Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Transpiration rate relates to within‐ and across‐species variations in effective path length in a leaf water model of oxygen isotope enrichment

Stable oxygen isotope ratio of leaf water (δ¹⁸O_L) yields valuable information on many aspects of plant–environment interactions. However, current understanding of the mechanistic controls on δ¹⁸O_L does not provide complete characterization of effective path length (L) of the Péclet effect, – a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field‐grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within‐ and across‐species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species‐specific constant in leaf water or cellulose isotope (δ¹⁸O_p) modelling; and (2) the use of δ¹⁸O_p as a proxy for g_s or E under low E conditions. Further, we show that incorporation of a multi‐species L‐E scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.     

Do pathways of water movement and leaf anatomical dimensions allow development of gradients in H218O between veins and the sites of evaporation within leaves?

The oxygen isotope enrichment of bulk leaf water (Δ_L) is often observed to be poorly predicted by the Craig–Gordon‐type models developed for evaporative enrichment from a body of water (Δ_e). The discrepancy between Δ_L and Δ_e may be explained by gradients in enrichment within the leaf as a result of convection of unenriched water to the sites of evaporation opposing the diffusion of enrichment away from the sites; a Péclet effect. However, this effect is difficult to quantify because the velocities of water movement within the leaf are unknown. This paper attempts to model the complex anatomy of a leaf, and hence such velocities, to assess if the gradients in H₂₁₈O required for a significant Péclet effect between the vein and the evaporation sites are possible within a leaf. Published dimensions of cells in wheat leaves are used to calculate the cross‐sectional areas perpendicular to the flow velocities of water through assumed pathways. By combining the ratio of actual to ‘slab’ velocities with anatomical lengths, equivalent lengths (L) emerge. In this way, it is concluded that if water moves only through the cell walls, or from cell to cell via either aquaporins or plasmodesmata, and evaporates from mesophyll cells, or the substomatal cells, or from the peristomatal region (a total of 15 combinations of assumptions), then the 15 central estimates of the values of L are between 9 and 200 mm. Each of these central estimates is subject to uncertainty, but overall their magnitude is important and estimates of L are comparable with those made from fitting to isotopic data (8 mm for wheat). It is concluded that significant gradients in enrichment between the vein and the evaporation sites are likely.     

Leaf Gas Exchange of Pachyrhizus ahipa and P. erosus Under Water and Temperature Stress

Gas exchange, water relations, and leaf traits were studied in the tuberous-root producing legumes ahipa (Pachyrhizus ahipa) and yambean (P. erosus) under different environmental conditions. Differences in leaf traits (hairiness, leaf area, areal leaf mass, stomatal density) and paraheliotropism were found between ahipa and yambean. Under sufficient water supply, the increase in air temperature and decrease in air humidity increased stomatal conductance (g _s) and net photosynthetic rate (P _N) in yambean but reduced them in ahipa. In a drying soil (14 d after irrigation), inter-specific variation in gas exchange was only observed in the early morning, and yambean showed a greater sensitivity to water restriction than ahipa. High g _s at low humidity increased P _N of P. erosus but resulted in lower water-use efficiency (WUE). However, long-term WUE, estimated by leaf carbon isotope discrimination, showed little variation between species. Daily-irrigated ahipa and yambean grown in the greenhouse did not show significant differences in gas exchange. However, leaf temperature was significantly greater in yambean than in ahipa while a steepper relationship between E and P _N and g _s was observed in ahipa.     

Water resource partitioning,stem xylem hydraulic properties,and plant water use strategies in a seasonally dry riparian tropical rainforest

This study investigated seasonal variation in the origin of water used by plants in a riparian tropical rainforest community and explored linkages between plant water source, plant xylem hydraulic conductivity and response to the onset of dry conditions. The study focused on five co-dominant canopy species, comprising three tree species (Doryphora aromatica, Argyrodendron trifoliolatum, Castanospora alphandii) and two climbing palms (Calamus australis and Calamus caryotoides). Stable isotope ratios of oxygen in water (¹⁸O) from soil, groundwater, stream water and plant xylem measured in the wet season and the subsequent dry season revealed water resource partitioning between species in the dry season. Measurement of stem-area-specific hydraulic conductivity (K_S) in the wet season and subsequent dry season showed a significant dry-season loss of K_S in three of the five species (Castanospora alphandii, Calamus australis and C. caryotoides) and a decrease in mean K_S for all species. This loss of hydraulic conductivity was positively correlated with the difference between wet-season and dry-season midday leaf water potentials and with leaf carbon isotope discrimination, indicating that plants that were less susceptible to loss of conductivity had greater control over transpiration rate and were more water-use efficient.     

Thermal acclimation of photosynthesis: a comparison of boreal and temperate tree species along a latitudinal transect

Common gardens were established along a ～900 km latitudinal transect to examine factors limiting geographical distributions of boreal and temperate tree species in eastern North America. Boreal representatives were trembling aspen (Populus tremuloides Michx.) and paper birch (Betula papyrifera Marsh.), while temperate species were eastern cottonwood (Populus deltoides Bartr ex. Marsh var. deltoides) and sweetgum (Liquidambar styraciflua L.). The species were compared with respect to adjustments of leaf photosynthetic metabolism along the transect, with emphasis on temperature sensitivities of the maximum rate of ribulose bisphosphate (RuBP) carboxylation (E_V) and regeneration (E_J). During leaf development, the average air temperature (T_growth) differed between the coolest and warmest gardens by 12 °C. Evidence of photosynthetic thermal acclimation (metabolic shifts compensating for differences in T_growth) was generally lacking in all species. Namely, neither E_V nor E_J was positively related to T_growth. Correspondingly, the optimum temperature (T_opt) of ambient photosynthesis (A_sat) did not vary significantly with T_growth. Modest variation in T_opt was explained by the combination of E_V plus the slope and curvature of the parabolic temperature response of mesophyll conductance (g_m). All in all, species differed little in photosynthetic responses to climate. Furthermore, the adaptive importance of photosynthetic thermal acclimation was overshadowed by g_m's influence on A_sat's temperature response.     

Variation in bulk‐leaf 13C discrimination,leaf traits and water‐use efficiency–trait relationships along a continental‐scale climate gradient in Australia

Large spatial and temporal gradients in rainfall and temperature occur across Australia. This heterogeneity drives ecological differentiation in vegetation structure and ecophysiology. We examined multiple leaf‐scale traits, including foliar ¹³C isotope discrimination (Δ¹³C), rates of photosynthesis and foliar N concentration and their relationships with multiple climate variables. Fifty‐five species across 27 families were examined across eight sites spanning contrasting biomes. Key questions addressed include: (i) Does Δ¹³C and intrinsic water‐use efficiency (WUE_i) vary with climate at a continental scale? (ii) What are the seasonal and spatial patterns in Δ¹³C/WUE_i across biomes and species? (iii) To what extent does Δ¹³C reflect variation in leaf structural, functional and nutrient traits across climate gradients? and (iv) Does the relative importance of assimilation and stomatal conductance in driving variation in Δ¹³C differ across seasons? We found that MAP, temperature seasonality, isothermality and annual temperature range exerted independent effects on foliar Δ¹³C/WUE_i. Temperature‐related variables exerted larger effects than rainfall‐related variables. The relative importance of photosynthesis and stomatal conductance (g_s) in determining Δ¹³C differed across seasons: Δ¹³C was more strongly regulated by g_s during the dry‐season and by photosynthetic capacity during the wet‐season. Δ¹³C was most strongly correlated, inversely, with leaf mass area ratio among all leaf attributes considered. Leaf N_mass was significantly and positively correlated with MAP during dry‐ and wet‐seasons and with moisture index (MI) during the wet‐season but was not correlated with Δ¹³C. Leaf P_mass showed significant positive relationship with MAP and Δ¹³C only during the dry‐season. For all leaf nutrient‐related traits, the relationships obtained for Δ¹³C with MAP or MI indicated that Δ¹³C at the species level reliably reflects the water status at the site level. Temperature and water availability, not foliar nutrient content, are the principal factors influencing Δ¹³C across Australia.     

Temperature responses of mesophyll conductance differ greatly between species

The temperature responses of mesophyll conductance (g_m) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. Species included the evergreen trees Eucalyptus pauciflora and Quercus engelmannii; the tropical evergreen tree Lophostemon confertus; as well as the herbaceous species Nicotiana tabacum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Glycine max and Arabidopsis thaliana. Responses varied from a two‐ to threefold increase in mesophyll conductance between 15 and 40 °C observed for N. tabacum, G. hirsutum, G. max and E. pauciflora to almost no change in L. confertus and T. aestivum. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were Eucalyptus and Gossypium, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.     

Stable isotope fractionation of strontium in coccolithophore calcite: Influence of temperature and carbonate chemistry

Marine calcifying eukaryotic phytoplankton (coccolithophores) is a major contributor to the pelagic production of CaCO₃ and plays an important role in the biogeochemical cycles of C, Ca and other divalent cations present in the crystal structure of calcite. The geochemical signature of coccolithophore calcite is used as palaeoproxy to reconstruct past environmental conditions and to understand the underlying physiological mechanisms (vital effects) and precipitation kinetics. Here, we present the stable Sr isotope fractionation between seawater and calcite (Δ^88/86Sr) of laboratory cultured coccolithophores in individual dependence of temperature and seawater carbonate chemistry. Coccolithophores were cultured within a temperature and a pCO₂ range from 10 to 25°C and from 175 to 1,240 μatm, respectively. Both environmental drivers induced a significant linear increase in coccolith stable Sr isotope fractionation. The temperature correlation at constant pCO₂ for Emiliania huxleyi and Coccolithus braarudii is expressed as Δ^88/86Sr = ?7.611 × 10^?3 T + 0.0061. The relation of Δ^88/86Sr to pCO₂ was tested in Emiliania huxleyi at 10 and 20°C and resulted in Δ^88/86Sr = ?5.394 × 10^?5 pCO₂ – 0.0920 and Δ^88/86Sr = ?5.742 × 10^?5 pCO₂ – 0.1351, respectively. No consistent relationship was found between coccolith Δ^88/86Sr and cellular physiology impeding a direct application of fossil coccolith Δ^88/86Sr as coccolithophore productivity proxy. An overall significant correlation was detected between the elemental distribution coefficient (D_Sr) and Δ^88/86Sr similar to inorganic calcite with a physiologically induced offset. Our observations indicate (i) that temperature and pCO₂ induce specific effects on coccolith Δ^88/86Sr values and (ii) that strontium elemental ratios and stable isotope fractionation are mainly controlled by precipitation kinetics when embedded into the crystal lattice and subject to vital effects during the transmembrane transport from seawater to the site of calcification. These results provide an important step to develop a coccolith Δ^88/86Sr palaeoproxy complementing the existing toolbox of palaeoceanography.     

Surrounding species diversity improves subtropical seedlings’ carbon dynamics

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two‐ and four‐species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant–soil–atmosphere continuum. We measured gas‐exchange variables (C assimilation, transpiration and above‐ and belowground respiration) and δ¹³C in leaf biomass, phloem, soil microbial biomass, leaf‐ and soil‐respired CO₂ as well as δ¹⁸O in leaf and xylem water. The ¹³C signal in phloem and respired CO₂ in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf‐water ¹⁸O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water‐use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth‐related C sinks in monoculture. These ecophysiological responses of L. glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.     

Elevated temperature is more effective than elevated [CO2] in exposing genotypic variation in Telopea speciosissima growth plasticity: implications for woody plant populations under climate change

Intraspecific variation in phenotypic plasticity is a critical determinant of plant species capacity to cope with climate change. A long‐standing hypothesis states that greater levels of environmental variability will select for genotypes with greater phenotypic plasticity. However, few studies have examined how genotypes of woody species originating from contrasting environments respond to multiple climate change factors. Here, we investigated the main and interactive effects of elevated [CO₂] (C_E) and elevated temperature (T_E) on growth and physiology of Coastal (warmer, less variable temperature environment) and Upland (cooler, more variable temperature environment) genotypes of an Australian woody species Telopea speciosissima. Both genotypes were positively responsive to C_E (35% and 29% increase in whole‐plant dry mass and leaf area, respectively), but only the Coastal genotype exhibited positive growth responses to T_E. We found that the Coastal genotype exhibited greater growth response to T_E (47% and 85% increase in whole‐plant dry mass and leaf area, respectively) when compared with the Upland genotype (no change in dry mass or leaf area). No intraspecific variation in physiological plasticity was detected under C_E or T_E, and the interactive effects of C_E and T_E on intraspecific variation in phenotypic plasticity were also largely absent. Overall, T_E was a more effective climate factor than C_E in exposing genotypic variation in our woody species. Our results contradict the paradigm that genotypes from more variable climates will exhibit greater phenotypic plasticity in future climate regimes.     

Provenance effect on carbon assimilation,photochemistry and leaf morphology in Mediterranean Cistus species under chilling stress

• The potential resilience of shrub species to environmental change deserves attention in those areas threatened by climate change, such as the Mediterranean Basin. We asked if leaves produced under different climate conditions through the winter season to spring can highlight the leaf traits involved in determining potential resilience of three Cistus spp. to changing environmental conditions and to what extent intraspecific differences affect such a response.
• We analysed carbon assimilation, maximum quantum efficiency of PSII photochemistry (F_v/F_m) and leaf morphological control of the photosynthetic process in leaves formed through the winter season into spring in C. creticus subsp. eriocephalus (CE), C. salvifolius (CS) and C. monspeliensis (CM) grown from seed of different provenances under common garden conditions.
• Intraspecific differences were found in F_v/F_m for CE and CS. Carbon assimilation‐related parameters were not affected by provenance. Moreover, our analysis highlighted that the functional relationships investigated can follow seasonal changes and revealed patterns originating from species‐specific differences in LMA arising during the favourable period.
• Cistus spp. have great ability to modify the structure and function of their leaves in the mid‐term in order to cope with changing environmental conditions. The F_v/F_m response to chilling reveals that susceptibility to photoinhibition is a trait under selection in Cistus species. Concerning carbon assimilation, differing ability to control stomatal opening was highlighted between species. Moreover, seasonal changes of the functional relationships investigated can have predictable consequences on species leaf turnover strategies.

     

A unifying explanation for variation in ozone sensitivity among woody plants

Tropospheric ozone is considered the most detrimental air pollutant for vegetation at the global scale, with negative consequences for both provisioning and climate regulating ecosystem services. In spite of recent developments in ozone exposure metrics, from a concentration‐based to a more physiologically relevant stomatal flux‐based index, large‐scale ozone risk assessment is still complicated by a large and unexplained variation in ozone sensitivity among tree species. Here, we explored whether the variation in ozone sensitivity among woody species can be linked to interspecific variation in leaf morphology. We found that ozone tolerance at the leaf level was closely linked to leaf dry mass per unit leaf area (LMA) and that whole‐tree biomass reductions were more strongly related to stomatal flux per unit leaf mass (r² = 0.56) than to stomatal flux per unit leaf area (r² = 0.42). Furthermore, the interspecific variation in slopes of ozone flux–response relationships was considerably lower when expressed on a leaf mass basis (coefficient of variation, CV = 36%) than when expressed on a leaf area basis (CV = 66%), and relationships for broadleaf and needle‐leaf species converged when using the mass‐based index. These results show that much of the variation in ozone sensitivity among woody plants can be explained by interspecific variation in LMA and that large‐scale ozone impact assessment could be greatly improved by considering this well‐known and easily measured leaf trait.     

三种温带树种叶片呼吸的时间动态及其影响因子

为认知叶片呼吸(RL)的季节变化格局及其影响因子,以东北东部山区3个主要树种(红松Pinus koraiensis、樟子松P.sylvestris var.mongolica和白桦Betula platyphylla)为对象,采用红外气体分析法在2011年生长季(常绿树4月至10月;落叶树6月至9月)测定了自然条件下叶片气体交换及其相关生理特征的季节变化,探索了RL与空气温度(Tair)和相关叶片特征之间的关系.结果表明:红松和樟子松基于叶面积的RL(RL-area)表现为生长季初期和末期较大,而白桦RL-area则随生长季进程而逐渐减小.在生长季中,RL-area与叶片总光合之比的时间动态明显.红松、樟子松RL-area与Tair关系显著,而白桦RL-area与Tair关系不显著;但3种树种基于叶质量的RL(RL-mass)与Tair均呈显著的指数函数关系.叶片特征(包括可溶性糖、淀粉、氮、比叶面积等参数)也有明显的季节变化.影响RL的叶片特征参数因树种而异,其中可溶性糖浓度对3种树种的RL均有显著影响.可见,RL的季节变化格局受树木的生长节律、温度和叶片特征的联合控制.     

Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access

High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (A_net) and stomatal conductance (g_s). However, evidence for decoupling of g_s from A_net under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum T_air > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced A_net and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced—g_s was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.     

The role of effective leaf mixing length in the relationship between the δ18O of stem cellulose and source water across a salinity gradient

Previous mangrove tree ring studies attempted, unsuccessfully, to relate the δ¹⁸O of trunk cellulose (δ¹⁸O_CELL) to the δ¹⁸O of source water (δ¹⁸O_SW). Here, we tested whether biochemical fractionation associated with one of the oxygen in the cellulose glucose moiety or variation in leaf water oxygen isotope fractionation (Δ_LW) can interfere with the δ¹⁸O_SW signal as it is recorded in the δ¹⁸O_CELL of mangrove (saltwater) and hammock (freshwater) plants. We selected two transects experiencing a salinity gradient, located in the Florida Keys, USA. The δ¹⁸O_CELL throughout both transects did not show the pattern expected based on that of the δ¹⁸O_SW. We found that in one of the transects, biochemical fractionation interfered with the δ¹⁸O_SW signal, while in the other transect Δ_LW differed between mangrove and hammock plants. Observed differences in Δ_LW between mangroves and hammocks were caused by a longer effective leaf mixing length (L) of the water pathway in mangrove leaves compared to those of hammock leaves. Changes in L could have caused the δ¹⁸O_CELL to record not only variations in the δ¹⁸O_SW but also in Δ_LW making it impossible to isolate the δ¹⁸O_SW signal.     

Morphological and physiological responses of two contrasting Malus species to exogenous abscisic acid application

This study characterized morphological and physiological responses of two Malus species to exogenous abscisic acid (ABA) application under both well-watered and drought-stressed conditions. Exogenous ABA was sprayed onto the leaves of potted 1-year-old seedlings of M. sieversii and M. hupehensis, originated from regions with low annual rainfall and high annual rainfall, respectively. The results demonstrated that exogenous ABA application significantly decreased height growth (H), total biomass (TB), total leaf area (LA), net photosynthesis (A) and stomatal conductance (g _s), and significantly increased root/shoot ratio (RS), specific leaf area (SLA), endogenous ABA concentration, water use efficiency (WUE) and carbon isotope composition (δ¹³C) under both well-watered and drought-stressed conditions. However, distinct interspecific differences were found in ABA-induced morphological and physiological responses. Compared with M. hupehensis, M. sieversii was more responsive to exogenous ABA application, resulting in larger decreases in H, LA, A and g _s, and larger increases in RS, SLA, WUE_L, WUE_i, ABA and δ¹³C. These results suggest strong evidence for different maintenance of fitness under stressful conditions between species of Malus. In addition, application of exogenous ABA appears to enhance the tolerance of two Malus species to drought-stress.     

Explaining geographical variation in the isotope composition of mouse lemurs (Microcebus)

Aim We sought to quantify geographical variation in the stable isotope values of mouse lemurs (Microcebus) and to determine whether this variation reflects trophic differences among populations or baseline isotopic differences among habitats. If the latter pattern is demonstrated, then Microcebus can become a proxy for tracking baseline habitat isotopic variability. Establishing such a baseline is crucial for identifying niche partitioning in modern and ancient communities. Location We studied five species of Microcebus from eight distinct habitats across Madagascar. Methods We compared isotopic variation in C₃ plants and Microcebus fur within and among localities. We predicted that carbon and nitrogen isotope values of Microcebus should: (1) vary as a function of abiotic variables such as rainfall and temperature, and (2) covary with isotopic values in plants. We checked for trophic differences among Microcebus populations by comparing the average difference between mouse lemur and plant isotope values for each locality. We then used multiple regression models to explain spatial isotope variation in mouse lemurs, testing a suite of explanatory abiotic variables. Results We found substantial isotopic variation geographically. Ranges for mean isotope values were similar for both Microcebus and plants across localities (carbon 3.5–4.0‰; nitrogen 10.5–11.0‰). Mean mouse lemur and plant isotope values were lowest in cool, moist localities and highest in hot, dry localities. Rainfall explained 58% of the variation in Microcebus carbon isotope values, and mean plant nitrogen isotope values explained 99.7% of the variation in Microcebus nitrogen isotope values. Average differences between mouse lemur and plant isotope values (carbon 5.0‰; nitrogen 5.9‰) were similar across localities. Main conclusions Isotopic data suggest that trophic differences among Microcebus populations were small. Carbon isotope values in mouse lemurs were negatively correlated with rainfall. Nitrogen isotope values in Microcebus and plants covaried. Such findings suggest that nitrogen isotope values for Microcebus are a particularly good proxy for tracking baseline isotopic differences among habitats. Our results will facilitate future comparative research on modern mouse lemur communities, and ecological interpretations of extinct Holocene communities.     

The influence of atmospheric wave dynamics on interannual variation in the surface temperature of lakes in the English Lake District

Surface water temperatures in four lakes of the English Lake District (T_L) are shown to be sensitive to climate change and a large‐scale atmospheric phenomenon known as tropospheric Rossby wave breaking (RWB). RWB occurs frequently near the English Lake District, bringing warm and moist air, or cool and dry air, from distant sources. RWB case examples and composites are used to show three dimensional circulations and anomalies of near‐surface temperature and humidity associated with the two types of RWB (anticyclonic and cyclonic). Statistical models of lake surface temperature are developed for each season using objectively identified variability patterns of anticyclonic and cyclonic RWB, along with an index of Northern Hemisphere annual mean surface temperature (T_NH) to account for climate change. The statistical models, depending on season, account for 54–69% of T_L variance. RWB alone contributes significantly during each season, accounting for 37–52% of T_L variance after the effect of T_NH is removed. RWB is a key physical mechanism underlying the North Atlantic Oscillation (NAO), a regional‐scale weather‐pattern that is frequently related to coherent lake properties. RWB may therefore be a more fundamental driver than the NAO in controlling interannual variation in the properties of lakes such as ice duration, metabolic rates, phenology, species composition and, via effects on stratification, underwater light‐climate, nutrient‐cycling and oxygen‐depletion. Variation in other meteorological features that are linked to RWB, such as precipitation, may have additional effects. RWB is also likely to influence terrestrial and marine environments.     

The influence of water content and leaf anatomy on carbon isotope discrimination and photosynthesis in Sphagnum

The relative effect of diffusional resistance due to water films (r_wf) and leaf anatomy (r_p) on rates of net photosynthesis and on-line measures of carbon isotope discrimination (Δ=Δδ¹³C) was investigated in Sphagnum. Sphagnum species differ in the exposure of photosynthetic cells at the leaf surface. In S. affine, photosynthetic cells are widely exposed at the surface, whereas in S. magellanicum, photo-synthetic cells are enclosed within water-filled hyaline cells. This difference is expected to lead to variation in diffusive resistance within leaves (r_p). Net photosynthesis and on-line Δ were measured at two water contents: greenhouse water content (wet) and blotted dry (dry). Without correcting for respiration, on-line Δ values differed significantly between wet (23.7%_o) and dry (30.9%_o) plants. However, there was no significant difference between species means and no species × water content interaction. Corrections for respiration lowered Δ values by approximately 8.1%_o and reduced the mean difference to 3.1%_o, but did not alter the rank order of treatments. Net photosynthesis also decreased by 16% in wet plants, but there was no significant difference between the two species. In addition, five populations of S. affine and S. magellanicum grown in a common garden were analysed for their organic matter carbon isotope composition (δ¹³C). These values varied more within each species (0.9–1.2%_o) than between the two species (0.6%_o). Therefore, we conclude that variation in surface water films leads to a greater difference in resistance to CO₂ uptake and carbon isotope discrimination than that due to variation in leaf anatomical properties in Sphagnum.