Aim: Climate change affects Arctic vegetation directly through increasing temperatures and indirectly through changes in nutrient availability. Here, we tested the long-term effects of nitrogen (N) and phosphorus (P) additions on tundra plant communities across an elevational gradient. We aimed to explore the interactive effects of nutrient addition, elevation-associated changes in temperature and vegetation type for plant community dynamics in tundra landscapes. Location: Abisko, northern Sweden. Methods: We measured plant community properties in a 14-year-long fully factorial N and P addition experiment in subarctic heath and meadow at three elevations. We used linear mixed effects models and non-metric multidimensional scaling to analyse the main and interactive effects of nutrient addition treatments, vegetation type and elevation over time on community responses. Results: Plant community properties often responded to the interactions between N addition, P addition, vegetation type, elevation and time. Nitrogen was the main limiting nutrient, but N added with P generated the greatest change in plant community composition over time. Community responses to nutrient additions were largely driven by an increase in the abundance of graminoids and a decrease in species richness. The greatest change in plant community composition and species richness occurred in heath at the lower elevation and in the meadow at higher elevations, and the responses of species richness to N and N + P addition became stronger after 7 years. Conclusions: Our results show that significant and relatively fast context (vegetation type and elevation) dependent responses of plant communities to increased N and P availability persist, but in several instances become stronger with time. They highlight the need to account for drivers of spatial and temporal variability in plant community dynamics when predicting plant community and diversity patterns under changes in climate and nutrient supply in tundra landscapes.

There is a public debate on how boreal forests can deliver climate change mitigation benefits. While most debates regarding Fennoscandian forests have centered on the contrasting effects of actively managed and old-growth unmanaged forests on carbon uptake and storage, the impact of surface albedo has often been overlooked. According to the new EU forest strategy for 2030, with aim of improving quantity and quality of forests by promoting primary old-growth forests and avoiding clear-cutting, among others, we examined how albedo across a wide age range of boreal Pinus-dominated forests develops over time after wildfire (defined as unmanaged) and clear-cutting (defined as managed). We find that albedo decreases over time after disturbance, but mainly in managed forests. Annual mean albedo in young (<30 years) managed forests (0.36 +/- 0.04) is markedly larger than in young unmanaged forests (0.18 +/- 0.04). This difference is particularly prominent during winter, when snow-covered ground is present. The mean albedo over the entire unmanaged forest-age gradient (0.17 +/- 0.05) is significantly lower (p < 0.05) than that of the managed forest-age gradient (0.23 +/- 0.10). Considering the typically higher frequency of clear-cuts compared to wildfires in Fennoscandian forests, these albedo differences would be even larger over long time scales. Our findings reveal the importance of considering the climatic cooling potential of albedo when making decisions on how to optimize future forest management in northern boreal forests to mitigate climate change.

Warming temperatures can promote arctic tree seedling establishment and growth, but these effects depend on the environmental context and the seedlings' capacity to acquire limiting resources. Here, we tested how temperature, soil properties, and the presence of a neighbouring shrub influence seedling growth rates. We further explored how the environment‐induced shifts in growth rates are related to traits associated with resource acquisition strategy. Seedlings of mountain birch Betula pubescens subsp. czerepanovii were grown in a growth chamber experiment with a fully factorial design manipulating temperature (ambient, warm), soil origin (low elevation, high elevation), and the presence or absence of a common neighbour, crowberry Empetrum nigrum subsp. hermaphroditum . Growth rate, carbon dioxide (CO 2 ) assimilation rates, ectomycorrhizal fungal colonisation, and above‐ and belowground functional traits that are linked to resource acquisition strategies, were measured for the mountain birch seedlings. Warming significantly increased seedling growth, and growth rates were positively associated with several above‐ and belowground traits associated with resource acquisition. Surprisingly, growth rates were not correlated with leaf traits such as specific leaf area and leaf dry matter content. Soil origin and the presence of Empetrum did not affect the growth rate responses to warming, although seedlings grown in high‐elevation soils generally grew faster and had higher CO 2 assimilation rates. These seedlings also expressed traits, commonly associated with conservative growth strategy. Our results highlight that both above‐ and belowground plant traits adjust differently to warming as part of their growth response. While seedling functional traits can be highly plastic in response to changes in temperature and soil conditions, they are not always coupled with changes in seedling growth rates. This is likely because birch seedlings adopt different growth‐promoting strategies under different environmental conditions.

We compared three methods of estimating gross primary production (GPP) of a boreal forest dominated by spruce and pine with the goals of 1) converging on the best estimate and 2) disaggregating the GPP among the two canopy species and the understory stratum. The three methods were: 1) eddy covariance (EC), 2) a soil-vegetation-atmosphere transfer model, APES, driven by meteorological data, and 3) an ecophysiological approach (Iso/SF) based on sap flux and phloem delta C-13, where sap flux is used to estimate stomatal conductance and delta C-13 is used to estimate intrinsic water-use efficiency (WUEi). The EC and APES methods agreed rather well, which was expected because APES was developed to predict eddy covariance data. The Iso/SF method, which is based on independent data, yielded lower estimates. This was partly because it excluded understory vegetation from the GPP estimate. We also found that the measured sap flux/transpiration estimates for spruce in Iso/SF were much lower than those from APES. In contrast, the absolute values for Scots pines were very similar between the two methods, especially in the summer. In both species, the seasonal dynamics match well among all methods. This multi-method approach allowed us to detect possible problems in the spruce sap-flux measurements, but successfully upscaled pine data from ecophysiological traits to stand and ecosystem functioning.

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.

Alternative water uptake pathways through leaves and bark complement water supply with interception, fog or dew. Bark water-uptake contributes to embolism-repair, as demonstrated in cut branches. We tested whether bark water-uptake could also contribute to supplement xylem-water for transpiration. We applied bandages injected with 2 H-enriched water on intact upper-canopy branches of Pinus sylvestris and Fagus sylvatica in a boreal and in a temperate forest, in summer and winter, and monitored transpiration and online isotopic composition (δ2 H and δ18 O) of water vapour, before sampling for analyses of δ2 H and δ18 O in tissue waters. Xylem, bark and leaf waters from segments downstream from the bandages were 2 H-enriched whereas δ18 O was similar to controls. Transpiration was positively correlated with 2 H-enrichment. Isotopic compositions of transpiration and xylem water allowed us to calculate isotopic exchange through the bark via vapour exchange, which was negligible in comparison to estimated bark water-uptake, suggesting that water-uptake occurred via liquid phase. Results were consistent across species, forests and seasons, indicating that bark water-uptake may be more ubiquitous than previously considered. We suggest that water taken up through the bark could be incorporated into the transpiration stream, which could imply that sap-flow measurements underestimate transpiration when bark is wet.

Sunflecks are transient patches of direct radiation that provide a substantial proportion of the daily irradiance to leaves in the lower canopy. In this position, faster photosynthetic induction would allow for higher sunfleck-use efficiency, as is commonly reported in the literature. Yet, when sunflecks are too few and far between, it may be more beneficial for shade leaves to prioritize efficient photosynthesis under shade. We investigated the temporal dynamics of photosynthetic induction, recovery under shade, and stomatal movement during a sunfleck, in sun and shade leaves of Fagus sylvatica from three provenances of contrasting origin. We found that shade leaves complete full induction in a shorter time than sun leaves, but that sun leaves respond faster than shade leaves due to their much larger amplitude of induction. The core-range provenance achieved faster stomatal opening in shade leaves, which may allow for better sunfleck-use efficiency in denser canopies and lower canopy positions. Our findings represent a paradigm shift for future research into light fluctuations in canopies, drawing attention to the ubiquitous importance of sunflecks for photosynthesis, not only in lower-canopy leaves where shade is prevalent, but particularly in the upper canopy where longer sunflecks are more common due to canopy openness.

Carbon storage and cycling in boreal forests-the largest terrestrial carbon store-is moderated by complex interactions between trees and soil microorganisms. However, existing methods limit our ability to predict how changes in environmental conditions will alter these associations and the essential ecosystem services they provide. To address this, we developed a metatranscriptomic approach to analyze the impact of nutrient enrichment on Norway spruce fine roots and the community structure, function, and tree-microbe coordination of over 350 root-associated fungal species. In response to altered nutrient status, host trees redefined their relationship with the fungal community by reducing sugar efflux carriers and enhancing defense processes. This resulted in a profound restructuring of the fungal community and a collapse in functional coordination between the tree and the dominant Basidiomycete species, and an increase in functional coordination with versatile Ascomycete species. As such, there was a functional shift in community dominance from Basidiomycetes species, with important roles in enzymatically cycling recalcitrant carbon, to Ascomycete species that have melanized cell walls that are highly resistant to degradation. These changes were accompanied by prominent shifts in transcriptional coordination between over 60 predicted fungal effectors, with more than 5,000 Norway spruce transcripts, providing mechanistic insight into the complex molecular dialogue coordinating host trees and their fungal partners. The host-microbe dynamics captured by this study functionally inform how these complex and sensitive biological relationships may mediate the carbon storage potential of boreal soils under changing nutrient conditions.

Summary Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water‐use efficiency or past tree productivity requires in‐depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (gm). We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative gm descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated gm responses with weather variations. We further compared the simulated δ13C of new photosynthates among the different gm descriptions and against measured phloem sugar δ13C. Simulated gm estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem δ13C during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar δ13C indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition. We suggest short‐interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (Anet ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (gm ) and its effect on the estimation of carboxylation capacity ( VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over Anet . The seasonality of Anet was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited Anet in the spring, whereas daily temperatures and the gradual reduction of light availability limited Anet in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between gm and Anet , but not between gm and stomatal conductance.

In plants, the constant demand for water driven by transpiration is supplied by uptake from the soil through the roots. Alternative water-uptake pathways through the leaves and the bark have been demonstrated for some species, mainly conifers. Alternative water-uptake pathways could allow plants to complement their water supply with canopy interception, fog or dew, sources often assumed unavailable as they are lost via evaporation before they can contribute to soil water recharge. Bark water-uptake has been putatively linked to repair of xylem embolism, although this has only been demonstrated in cut branches and/or under artificial conditions. We hypothesized that besides embolism repair, bark water uptake might also contribute to maintaining the transpiration stream in upper canopy branches when the xylem water column is subject to excess negative pressure, either because temperature drops, and water viscosity increases, or under high vapour pressure deficit and low soil water availability. We used a novel labelling methodology combining online measurements of the isotope composition (δ2H and δ18O) of the transpiration stream with analyses of δ2H and δ18O from leaf, bark and xylem water in Pinus sylvestris and Fagus sylvatica. We conducted sampling campaigns in two study sites: a boreal (northern Sweden) and a temperate (northern Spain) forest. We applied semi-permeable bandages injected with 2H-enriched water (0.8% 2H2O), on intact upper canopy branches (7-13 m), and monitored δ2H and δ18O of the transpiration stream with a Cavity Ring-Down Spectrometer (CRDS) in three branches (only P. sylvestris in Sweden) for 24 h and then sampled branch segments 2 cm upstream and downstream of the bandage. We determined δ2H and δ18O of leaf, bark and xylem water from sampled segments with a CRDS after cryogenic extraction. Xylem, bark and leaf water from segments downstream of the bandage were enriched in δ2H with respect to their corresponding upstream segments. The δ2H and δ18O from leaf, bark and xylem water from upstream segments were similar to those of control branches (no bandages). Results were similar for both study species, sites and campaigns, indicating that bark water uptake is not restricted to gymnosperms and may be more ubiquitous than previously considered. Enrichment in δ2H in the transpiration stream was also detected in one out of the three continuously monitored pine branches within the 12 h following the bandage application. Our results demonstrate that water taken up through the bark may be incorporated into the transpiration stream and that transpiration might not solely rely on water absorbed through the roots and transported through the main stem. This could imply, for example, that sapflux measurements would underestimate canopy transpiration. Combining empirical flux measurements with stable isotopes and/or other atmospheric tracers could render more realistic estimates of transpiration and help constrain partitioning of evaporation and transpiration and its coupling to gross primary productivity.

Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi-empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPPiso/SF , at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water-use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPPiso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPPiso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPPiso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPPiso/SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met.

Stand-scale estimates of gross primary production (GPP) commonly depend on eddy-covariance or eddy-covariance derived models. Chamber-based methods provide an alternative, but they are tricky to scale up to the stand. We estimate GPP by combining isotopic δ13C of phloem sugars with sap-flow measurements. The method consists of calculating intrinsic water-use efficiency and transpiration to determine GPP. We have improved this approach by considering mesophyll conductance and seasonal variation in photosynthetic capacity and then compared our results to a semi-empirical eddy-covariance based model, PRELES. We compared a fertilised plot and an unfertilised plot in a monospecific Scots pine forest in northern Sweden. The method captured both the stand response to fertilisation and seasonal patterns, as PRELES did. Our results demonstrate the importance of considering a finite mesophyll conductance value to avoid an unreasonable overestimate of GPP. We have now applied the method in a mixed boreal forest where we will partition total stand GPP among the three dominant tree species (pine, spruce, and birch). This approach provides an independent test of GPP estimates and provides a means of estimating GPP where eddy-covariance assumptions are not met.

Background Fast growing invasive alien species are highly efficient with little investment in their tissues. They often outcompete slower growing species with severe consequences for diversity and community composition. The plant economics trait-based approach provides a theoretical framework, allowing the classification of plants with different performance characteristics. However, in multifaceted background, this approach needs testing. The evaluation and prediction of plant performance outcomes in ecologically relevant settings is among the most pressing topics to understand and predict ecosystem functioning, especially in a quickly changing environment. Temperature and nutrient availability are major components of the global environmental change and this study examines the response of growth economic traits, photosynthesis and respiration to such changes for an invasive fast-growing ( Bromus hordaceus ) and a slow-growing perennial ( Bromus erectus ) grass species. Results The fully controlled growth chamber experiment simulated temperature—and changes in nitrogen availability individually and in combination. We therefore provide maximum control and monitoring of growth responses allowing general growth trait response patterns to be tested. Under optimal nitrogen availability the slow growing B. erectus was better able to handle the lower temperatures (7 °C) whilst both species had problems at higher temperatures (30 °C). Stresses produced by a combination of heat and nutrient availability were identified to be less limiting for the slow growing species but the combination of chilling with low nutrient availability was most detrimental to both species. Conclusions For the fast-growing invader B. hordeaceus a reduction of nitrogen availability in combination with a temperature increase, leads to limited growth performance in comparison to the slow-growing perennial species B.erectus and this may explain why nutrient-rich habitats often experience more invasion than resource-poor habitats.

Global models of photosynthesis commonly use photosynthetic capacity parameters (Vcmax, Jmax) that are estimated based on Ci, the intercellular CO2 concentration. The underlying assumption of these models is that mesophyll conductance (gm) is infinite and therefore the CO2 concentration at the site of carboxylation (Cc) is equal to Ci, despite a growing body of literature acknowledging that these assumptions are incorrect. Because relatively few studies on gm have been conducted under natural conditions and with high enough resolution, it is currently unclear how significant the assumption of infinite Cc is for the accuracy of long-term predictions by large-scale photosynthesis models. In this study we investigated this question with data collected in a mature Scots pine stand, one of the dominant species of the boreal region. We conducted high-resolution gas-exchange and online 13C discrimination measurements over a whole growing season (May-October), and analysed the relative contribution of diffusional and biochemical limitations to photosynthesis. We hypothesised that diffusional limitation will be significant in this species, as conifers typically have low stomatal and mesophyll conductance (gs and gm respectively). Accordingly, we found that diffusional limitations were similar or stronger than biochemical limitation during May-July, and that all limitations reached minima around the end of June when Anet values were highest. However, during August-October biochemical limitation became increasingly dominant, as the diffusional limitations were relatively small and stable. Over the whole period, both gm and the relative mesophyll limitation were similar in magnitude to gs and the stomatal limitation, respectively, resulting in a 40-100 ppm reduction in CO2 concentration between Ci and Cc. This meant that Vcmax was under-estimated by 20-40% when calculated from Cicompared to Cc, highlighting the importance of accounting for the finite gm when determining photosynthetic capacity and modelling photosynthesis under natural conditions.

Mesophyll conductance (gm) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although gm is similar in magnitude to stomatal conductance (gs), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO2 analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic 13C discrimination (Δ13C) on mature Pinus sylvestris trees. This allowed the calculation of gm, gs, net photosynthesis (Anet), and WUE. These measurements highlighted the asynchronous diurnal behaviour of gm and gs. While gs declined from around 10:00, Anet declined first after 12:00, and gm remained near its maximum until 16:00. We suggest that high gm played a role in supporting an extended Anet peak despite stomatal closure. Comparing three models to estimate WUE from ∆13C, we found that a simple model, assuming constant net fractionation during carboxylation (27‰), predicted WUE well, but only for about 75% of the day. A more comprehensive model, accounting explicitly for gm and the effects of daytime respiration and photorespiration, gave reliable estimates of WUE, even in the early morning hours when WUE was more variable. Considering constant, finite gm or gm/gs yielded similar WUE estimates on the diurnal scale, while assuming infinite gm led to overestimation of WUE. These results highlight the potential of high-resolution gm measurements to improve modelling of Anet and WUE and demonstrate that such gm data can be acquired, even under field conditions.

High latitude forests will experience large changes in temperature and CO2 concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO2 (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO2 assimilation rates (Anet ), and acclimated dark respiration (Rdark ). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of Anet decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of Rdark was weak. Elevated CO2 tended to exacerbate these effects in warm-grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO2 treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism.

Reply to Adams et al. : Empirical versus process-based approaches to modeling temperature responses of leaf respiration

The central role that ectomycorrhizal (EM) symbioses play in the structure and function of boreal forests pivots around the common assumption that carbon (C) and nitrogen (N) are exchanged at rates favorable for plant growth. However, this may not always be the case. It has been hypothesized that the benefits mycorrhizal fungi convey to their host plants strongly depends upon the availability of C and N, both of which are rapidly changing as a result of intensified human land use and climate change. Using large-scale shading and N addition treatments, we assessed the independent and interactive effects of changes in C and N supply on the transfer of N in intact EM associations with -15 yr. old Scots pine trees. To assess the dynamics of N transfer in EM symbioses, we added trace amounts of highly enriched 5NO3(-) label to the EM-dominated mor-layer and followed the fate of the 15N label in tree foliage, fungal chitin on EM root tips, and EM sporocarps. Despite no change in leaf biomass, shading resulted in reduced tree C uptake, ca. 40% lower fungal biomass on EM root tips, and greater 15N label in tree foliage compared to unshaded control plots, where more 15N label was found in fungal biomass on EM colonized root tips. Short-term addition of N shifted the incorporation of 15N label from EM fungi to tree foliage, despite no significant changes in below-ground tree C allocation to EM fungi. Contrary to the common assumption that C and N are exchanged at rates favorable for plant growth, our results show for the first time that under N-limited conditions greater C allocation to EM fungi in the field results in reduced, not increased, N transfer to host trees. Moreover, given the ubiquitous nature of mycorrhizal symbioses, our results stress the need to incorporate mycorrhizal dynamics into process-based ecosystem models to better predict forest C and N cycles in light of global climate change.