Corrigendum to “Seasonal variations of Quercus pubescens isoprene emissions from an in natura forest under drought stress and sensitivity to futureclimate change in the Mediterranean area”

International audienc

Switching the light off: A break in photosynthesis and sap flow of forest trees under total solar eclipse

In mature trees of Picea abies, Fagus sylvatica and Quercus robur, photosynthesis and transpiration were assessed in response to the total solar eclipse that occurred in Central Europe during the late morning hours of August 11, 1999, a day with changing cloudiness. Measurements were conducted at three forest sites located in the totality zone and the 99% area of the eclipse within a radius of about 100 km around the city of Munich (southern Germany). The eclipse lasting 164 minutes lowered the photosynthetic photon flux density (PPFD) to about 1 μmol m-2 s-1 during the 2-minute totality period, when the sky was clear. During totality, photosynthesis was reduced to an extent that allowed CO2 release to dominate the gas exchange of leaves. Effects on transpiration were less pronounced as the totality was apparently too short to induce distinct stomatal closure in response to low PPFD. Transpiration was strongly reduced, however, by increased air humidity and wet leaf surfaces during sporadic rain showers which preceded or succeeded the eclipse during the same day, whereas low PPFD through intermittent cloudiness during rain only moderately reduced photosynthesis. Although transpiration was lowered to a minor extent only by the eclipse, the latter affected the water transport through the whole tree, as reflected in a decline in the sap flow rate through the basal stem part with a time delay depending on the species. Nevertheless, trees responded in a synchronous way, regardless of the site, species or the percent degree of the eclipse

The influence of soil carbonic anhydrase on the partitioning of gross CO2 fluxes using the oxygen isotopes of CO2 and water

International audienceMeasuring terrestrial gross CO$_2$ fluxes at large scales presents one of the main challenges in global carbon cycle research. The oxygen isotopic composition ($\delta^{18}$O) of atmospheric CO$_2$ offers the possibility to partition net CO$_2$ fluxes into photosynthesis and respiration at ecosystem, regional and global scales. This approach relies on a detailed knowledge of the $\delta^{18}$O signature of the terrestrial gross CO$_2$ fluxes. The latter reflects the $\delta^{18}$O of leaf and soil water because CO$_2$ exchanges isotopically with water. This exchange can be accelerated by the enzyme carbonic anhydrase (CA). The high CA content in leaves of plants amplifies the impact of leaf photosynthesis on the $\delta^{18}$O of atmospheric CO$_2$ (deltaa) by enhancing the equilibration of atmospheric CO$_2$ with isotopically enriched leaf water. Here, we report that the accelerated isotopic exchange between CO$_2$ and water due to CA activity may be a widespread phenomenon in soils as well. Across a range of ecosystems, we found that CO$_2$ hydration was 10 to 300 times faster than the uncatalysed rate, with highest values in the hottest ecosystems. At the global scale, accounting for soil CA activity dramatically shifts the influence of soil and leaf fluxes on $\delta_a$, thus changing the estimates of terrestrial gross CO$_2$ fluxes. At a time when new laser technologies are poised to deliver more extensive data coverage of variations in $\delta_a$, our finding indicates that deltaa signals should enable us to constrain CO$_2$ gross fluxes in regions where this information has been particularly difficult to obtain, such as in the tropics

Amplified drought induced by climate change reduces seedling emergence and increases seedling mortality for two Mediterranean perennial herbs

International audienc

Plant litter mixture partly mitigates the negative effects of extended drought on soil biota and litter decomposition in a Mediterranean oak forest

International audienceA major challenge of current ecological research is to determine the responses of plant and animal communities and ecosystem processes to future environmental conditions. Ecosystems respond to climate change in complex ways, and the outcome may significantly depend on biodiversity. We studied the relative effects of enhanced drought and of plant species mixture on soil biota and on litter decomposition in a Mediterranean oak forest. We experimentally reduced precipitation, accounting for seasonal precipitation variability, and created a single-species litter (Quercus pubescens), a two-species litter mixture (Q. pubescens + Acer monspessulanum) and a three-species litter mixture (Q. pubescens + A. monspessulanum + Cotinus coggygria).In general, drier conditions affected decomposers negatively, directly by reducing fungal biomass and detritivorous mesofauna, and also indirectly by increasing the predation pressure on detritivorous mesofauna by predatory mesofauna. This is reflected under drier conditions in that Collembola abundance decreased more strongly than Acari abundance. One Collembola group (i.e. Neelipleona) even disappeared completely. Increased drought strongly decreased litter decomposition rates. Mixed litter with two and three plant species positively affected soil biota communities and led to a more efficient litter decomposition process, probably through a greater litter quality. Faster decomposition in mixed litter can thus compensate slower decomposition rates under drier condition. Synthesis. Our results highlight that, within our study system, drier climate strongly impacts on soil biodiversity and hence litter decomposition. Species-rich litter may mitigate such a decline in decomposition rates. Diverse plant communities should hence be maintained to reduce shifts in ecosystem functioning under climate change

Retrieval of chlorophyll fluorescence from a large distance using oxygen absorption bands

The detection of solar induced chlorophyll fluorescence (SIF) in the field with spectrometers is based on the depth of the solar Fraunhofer or oxygen absorption lines in the upwelling radiance compared to that in the downwelling irradiance. This relative depth enables the differentiation of SIF from the reflected radiation. Recent studies have shown that if oxygen bands are used to retrieve SIF from tower-based measurements, then atmospheric correction is required. This study presents a band shape fitting (BSF) approach to retrieve both the relative optical path length (deepening) and SIF (infilling) from field measurements at the same time, using information in the measured spectral shape of the O2 feature. This approach is an alternative to using radiative transfer process models for estimating atmospheric transmittance. The method was applied to measurements taken from 100 m elevation above a forest, yielding plausible results for SIF in the O2A and O2B bands. The sensitivity to combined atmospheric and instrument characteristics prohibits application at much greater distances from the surface

First field record of 17O-excess of atmospheric vapor measured by cavity ring-down spectroscopy

International audienc