Abstract. Rain recharges soil water storages and either percolates
downward into aquifers and streams or is returned to the atmosphere through
evapotranspiration. Although it is commonly assumed that summer rainfall
recharges plant-available water during the growing season, the seasonal
origins of water used by plants have not been systematically explored. We
characterize the seasonal origins of waters in soils and trees by comparing
their midsummer isotopic signatures (δ2H) to seasonal isotopic
cycles in precipitation, using a new seasonal origin index. Across 182 Swiss
forest sites, xylem water isotopic signatures show that summer rain was not
the predominant water source for midsummer transpiration in any of the three
sampled tree species. Beech and oak mostly used winter precipitation, whereas
spruce used water of more diverse seasonal origins. Even in the same plots,
beech consistently used more winter precipitation than spruce, demonstrating
consistent niche partitioning in the rhizosphere. All three species' xylem
water isotopes indicate that trees used more winter precipitation in drier
regions, potentially mitigating their vulnerability to summer droughts. The
widespread occurrence of winter isotopic signatures in midsummer xylem
implies that growing-season rainfall may have minimally recharged the soil
water storages that supply tree growth, even across diverse humid climates
(690–2068 mm annual precipitation). These results challenge common
assumptions concerning how water flows through soils and is accessed by
trees. Beyond these ecological and hydrological implications, our findings
also imply that stable isotopes of δ18O and δ2H in plant
tissues, which are often used in climate reconstructions, may not reflect
water from growing-season climates.
a b s t r a c tTo derive O 3 doseeresponse relationships (DRR) for five European forest trees species and broadleaf deciduous and needleleaf tree plant functional types (PFTs), phytotoxic O 3 doses (PODy) were related to biomass reductions. PODy was calculated using a stomatal flux model with a range of cut-off thresholds (y) indicative of varying detoxification capacities. Linear regression analysis showed that DRR for PFT and individual tree species differed in their robustness. A simplified parameterisation of the flux model was tested and showed that for most non-Mediterranean tree species, this simplified model led to similarly robust DRR as compared to a species-and climate region-specific parameterisation. Experimentally induced soil water stress was not found to substantially reduce PODy, mainly due to the short duration of soil water stress periods. This study validates the stomatal O 3 flux concept and represents a step forward in predicting O 3 damage to forests in a spatially and temporally varying climate.Crown
The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day-night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly-resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 years. We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Speciesspecific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.
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