Gas exchange analysers exhibit large measurement error driven by internal thermal gradients.

Portable gas exchange analysers provide critical data for understanding plant-atmosphere carbon and water fluxes, and for parameterising Earth system models that forecast climate change effects and feedbacks. We characterised temperature measurement errors in the Li-Cor LI-6400XT and LI-6800, and estimated downstream errors in derived quantities, including stomatal conductance (gsw ) and leaf intercellular CO2 concentration (Ci ). The LI-6400XT exhibited air temperature errors (differences between reported air temperature and air temperature measured near the leaf) up to 7.2°C, leaf temperature errors up to 5.3°C, and relative errors in gsw and Ci that increased as temperatures departed from ambient. This caused errors in leaf-to-air temperature relationships, assimilation-temperature curves and CO2 response curves. Temperature dependencies of maximum Rubisco carboxylation rate (Vcmax ) and maximum RuBP regeneration rate (Jmax ) showed errors of 12% and 35%, respectively. These errors are likely to be idiosyncratic and may differ among machines and environmental conditions. The LI-6800 exhibited much smaller errors. Earth system model predictions may be erroneous, as much of their parametrisation data were measured on the LI-6400XT system, depending on the methods used. We make recommendations for minimising errors and correcting data in the LI-6400XT. We also recommend transitioning to the LI-6800 for future data collection.

Precipitation mediates sap flux sensitivity to evaporative demand in the neotropics.

Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

Reductions in tree performance during hotter droughts are mitigated by shifts in nitrogen cycling.

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.

Tree water dynamics in a drying and warming world.

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (FD ), stomatal conductance (Gs ), hydraulic conductivity (KL ) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower Gs and FD were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of Gs and FD to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced FD , Gs and KL . Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in Gs , FD and KL took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi-arid regions.

Prolonged warming and drought modify belowground interactions for water among coexisting plants.

Understanding how climate alters plant-soil water dynamics, and its impact on physiological functions, is critical to improved predictions of vegetation responses to climate change. Here we analyzed how belowground interactions for water shift under warming and drought, and associated impacts on plant functions. In a semi-arid woodland, adult trees (piñon and juniper) and perennial grasses (blue grama) were exposed to warming and precipitation reduction. After 6 years of continuous treatment exposure, soil and plant water isotopic composition was measured to assess plant water uptake depths and community-level water source partitioning. Warming and drought modified plant water uptake depths. Under warming, contrasting changes in water sources between grasses and trees reduced belowground water source partitioning, resulting in higher interspecific competition for water. Under drought, shifts in trees and grass water sources to deeper soil layers resulted in the maintenance of the naturally occurring water source partitioning among species. Trees showed higher water stress, and reduced water use and photosynthesis in response to warming and drought. This case study demonstrates that neighboring plants shift their competitive interactions for water under prolonged warming and drought, but regardless of whether changes in moisture sources will result in increased competition among species or maintained partitioning of water resources, these competitive adaptations may easily be overridden by climate extremes.

Lack of acclimation of leaf area:sapwood area ratios in piñon pine and juniper in response to precipitation reduction and warming.

The leaf area to sapwood area ratios of trees (Al:AS) can shift to maintain homeostatic gas exchange per unit leaf area in response to climate variability. We tested the hypothesis that trees alter their Al:AS ratios in response to long-term warming and reduced precipitation in order to maintain leaf-specific gas exchange rates under more stressful conditions. Whole-tree Al:AS was measured on mature piñon pine (Pinus edulis Engelm.) and one-seed juniper (Juniperus monosperma (Engelm.) Sarg.) trees after 5 years (2012-16) of chronic exposure to increased temperature (+4.8 °C), precipitation reduction (-45%), or both simultaneously. No difference was found in Al:As among treatments for either species. Associated with this lack of shift in Al:As were large changes in pre-dawn leaf water potential and stomatal conductance, consistent with theoretical expectations of interactions between leaf and whole-tree hydraulic supply. Our results suggest that a lack of whole-tree acclimation in Al:As results in the reductions in plant gas exchange and water status associated with long-term warming and reduced precipitation in semi-arid woodlands.

Activación del sistema complemento e inmunocomplejos circulantes en la anemia drepanocítica / Activation of coplement system and circulating immune complexes in sickle cell anemia

Se estudió la actividad del sistema complemento así como la presencia de inmunocomplejos circulantes (ICC) en 61 pacientes con anemia drepanocítica (AD) distribuidos en 4 grupos según sus manifestaciones clínicas: condiciones basales, crisis vasooclusivas, crisis hepáticas y crisis aplástica. Se encintraron diferencias significativas entre la actividad hemolítica de la vía alternativa y el factor B en todos los enfermos en relación con los sujetos sanos; se correlacionaron estos parámetros con niveles bajos de C3. Se demostraron ICC en los grupos en crisis, no asociados con la actividad de la vía clásica, excepto en el grupo de crisis aplástica. En este último, la actividad de la vía clásica se correlacionó significativamente con las concentraciones de C3 y C4. Se discuten las posibles causas de las alteraciones observadas en el sistema complemento, así como la participación de los ICC en los diferentes estadios estudiados en la AD

Activación del sistema complemento e inmunocomplejos circulantes en la anemia drepanocítica / Activation of complement system and circulating immune complexes in sickle cell anemia

Se estudió la actividad del sistema complemento así como la presencia de inmunocomplejos circulantes (ICC) en 61 pacientes con anemia drepanocítica (AD) distribuidos en 4 grupos según sus manifestaciones clínicas: condiciones basales, crisis vasooclusivas, crisis hepáticas y crisis aplástica. Se encintraron diferencias significativas entre la actividad hemolítica de la vía alternativa y el factor B en todos los enfermos en relación con los sujetos sanos; se correlacionaron estos parámetros con niveles bajos de C3. Se demostraron ICC en los grupos en crisis, no asociados con la actividad de la vía clásica, excepto en el grupo de crisis aplástica. En este último, la actividad de la vía clásica se correlacionó significativamente con las concentraciones de C3 y C4. Se discuten las posibles causas de las alteraciones observadas en el sistema complemento, así como la participación de los ICC en los diferentes estadios estudiados en la AD