Growth cessation uncouples isotopic signals in leaves and tree rings of drought-exposed oak trees.

An increase in temperature along with a decrease in summer precipitation in Central Europe will result in an increased frequency of drought events and gradually lead to a change in species composition in forest ecosystems. In the present study, young oaks (Quercus robur L. and Quercus petraea (Matt.) Liebl.) were transplanted into large mesocosms and exposed for 3 years to experimental warming and a drought treatment with yearly increasing intensities. Carbon and oxygen isotopic (δ(13)C and δ(18)O) patterns were analysed in leaf tissue and tree-ring cellulose and linked to leaf physiological measures and tree-ring growth. Warming had no effect on the isotopic patterns in leaves and tree rings, while drought increased δ(18)O and δ(13)C. Under severe drought, an unexpected isotopic pattern, with a decrease in δ(18)O, was observed in tree rings but not in leaves. This decrease in δ(18)O could not be explained by concurrent physiological analyses and is not supported by current physiological knowledge. Analysis of intra-annual tree-ring growth revealed a drought-induced growth cessation that interfered with the record of isotopic signals imprinted on recently formed leaf carbohydrates. This missing record indicates isotopic uncoupling of leaves and tree rings, which may have serious implications for the interpretation of tree-ring isotopes, particularly from trees that experienced growth-limiting stresses.

Nitrogen dynamics in oak model ecosystems subjected to air warming and drought on two different soils.

Being tolerant to heat and drought, oaks are promising candidates for future forestry in view of climate change in Central Europe. Air warming is expected to increase, and drought decrease soil N availability and thus N supply to trees. Here, we conducted a model ecosystem experiment, in which mixed stands of young oaks (Quercus robur, Q. petraea and Q. pubescens) were grown on two different soils and subjected to four climate treatments during three growing seasons: air warming by 1-2 °C, drought periods (average precipitation reduction of 43-60%), a combination of these two treatments, and a control. In contrast to our hypotheses, neither air warming nor drought significantly affected N availability, whereas total amounts, vertical distribution and availability of soil N showed substantial differences between the two soils. While air warming had no effect on tree growth and N accumulation, the drought treatment reduced tree growth and increased, or tended to increase, N accumulation in the reduced biomass, indicating that growth was not limited by N. Furthermore, (15) N-labelling revealed that this accumulation was associated with an increased uptake of nitrate. On the basis of our results, climate change effects on N dynamics are expected to be less important in oak stands than reduced soil water availability.

Nitrogen partitioning in oak leaves depends on species, provenance, climate conditions and soil type.

Climate-tolerant tree species and/or provenances have to be selected to ensure the high productivity of managed forests in Central Europe under the prognosticated climate changes. For this purpose, we studied the responses of saplings from three oak species (i.e. Quercus robur, Q. petraea and Q. pubescens) and provenances of different climatic origin (i.e. low or high rainfall, low or high temperature habitats) with regard to leaf nitrogen (N) composition as a measure of N nutrition. Saplings were grown in model ecosystems on either calcareous or acidic soil and subjected to one of four treatments (control, drought, air warming or a combination of drought and air warming). Across species, oak N metabolism responded to the influence of drought and/or air warming with an increase in leaf amino acid N concentration at the expense of structural N. Moreover, provenances or species from drier habitats were more tolerant to the climate conditions applied, as indicated by an increase in amino acid N (comparing species) or soluble protein N (comparing provenances within a species). Furthermore, amino acid N concentrations of oak leaves were significantly higher on calcareous compared to acidic soil. From these results, it can be concluded that seeds from provenances or species originating from drier habitats and - if available - from calcareous soil types may provide a superior seed source for future forest establishment.

Physiological and biochemical responses of Quercus pubescens to air warming and drought on acidic and calcareous soils.

The drought- and thermo-tolerant Quercus pubescens, a tree species growing on both acidic and calcareous soils in the sub-Mediterranean region, was exposed to soil drought (-60% to -80% soil water content) and air warming (+1.2 °C daytime temperature), singly and in combination. The experiment was conducted on two natural forest soils with similar texture but different pH (acidic and calcareous soils). The physiological (photosynthesis) and biochemical (antioxidant system) responses of Q. pubescens were investigated. On acidic soil, Q. pubescens had a higher reactive oxygen species (ROS) content than on calcareous soil, confirming that this species is better adapted to the latter soil type. A down-regulation of ascorbate-glutathione cycle enzymes suggests that ROS were used as signalling molecules. Air warming stimulated stomatal opening, while soil drought induced stomatal closure in the late afternoon and reduced Rubisco carboxylation efficiency. Photosynthetic performance in the combined treatment was higher than under single drought stress and similar to control and air warming. Q. pubescens biochemical responses depended on soil pH. On acidic soil, Q. pubescens trees exposed to air warming used ROS as signalling molecules. On calcareous soil, these trees were able to balance both soil drought and air warming stress, avoiding ROS toxic effects by increasing antioxidant enzyme activitiy and maintaining a high enzymatic antioxidant defence. When combined, drought and air warming induced either more severe (higher oxidative pressure and impairment of the light-harvesting complex) or different responses (decline of the thermal energy dissipation capacity) relative to the single stressors. Overall, however, Q. pubescens preserved the functionality of the photosynthetic apparatus and controlled the antioxidant system response, thus confirming its drought and thermo-tolerance and therefore its potential to adapt to the ongoing climate change.

Water regime and growth of young oak stands subjected to air-warming and drought on two different forest soils in a model ecosystem experiment.

Global climate change is expected to increase annual temperatures and decrease summer precipitation in Central Europe. Little is known of how forests respond to the interaction of these climate factors and if their responses depend on soil conditions. In a 3-year lysimeter experiment, we investigated the growth response of young mixed oak stands, on either acidic or calcareous soil, to soil water regime, air-warming and drought treatments corresponding to an intermediate climate change scenario. The air-warming and drought treatments were applied separately as well as in combination. The air-warming treatment had no effect on soil water availability, evapotranspiration or stand biomass. Decreased evapotranspiration from the drought-exposed stands led to significantly higher air and soil temperatures, which were attributed to impaired transpirational cooling. Water limitation significantly reduced the stand foliage, shoot and root biomass as droughts were severe, as shown in low leaf water potentials. Additional air warming did not enhance the drought effects on evapotranspiration and biomass, although more negative leaf water potentials were observed. After re-watering, evapotranspiration increased within a few days to pre-drought levels. Stands not subjected to the drought treatment produced significantly less biomass on the calcareous soil than on the acidic soil, probably due to P or Mn limitation. There was no difference in biomass and water regime between the two soils under drought conditions, indicating that nutrient availability was governed by water availability under these conditions. The results demonstrate that young oak stands can cope with severe drought and therefore can be considered for future forestry.

Foliage response of young central European oaks to air warming, drought and soil type.

Three Central European oak species, with four provenances each, were experimentally tested in 16 large model ecosystem chambers for their response to passive air warming (AW, ambient +1-2 °C), drought (D, -43 to -60% irrigation) and their combination (AWD) for 3 years on two forest soil types of pH 4 or 7. Throughout the entire experiment, the influence of the different ambient and experimental climates on the oak trees was strong. The morphological traits of the Quercus species were affected in opposing ways in AW and D treatments, with a neutral effect in the AWD treatment. Biochemical parameters and LMA showed low relative plasticity compared to the morphological and growth parameters. The high plasticity in physiologically important parameters of the three species, such as number of intercalary veins or leaf size, indicated good drought acclimation properties. The soil type influenced leaf chlorophyll concentration, C/N and area more than drought, whereas foliage mass was more dependent on drought than on soil type. Through comparison of visible symptom development with the water deficits, a drought tolerance threshold of -1.3 MPa was determined. Although Q. pubescens had xeromorphic leaf characteristics (small leaf size, lower leaf water content, high LMA, pilosity, more chlorophyll, higher C/N) and less response to the treatments than Q. petraea and Q. robur, it suffered more leaf drought injury and shedding of leaves than Q. petraea. However, if foliage mass were used as the criterion for sustainable performance under a future climate, Q. robur would be the most appropriate species.

Seasonal photosynthetic responses of European oaks to drought and elevated daytime temperature.

Oaks are commonly considered as drought- and heat-tolerant trees that might benefit from a warmer and drier climate. Their tolerance to drought has been frequently studied in the past, whereas studies dealing with elevated temperature or its combination with drought are very limited in number. In this study we investigated seasonal photosynthetic patterns in three European oak species (Quercus robur, Q. petraea, Q. pubescens) exposed in lysimeter-based open-top chambers (OTC) to elevated daytime temperature, drought and their combination. Stomatal and non-stomatal traits of photosynthesis were followed over an entire growing season and related to changes in daytime temperature, soil moisture and pre-dawn leaf water potential (Ψ(PD) ). Elevated daytime temperature enhanced net photosynthesis (P(N) ) in a season-dependent manner, with higher mid-summer rates than in controls exposed to ambient temperature. Drought imposed in early and mid-summer reduced the soil moisture content and caused a gradual decline in Ψ(PD) , stomatal conductance (g(S) ) and P(N) . Drought effects on Ψ(PD) and P(N) were exacerbated when drought was combined with elevated daytime temperature. In general, P(N) tended to be more affected by low soil moisture content or low Ψ(PD) in Q. robur than in Q. petraea and Q. pubescens. Non-stomatal limitations may have contributed to the drought-induced decline of P(N) in Q. robur, as indicated by a down-regulation of PSII photochemistry (F(V) /F(M) ) and decreased chlorophyll content. Taken together, our findings show that European oaks may benefit from elevated temperature, but detrimental effects can be expected when elevated temperature occurs simultaneously with drought.