Search

Plant water stress due to climate change is posing a threat to various ecosystem services such as carbon sequestration, food and wood production, and climate regulation. To address this issue, methods are needed to assess and monitor plant water stress at various spatial and temporal scales. Passive microwave emission observations from satellites have proven useful in monitoring changes in vegetation water content and assessing plant water stress at a low spatial resolution (> 9 km). In this study, we used vegetation optical depth (VOD) and measurements of hydraulic vulnerability to create a novel model for assessing ecosystem-level water stress. We used L-band VOD and global measurements of xylem water potential at 88% loss of stem hydraulic conductivity (P88) from the TRY database (including 1103 measurements of P88 from 463 species and nine different vegetation biomes) to create a linear regression model between L-band VOD and biome-level P88. We used monthly mean values of L-band VOD and calculated ratios of yearly minimum and maximum VOD (L-VODmin/max) for each pixel to describe average variability in ecosystem-level water content. The developed L-VODmin/max metric explained 75% of the variation in P88 at the biome level (R2=0.75) indicating that the novel L-VODmin/max metric is capable of capturing changes in plant water status. We then used the L-VODmin/max metric and daily climate data from the ERA5 to see if water stress has increased over time in the world's forests that are more water limited (aridity index below 1.5). For these areas, we found a positive trend in maximum daily vapour pressure deficit, which correlated negatively (p

Climate change is causing rapid changes in sub-Arctic and Arctic terrestrial ecosystems compared to the other biomes. As these ecosystems are so sensitive to climate change, more research on how the rising of temperature affects these ecosystems is essential.This study is implemented in the ForHot research site located in Iceland encompassing geothermally heated natural grasslands. This research site contains two areas, one that has been warmed geothermally for over 60 years (long-term warming; LTW) and the other since 2008 (medium-term warming; MTW). The LTW area contains 24 plots and the MTW contains 30 plots, with the mean annual soil temperature ranging between 5 to 21 °C for LTW and 6 to 46 °C for MTW in 2022. The main goal of this study was to understand how the warming level and the duration of warming (MTW vs. LTW) have affected the average seasonal Normalized Difference Vegetation Index (NDVI) during 2022.For this study, we repeatedly collected high-resolution multispectral data using Micasense dual camera system and DJI Matrice 600 drone each month from April 2022 to October 2022 as well as soil temperature data of each plot, and some other parameters such as precipitation, air temperature, wind speed, and soil water content.Here we present the results on how the warming level and the duration of warming affected the monthly and seasonal average NDVI in the MTW and LTW grassland ecosystems.

Climate change affects ecosystems considerably worldwide, but as warming is happening at an accelerated pace at higher latitudes, it is essential to study how warming affects ecosystem structure and function in Arctic and sub-Arctic ecosystems.In this research, we looked at changes in the aboveground biomass (AGB) of two Icelandic sub-Arctic grasslands located at the ForHot site. The ForHot site is an exceptional studying site where the soils are naturally warmed. Thus, making it an important natural laboratory to assess and learn more about the long-term effect of global warming. At the research site, one grassland ecosystem has soils that have been warmed for over 60 years (long-term warming; LTW) and the other since 2008 (medium-term warming; MTW), when an earthquake disrupted geothermal channels in the underlying bedrock. Fifty permanent survey plots were established in the autumn of 2012 along the two grassland soil temperature gradients (ranging from 0 to +18°C).We assessed how vegetation structure (non-vascular; AGBnon-vasc and vascular plants; AGBvasc) and the ecosystem's maximum aboveground total biomass(AGBtot) were affected by different levels of soil warming over multiple studied years (i.e. 2013, 2016, 2018, 2020, 2021 and 2022).Our preliminary results showed unexpectedly relatively small changes in AGBtot with warming. We hypothesise that changes in AGBvasc would typically induce opposite changes in AGBnon-vasc, probably because of light competition. When looking separately at the vegetation structures, for AGBvasc, the duration of soil warming induced contrasted responses between MTW and LTW grasslands. That is, in the MTW grassland, there were no changes for most years (p > .05) and strong negative responses (p < .05) with warming in seasonally maximum AGBvasc for other years. Whereas in the LTW grassland, warming generally increased AGB for most years (p < .05), and also a strong negative response as observed in the MTW for the respective years despite statistically not significant. This strong negative response could be because of untypically high AGB production in control (unwarmed) plots during those years and less positive reactions with different levels of soil warming. We will show some potential drivers (environmental variables) for those unexpected temporal variations in the warming response. AGBnon-vasc, such as lichens and mosses, have an unclear pattern across the soil warming gradient in both grassland ecosystems.

In response to changes in environmental factors (e.g., temperature, radiation, soil moisture), plants emit biogenic volatile organic compounds (BVOCs). Once released in the atmosphere, BVOCs influence levels of greenhouse gases and air pollutants (e.g., methane, ozone and aerosols), thus affecting both climate and air quality. In turn, climate change may alter BVOC emissions by modifying the driving environmental conditions and by increasing the occurrence and intensity of severe stresses that alter plant functioning. To understand and better constrain the evolution of BVOC emissions under future climates, it is important to reduce the uncertainties in global and regional estimates of BVOC emissions under present climate. Part of the uncertainty in the estimates of BVOC emissions is related to the impact that water stress might have on BVOC emissions. Field campaign, in-situ and laboratory experiments investigated the effect of different regimes of water stress (short- vs. long-term) on BVOC emissions. However, these studies provide geographically scattered and uneven results. To explore the relationship between BVOC emissions and water stress globally, we use remotely sensed soil moisture and formaldehyde, a proxy of BVOC emissions. As BVOCs include a multitude of gas tracers with lifetime ranging from few hours to days, a fully characterisation of these components is virtually impossible. Nevertheless, in the continental boundary layer, formaldehyde is an intermediate by-product of the oxidation of BVOCs, it thus provides a proxy for probing local BVOC emissions, and in particular isoprene, which accounts for about 50% of the total BVOC emissions.In the present study, retrievals of formaldehyde from the Ozone Monitoring Instrument (OMI) are combined with observations of soil moisture, biomass, aerosols, evapotranspiration, drought index, temperature and precipitation. Firstly, we look into the linear annual trend of the selected fields. Secondly, assuming formaldehyde as the dependent variable, we apply a linear mixed model analysis that extends the application of a simple linear regression model by accounting for both fixed (i.e., explained by the independent variables) and random (i.e., due to dependence in the data) effects. The analysis of the linear trend of formaldehyde concentrations shows a positive trend over the Amazon and Central Africa and a negative trend over South Africa and Australia. Over the Amazon, formaldehyde is negatively correlated with the Standardised Precipitation-Evapotranspiration Index (SPEI), a drought index that accounts for both changes in temperature and precipitation, with positive and negative values identifying wet and dry events, respectively. The outcomes of this analysis might provide new insights in the relationship between BVOC emissions and water stress and might help in improving parameterizations that link soil moisture to BVOC emissions in numerical models.

Landscape-scale fluxes of biogenic gases were surveyed by deploying a 100 m Teflon tube attached to a tethered balloon as a sampling inlet for a fast-response proton-transfer-reaction mass spectrometer (PTRMS). Along with meteorological instruments deployed on the tethered balloon and a 3 m tripod and outputs from a regional weather model, these observations were used to estimate landscape-scale biogenic volatile organic compound fluxes with two micrometeorological techniques: mixed layer variance and surface layer gradients. This highly mobile sampling system was deployed at four field sites near Barcelona to estimate landscape-scale biogenic volatile organic compound (BVOC) emission factors in a relatively short period (3 weeks). The two micrometeorological techniques were compared with emissions predicted with a biogenic emission model using site-specific emission factors and land-cover characteristics for all four sites. The methods agreed within the uncertainty of the techniques in most cases, even though the locations had considerable heterogeneity in species distribution and complex terrain. Considering the wide range in reported BVOC emission factors for individual vegetation species (more than an order of magnitude), this temporally short and inexpensive flux estimation technique may be useful for constraining BVOC emission factors used as model inputs.

43 páginas, 9 figuras, 2 tablas., Atmospheric volatile organic compounds (VOCs) are involved in ozone and aerosol generation, thus having implications for air quality and climate. VOCs and their emissions by vegetation also have important ecological roles as they can protect plants from 5 stresses and act as communication cues between plants and between plants and animals. In spite of these key environmental and biological roles, the reports on seasonal and daily VOC mixing ratios in the literature for Mediterranean natural environments are scarce. We conducted seasonal (winter and summer) measurements of VOC mixing ratios 10 in an elevated (720 m a.s.l.) holm oak Mediterranean forest site near the metropolitan area of Barcelona (NE Iberian peninsula). Methanol was the most abundant compound among all the VOCs measured in both seasons. While aromatic VOCs showed almost no seasonal variability, short-chain oxygenated VOCs presented higher mixing ratios in summer, presumably due to greater emission by vegetation and increased photo- 15 chemistry, both enhanced by the high temperatures and solar radiation in summer. Isoprenoid VOCs showed the biggest seasonal change in mixing ratios: they increased by one order of magnitude in summer, as a result of the vegetation’s greater physiological activity and emission rates. The maximum diurnal concentrations of ozone increased in summer too, most likely due to more intense photochemical activity and the higher 20 levels of VOCs in the air. The daily variation of VOC mixing ratios was mainly governed by the wind regime of the mountain, as the majority of the VOC species analyzed followed a very similar diel cycle. Mountain and sea breezes that develop after sunrise advect polluted air masses to the mountain. These polluted air masses had previously passed over the urban 25 and industrial areas surrounding the Barcelona metropolitan area, where they were enriched in NOx and in VOCs of biotic and abiotic origin. Moreover, these polluted air masses receive additional biogenic VOCs emitted in the local valley by the vegetation, thus enhancing O3 formation in this forested site. The only VOC species that showed a somewhat different daily pattern were monoterpenes because of their local biogenic emission. Isoprene also followed in part the daily pattern of monoterpenes, but only in summer when its biotic sources were stronger. The increase by one order of magnitude in the concentrations of these volatile isoprenoids highlights the importance of local 5 biogenic summer emissions in these Mediterranean forested areas which also receive polluted air masses from nearby or distant anthropic sources., This study was supported by the Spanish Government grants CGL2006- 04025/BOS, CGL2010-17172, Consolider-Ingenio Montes CSD2008-00040 and Accion Com- ´ plementaria DAURE CGL2007-30502-E/CLI, and the Catalan Government grant SGR 2009- 10 458. Roger Seco gratefully acknowledges an FPI fellowship (BES-2005-6989) from the Spanish Government. This research has received funding from the EC Seventh Framework Programme under grant agreement no. 215072 (Marie Curie Initial Training Network, “CLOUD-ITN”). Helpful discussion concerning the PTR-TOF evaluation with Martin Graus and Ralf Schnitzhofer from the Univer- 15 sity of Innsbruck was much appreciated.