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Abstract: 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. [ABSTRACT FROM AUTHOR]

The partitioning of photosynthates toward biomass compartments plays a crucial role in the carbon (C) sink function of forests. Few studies have examined how carbon is allocated toward plant compartments in drought-prone forests. We analyzed the fate of gross primary production (GPP) in relation to yearly water deficit in an old evergreen Mediterranean Quercus ilex coppice severely affected by water limitations. Carbon fluxes between the ecosystem and the atmosphere were measured with an eddy co-variance flux tower running continuously since 2001. Discrete measurements of litterfall, stem growth and FAPAR allowed us to derive annual productions of leaves, wood, flowers and acorns, and an isometric relationship between stem and belowground biomass has been used to estimate perennial belowground growth. By combining eddy covariance fluxes with annual net primary productions (NPP), we managed to close a C budget and derive values of autotrophic, heterotrophic respirations and carbon-use efficiency (CUE; the ratio between NPP and GPP). Average values of yearly net ecosystem production (NEP), GPP and Reco were 282, 1259 and 977g C m-2. The corresponding aboveground net primary production (ANPP) components were 142.5, 26.4 and 69.6 g C m-2 for leaves, reproductive effort (flowers and fruits) and stems, respectively. NEP, GPP and Reco were affected by annual water deficit. Partitioning to the different plant compartments was also impacted by drought, with a hierarchy of responses going from the most affected - the stem growth - to the least affected - the leaf production. The average CUE was 0.40, which is well in the range for Mediterranean-type forest ecosystems. CUE tended to decrease less drastically in response to drought than GPP and NPP did, probably due to drought acclimation of autotrophic respiration. Overall, our results provide a baseline for modeling the inter-annual variations of carbon fluxes and allocation in this widespread Mediterranean ecosystem, and they highlight the value of maintaining continuous experimental measurements over the long term. [ABSTRACT FROM AUTHOR]

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. 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 FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]