Canopy structure and the impact of drought on a Quercus suber L. woodland in Portugal

The thesis covers two topics, a performance evaluation of indirect leaf area index L and gap probability Pgap observation with the new digital cover photography DCP with special focus on the application in sparse canopies and the analysis of drought effects on water / carbon fluxes in a typical Mediterranean Quercus suber L. ecosystem. L and Pgap are important ecosystem parameters. Their indirect, height and angular distributed measurement remains a challenging task in open canopies. DCP was successfully applied here for the first time height and angular dependent. Results show similar Pgap compared to an established method. Clumping index Omega could be successfully derived by DCP for calculating L. DCP yielded precise L matching observations with litter traps. Woody component exclusion by object-based image analysis improved results. Ground-based crown observations yielded reasonable L height distributions compared with direct measurements. Plant species developed vast structural and functional adaptations to regulate carbon assimilation and respiratory water loss under drought. In the context of the extreme drought year 2012, drought effects on the entire ecosystem functioning are reported in the second part of the thesis. Therefore, multi-year observations of climate forcing, soil properties as well as ecosystem flux observations were conducted and combined stomatal conductance-photosynthesis modeling was applied and evaluated. Results show that precipitation effectiveness ET/P increased up to 122\% in the dry year 2012 due to the ground water access of trees. Understorey and overstorey gross primary productivity and were reduced by 53\% and 28\% in 2012. Modelling results showed simultaneous reduction of maximum carboxylation rate and stomatal conductance. However, the ecosystem remained a carbon sink in both years with with 38\% reduced sink strength

Stable oxygen isotope and flux partitioning demonstrates understory of an oak savanna contributes up to half of ecosystem carbon and water exchange

Semi-arid ecosystems contribute about 40% to global net primary production (GPP) even though water is a major factor limiting carbon uptake. Evapotranspiration (ET) accounts for up to 95% of the water loss and in addition, vegetation can also mitigate drought effects by altering soil water distribution. Hence, partitioning of carbon and water fluxes between the soil and vegetation components is crucial to gain mechanistic understanding of vegetation effects on carbon and water cycling. However, the possible impact of herbaceous vegetation in savanna type ecosystems is often overlooked. Therefore, we aimed at quantifying understory vegetation effects on the water balance and productivity of a Mediterranean oak savanna. ET and net ecosystem CO2 exchange (NEE) were partitioned based on flux and stable oxygen isotope measurements and also rain infiltration was estimated. The understory vegetation contributed importantly to total ecosystem ET and GPP with a maximum of 43 and 51%, respectively. It reached water-use efficiencies (WUE; ratio of carbon gain by water loss) similar to cork-oak trees. The understory vegetation inhibited soil evaporation (E) and, although E was large during wet periods, it did not diminish WUE during water-limited times. The understory strongly increased soil water infiltration, specifically following major rain events. At the same time, the understory itself was vulnerable to drought, which led to an earlier senescence of the understory growing under trees as compared to open areas, due to competition for water. Thus, beneficial understory effects are dominant and contribute to the resilience of this ecosystem. At the same time the vulnerability of the understory to drought suggests that future climate change scenarios for the Mediterranean basin threaten understory development. This in turn will very likely diminish beneficial understory effects like infiltration and ground water recharge and therefore ecosystem resilience to drought

Effects of an extremely dry winter on net ecosystem carbon exchange and tree phenology at cork oak woodland

In seasonally dry climates, such as the Mediterranean, lack of rainfall in the usually wet winter may originate severe droughts which are a main cause of inter-annual variation in carbon sequestration. Leaf phenology variability may alter the seasonal pattern of photosynthetic uptake, which in turn is determined by leaf gas exchange limitations. The current study is based on the monitoring of an extremely dry winter in an evergreen cork oak woodland under the Mediterranean climate of central Portugal. Results are focused on net ecosystem CO2 exchange (NEE), phenology and tree growth measurements during two contrasting years: 2011, a wet year with a typical summer drought pattern and 2012, with an extremely unusual dry winter (only 10mmof total rainfall) that exacerbated the following summer drought effects. Main aims of this study were to assess the effects of an extreme dry winter in (1) annual and seasonal net ecosystem CO2 exchange, and in (2) cork oak phenology. The dry year 2012 was marked by a 45% lower carbon sequestration (−214 vs. −388gCm−2 year−1) and a 63% lower annual tree diameter growth but only a 9% lower leaf area index compared to the wet year 2011. A significant reduction of 15% in yearly carbon sequestration was associated with leaf phenological events of canopy renewal in the early spring. In contrast to male flower production, fruit setting was severely depressed by water stress with a 54% decrease during the dry year. Our results suggest that leaf growth and leaf area maintenance are resilient ecophysiological processes under winter drought and are a priority carbon sink for photoassimilates in contrast to tree diameter growth. Thus, carbon sequestration reductions under low water availabilities in cork oak woodland should be ascribed to stomatal regulation or photosynthetic limitations and to a lesser extent to leaf area reductionsinfo:eu-repo/semantics/publishedVersio

Modelling CO2 and CH4 emissions from drained peatlands with grass cultivation by the BASGRA-BGC model

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The FACCE-ERA-Net Plus project “Climate smart Agriculture on Organic Soils” (CAOS)

The FACCE-ERA-Net Plus project “Climate smart Agriculture on Organic Soils” (CAOS) focuses on farmed organic soils, hotspots of vulnerability and GHG emissions in Europe. We propose to use wet organic soils as risk insurance in dry periods on farm/regional level, while water and soil management assures trafficability in wet conditions. Wet management systems abate peat degradation and therefore foster higher infiltration rates and ease subirrigation. Economically, wetness-adapted crops with stable yield quantity/quality for food, feed and bioenergy are needed. Convincing farmers and decision makers of profitable and resilient wet management systems on organic soils under climate change needs proof by on-farm experiments, historical evidence and bi-directional involvement.Overall, we aim to generate knowledge of climate smart agricultural system design on organic soils adapted to regional European conditions. CAOS will provide and distribute evidence that active management with control of groundwater levels, improved trafficability and alternative high productivity crops improves yield stability/quality and climate change resilience while mitigating GHG emissions and improving soil/water quality. We hypothesize that the strong potential for adaptation to increased climatic variability on farmed organic soil will facilitate mitigation of the largest GHG source from agriculture in Central/Northern Europe. At MACSUR conference, we present the project concept and first results

Effects of birch encroachment, water table and vegetation on methane emissions from peatland microforms in a rewetted bog

Abstract This study investigated the influence of vegetation and microforms on methane (CH4) balances of a rewetted bog in north-west Germany. The two study sites are in close proximity on the same former peat extraction area, one dominated by Sphagnum-mosses and the other one by a dense Betula pubescens stand with a high Eriophorum vaginatum cover. The contribution of microforms (hummocks/hollows) to CH4 emissions and the effect of Betula encroachment has been studied. Transparent and opaque chambers were used to measure CH4 fluxes every 3–4 weeks during daytime for one year. For the estimation of annual balances, three methods were compared and the method using water level and soil temperature as explanatory variables was selected. Fluxes were scaled to the site level. The annual emissions per site are and 7.1 ± 1.5 g CH4-C m−2 year−1 at the treed site and 36.1 ± 3.5 g CH4-C m−2 year−1 at the open site, mainly controlled by higher water levels. Highest annual emissions originated from hollows at the open site, but in the vegetation period, hummock emissions tend to be higher. At the tree site, emission differences between the microforms were less pronounced. There were no differences between fluxes from transparent and opaque chambers

Comparing methods for measuring water retention of peat near permanent wilting point

Peat soils shrink and become very hydrophobic when dried. Both properties may cause inaccuracies when applying laboratory methods for soil hydraulic properties that have been developed and tested for mineral soils. This study aimed to compare different methods for the determination of the water retention of peat soils near permanent wilting point (pF 3.5 to 4.2). Three common methods were tested: two pressure apparatus (ceramic plate [Soilmoisture] vs. membrane [eijkelkamp]) and a dew-point potentiameter (WP4C, Decagon Devices, Inc.), which is based on the equilibrium of soil water potential with air humidity. We used both field-moist peat samples and samples that had been rewetted after oven-drying. We found that there was no systematic difference between the two pressure apparatus. Low moisture variability among replicates and dew-point potentiameter measurements that indicated a drainage to pF 4.2 support the use of pressure apparatus for the determination of water retention near permanent wilting point. Despite a rewetting time of 2 wk including periodic mixing, rewetted oven-dried samples showed lower soil moistures at pF 3.5 and 4.2 than field-moist ones. This severe and long-lasting hysteresis effect was strongest for less decomposed peat samples. Thus, field-moist samples should be used. This makes the classical dew-point potentiameter measurement protocol, which is based on defined water additions to oven-dried samples, unsuitable for peat samples

Evaporation experiments for the determination of hydraulic properties of peat and other organic soils: An evaluation of methods based on a large dataset

status: Published onlin

Evaporation experiments for the determination of hydraulic properties of peat and other organic soils: An evaluation of methods based on a large dataset

edition: Geophysical Research Abstractsstatus: publishe