Climate projections predict higher precipitation variability with more frequent dry extremes(1). CO2 assimilation of forests decreases during drought, either by stomatal closure(2) or by direct environmental control of sink tissue activities(3). Ultimately, drought effects on forests depend on the ability of forests to recover, but the mechanisms controlling ecosystem resilience are uncertain(4). Here, we have investigated the effects of drought and drought release on the carbon balances in beech trees by combining CO2 flux measurements, metabolomics and (13)CO2 pulse labelling. During drought, net photosynthesis (AN), soil respiration (RS) and the allocation of recent assimilates below ground were reduced. Carbohydrates accumulated in metabolically resting roots but not in leaves, indicating sink control of the tree carbon balance. After drought release, RS recovered faster than AN and CO2 fluxes exceeded those in continuously watered trees for months. This stimulation was related to greater assimilate allocation to and metabolization in the rhizosphere. These findings show that trees prioritize the investment of assimilates below ground, probably to regain root functions after drought. We propose that root restoration plays a key role in ecosystem resilience to drought, in that the increased sink activity controls the recovery of carbon balances.

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Forest ecosystems in central Europe are predicted to face an increasing frequency and severity of summer droughts because of global climate change. European beech and Norway spruce often coexist in these forests with mostly positive effects on their growth. However, their different below-ground responses to drought may lead to differences in ectomycorrhizal (ECM) fungal community composition and functions which we examined at the individual root and ecosystem levels. We installed retractable roofs over plots in Kranzberg Forest (11°39'42″E, 48°25'12″N; 490 m a.s.l.) to impose repeated summer drought conditions and assigned zones within each plot where trees neighboured the same or different species to study mixed species effects. We found that ECM fungal community composition changed and the numbers of vital mycorrhizae decreased for both tree species over 3 drought years (2014-2016), with the ECM fungal community diversity of beech exhibiting a faster and of spruce a stronger decline. Mixed stands had a positive effect on the ECM fungal community diversity of both tree species after the third drought year. Ectomycorrhizae with long rhizomorphs increased in both species under drought, indicating long-distance water transport. However, there was a progressive decline in the number of vital fine roots during the experiment, resulting in a strong reduction in enzyme activity per unit volume of soil. Hydrolytic enzyme activities of the surviving ectomycorrhizae were stable or stimulated upon drought, but there was a large decline in ECM fungal species with laccase activity, indicating a decreased potential to exploit nutrients bound to phenolic compounds. Thus, the ectomycorrhizae responded to repeated drought by maintaining or increasing their functionality at the individual root level, but were unable to compensate for quantitative losses at the ecosystem level. These findings demonstrate a strong below-ground impact of recurrent drought events in forests.

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Tropospheric ozone, one of the most phytotoxic air pollutants, may specially impose in long-lived forest trees substantial reduction in productivity and biomass. European beech saplings grown in lysimeter around areas were used to monitor proteomic changes upon elevated ozone concentrations following four vegetation periods of exposure. A proteome study based on highly sensitive two-dimensional fluorescence difference gel electrophoresis (2-D DIGE) was performed to identify protein changes in European beech, the most important deciduous tree in Central Europe. Main emphasis was on identifying differentially expressed proteins after long-time period of ozone exposure under natural conditions rather than short-term responses or reactions under controlled conditions. Our results clearly demonstrate a response of European beech saplings to long-term ozone fumigation at the protein level. We indicate changes in the protein abundance of 142 protein spots; among them 59 were increased and 83 decreased following three years of elevated ozone exposure. As the first step, 40 proteins were identified by a homology driven mass spectrometric approach. Some of the identified proteins have been previously described in the context of short-term ozone responses in plants, indicating, at least for certain cellular functions, the congruence of plant reactions following short- and long-term ozone exposure. Under elevated ozone exposure, abundance of proteins related to the Calvin cycle and photosynthetic electron transport chain were decreased whereas the abundance of proteins regarding the carbon metabolism/catabolism were increased