthermore, we found that the responses of N2 -fixation to elevated CO2 were generally consistent between models and very limited empirical data. In contrast, empirical-model comparisons suggest that all models we assessed, and particularly those that scale N2 -fixation with net primary productivity or evapotranspiration, may be overestimating N2 -fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N2 -fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N2 -fixation and that field studies include more targeted measurements to improve model structures and parameterization.

Soil biotic and abiotic effects on seedling growth exhibit context-dependent interactions: evidence from a multi-country experiment on Pinus contorta invasion.

The success of invasive plants is influenced by many interacting factors, but evaluating multiple possible mechanisms of invasion success and elucidating the relative importance of abiotic and biotic drivers is challenging, and therefore rarely achieved. We used live, sterile or inoculated soil from different soil origins (native range and introduced range plantation; and invaded plots spanning three different countries) in a fully factorial design to simultaneously examine the influence of soil origin and soil abiotic and biotic factors on the growth of invasive Pinus contorta. Our results displayed significant context dependency in that certain soil abiotic conditions in the introduced ranges (soil nitrogen, phosphorus or carbon content) influenced responses to inoculation treatments. Our findings do not support the enemy release hypothesis or the enhanced mutualism hypothesis, as biota from native and plantation ranges promoted growth similarly. Instead, our results support the missed mutualism hypothesis, as biota from invasive ranges were the least beneficial for seedling growth. Our study provides a novel perspective on how variation in soil abiotic factors can influence plant-soil feedbacks for an invasive tree across broad biogeographical contexts.

Boreal forest soil carbon fluxes one year after a wildfire: Effects of burn severity and management.

The extreme 2018 hot drought that affected central and northern Europe led to the worst wildfire season in Sweden in over a century. The Ljusdal fire complex, the largest area burnt that year (8995 ha), offered a rare opportunity to quantify the combined impacts of wildfire and post-fire management on Scandinavian boreal forests. We present chamber measurements of soil CO2 and CH4  fluxes, soil microclimate and nutrient content from five Pinus sylvestris sites for the first growing season after the fire. We analysed the effects of three factors on forest soils: burn severity, salvage-logging and stand age. None of these caused significant differences in soil CH4 uptake. Soil respiration, however, declined significantly after a high-severity fire (complete tree mortality) but not after a low-severity fire (no tree mortality), despite substantial losses of the organic layer. Tree root respiration is thus key in determining post-fire soil CO2 emissions and may benefit, along with heterotrophic respiration, from the nutrient pulse after a low-severity fire. Salvage-logging after a high-severity fire had no significant effects on soil carbon fluxes, microclimate or nutrient content compared with leaving the dead trees standing, although differences are expected to emerge in the long term. In contrast, the impact of stand age was substantial: a young burnt stand experienced more extreme microclimate, lower soil nutrient supply and significantly lower soil respiration than a mature burnt stand, due to a thinner organic layer and the decade-long effects of a previous clear-cut and soil scarification. Disturbance history and burn severity are, therefore, important factors for predicting changes in the boreal forest carbon sink after wildfires. The presented short-term effects and ongoing monitoring will provide essential information for sustainable management strategies in response to the increasing risk of wildfire.

Mosses modify effects of warmer and wetter conditions on tree seedlings at the alpine treeline.

Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic-alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic-alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open-top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate-change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.

Effects of plant functional group removal on structure and function of soil communities across contrasting ecosystems.

Loss of plant diversity has an impact on ecosystems worldwide, but we lack a mechanistic understanding of how this loss may influence below-ground biota and ecosystem functions across contrasting ecosystems in the long term. We used the longest running biodiversity manipulation experiment across contrasting ecosystems in existence to explore the below-ground consequences of 19 years of plant functional group removals for each of 30 contrasting forested lake islands in northern Sweden. We found that, against expectations, the effects of plant removals on the communities of key groups of soil organisms (bacteria, fungi and nematodes), and organic matter quality and soil ecosystem functioning (decomposition and microbial activity) were relatively similar among islands that varied greatly in productivity and soil fertility. This highlights that, in contrast to what has been shown for plant productivity, plant biodiversity loss effects on below-ground functions can be relatively insensitive to environmental context or variation among widely contrasting ecosystems.

Comparison of plant-soil feedback experimental approaches for testing soil biotic interactions among ecosystems.

The study of interactions and feedbacks between plants and soils is a rapidly expanding research area, and a primary tool used in this field is to perform glasshouse experiments where soil biota are manipulated. Recently, there has been vigorous debate regarding the correctness of methods for carrying out these types of experiment, and specifically whether it is legitimate to mix soils from different sites or plots (mixed soil sampling, MSS) or not (independent soil sampling, ISS) to create either soil inoculum treatments or subjects. We performed the first empirical comparison of MSS vs ISS approaches by comparing growth of two boreal tree species (Picea abies and Pinus sylvestris) in soils originating from 10 sites near the boreal forest limit in northern Sweden, and 10 sites in the subarctic region where boreal forests may potentially expand as a result of climate change. We found no consistent differences in the conclusions that we reached whether we used MSS or ISS approaches. We propose that researchers should not choose a soil handling method based on arguments that one method is inherently more correct than the other, but rather that method choice should be based on correct alignment with specific research questions and goals.

Consistent effects of biodiversity loss on multifunctionality across contrasting ecosystems.

Understanding how loss of biodiversity affects ecosystem functioning, and thus the delivery of ecosystem goods and services, has become increasingly necessary in a changing world. Considerable recent attention has focused on predicting how biodiversity loss simultaneously impacts multiple ecosystem functions (that is, ecosystem multifunctionality), but the ways in which these effects vary across ecosystems remain unclear. Here, we report the results of two 19-year plant diversity manipulation experiments, each established across a strong environmental gradient. Although the effects of plant and associated fungal diversity loss on individual functions frequently differed among ecosystems, the consequences of biodiversity loss for multifunctionality were relatively invariant. However, the context-dependency of biodiversity effects also worked in opposing directions for different individual functions, meaning that similar multifunctionality values across contrasting ecosystems could potentially mask important differences in the effects of biodiversity on functioning among ecosystems. Our findings highlight that an understanding of the relative contribution of species or functional groups to individual ecosystem functions among contrasting ecosystems and their interactions (that is, complementarity versus competition) is critical for guiding management efforts aimed at maintaining ecosystem multifunctionality and the delivery of multiple ecosystem services.

Trophic cascades in the bryosphere: the impact of global change factors on top-down control of cyanobacterial N2 -fixation.

Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem-level fluxes and processes, and the conditions under which top-down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss-inhabiting microfauna exerted top-down control of N2 -fixation by moss-associated cyanobacteria. However, the presence of higher trophic levels alleviated this top-down control, likely through feeding on bacterivorous microfauna. These effects of food-web structure on cyanobacterial N2 -fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N-limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning.

Shifts in Aboveground Biomass Allocation Patterns of Dominant Shrub Species across a Strong Environmental Gradient.

Most plant biomass allocation studies have focused on allocation to shoots versus roots, and little is known about drivers of allocation for aboveground plant organs. We explored the drivers of within-and between-species variation of aboveground biomass allocation across a strong environmental resource gradient, i.e., a long-term chronosequence of 30 forested islands in northern Sweden across which soil fertility and plant productivity declines while light availability increases. For each of the three coexisting dominant understory dwarf shrub species on each island, we estimated the fraction of the total aboveground biomass produced year of sampling that was allocated to sexual reproduction (i.e., fruits), leaves and stems for each of two growing seasons, to determine how biomass allocation responded to the chronosequence at both the within-species and whole community levels. Against expectations, within-species allocation to fruits was least on less fertile islands, and allocation to leaves at the whole community level was greatest on intermediate islands. Consistent with expectations, different coexisting species showed contrasting allocation patterns, with the species that was best adapted for more fertile conditions allocating the most to vegetative organs, and with its allocation pattern showing the strongest response to the gradient. Our study suggests that co-existing dominant plant species can display highly contrasting biomass allocations to different aboveground organs within and across species in response to limiting environmental resources within the same plant community. Such knowledge is important for understanding how community assembly, trait spectra, and ecological processes driven by the plant community vary across environmental gradients and among contrasting ecosystems.

Seasonal variation in nifH abundance and expression of cyanobacterial communities associated with boreal feather mosses.

Dinitrogen (N2)-fixation by cyanobacteria living in symbiosis with pleurocarpous feather mosses (for example, Pleurozium schreberi and Hylocomium splendens) represents the main pathway of biological N input into N-depleted boreal forests. Little is known about the role of the cyanobacterial community in contributing to the observed temporal variability of N2-fixation. Using specific nifH primers targeting four major cyanobacterial clusters and quantitative PCR, we investigated how community composition, abundance and nifH expression varied by moss species and over the growing seasons. We evaluated N2-fixation rates across nine forest sites in June and September and explored the abundance and nifH expression of individual cyanobacterial clusters when N2-fixation is highest. Our results showed temporal and host-dependent variations of cyanobacterial community composition, nifH gene abundance and expression. N2-fixation was higher in September than June for both moss species, explained by higher nifH gene expression of individual clusters rather than higher nifH gene abundance or differences in cyanobacterial community composition. In most cases, 'Stigonema cluster' made up less than 29% of the total cyanobacterial community, but accounted for the majority of nifH gene expression (82-94% of total nifH expression), irrespective of sampling date or moss species. Stepwise multiple regressions showed temporal variations in N2-fixation being greatly explained by variations in nifH expression of the 'Stigonema cluster'. These results suggest that Stigonema is potentially the most influential N2-fixer in symbiosis with boreal forest feather mosses.

Interactions with soil biota shift from negative to positive when a tree species is moved outside its native range.

Studies evaluating plant-soil biota interactions in both native and introduced plant ranges are rare, and thus far have lacked robust experimental designs to account for several potential confounding factors. Here, we investigated the effects of soil biota on growth of Pinus contorta, which has been introduced from Canada to Sweden. Using Swedish and Canadian soils, we conducted two glasshouse experiments. The first experiment utilized unsterilized soil from each country, with a full-factorial cross of soil origin, tree provenance, and fertilizer addition. The second experiment utilized gamma-irradiated sterile soil from each country, with a full-factorial cross of soil origin, soil biota inoculation treatments, tree provenance, and fertilizer addition. The first experiment showed higher seedling growth on Swedish soil relative to Canadian soil. The second experiment showed this effect was due to differences in soil biotic communities between the two countries, and occurred independently of all other experimental factors. Our results provide strong evidence that plant interactions with soil biota can shift from negative to positive following introduction to a new region, and are relevant for understanding the success of some exotic forest plantations, and invasive and range-expanding native species.

Boreal feather mosses secrete chemical signals to gain nitrogen.

The mechanistic basis of feather moss-cyanobacteria associations, a main driver of nitrogen (N) input into boreal forests, remains unknown. Here, we studied colonization by Nostoc sp. on two feather mosses that form these associations (Pleurozium schreberi and Hylocomium splendens) and two acrocarpous mosses that do not (Dicranum polysetum and Polytrichum commune). We also determined how N availability and moss reproductive stage affects colonization, and measured N transfer from cyanobacteria to mosses. The ability of mosses to induce differentiation of cyanobacterial hormogonia, and of hormogonia to then colonize mosses and re-establish a functional symbiosis was determined through microcosm experiments, microscopy and acetylene reduction assays. Nitrogen transfer between cyanobacteria and Pleurozium schreberi was monitored by secondary ion mass spectrometry (SIMS). All mosses induced hormogonia differentiation but only feather mosses were subsequently colonized. Colonization on Pleurozium schreberi was enhanced during the moss reproductive phase but impaired by elevated N. Transfer of N from cyanobacteria to their host moss was observed. Our results reveal that feather mosses likely secrete species-specific chemo-attractants when N-limited, which guide cyanobacteria towards them and from which they gain N. We conclude that this signalling is regulated by N demands of mosses, and serves as a control of N input into boreal forests.

Synergistic, additive and antagonistic impacts of drought and herbivory on Pinus sylvestris: leaf, tissue and whole-plant responses and recovery.

Forests typically experience a mix of anthropogenic, natural and climate-induced stressors of different intensities, creating a mosaic of stressor combinations across the landscape. When multiple stressors co-occur, their combined impact on plant growth is often greater than expected based on single-factor studies (i.e., synergistic), potentially causing catastrophic dysfunction of physiological processes from an otherwise recoverable situation. Drought and herbivory are two stressors that commonly co-occur in forested ecosystems, and have the potential to 'overlap' in their impacts on various plant traits and processes. However, the combined impacts from these two stressors may not be predictable based on additive models from single-stressor studies. Moreover, the impacts and subsequent recovery may be strongly influenced by the relative intensities of each stressor. Here, we applied drought stress and simulated bark-feeding herbivory at three levels of intensity (control, moderate and severe) in a full factorial design on young Pinus sylvestris L. seedlings. We assessed if the combined effects from two stressors were additive (responses were equal to the sum of the single-factor effects), synergistic (greater than expected) or antagonistic (less than expected) on a suite of morphological and physiological traits at the leaf-, tissue- and whole-plant level. We additionally investigated whether recovery from herbivory was dependent on relief from drought. The two stressors had synergistic impacts on specific leaf area and water-use efficiency, additive effects on height and root-to-shoot ratios, but antagonistic effects on photosynthesis, conductance and, most notably, on root, shoot and whole-plant biomass. Nevertheless, the magnitude and direction of the combined impacts were often dependent on the relative intensities of each stressor, leading to many additive or synergistic responses from specific stressor combinations. Also, seedling recovery was far more dependent on the previous year's drought compared with the previous year's herbivory, demonstrating the influence of one stressor over another during recovery. Our study reveals for the first time, the importance of not only the presence or absence of drought and herbivory stressors, but also shows that their relative intensities are critical in determining the direction and magnitude of their impacts on establishing seedlings.

Bryophyte-cyanobacteria associations as regulators of the northern latitude carbon balance in response to global change.

Ecosystems in the far north, including arctic and boreal biomes, are a globally significant pool of carbon (C). Global change is proposed to influence both C uptake and release in these ecosystems, thereby potentially affecting whether they act as C sources or sinks. Bryophytes (i.e., mosses) serve a variety of key functions in these systems, including their association with nitrogen (N2 )-fixing cyanobacteria, as thermal insulators of the soil, and producers of recalcitrant litter, which have implications for both net primary productivity (NPP) and heterotrophic respiration. While ground-cover bryophytes typically make up a small proportion of the total biomass in northern systems, their combined physical structure and N2 -fixing capabilities facilitate a disproportionally large impact on key processes that control ecosystem C and N cycles. As such, the response of bryophyte-cyanobacteria associations to global change may influence whether and how ecosystem C balances are influenced by global change. Here, we review what is known about their occurrence and N2 -fixing activity, and how bryophyte systems will respond to several key global change factors. We explore the implications these responses may have in determining how global change influences C balances in high northern latitudes.

The role of biotic interactions in shaping distributions and realised assemblages of species: implications for species distribution modelling.

Predicting which species will occur together in the future, and where, remains one of the greatest challenges in ecology, and requires a sound understanding of how the abiotic and biotic environments interact with dispersal processes and history across scales. Biotic interactions and their dynamics influence species' relationships to climate, and this also has important implications for predicting future distributions of species. It is already well accepted that biotic interactions shape species' spatial distributions at local spatial extents, but the role of these interactions beyond local extents (e.g. 10 km(2) to global extents) are usually dismissed as unimportant. In this review we consolidate evidence for how biotic interactions shape species distributions beyond local extents and review methods for integrating biotic interactions into species distribution modelling tools. Drawing upon evidence from contemporary and palaeoecological studies of individual species ranges, functional groups, and species richness patterns, we show that biotic interactions have clearly left their mark on species distributions and realised assemblages of species across all spatial extents. We demonstrate this with examples from within and across trophic groups. A range of species distribution modelling tools is available to quantify species environmental relationships and predict species occurrence, such as: (i) integrating pairwise dependencies, (ii) using integrative predictors, and (iii) hybridising species distribution models (SDMs) with dynamic models. These methods have typically only been applied to interacting pairs of species at a single time, require a priori ecological knowledge about which species interact, and due to data paucity must assume that biotic interactions are constant in space and time. To better inform the future development of these models across spatial scales, we call for accelerated collection of spatially and temporally explicit species data. Ideally, these data should be sampled to reflect variation in the underlying environment across large spatial extents, and at fine spatial resolution. Simplified ecosystems where there are relatively few interacting species and sometimes a wealth of existing ecosystem monitoring data (e.g. arctic, alpine or island habitats) offer settings where the development of modelling tools that account for biotic interactions may be less difficult than elsewhere.

Nitrogen niches revealed through species and functional group removal in a boreal shrub community.

Most theories attempting to explain the coexistence of species in local communities make fundamental assumptions regarding whether neighbors exhibit competitive, neutral, or positive resource-use interactions; however, few long-term data from naturally assembled plant communities exist to test these assumptions. We utilized a 13-year experiment consisting of factorial removal of three shrub species (Vaccinium myrtillus, V. vitis-idaea, and Empetrum hermaphroditum) and factorial removal of two functional groups (tree roots and feather mosses) to assess how neighbors affect N acquisition and growth of each of the three shrub species. The removal plots were established on each of 30 lake islands in northern Sweden that form a natural gradient of resource availability. We tested the hypotheses that: (1) the presence of functionally similar neighbors would reduce shrub N acquisition through competition for a shared N resource; (2) the removal of functional groups would affect shrub N acquisition by altering the breadth of their niches; and (3) soil fertility would influence the effects of neighbor removals. We found that the removal of functionally similar neighbors (i.e., other shrub species) usually resulted in higher biomass and biomass N, with the strength of these effects varying strongly with site fertility. Shrub species removals never resulted in altered stable N isotope ratios (delta(15)N), suggesting that the niche breadth of the three shrubs was unaffected by the presence of neighboring shrub species. In the functional group removal experiment, we found positive effects of feather moss removal on V. myrtillus biomass and biomass N, and negative effects on E. hermaphrotium N concentration and V. vitis-idaea biomass and biomass N. Tree root removal also caused a significant shift in foliar delta(15)N of V. myrtillus and altered the delta(15)N, biomass, and biomass N of E. hermaphroditum. Collectively, these results show that the resource acquisition and niche breadth of the three shrub species are often affected by neighbors, and further that both the identity of neighbors and site fertility strongly determine whether these interactions are positive, negative, or neutral. These findings have implications for understanding species coexistence and the reciprocal relationships between productivity and species diversity in this ecosystem.

The effect of altered macroclimate on N-fixation by boreal feather mosses.

Plant productivity is predicted to increase in boreal forests owing to climate change, but this may depend on whether N inputs from biological N-fixation also increases. We evaluated how alteration of climatic factors affects N input from a widespread boreal N-fixer, i.e. cyanobacteria associated with the feather moss Pleurozium schreberi. In each of 10 forest stands in northern Sweden, we established climate-change plots, including a control (ambient climate) plot and three plots experiencing a +2°C temperature increase, an approximately threefold reduction in precipitation frequency, and either 0.07, 0.29 or 1.16 times normal summer precipitation. We monitored N-fixation in these plots five times between 2007 and 2009, and three times in 2010 after climate treatments ended to assess their recovery. Warmer temperatures combined with less frequent precipitation reduced feather moss moisture content and N-fixation rates regardless of total precipitation. After climate treatments ended, recovery of N-fixation rates occurred on the scale of weeks to months, suggesting resilience of N-fixation to changes in climatic conditions. These results suggest that modelling of biological N-inputs in boreal forests should emphasize precipitation frequency and evaporative water loss in conjunction with elevated temperature rather than absolute changes in mean precipitation.