Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest ...die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land–atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global monitoring network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.
The terminal branch orders of plant root systems have been proposed as short-lived ‘ephemeral’ modules specialized for resource absorption. The occurrence of ephemeral root modules has so far only ...been reported for a temperate tree species and it is unclear if the concept also applies to other woody (shrub, tree) and herb species.
Fine roots of 12 perennial dicotyledonous herb, shrub and tree species were monitored for two growing seasons using a branch-order classification, sequential sampling and rhizotrons in the Taklamakan desert.
Two root modules existed in all three plant functional groups. Among the first five branch orders, the first two (perennial herbs, shrubs) or three (trees) root orders were ephemeral and had a primary anatomical structure, high nitrogen (N) concentrations, high respiration rates and very short life spans of 1–4 months, whereas the last two branch orders in all functional groups were perennial, with thicker diameters, no or collapsed cortex, distinct secondary growth, low N concentrations, low respiration rates, but much longer life spans.
Ephemeral, short-lived root modules and long-lived, persistent root modules seem to be a general feature across many plant functional groups and could represent a basic root system design.
Relative water content (RWC) and the osmotic potential (π) of plant leaves are important plant traits that can be used to assess drought tolerance or adaptation of plants. We estimated the magnitude ...of errors that are introduced by dilution of π from apoplastic water in osmometry methods and the errors that occur during rehydration of leaves for RWC and π in 14 different plant species from trees, grasses and herbs. Our data indicate that rehydration technique and length of rehydration can introduce significant errors in both RWC and π. Leaves from all species were fully turgid after 1–3 h of rehydration and increasing the rehydration time resulted in a significant underprediction of RWC. Standing rehydration via the petiole introduced the least errors while rehydration via floating disks and submerging leaves for rehydration led to a greater underprediction of RWC. The same effect was also observed for π. The π values following standing rehydration could be corrected by applying a dilution factor from apoplastic water dilution using an osmometric method but not by using apoplastic water fraction (AWF) from pressure volume (PV) curves. The apoplastic water dilution error was between 5 and 18%, while the two other rehydration methods introduced much greater errors. We recommend the use of the standing rehydration method because (1) the correct rehydration time can be evaluated by measuring water potential, (2) overhydration effects were smallest, and (3) π can be accurately corrected by using osmometric methods to estimate apoplastic water dilution.
Tree stems are an important and unconstrained source of methane, yet it is uncertain whether internal microbial controls (i.e. methanotrophy) within tree bark may reduce methane emissions. Here we ...demonstrate that unique microbial communities dominated by methane-oxidising bacteria (MOB) dwell within bark of Melaleuca quinquenervia, a common, invasive and globally distributed lowland species. In laboratory incubations, methane-inoculated M. quinquenervia bark mediated methane consumption (up to 96.3 µmol m
bark d
) and reveal distinct isotopic δ
C-CH
enrichment characteristic of MOB. Molecular analysis indicates unique microbial communities reside within the bark, with MOB primarily from the genus Methylomonas comprising up to 25 % of the total microbial community. Methanotroph abundance was linearly correlated to methane uptake rates (R
= 0.76, p = 0.006). Finally, field-based methane oxidation inhibition experiments demonstrate that bark-dwelling MOB reduce methane emissions by 36 ± 5 %. These multiple complementary lines of evidence indicate that bark-dwelling MOB represent a potentially significant methane sink, and an important frontier for further research.
Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range ...of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ18O and δ2H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.
Brief Summary Statement
Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of evaporative enrichment of heavy isotopes in leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases.
•Leaf wax alkanes of 2 plant species along a hydrological gradient investigated.•Composition clearly affected by hydroclimate drivers.•Different plants may show opposite effects, i.e. n-alkanes may ...increase or decrease vs. aridity.•Difficult to use ACL as a palaeo proxy in absence of other data.
Environmental parameters such as rainfall, temperature and relative humidity can affect the composition of higher plant leaf wax. The abundance and distribution of leaf wax biomarkers, such as long chain n-alkanes, in sedimentary archives have therefore been proposed as proxies reflecting climate change. However, a robust palaeoclimatic interpretation requires a thorough understanding of how environmental changes affect leaf wax n-alkane distributions in living plants. We have analysed the concentration and chain length distribution of leaf wax n-alkanes in Acacia and Eucalyptus species along a 1500km climatic gradient in northern Australia that ranges from subtropical to arid. We show that aridity affected the concentration and distribution of n-alkanes for plants in both genera. For both Acacia and Eucalyptus n-alkane concentration increased by a factor of ten to the dry centre of Australia, reflecting the purpose of the wax in preventing water loss from the leaf. Furthermore, Acacian-alkanes decreased in average chain length (ACL) towards the arid centre of Australia, whereas Eucalyptus ACL increased under arid conditions. Our observations demonstrate that n-alkane concentration and distribution in leaf wax are sensitive to hydroclimatic conditions. These parameters could therefore potentially be employed in palaeorecords to estimate past environmental change. However, our finding of a distinct response of n-alkane ACL values to hydrological changes in different taxa also implies that the often assumed increase in ACL under drier conditions is not a robust feature for all plant species and genera and as such additional information about the prevalent vegetation are required when ACL values are used as a palaeoclimate proxy.
OzFlux is the regional Australian and New Zealand flux tower network that aims to provide a continental-scale national research facility to monitor and assess trends, and improve predictions, of ...Australia's terrestrial biosphere and climate. This paper describes the evolution, design, and current status of OzFlux as well as provides an overview of data processing. We analyse measurements from all sites within the Australian portion of the OzFlux network and two sites from New Zealand. The response of the Australian biomes to climate was largely consistent with global studies except that Australian systems had a lower ecosystem water-use efficiency. Australian semi-arid/arid ecosystems are important because of their huge extent (70 %) and they have evolved with common moisture limitations. We also found that Australian ecosystems had a similar radiation-use efficiency per unit leaf area compared to global values that indicates a convergence toward a similar biochemical efficiency. The two New Zealand sites represented extremes in productivity for a moist temperate climate zone, with the grazed dairy farm site having the highest GPP of any OzFlux site (2620 gC m−2 yr−1) and the natural raised peat bog site having a very low GPP (820 gC m−2 yr−1). The paper discusses the utility of the flux data and the synergies between flux, remote sensing, and modelling. Lastly, the paper looks ahead at the future direction of the network and concludes that there has been a substantial contribution by OzFlux, and considerable opportunities remain to further advance our understanding of ecosystem response to disturbances, including drought, fire, land-use and land-cover change, land management, and climate change, which are relevant both nationally and internationally. It is suggested that a synergistic approach is required to address all of the spatial, ecological, human, and cultural challenges of managing the delicately balanced ecosystems in Australasia.
In order to realize a chemical sensor with a high signal reproducibility, a thermo-shrinking photo cross-linkable co- and terpolymer of
N-isopropylacrylamide (NIPAAm), 2-(dimethyl maleimido)-
...N-ethylacrylamide (DMIAAm) as the chromophore and poly(2-vinylpyridine) (P2VP) (or
N,
N-dimethylacrylamide (DMAAm), respectively) has been used as chemo-mechanical transducer. Commercially available pressure sensor chips with a flexible thin silicon bending plate have been employed as mechano-electrical transducer for the transformation of bending plate deflection into an corresponding electrical output signal. A thin film of the photo cross-linkable hydrogel has been deposited onto the backside of the silicon bending plate and irradiated with UV light. The aqueous solution to be measured has been pumped through inlet tubes into the silicon chip cavity and induces swelling or shrinking processes of the hydrogel.
Changes in the polymer composition have allowed to vary the gel volume phase transition temperature (
T
c) from 25
°C for P2VP-block-P(NIPAAm-co-DMIAAm) to 43
°C for NIPAAm-DMIAAm-DMAAm terpolymer. The combination of the temperature-sensitive PNIPAAm with a pH-sensitive P2VP component allowed us to realize a pH sensor with a long-term stable signal in the pH range between 2 and 5. In addition, the sensitivity of the sensors with regard to concentration changes of different inorganic solutes has been studied. The influence of an initial gel-conditioning procedure on the signal value as well as the sensitivity of the proposed chemical sensors was investigated. Measurement conditions necessary for high signal reproducibility and a long-term stability were determined.
•Flux tower sites often experience inhomogeneous surface characteristics.•CO2 fluxes are affected by variable climatic conditions with wind direction.•Carbon uptake ‘hotspots’ were most distinctive ...during the summer months.•Wind sector contributions affected estimates of annual CO2 budgets by up to 25%.•A method is presented to adjust for variations in wind sector contributions.
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.
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