•Inter-individual variability of tree drought responses within a stand has received little attention. Here we explore whether the spatial variations in soil/subsoil properties assessed through ...Electrical Resistivity Tomography (ERT) could explain variations in drought response traits among trees.•We used ERT to compute the percent variation in resistivity (PVR) between dry and wet conditions as an indicator of spatial variability in total available water content. PVR was computed in two different depth ranges (0–2 and 2–5 m) for eleven Quercus ilex stools in a Mediterranean forest stand. PVR values were compared to biological traits, including tree water status (predawn water potential (Ψ)), leaf traits (δ13C, leaf mass area (LMA)), and canopy defoliation measured after intense drought.•We found significant correlations between PVR and biological variables. For Ψ, the nature and strength of the correlations vary according to the level of drought intensity. The correlation between Ψ and PVR was positive during well-watered conditions in the upper layer (0–2 m) and during water-limited conditions in the deeper layer (2–5 m). During most severe droughts, however, the Ψ was negatively correlated with PVR in the upper layer. Trees with lower PVR in the upper layer were also associated with water use efficiency (higher δ13C), higher LMA, and a lower level of defoliation after extreme drought.•Overall, our results indicate that local differences in soil/subsoil properties affect tree response to drought and suggest that less favorable soil/subsoil conditions (lower PVR) can lead to lower water stress during the driest period and to lower defoliation after extreme drought. Plausible explanations for this better acclimation include higher stomatal regulation and improved deep soil and subsoil water exploration by trees located in more adverse conditions. We encourage the development of ERT in ecological studies to further explore the interrelated relationships between soil/subsoil, climate, and tree functioning.
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•ERT is used to assess spatial variability in soil/subsoil conditions in a forest.•Soil/subsoil conditions induce contrasted drought responses between individual trees.•Low TAW in shallow layer induce a strong water use efficiency.•High TAW in shallow layer induce more stress during severe drought.
Tree species distribution, and hence forest biodiversity, relies on the reproductive capacity of trees, which is currently affected by climate change. Drought-induced pollen sterility could increase ...as a consequence of more intense and more frequent droughts projected for temperate and Mediterranean regions, and threaten the sexual regeneration of trees in these regions. To evaluate this possibility, we examined the effect of long-term partial rainfall exclusion (-27% precipitation) on male reproductive development in holm oak, Quercus ilex, one of the most important and widespread tree species of the Mediterranean region. We examined anther area, pollen production, pollen abortion as well as viable pollen production in control and dry treatments. Microscopic examinations revealed significant differences in pollen development between trees in the dry and the control treatments, even though anthesis occurred before the onset of annual drought. Our results demonstrate that anthers collected from Q. ilex trees in the dry treatment, which experienced long-term increased drought stress especially during the summer, were the same size as anthers in the control treatment, but displayed 25% pollen abortion and almost 20% reduction in pollen production. Subsequently, the number of viable pollen grains in anthers from dry treatment was 35% less than in control. These results suggest a carry-over effect of drought stress on pollen production that could reduce the reproductive success of Q. ilex. The results have broad implications for better understanding of the determinants of tree reproduction by masting and anticipate the outcomes of expected drought increase in the Mediterranean on forest dynamics.
Mast seeding, the synchronous, highly variable seed production among years, is very common in tree species, but there is no consensus about its main causes and the main environmental factors ...affecting it. In this study, we first analyze a long-term data set on reproductive and vegetative growth of
Quercus ilex
in a mediterranean woodland in order to identify the main environmental drivers of interannual variation in flower and seed production and contrast the impact of climate vs
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adaptive factors as main causes of masting. Second, we conducted an experiment of rainfall exclusion to evaluate the effects of an increasing drought (simulating predictions of global change models) on both reproductive processes. The annual seed crop was always affected by environmental factors related to the precipitation pattern, these abiotic factors disrupting the fruiting process at different periods of time. Seed production was strongly dependent upon water availability for the plant at initial (spring) and advanced (summer) stages of the acorn maturation cycle, whereas the final step of seed development was negatively affected by the frequency of torrential-rain events. We also found clear evidence that seed masting in the study species is not only regulated by selective endogenous rhythms, but is mainly a physiological response to the variable environment. Our results from the rainfall exclusion experiment corroborated the conclusions obtained from the 26-year fruiting record and demonstrated that the high interannual variation in seed crop was mainly determined by the success in seed development rather than by the flowering effort. Under a global change scenario, it could be expected that the drier conditions predicted by climate models reinforce the negative effects of summer drought on seed production, leading to negative consequences for tree recruitment and forest dynamics.
Abstract
Gravitational lensing by clusters of galaxies offers a powerful probe of their structure and mass distribution. Several research groups have developed techniques independently to achieve ...this goal. While these methods have all provided remarkably high-precision mass maps, particularly with exquisite imaging data from the Hubble Space Telescope (HST), the reconstructions themselves have never been directly compared. In this paper, we present for the first time a detailed comparison of methodologies for fidelity, accuracy and precision. For this collaborative exercise, the lens modelling community was provided simulated cluster images that mimic the depth and resolution of the ongoing HST Frontier Fields. The results of the submitted reconstructions with the un-blinded true mass profile of these two clusters are presented here. Parametric, free-form and hybrid techniques have been deployed by the participating groups and we detail the strengths and trade-offs in accuracy and systematics that arise for each methodology. We note in conclusion that several properties of the lensing clusters are recovered equally well by most of the lensing techniques compared in this study. For example, the reconstruction of azimuthally averaged density and mass profiles by both parametric and free-form methods matches the input models at the level of ∼10 per cent. Parametric techniques are generally better at recovering the 2D maps of the convergence and of the magnification. For the best-performing algorithms, the accuracy in the magnification estimate is ∼10 per cent at μtrue = 3 and it degrades to ∼30 per cent at μtrue ∼ 10.
We present a high-precision mass model of galaxy cluster Abell 2744, based on a strong gravitational-lensing analysis of the Hubble Space Telescope Frontier Fields (HFF) imaging data, which now ...include both Advanced Camera for Surveys and Wide Field Camera 3 observations to the final depth. Taking advantage of the unprecedented depth of the visible and near-infrared data, we identify 34 new multiply imaged galaxies, bringing the total to 61, comprising 181 individual lensed images. In the process, we correct previous erroneous identifications and positions of multiple systems in the northern part of the cluster core. With the lenstool software and the new sets of multiple images, we model the cluster using two cluster-scale dark matter haloes plus galaxy-scale haloes for the cluster members. Our best-fitting model predicts image positions with an rms error of 0.79 arcsec, which constitutes an improvement by almost a factor of 2 over previous parametric models of this cluster. We measure the total projected mass inside a 200 kpc aperture as (2.162 ± 0.005) × 1014 M⊙, thus reaching 1 per cent level precision for the second time, following the recent HFF measurement of MACSJ0416.1−2403. Importantly, the higher quality of the mass model translates into an overall improvement by a factor of 4 of the derived magnification factor. Together with our previous HFF gravitational lensing analysis, this work demonstrates that the HFF data enables high-precision mass measurements for massive galaxy clusters and the derivation of robust magnification maps to probe the early Universe.
•An holm oak coppice was treated by thinning and rainfall exclusion over 15 years.•Drought increased the mortality of large trees and slowed stand basal area increment.•Thinning improved growth and ...survival even under rainfall exclusion.•Size selection and competition release both accounted for thinning effects.•Thinning increased stand basal area increment without further losses of unique genotypes.
In northern Mediterranean forests, increasing drought stress due to the on-going climate change is combined with stand ageing due to the lack of management. Management by thinning may alleviate drought stress by reducing competition, but its application is challenging in coppices of resprouting species where its long-term consequences for tree demography and stand dynamics are difficult to evaluate. In this study, we investigate the long-term (15 years) demographic responses of holm oak (Quercus ilex L.) to a combination of thinning from below (−30% basal area) and experimental rainfall exclusion (−27% precipitation). Stem growth, survival and resistance to an extreme drought event were positively linked to both stem size and local competition release after thinning. Thinning improvement of growth and survival were thus due to both a selection of the biggest, most vigorous, trees and to a release of competition for water. Rainfall exclusion, on the other hand, led to a shift of the tree size-mortality relationship, which resulted in the death of bigger trees, in a faster loss of stool density and in a slower evolution of the stand basal area compared to the control. Thinning was beneficial by cancelling the rainfall exclusion effects on growth and mortality, and by doubling the stand basal area increment compared to unthinned control. The initial loss of stools due to thinning was compensated by a lower mortality, suggesting that thinning do not reduce further the amount of unique genotypes on the long-term. Positive thinning effects on stem growth decreased over time but remained significant 15 years after thinning, while resprouting dynamics strongly decreased with time. These results indicate that moderate thinning from below is a relevant strategy to increase stem vitality and stand production in old coppices, particularly in a context of a chronic rise in drought stress and more frequent extreme drought episodes.
We present a high-precision mass model of the galaxy cluster MACSJ0416.1-2403, based on a strong-gravitational-lensing analysis of the recently acquired Hubble Space Telescope Frontier Fields (HFF) ...imaging data. Taking advantage of the unprecedented depth provided by HST/Advanced Camera for Survey observations in three passbands, we identify 51 new multiply imaged galaxies, quadrupling the previous census and bringing the grand total to 68, comprising 194 individual lensed images. Having selected a subset of the 57 most securely identified multiply imaged galaxies, we use the lenstool software package to constrain a lens model comprised of two cluster-scale dark-matter haloes and 98 galaxy-scale haloes. Our best-fitting model predicts image positions with an rms error of 0.68 arcsec, which constitutes an improvement of almost a factor of 2 over previous, pre-HFF models of this cluster. We find the total projected mass inside a 200 kpc aperture to be (1.60 ± 0.01) x 10^sup 14^ M.., a measurement that offers a three-fold improvement in precision, reaching the per cent level for the first time in any cluster. Finally, we quantify the increase in precision of the derived gravitational magnification of high-redshift galaxies and find an improvement by a factor of ~2.5 in the statistical uncertainty. Our findings impressively confirm that HFF imaging has indeed opened the domain of high-precision mass measurements for massive clusters of galaxies.(ProQuest: ... denotes formulae/symbols omitted.)
In the Mediterranean basin, precipitation is expected to decline as a consequence of climate change. The response of a Quercus ilex forest in southern France to such a decline in water availability ...was studied using a 4-year throughfall exclusion experiment. Seasonal courses of sap flow and leaf water potential were obtained from 2004 to 2007 and used to characterize tree water relations in a control and a dry treatment. The experiment reduced the average precipitation input to the soil by 29%, and resulted in a 23% reduction in annual transpiration. Soil water potential was significantly lower in the dry treatment only during summer drought, but transpiration was reduced all year round even during well-watered periods. Despite a tight stomatal control over transpiration, whole-tree hydraulic conductance was found to be lower in the trees growing in the driest conditions. This reduction in water transport capacity was observed jointly with a reduction in leaf transpiring area. Canopy leaf area decreased by 18% in the dry treatment as a consequence of the throughfall exclusion, which was found to validate the ecohydrological equilibrium theory.
We present a joint optical/X-ray analysis of the massive galaxy cluster Abell 2744 (z = 0.308). Our strong- and weak-lensing analysis within the central region of the cluster, i.e. at R < 1 Mpc from ...the brightest cluster galaxy, reveals eight substructures, including the main core. All of these dark matter haloes are detected with a significance of at least 5s and feature masses ranging from 0.5 to 1.4 x 10 super( 14) M... within R < 150 kpc. Merten et al. and Medezinski et al. substructures are also detected by us. We measure a slightly higher mass for the main core component than reported previously and attribute the discrepancy to the inclusion of our tightly constrained strong-lensing mass model built on Hubble Frontier Fields data. X-ray data obtained by XMM-Newton reveal four remnant cores, one of them a new detection, and three shocks. Unlike Merten et al., we find all cores to have both dark and luminous counterparts. A comparison with clusters of similar mass in the Millennium XXL simulations yields no objects with as many massive substructures as observed in Abell 2744, confirming that Abell 2744 is an extreme system. We stress that these properties still do not constitute a challenge to ... cold dark matter, as caveats apply to both the simulation and the observations: for instance, the projected mass measurements from gravitational lensing and the limited resolution of the subhaloes finders. We discuss implications of Abell 2744 for the plausibility of different dark matter candidates and, finally, measure a new upper limit on the self-interaction cross-section of dark matter of sDM < 1.28 cm super( 2) g super( -1) (68 per cent CL), in good agreement with previous results from Harvey et al. (ProQuest: ... denotes formulae/symbols omitted.)
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