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Circadian regulation is an endogenous self-sustaining mechanism that drives temporal gene expression and, amongst others, affects the diurnal patterns of photosynthesis (A) and stomatal conductance (gs). Here we review current knowledge on how circadian regulation drives diurnal gas exchange from genes to ecosystems in the field. Molecular mechanisms underlying the structure of circadian clocks and how they regulate A and gs in a few model species are starting to be elucidated but additional data are required to understand regulation across phylogenies, especially within the gymnosperms, and across environments and scales. Circadian rhythms were responsible for 15–25% and for 30–35% of the daytime oscillations in A and gs, respectively, across the C3 and C4 species for which data are available. Consequently, circadian effects over diurnal gas exchange are of similar magnitude to the effects of temperature or vapor pressure deficit. Moreover, recent findings indicate how circadian rhythms could exert significant impacts on ecosystem patterns of gas exchange, which would challenge conventional approaches to derive the environmental flux dependences. Progress in transferring laboratory findings to the field is being hampered by lack of suitable experimental and modeling facilities that can disentangle circadian effects from environmental responses in the field and in ecosystems, and methodological recommendations are offered. The effects of environmental stressors on circadian regulation of gas exchange are also poorly understood. We document how circadian control of gas exchange may be adaptive by allowing plants to anticipate highly predictable environmental cues, but also by increasing the diversity of potential gas exchange responses to environmental variation in plant populations. ; VRD is funded by the Spanish Government (RYC-2012-10970). We acknowledge useful discussions with many colleagues over the years, including A Hall, J Hartwell, M Goulden, DT Tissue, J Roy and many others.

Received: 1 July 2014 / Revised: 13 November 2014 / Accepted: 4 December 2014 / Published online: 11 December 2014 ; Recent research has shown that interactions between species with different functional traits can promote forest ecosystem processes. In the context of climate change, understanding whether species interactions in mixed-species ecosystems can improve the adaptation of these ecosystems to extreme climatic events is crucial to developing new management strategies. In this study, we investigated the impact of species interactions on the sap flux density of three Mediterranean tree species (Quercus faginea, Pinus nigra and Pinus sylvestris) during a summer drought. Measurements of foliar carbon isotopic composition (δ 13C) were also conducted on the same trees. The decline in transpiration during drought was the greatest for P. sylvestris and the least pronounced for Q. faginea. For P. nigra and Q. faginea, the decrease in transpiration as the drought progressed was lower when these species where interacting with another tree species, particularly with P. sylvestris. In contrast, the decrease for P. sylvestris was higher when this species was interacting with another species. Differing drought effects were consistent with the δ 13C values. We showed that the identity of the species present in the direct neighbourhood of a given tree can differentially influence water availability and water-use of these three co-existing Mediterranean tree species during a summer drought. Our findings suggest that species interactions play an important role in modulating the response of tree species to drought. Favouring tree species diversity in this region does not seem to be systematically beneficial in terms of soil water availability and water-use for all the interacting species. ; A.F. was supported by JAE-PREDOC from CSIC and co-funded by the European Union (Fondo Social Europeo). ; The research leading to these results was conducted within the FunDiv EUROPE project and has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under Grant agreement No 265171. ; Peer reviewed

Climate warming should result in hotter droughts of unprecedented severity in this century. Such droughts have been linked with massive tree mortality, and data suggest that warming interacts with drought to aggravate plant performance. Yet how forests will respond to hotter droughts remains unclear, as does the suite of mechanisms trees use to deal with hot droughts. We used an ecosystem‐scale manipulation of precipitation and temperature on piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees to investigate nitrogen (N) cycling‐induced mitigation processes related to hotter droughts. We found that while negative impacts on plant carbon and water balance are manifest after prolonged drought, performance reductions were not amplified by warmer temperatures. Rather, increased temperatures for 5 years stimulated soil N cycling under piñon trees and modified tree N allocation for both species, resulting in mitigation of hotter drought impacts on tree water and carbon functions. These findings suggest that adjustments in N cycling are likely after multi‐year warming conditions and that such changes may buffer reductions in tree performance during hotter droughts. The results highlight our incomplete understanding of trees' ability to acclimate to climate change, raising fundamental questions about the resistance potential of forests to long‐term, compound climatic stresses. ; Pacific Northwest National Laboratories; Swiss Federal Research Institute; Swiss Forest Lab; Swiss National Science Foundation SNF, Grant/Award Number: 31003A_159866; U.S. Geological Survey; U.S. Department of Energy Office of Science, Grant/Award Number: DESC‐0008168; Spanish Government, Grant/Award Number: CGL2015‐69773‐C2‐2‐P; Generalitat Valenciana, Grant/Award Number: BEST/2016/289; Los Alamos National Laboratory.

There is increasing evidence that the circadian clock is a signif- icant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30–48 h. Photosynthesis and quantum yield peaked at subjective noon, and non-photochemical quenching peaked at night. These oscillations were not associ- ated with parallel changes in carbohydrate content or xantho- phyll cycle activity. W e observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night dur- ing the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of tri- meric light-harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non-stressed plants maximize photosynthesis. ; The authors acknowledge the support of the following re- search grants: UPV/EHU-GV IT-624-13 and IT-1018-16 from the Basque Government and CTM2014-53902-C2-2-P from the Spanish Ministry of Economy and Competitiveness (MINECO) and the ERDF (FEDER). This study benefited from the CNRS human and technical resources allocated to the Ecotrons Research Infrastructures as well as from the state allocation 'Investissement d'Avenir' ANR-11-INBS- 0001, ExpeER FP7 Transnational Access programme, Ramón y Cajal fellowships (RYC-2012-10970 to V.R.D. and RYC-2008-02050 to J.P.F.), the Erasmus Mundus Master Course Mediterranean Forestry and Natural Resources Management (MEDfOR) and internal grants from UWS- HIE to V.R.D. and ZALF to A.G. EHU postdoctora ellowship and JdC-Incorporation fellowship (IJCI-2014- 22489) to B.F.M. are also acknowledged. We remain indebted to E. Gerardeau, D. Dessauw, J. Jean, P. Prudent (Aïda CIRAD), J.-J. Drevon, C. Pernot (Eco&Sol INRA), B. Buatois, A. Rocheteau (CEFE CNRS), S. Devidal, C. Piel, O. Ravel and the full Ecotron team, J. del Castillo, P. Martín, A. Mokhtar, A. Pra, S. Salekin (UdL), S. García-Muñoz (IMIDRA), Z. Kayler and K. Pirhofer-Walzl (ZALF) for out- standing technical assistance during experiment setup, plant cultivation or subsequent measurements.

Abstract
Linking individual and stand-level dynamics during forest development reveals a scaling relationship between mean tree size and tree density in forest stands, which integrates forest structure and function. However, the nature of this so-called scaling law and its variation across broad spatial scales remain unquantified, and its linkage with forest demographic processes and carbon dynamics remains elusive. In this study, we develop a theoretical framework and compile a broad-scale dataset of long-term sample forest stands (n = 1,433) from largely undisturbed forests to examine the association of temporal mean tree size vs. density scaling trajectories (slopes) with biomass accumulation rates and the sensitivity of scaling slopes to environmental and demographic drivers. The results empirically demonstrate a large variation of scaling slopes, ranging from −4 to −0.2, across forest stands in tropical, temperate, and boreal forest biomes. Steeper scaling slopes are associated with higher rates of biomass accumulation, resulting from a lower offset of forest growth by biomass loss from mortality. In North America, scaling slopes are positively correlated with forest stand age and rainfall seasonality, thus suggesting a higher rate of biomass accumulation in younger forests with lower rainfall seasonality. These results demonstrate the strong association of the transient mean tree size vs. density scaling trajectories with forest demography and biomass accumulation rates, thus highlighting the potential of leveraging forest structure properties to predict forest demography, carbon fluxes, and dynamics at broad spatial scales.

Molecular clocks drive oscillations in leaf photosynthesis, stomatal conductance, and other cell and leaf-level processes over ~24 h under controlled laboratory conditions. The influence of such circadian regulation over whole-canopy fluxes remains uncertain; diurnal CO2 and H2O vapor flux dynamics in the field are currently interpreted as resulting almost exclusively from direct physiological responses to variations in light, temperature and other environmental factors. We tested whether circadian regulation would affect plant and canopy gas exchange at the Montpellier European Ecotron. Canopy and leaf-level fluxes were constantly monitored under field-like environmental conditions, and under constant environmental conditions (no variation in temperature, radiation, or other environmental cues). ; This study benefited from the CNRS human and technical resources allocated to the Research Infrastructure Ecotrons, as well as from the state allocation 'Investissement d'Avenir' ANR-11-INBS-0001; ExpeER Transnational Access program; Ramón y Cajal fellowships (RYC-2012-10970 to VRD and RYC-2008-02050 to JPF); the Erasmus Mundus Master Course Mediterranean Forestry and Natural Resources Management (MEDfOR); and internal grants from the Leibniz Centre for Agricultural Landscape Research to AG, and from the Western Sydney University's Hawkesbury Institute for the Environment and the Spanish Government (AGL2015-69151-R) to VRD.

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality. ; We thank the Hainich National Park administration as well as Felix Berthold and Carsten Beinhoff for support of this study and Gerald Kaendler and the Johann Heinrich von Thünen-Institut for providing access to the German National Forest Inventory data. The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under Grant Agreement 265171. ; This is the final version of the article. It first appeared from the National Academy of Sciences via https://doi.org//10.1073/pnas.1517903113

There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodiversity effects on multifunctionality into three mechanisms and apply this to European forest data. We show that throughout Europe, tree diversity is positively related with multifunctionality when moderate levels of functioning are required, but negatively when very high function levels are desired. For two well-known mechanisms, 'complementarity' and 'selection', we detect only minor effects on multifunctionality. Instead a third, so far overlooked mechanism, the 'jack-of-all-trades' effect, caused by the averaging of individual species effects on function, drives observed patterns. Simulations demonstrate that jack-of-all-trades effects occur whenever species effects on different functions are not perfectly correlated, meaning they may contribute to diversity-multifunctionality relationships in many of the world's ecosystems. ; The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 265171. ; This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms11109