Temperate tree species differ in their physiological sensitivity to declining soil moisture and drought. Although species-specific responses to drought have often been suggested to be the result of ...different water uptake depths, empirical evidence for such a mechanism is scarce. Here we test if differences in water uptake depths can explain previously observed species-specific physiological responses of temperate trees to drought and if the water uptake depth of different species varies in response to declining soil moisture. For this purpose, we employed stable oxygen and hydrogen isotopes of soil and xylem water that we collected over the course of three growing seasons in a mature temperate forest in Switzerland. Our data show that all investigated species utilise water from shallow soil layers during times of sufficient soil water supply. However, Fraxinus excelsior, Fagus sylvatica and Acer pseudoplatanus were able to shift their water uptake to deeper soil layers when soil water availability decreased in the topsoil. In contrast, Picea abies, was not able to shift its water uptake to deeper soil layers. We conclude from our data that more drought-resistant tree species are able to shift their water uptake to deeper soil layers when water availability in the topsoil is becoming scarce. In addition, we were able to show that water uptake depth of temperate tree species is a trait with high plasticity that needs to be characterised across a range of environmental conditions.
Sun-induced chlorophyll fluorescence (SIF) is a radiation flux emitted from chlorophyll molecules and is considered an indicator of the actual functional state of plant photosynthesis. The remote ...measurement of SIF opens a new perspective to assess actual photosynthesis at larger, ecologically relevant scales and provides an alternative approach to study the terrestrial carbon cycle. Recent studies demonstrated the reliability of measured SIF signals and showed significant relationships between SIF and gross primary production (GPP) at ecosystem and global scales. Despite these encouraging results, understanding the complex mechanisms between SIF and GPP remains challenging before SIF can be finally utilized to constrain estimates of GPP. In this study, we present a comprehensive assessment of the relationship between far-red SIF retrieved at 760nm (SIF760) and GPP, and its transferability across three structurally and physiologically contrasting ecosystems: perennial grassland, cropland and mixed temperate forest. We use multi-temporal imaging spectroscopy (IS) data acquired with the Airborne Prism EXperiment (APEX) sensor as well as eddy covariance (EC) flux tower data to evaluate the relationship between SIF760 and GPPEC. We use simulations performed with the coupled photosynthesis–fluorescence model SCOPE to prove trends obtained from our observational data and to assess apparent confounding factors such as physiological and structural interferences or temporal scaling effects. Observed relationships between SIF760 and GPPEC were asymptotic and ecosystem-specific, i.e., perennial grassland (R2=0.59, rRMSE=27.1%), cropland (R2=0.88, rRMSE=3.5%) and mixed temperate forest (R2=0.48, rRMSE=15.88%). We demonstrate that asymptotic leaf level relationships between SIF760 and GPPEC became more linear at canopy level and scaled with temporal aggregation. We conclude that remote sensing of SIF provides a new observational approach to decrease uncertainties in estimating GPP across ecosystems but requires dedicated strategies to compensate for the various confounding factors impacting SIF–GPP relationships. Our findings help in bridging the gap between mechanistic understanding at leaf level and ecosystem-specific observations of the relationships between SIF and GPP.
•SIF760 shows ecosystem-specific and asymptotic relationships to GPP.•Compared to greenness based indices, SIF760 is more consistently related to GPP.•Canopy structure and competing energy pathways confound SIF760–GPP relationships.•SIF760–GPP relationships scale with temporal aggregation.•Complementary environmental and vegetation information is needed to use SIF760.
A profound understanding of temporal and spatial variabilities of soil carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N2 O) fluxes between terrestrial ecosystems and the atmosphere is ...needed to reliably quantify these fluxes and to develop future mitigation strategies. For managed grassland ecosystems, temporal and spatial variabilities of these three soil greenhouse gas (GHG) fluxes occur due to changes in environmental drivers as well as fertilizer applications, harvests and grazing. To assess how such changes affect soil GHG fluxes at Swiss grassland sites, we studied three sites along an altitudinal gradient that corresponds to a management gradient: from 400 m a.s.l. (intensively managed) to 1000 m a.s.l. (moderately intensive managed) to 2000 m a.s.l. (extensively managed). The alpine grassland was included to study both effects of extensive management on CH4 and N2 O fluxes and the different climate regime occurring at this altitude. Temporal and spatial variabilities of soil GHG fluxes and environmental drivers on various timescales were determined along transects of 16 static soil chambers at each site. All three grasslands were N2 O sources, with mean annual soil fluxes ranging from 0.15 to 1.28 nmol m-2 s-1 . Contrastingly, all sites were weak CH4 sinks, with soil uptake rates ranging from -0.56 to -0.15 nmol m-2 s-1 . Mean annual soil and plant respiration losses of CO2 , measured with opaque chambers, ranged from 5.2 to 6.5 μmol m-2 s-1 . While the environmental drivers and their respective explanatory power for soil N2 O emissions differed considerably among the three grasslands (adjusted r2 ranging from 0.19 to 0.42), CH4 and CO2 soil fluxes were much better constrained (adjusted r2 ranging from 0.46 to 0.80) by soil water content and air temperature, respectively. Throughout the year, spatial heterogeneity was particularly high for soil N2 O and CH4 fluxes. We found permanent hot spots for soil N2 O emissions as well as locations of permanently lower soil CH4 uptake rates at the extensively managed alpine site. Including hot spots was essential to obtain a representative mean soil flux for the respective ecosystem. At the intensively managed grassland, management effects clearly dominated over effects of environmental drivers on soil N2 O fluxes. For CO2 and CH4 , the importance of management effects did depend on the status of the vegetation (LAI).
The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants ...and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO2 and the soil matrix, such as CO2 diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps.
Drought has been frequently discussed as a trigger for forest decline. Today, large-scale Scots pine decline is observed in many dry inner-Alpine valleys, with drought discussed as the main causative ...factor. This study aimed to analyse the impact of drought on wood formation and wood structure. To study tree growth under contrasting water supply, an irrigation experiment was installed in a mature Scots pine (Pinus sylvestris L.) forest at a xeric site in a dry inner-Alpine valley. Inter- and intra-annual radial increments as well as intra-annual variations in wood structure of pine trees were studied. It was found that non-irrigated trees had a noticeably shorter period of wood formation and showed a significantly lower increment. The water conduction cells were significantly enlarged and had significantly thinner cell walls compared with irrigated trees. It is concluded that pine trees under drought stress build a more effective water-conducting system (larger tracheids) at the cost of a probably higher vulnerability to cavitation (larger tracheids with thinner cell walls) but without losing their capability to recover. The significant shortening of the growth period in control trees indicated that the period where wood formation actually takes place can be much shorter under drought than the 'potential' period, meaning the phenological growth period.
Atmospheric methane concentrations were quantified along transects in Switzerland, using a mobile laser spectrometer combined with a GPS, to identify their spatio-temporal patterns and their ...controlling factors. Based on these measurements in complex terrain dominated by agriculture, three main factors were found to be responsible for the diurnal and regional patterns of atmospheric methane: (1) magnitude and distribution of methane sources within the region, (2) efficiency of vertical exchange, and (3) local wind patterns within the complex topography. An autocorrelation analysis of measured methane concentrations showed that nighttime measurements close to the ground provide information about regional sources (up to 8.3 km), while daytime measurements only carry information about sources located up to 240 m away in the upwind fetch. Compared to daytime concentrations, nighttime methane concentrations do also better reflect emissions obtained from a spatially explicit methane emission inventory and allowed the investigation of inconsistencies in this emission inventory.
•Vertical mixing and local wind systems affect spatio-temporal patterns of methane.•Nighttime concentration measurements of methane carry regional information.•Use of methane concentrations to identify inconsistencies in emission inventories.
Nighttime measurements close to the ground provide information about regional sources, while daytime measurements carry very local information.
The response of carbon allocation to drought has often been studied in terms of short-term transport velocity of recently fixed carbon from leaves to roots and root respiration. However, its dynamic ...response to other environmental conditions, e.g., to changes in temperature, is less clear. Here, we investigated the effects of drought, increased temperatures and their combination on transport velocity as well as on distribution of recent photoassimilates for different compounds, such as sugars, starch, organic acids and amino acids. We used a (13)CO(2) pulse-labelling approach and studied the recovery of (13)C in different plant tissues and compounds of beech saplings (Fagus sylvatica L.) during a 9-day chase period. Neither total dry biomass nor dry weights of leaves or roots were affected by drought or increased temperatures. Generally, the fast transfer of recently fixed assimilates from leaves to roots took about 1 day, while (13)C enrichment in soil CO(2) efflux peaked only 2 days after labelling. Increased temperatures prolonged mean transfer times of recent photoassimilates from the leaves to the roots, probably caused by enhanced intermediate storage alongside basipetal transfer, clearly impacting short-term carbon allocation. This temperature effect was seen in the delayed peak in (13)C excess of root sugars, decoupling the roots from the leaves in the short term. On average, ∼40% of the (13)C label initially present in the plant was recovered in the roots (over all treatment combinations), providing strong evidence for preferred carbon allocation into the roots at the end of the growing season. Root starch was the principal compound for long-term storage of carbon, whereas leaf (transitory) starch was remobilized again after some days, exhibiting the longest mean residence times under dry and warm conditions. These observation clearly point to different functionalities of the same compound (i.e., starch) in different plant tissues and the crucial role of roots for long-term carbon storage.
Climate models suggest that enhanced greenhouse gas concentrations and aerosols have major impacts on the land energy and water cycles, and in particular on evapotranspiration (ET). Here we analyze ...how the main external drivers of ET (incident solar radiation and precipitation) vary regionally, using recent data from a eddy‐covariance flux tower network (FLUXNET) and a multi‐model re‐analysis (GSWP‐2). Trends in radiation (global “dimming” and “brightening”) are expected to impact ET only in regions where ET correlates with radiation. In central Europe this correlation is particularly strong, and trends derived from weighing lysimeters and river‐basin water budgets follow trends in radiation. In central North America the correlation is weak, and trends in precipitation rather than radiation explain trends in ET. Our results reconcile previous hypotheses by demonstrating the strongly regional and temporal differentiation of trends in evaporation.
Half‐hourly measurements of the net exchanges of carbon dioxide and water vapor between terrestrial ecosystems and the atmosphere provide estimates of gross primary production (GPP) and ...evapotranspiration (ET) at the ecosystem level and on daily to annual timescales. The ratio of these quantities represents ecosystem water use efficiency. Its multiplication with mean daylight vapor pressure deficit (VPD) leads to a quantity which we call “inherent water use efficiency” (IWUE*). The dependence of IWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment. IWUE* is analyzed for 43 sites across a range of plant functional types and climatic conditions. IWUE* increases during short‐term moderate drought conditions. Mean annual IWUE* varied by a factor of 3 among all sites. This is partly explained by soil moisture at field capacity, particularly in deciduous broad‐leaved forests. Canopy light interception sets the upper limits to canopy photosynthesis, and explains half the variance in annual IWUE* among herbaceous ecosystems and evergreen needle‐leaved forests. Knowledge of IWUE* offers valuable improvement to the representation of carbon and water coupling in ecosystem process models.
Drought impairs tree growth in the inner-Alpine valleys of Central Europe. We investigated species-specific responses to contrasting water supply, with Scots pine (Pinus sylvestris L.), threatened by ...drought-induced mortality, and pubescent oak (Quercus pubescens Willd.), showing no connection between drought events and mortality. The two co-occurring tree species were compared, growing either along an open water channel or at a site with naturally dry conditions. In addition, the growth response of Scots pine to a draining of a water channel was studied. We analysed the radial increment for the last 100 years and wood anatomical parameters for the last 45 years. Drought reduced the conduit area of pubescent oak, but increased the radial lumen diameter of the conduits in Scots pine. Both species decreased their radial increment under drought. In Scots pine, radial increment was generally more dependent on water availability than that in pubescent oak. Irrigated trees responded less negatively to high temperature as seen in the increase in the conduit area in pubescent oak and the removal of the limitation of cell division by high temperatures. After irrigation stopped, tree-ring width for Scots pine decreased within 1-year delay, whereas lumen diameter and cell-wall thickness responded with a 4-year delay. Scots pine seemed to optimize the carbon-per-conduit-costs under drought by increasing conduits diameter while decreasing cell numbers. This strategy might lead to a complete loss of tree rings under severe drought and thus to an impairment of water transport. In contrast, in pubescent oak tree-ring width is less affected by summer drought because parts of the earlywood are built in early spring. Thus, pubescent oak might have gradual advantages over pine in today's climate of the inner-Alpine valley.
PDF
