Generating spatially robust carbon budgets from flux tower observations

Estimating global terrestrial productivity is typically achieved by rescaling individual flux tower measurements, traditionally assumed to represent homogeneous areas, using gridded remote sensing and climate data. Using 154 locations from the FLUXNET2015 database, we demonstrate that variations in spatial homogeneity and nonuniform sampling patterns introduce variability in carbon budget estimates that propagate to the biome scale. We propose a practical solution to quantify the variability of vegetation characteristics and uniformity of sampling patterns and, moreover, account for contributions of sampling variations over heterogeneous surfaces to carbon budgets from flux towers. Our proposed space‐time‐equitable budgets reduce uncertainty related to heterogeneities, allow for more accurate attribution of physiological variations in productivity trends, and provide more representative grid cell averages for linking fluxes with gridded data products

TAVR : nemesis of NOACs?

Data on non-vitamin K antagonist oral anticoagulants (NOACs) in transcatheter aortic valve replacement (TAVR) patients are controversial. In patients without atrial fibrillation (AF), rivaroxaban showed enhanced ischemia and bleeding as compared to standard of care. ENVISAGE showed enhanced bleeding in AF patients as compared to vitamin K antagonist (VKA). Only apixaban was non-inferior but failed superiority regarding bleeding in AF patients after TAVR. One could hypothesize that this might be due to pharmacokinetics of NOACs. Therefore, we compared outcome in rivaroxaban/edoxaban (once-daily) and apixaban (twice-daily) treated patients. 568 patients with indication for permanent oral anticoagulation due to AF undergoing TAVR were analyzed via inverse probability of treatment weighting. Valve academic research consortium complications during 30-day follow-up were assessed. Bleeding complications were similar in once-daily and twice-daily NOACs (major: 22 (7.5%) vs. 14 (5.3%), p = 0.285; minor: 66 (22.4%) vs. 46 (17.4%), p = 0.133). Complications did not change when splitting the cohort in the different agents apixaban, rivaroxaban and edoxaban. These findings remained robust after multivariate analysis. In summary, twice-daily and once-daily NOACs did not differ regarding bleeding complications in a hypothesis generating real-world cohort of TAVR patients with AF

Effects of inhomogeneities within the flux footprint on the interpretation of seasonal, annual, and interannual ecosystem carbon exchange

Carbon flux measurements using the eddy covariance method rely on several assumptions, including reasonably flat terrain and homogeneous vegetation cover. An increasing number of flux sites are located over partially or completely inhomogeneous areas, but the implication of such inhomogeneities on carbon budgets, and particularly the influence of year-to-year variations in wind patterns on annual budgets, remains unclear. Moreover, directional homogeneity of climatic drivers of carbon fluxes is often assumed, although climatic variables vary with wind direction at many locations. In this study, we examined the directional flux characteristics, incorporating the combined effects of variable surface characteristics and climatic drivers on the annual carbon budgets of an evergreen forest. Our study area was characterized by moderate variation in surface characteristics (leaf area index: 1.5-2; topographic wetness index: 4-16), and significant variation in the key drivers of carbon fluxes with wind direction (such as temperature, VPD and turbulence). Interactions among surface characteristics and climatic variables resulted in carbon uptake 'hotspots'. These localized hotspots influenced mean CO2 fluxes from several wind directions, and were most distinctive during the summer months. Hotspot contributions to yearly budgets varied from year to year, depending on prevailing weather conditions. Consequently, directional variations in flux characteristics affected quarterly estimates of carbon budgets by up to 22%, and annual budgets by up to 25%. We present a procedure to quantify and adjust for the effects of year-to-year variations in directional flux characteristics on interannual comparisons of carbon budgets. Any remaining differences in budgets (after the adjustment) can then be linked more accurately to variations in ecophysiological drivers. Our study clearly highlights that directional variations in flux characteristics can have a significant influence on annual carbon budgets, and that these should be accounted for in interannual comparisons

Spatio-temporal transpiration patterns reflect vegetation structure in complex upland terrain

Topography exerts control on eco-hydrologic processes via alteration of energy inputs due to slope angle and orientation. Further, water availability varies with drainage position in response to topographic water redistribution and the catena effect on soil depth and thus soil water storage capacity. Our understanding of the spatio-temporal dynamics and drivers of transpiration patterns in complex terrain is still limited by lacking knowledge of how systematic interactions of energy and moisture patterns shape ecosystem state and water fluxes and adaptation of the vegetation to these patterns. To untangle the effects of slope orientation and hillslope position on forest structure and transpiration patterns, we measured forest structure, sap flux, soil moisture, throughfall and incoming shortwave radiation along two downslope transects in a forested head water catchment in south-east Australia. Our plot locations controlled for three systematically varying drainage position levels (topographic wetness index: 5.0, 6.5 and 8.0) and two levels of energy input (aridity index: 1.2 and 1.8). Vegetation patterns were generally stronger related to drainage position than slope orientation, whereas sap velocity variations were less pronounced. However, in combination with stand sapwood area, consistent spatio-temporal transpiration patterns emerged in relation to landscape position, where slope orientation was the primary and drainage position the secondary controlling factor. On short temporal scales, radiation and vapor pressure deficit were most important in regulating transpiration rates, whereas soil water limitation only occurred on shallow soils during summer. The importance of stand structural parameters increased on longer time scales, indicating optimization of vegetation in response to the long-term hydro-climatic conditions at a given landscape position. Thus, vegetation patterns can be conceptualized as a ‘time-integrated’ predictor variable that captures large fractions of other factors contributing to transpiration patterns

Trading water for carbon : maintaining photosynthesis at the cost of increased water loss during high temperatures in a temperate forest

Carbon and water fluxes are often assumed to be coupled as a result of stomatal regulation during dry conditions. However, recent observations evidenced increased transpiration rates during isolated heatwaves across a range of eucalypt species under experimental and natural conditions, with inconsistent effects on photosynthesis (ranging from increases to stark declines). To improve the empirical basis for understanding carbon and water fluxes in forests under hotter and drier climates, we measured the water use of dominant trees and ecosystem‐scale carbon and water exchange in a temperate eucalypt forest over three summer seasons. The forest maintained photosynthesis within 16% of baseline rates during hot and dry conditions, despite ~70% reductions in canopy conductance during a 5‐day heatwave. While carbon and water fluxes both decreased by 16% on exceptionally dry days, gross primary productivity only decreased by 5% during the hottest days and increased by 2% during the heatwave. However, evapotranspiration increased by 43% (hottest days) and 74% (heatwave), leading to ~40% variation in traditional water use efficiency (water use efficiency = gross primary productivity/evapotranspiration) across conditions and approximately two‐fold differences between traditional and underlying or intrinsic water use efficiency on the same days. Furthermore, the forest became a net source of carbon following a 137% increase in ecosystem respiration during the heatwave, highlighting that the potential for temperate eucalypt forests to act as net carbon sinks under hotter and drier climates will depend not only on the responses of photosynthesis to higher temperatures and changes in water availability, but also on the concomitant responses of ecosystem respiration

Recovery from severe mistletoe infection after heat- and drought-induced mistletoe death

Mistletoes are emerging as important co-contributors to tree mortality across terrestrial ecosystems, particularly when infected trees are stressed by water limitations during drought. While the mechanistic effects of mistletoe infection on host physiology are reasonably well understood, quantifying the effects of mistletoe infection on stand productivity, canopy turnover and ecosystem structure remains challenging. Moreover, the potential devastating effect of mistletoe infection on host survival has distracted from the challenges that mistletoe populations are facing when increasing drought and heat stress threaten their survival in healthy populations. We coupled intensive observations of mistletoe population dynamics with measurements of host tree stem growth, canopy turnover and stand structure in a severely infected temperate eucalypt woodland to monitor how mistletoe infection alters aboveground biomass distribution and to assess ecosystem recovery from severe mistletoe infection during and after a three-year drought. We show that severe mistletoe infection reduces live standing biomass and canopy volume, with mistletoe leaves contributing up to 43% to total stand litter fall. We further identified that a mistletoe:host leaf area ratio above 60% significantly reduced basal area growth, which provides a threshold for productivity losses due to mistletoe infection in eucalypts. Yet, concurrent increases in basal area and the thickening of canopy volume indicate that host trees recover rapidly after the three-year drought combined with record summer heat nearly extinguished the mistletoe population. How common, or how widespread such dynamic changes in mistletoe population dynamics are within Australian or global ecosystems remains subject to further exploration

Using a paired tower approach and remote sensing to assess carbon sequestration and energy distribution in a heterogeneous sclerophyll forest

The critically endangered Cumberland Plain woodland within the greater Sydney metropolitan area hosts a dwindling refuge for melaleuca trees, an integral part of Australia's native vegetation. Despite their high carbon stocks, melaleucas have not explicitly been targeted for studies assessing their carbon sequestration potential, and especially little is known about their energy cycling or their response to increasing climate stress, precluding a holistic assessment of the resilience of Australia's forests to climate change. To improve our understanding of the role of melaleuca forest responses to climate stress, we combined forest inventory and airborne LiDAR data to identify species distribution and associated variations in forest structure, and deployed flux towers in a melaleuca-dominated (AU-Mel) and in a eucalypt-dominated (AU-Cum) stand to simultaneously monitor carbon and energy fluxes under typical growing conditions, as well as during periods with high atmospheric demand and low soil water content. We discovered that the species distribution at our study site affected the vertical vegetation structure, leading to differences in canopy coverage (75% at AU-Cum vs. 84% at AU-Mel) and plant area index (2.1 m2 m−2 at AU-Cum vs. 2.6 m2 m−2 at AU-Mel) that resulted in a heterogeneous forest landscape. Furthermore, we identified that both stands had comparable net daytime carbon exchange and sensible heat flux, whereas daytime latent heat flux (115.8 W m−2 at AU-Cum vs 119.4 W m−2 at AU-Mel, respectively) was higher at the melaleuca stand, contributing to a 0.3 °C decrease in air temperature and reduced vapor pressure deficit above the melaleuca canopy. However, increased canopy conductance and higher latent heat flux during moderate VPD or when soil moisture was low indicated a lack of water preservation at the melaleuca stand, highlighting the potential for increased vulnerability of melaleucas to projected hotter and drier future climates

[In Press] Tapping into the physiological responses to mistletoe infection during heat and drought stress

Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts experienced unprecedented heat and drought stress in recent years. We investigated whether the excessive water use of mistletoes increased the probability of xylem emboli in a mature woodland during the recent record drought that was compounded by multiple heatwaves. We continuously recorded transpiration (⁠TSLA⁠) of infected and uninfected branches from two eucalypt species over two summers, monitored stem and leaf water potentials (⁠Ψ⁠), and used hydraulic vulnerability curves to estimate percent loss in conductivity (PLC) for each species. Variations in weather (vapour pressure deficit, photosynthetic active radiation, soil water content), host species and % mistletoe foliage explained 78% of hourly TSLA⁠. While mistletoe acted as an uncontrollable sink for water in the host even during typical summer days, daily TSLA increased up to 4-fold in infected branches on hot days, highlighting the previously overlooked importance of temperature stress in amplifying water loss in mistletoes. The increased water use of mistletoes resulted in significantly decreased host Ψleaf and Ψtrunk⁠. It further translated to an estimated increase of up to 11% PLC for infected hosts, confirming greater hydraulic dysfunction of infected trees that place them at higher risk of hydraulic failure. However, uninfected branches of Eucalyptus fibrosa had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana, which shifted the risk of hydraulic failure towards an increased risk of carbon starvation for E. fibrosa. The contrasting mechanistic responses to heat and drought stress between both co-occurring species demonstrates the complexity of host–parasite interactions and highlights the challenge in predicting species-specific responses to biotic agents in a warmer and drier climate