Plant respiration constitutes a massive carbon flux to the atmosphere, and a major control on the evolution of the global carbon cycle. It therefore has the potential to modulate levels of climate change due to the human burning of fossil fuels. Neither current physiological nor terrestrial biosphere models adequately describe its short-term temperature response, and even minor differences in the shape of the response curve can significantly impact estimates of ecosystem carbon release and/or storage. Given this, it is critical to establish whether there are predictable patterns in the shape of the respiration-temperature response curve, and thus in the intrinsic temperature sensitivity of respiration across the globe. Analyzing measurements in a comprehensive database for 231 species spanning 7 biomes, we demonstrate that temperature-dependent increases in leaf respiration do not follow a commonly used exponential function. Instead, we find a decelerating function as leaves warm, reflecting a declining sensitivity to higher temperatures that is remarkably uniform across all biomes and plant functional types. Such convergence in the temperature sensitivity of leaf respiration suggests that there are universally applicable controls on the temperature response of plant energy metabolism, such that a single new function can predict the temperature dependence of leaf respiration for global vegetation. This simple function enables straightforward description of plant respiration in the land-surface components of coupled earth system models. Our cross-biome analyses shows significant implications for such fluxes in cold climates, generally projecting lower values compared with previous estimates.temperature sensitivity | climate models | carbon exchange | Q 10 | thermal response P lant respiration provides continuous metabolic support for growth and maintenance of all tissues and contributes ∼60 Pg C y −1 to the atmosphere (1, 2), with ∼50% of the carbon (C) released by whole-plant respiration from leaves (3). As rates of leaf respiration (R) vary substantially with changes in temperature (T) (4, 5), even slight increases in ambient T can lead to increases in the flux of carbon dioxide (CO 2 ) from leaves to the atmosphere. This has the potential to create concomitant decreases in net primary productivity, and affect the implications of fossil fuel burning by contributing additionally to atmospheric CO 2 levels due to any imposed surface-level global warming. Hence, quantification of the T response of leaf R, and how this response may vary across diverse ecosystems and plant species, is critical to current estimations and future projections of the global carbon cycle (6-8). Evaluating how leaf R relates to T in terrestrial plants will clarify fundamental controls on energy metabolism and enable more accurate parameterization, as leaf R, in addition to photosynthesis (9, 10), has been identified as a major source of uncertainty in models of the global carbon cycle (8, 11). The response of leaf R to T differs in both ma...

Mesophyll conductance ( g m ) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although g m is similar in magnitude to stomatal conductance ( g s ), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO 2 analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic 13 C discrimination (Δ 13 C) on mature Pinus sylvestris trees. This allowed the calculation of g m , g s , net photosynthesis ( A net ), and WUE. These measurements highlighted the asynchronous diurnal behaviour of g m and g s . While g s declined from around 10:00, A net declined first after 12:00, and g m remained near its maximum until 16:00. We suggest that high g m played a role in supporting an extended A net peak despite stomatal closure. Comparing three models to estimate WUE from ∆ 13 C, 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 g m 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 g m or g m / g s yielded similar WUE estimates on the diurnal scale, while assuming infinite g m led to overestimation of WUE. These results highlight the potential of high-resolution g m measurements to improve modelling of A net and WUE and demonstrate that such g m data can be acquired, even under field conditions. Electronic supplementary material The online version of this article (10.1007/s11120-019-00645-6) contains supplementary material, which is available to authorized users.

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 15 NO 3 label to the EM-dominated mor-layer and followed the fate of the 15 N 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 15 N label in tree foliage compared to unshaded control plots, where more 15 N label was found in fungal biomass on EM colonized root tips. Short-term addition of N shifted the incorporation of 15 N 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.

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 15 NO 3 label to the EM-dominated mor-layer and followed the fate of the 15 N 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 15 N label in tree foliage compared to unshaded control plots, where more 15 N label was found in fungal biomass on EM colonized root tips. Short-term addition of N shifted the incorporation of 15 N 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.

High latitude forests will experience large changes in temperature and CO 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 CO (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 CO assimilation rates (A ), and acclimated dark respiration (R ). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of A decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of R was weak. Elevated CO tended to exacerbate these effects in warm-grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO 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.

 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 (g m).  We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g m descriptions and a simple transport of sugars from foliage down the trunk. We parameterized the model for a boreal Scots pine stand and compared simulated g m responses to weather variations. We further compared the simulated δ 13 C of new photosynthates among the different g m descriptions and against measured phloem sugar δ 13 C.  Simulated g m estimates of the seven descriptions varied according to weather conditions resulting in varying estimates of phloem δ 13 C during cold / moist and warm / dry periods. The model succeeded in predicting a drought response and a post-drought release in phloem sugar δ 13 C 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.

Using an empirical approach, we report that the slope of the short-term log-transformed leaf respiration (R)-temperature (T) curves declines with increasing leaf T in a manner that is uniform across biomes (1); the results have utility for modeling carbon fluxes in terrestrial biosphere models (TBMs). The use of an empirical approach reflects the fact that, despite advances in understanding of factors regulating R (2-4) and its T-response (5), basic information on key determinants of R remains lacking, hindering development of a process-based model with utility for TBMs. Some, including Adams et al. (6), view Arrhenius theory as providing a way forward and argue that it is both predictive and mechanistic. As noted (6), this approach provides equivalent predictive power as the log-polynomial function (1), a finding that we do not dispute, and that was recently noted in a separate paper comparing several approaches to fitting shortterm T-function of R (7). We also agree that global convergence in the shape of R-T curves is an indication that respiratory regulation is likely to be common across plants (6). Where we differ, however, is whether the applied Arrhenius approach (6) is mechanistic. Arrhenius theory is applicable to reactions catalyzed by single enzymes that are substrate-saturated. For respiratory metabolism in plants, neither assumption holds, because the respiratory system is made up of numerous, highly-regulated reactions that are rarely substrate-saturated (8). Thus, although activation energy (E a ) values of R, including temperature-dependent ones (9, 10), provide estimates of the temperature coefficient of the overall respiratory system, they can be viewed as outputs of a statistical fit, because they do not necessarily provide insights into the individual mechanisms underpinning variation in R-T curves.In the temperature-modified Arrhenius approach (9), changes in the slope of log-transformed R-T curves are achieved via adding a factor (δ) to account for T-dependent changes in the activation energy (i.e., T-sensitivity) estimated at 0°C (E 0 ). Similarly, we (1) provide estimates of the T-sensitivity at 0°C, and how the T-sensitivity of R declines with increasing leaf T (i.e., b-and c-parameters in the polynomial). Thus, in general terms, b and E 0 describe the T-sensitivity at 0°C, with c and δ accounting for deceleration in R as leaves warm. Congruence in the two approaches therefore reflects their underlying operational similarities, raising the possibility that TBMs can indeed use either approach (1, 9).Looking forward, development of a process-based model to account for the complexity of taxa-and environment-driven variations in R (11) remains a high priority. Notable advances are clarifying individual and collective mechanistic controls of R through models and experiments (2-5). Ideally in the future, a truly mechanistic approach based on these advances will emerge that meets the TBM integration requirements of being parameter-sparse, scalable, and spatially robust; however, current knowled...

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 GPP iso/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 GPP iso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPP iso/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 GPP iso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPP iso/ SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met. K E Y W O R D S boreal forest, intrinsic water-use efficiency, mesophyll conductance, nitrogen fertilization, phloem δ 13 C, PRELES, sap flux, stand transpiration 1 | INTRODUCTION Gross primary production (GPP) represents a key flux in the carbon (C) budget of a forest ecosystem. GPP has been commonly estimated using many approaches, such as eddy covariance (EC), empirical models and upscaling ecophysiological measurements at stand scale