Abstract1. More frequent drought episodes are expected to cause higher mortality in isohydric tree species such as pines, because individuals close their stomata early during drought in order to maintain constant needle water potentials. It has been suggested that trees delay the ensuing carbon starvation by actively storing carbon at the expense of growth ("bet hedging"). Because such a strategy is only adaptive in drought-prone regions, we hypothesise that the degree of carbon "bet hedging"should differ between ecotypes.2. We repeatedly measured the allocation of biomass, starch and soluble sugars to needles, stem and roots in seedlings of nine populations of Pinus sylvestris and Pinus nigra along a gradient from Central Europe to the Mediterranean. During two consecutive growing seasons, seedlings grown from seed were exposed to factorial combinations of 4 months of drought (D1, D2) and ambient/elevated CO 2 (aCO 2 /eCO 2 ).3. Drought-stressed pine seedlings did neither increase starch concentrations, nor change biomass production or experience lower mortality under eCO 2 compared to aCO 2 . By the end of D2, seedlings from drier origin had accumulated more starch but at the same time also more biomass than seedlings from wetter origin. 4. Surprisingly, seedlings acclimatised to dry conditions after D1 so that mortality dropped to zero and drought effects on needle starch (P. sylvestris) and overall starch (P. nigra), respectively, disappeared after D2. 5.Synthesis. The absence of a trade-off between carbon storage (starch) and growth (biomass), and the patterns of mortality observed in seedlings growing under combined drought and eCO 2 do not support the theory of carbon "bet hedging" in isohydric Pinus sylvestris and Pinus nigra. Results suggest that reduced growth and acclimatisation minimised seedling mortality in the second year. Acclimatisation might thus enable pine seedlings to resist a moderate increase in summer drought frequency expected in the future. K E Y W O R D SC-allocation, C-storage, CO 2 , drought mortality, ecophysiology, non-structural carbohydrates, P. nigra, P. sylvestris, stomatal conductancePaper previously published as Standard Paper

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-To investigate whether landscape structures act as insurmountable barriers for foraging bees, we conducted mark-recapture studies with two pollen-specialist solitary species. Foraging options of the bees were confined to host plant stands across different landscape structures. Differences in altitude of more than 130 m were overcome and forests covering a distance of up to 480 m were crossed by Chelostoma florisomne. A broad river and a motorway with intense traffic did not represent insurmountable barriers for Hoplitis adunca. For C. florisomne, total foraging distances of up to 650 m were measured, but foraging females were recorded predominantly on host plant patches available in relatively close vicinity to their nesting site. While landscape structures might impede foraging in endangered bees, the investigated landscape structures clearly did not act as insurmountable physical barriers for the two common solitary bee species tested in our study. landscape barrier / fragmentation / foraging distance / Chelostoma florisomne / Hoplitis adunca

Climate change alters both water and CO2 availability for plants, but it is largely unknown how they interact with light to affect tree seedling establishment and early growth. Light availability is often regulated by forest management, thus understanding how these resources co‐limit the regeneration success of tree species and populations with contrasting drought tolerances is essential for adaptive forest management and particularly for assisted migration. We studied biomass partitioning of 3‐year‐old Scots pine (Pinus sylvestris) and European black pine (Pinus nigra) seedlings in response to combined effects of light (22% and 40% shade), soil water availability (moist and dry conditions) and CO2 (ambient and elevated), and examined the responses of seedlings from Central Alpine and Mediterranean origin. Seedlings of nine populations with varying drought tolerances were grown in a common garden in the European Central Alps. Shoot height, vertical root length, shoot and root biomass of the plants were assessed at the end of the third growing season. Under 40% shade and dry conditions, P. sylvestris seedlings severely reduced shoot biomass, resulting in an increased specific shoot height (SSH) compared to seedlings under 22% shade and moist conditions. In contrast, P. nigra seedlings retained a constant shoot biomass under all treatment combinations. Seedlings from drier origin were generally larger, heavier and had longer vertical roots than those from wetter locations. In order to keep up shoot height, seedlings from wetter origins disproportionately increased SSH under shaded conditions compared to populations from drier origin. Synthesis and applications. Under high light availability, Scots pine (Pinus sylvestris) and European black pine (Pinus nigra) seedlings were well adapted to dry conditions. Moderate shading, however, substantially reduced Scots pine but not black pine growth, and potentially amplified the vulnerability of Scots pine seedlings to drought. Optimising light conditions in forests, for example by thinning, may thus enhance early Scots pine regeneration in a drier future climate.

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1. Warming and drought alter plant phenology, photosynthesis and growth with important consequences for the global carbon cycle and the earth's climate. Yet, few studies have attempted to tease apart their effects on tree phenology, particularly leaf senescence, and on source and sink activity.2. We experimentally assessed the single and combined effects of warming and reduced soil moisture on the phenology (leaf-out and senescence date, growing season length) and above-ground sink (height and diameter growth, leaf area and Huber values) and source activity (net photosynthesis, photosynthetic efficiency, chlorophyll concentration and total carbon [C] uptake) of two tree species with distinct strategies to deal with drought: European beech and pubescent oak.3. Warming advanced leaf-out, irrespective of soil moisture levels, particularly in oak and to a lower extent in beech, leading to a prolonged growing season in oak but not beech. No impacts of warming on senescence timing were found for both species. Reduced moisture had little impact on the phenology of both species. Warming-induced advances in phenology and higher photosynthetic efficiency increased the annual C uptake for oak and compensated for the reduced photosynthetic activity in the presence of reduced moisture. Conversely, for beech, source activity, including yearly C uptake, was lower in all treatments than the control, indicating no compensation of the C budget by phenological shifts. Synthesis.Our results demonstrate that a warming-driven earlier activity and higher photosynthetic efficiency compensates for reduced photosynthesis during hot and dry periods, but only for pubescent oak, which is a rather droughttolerant species. Current predictions of warming-induced mitigation effects through extended C uptake seem incorrect for beech.

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Summary Bioclimatic envelope models have predicted latitudinal range shifts of tree species in Europe following climate change. Accordingly, Mediterranean species will be able to migrate northwards if climatic conditions become warmer and dryer. Frost, on the other hand, is an important and recurring factor in temperate and boreal regions causing damage to buds and leaves, and potentially limiting the survival of Mediterranean tree species or populations at higher latitudes. Since species distribution models rely on average climatic parameters, they may underestimate the risk of frost damage from low temperature extremes. We measured the cold hardiness of Pinus sylvestris, Pinus nigra and Pinus halepensis seedlings from a total of 11 European populations growing in a common garden in a cold Central Alpine valley on seven dates between February and July 2013. On each date, needles were artificially frozen at several temperatures and the temperature estimated at which 50% of the needle tissue is damaged (LT50; relative electrolyte leakage). Cold hardiness did not differ between populations of the same species and was not related to the minimum temperatures at the seed origin. In comparison with deciduous trees, Mediterranean P. sylvestris and P. nigra maintained extremely wide safety margins against frost throughout late winter and spring. By contrast, safety margins of P. halepensis were much narrower until March and winter cold hardiness was in the range of regularly recurring low temperature events in Central Europe. According to the measured LT50 values, the migration of a wide range of drought‐tolerant populations of P. sylvestris and P. nigra from the Mediterranean to Central and Western Europe is not limited by intermittent cold temperature extremes in spring. They are notably as well adapted to frost as populations from Central Alpine origin. Differences in dehardening patterns between species demonstrate the importance of analysing cold hardiness repeatedly during potentially sensitive periods in order to predict species range shifts in the context of climatic change.