Deuterium labelling of roots provides evidence of deep water access and hydraulic lift by Pinus nigra in a Mediterranean forest of NE Spain

We studied the tree access to deep water sources and the possibility of hydraulic lift from the deep roots of a Pinus nigra tree to the shallow soil layers in a Mediterranean forest of NE Spain. We also studied the use of hydraulically lifted water by neighboring trees, shrubs, and sprouts. We enriched the roots of a large P. nigra (10 m tall) with deuterium by accessing them from a below ground cave. During the next 3 days we measured stable deuterium isotopic composition of xylem sap, shoot predawn and midday water potentials, and the leaf δ¹³C and δ¹⁵N of the P. nigra tree, neighboring Quercus ilex ballota trees and sprouts, and Juniperus oxycedrus shrubs. The study was conducted both in dry summer and in wet spring. In summer, deuterated water absorbed by deep roots of P. nigra appeared in the stem water of neighboring plants and in surface soil. The most δD-enriched plant xylem sap was found in the enriched P. nigra tree, followed by the Q. ilex sprouts, the small Q. ilex trees and the surface soil (15 cm). All these trends disappeared in the wet spring season, when HDO only slightly appeared in the surface soil. The results show that the studied P. nigra tree accesses deep water source and conducts hydraulic lift in this Mediterranean forest in dry summer but not necessarily in wet spring.     

Hydraulic Lift in Cork Oak Trees in a Savannah-Type Mediterranean Ecosystem and its Contribution to the Local Water Balance

The aim of this study was to identify the sources and depth of water uptake by 15-years old Quercus suber L. trees in southern Portugal under a Mediterranean climate, measuring δ¹⁸O and δD in the soil–plant-atmosphere continuum. Evidence for hydraulic lift was substantiated by the daily fluctuations observed in Ψ_s at 0.4 and 1 m depth and supported by similar δ¹⁸O values found in tree xylem sap, soil water in the rhizosphere and groundwater. From 0.25 m down to a depth of 1 m, δD trends differed according to vegetation type, showing a more depleted value in soil water collected under the evergreen trees (−47‰) than under dead grasses (−35‰). The hypothesis of a fractionation process occurring in the soil due to diffusion of water vapour in the dry soil is proposed to explain the more depleted soil δD signature observed under trees. Hydraulically lifted water was estimated to account for 17–81% of the water used during the following day by tree transpiration at the peak of the drought season, i.e., 0.1–14 L tree⁻¹ day⁻¹. Significant relationships found between xylem sap isotopic composition and leaf water potential in early September emphasized the positive impact of the redistribution of groundwater in the rhizosphere on tree water status.     

Indications of Hydraulic Lift by Pinus halepensis and Its Effects on the Water Relations of Neighbour Shrubs

We measured the stable deuterium isotopic composition of xylem sap, the shoot predawn and midday water potentials, and the leaf δ¹³C of Mediterranean shrubs Pistacia lentiscus, Globularia alypum and Rosmarinus officinalis in a south-oriented transect from a large (12 m tall) Aleppo pine tree, Pinus halepensis. We aimed to study the possibility of hydraulic lift from the deep roots of this pine tree to the shallow soil layers and its influence on these neighbour shrubs. These same traits were also studied in several individuals of the shrub Pistacia lentiscus growing with different types of neighbours: just shrubs, a small (3 – 4 m) pine tree, and the above mentioned large pine tree. The greater the distance from P. halepensis the plants grew, the higher xylem water δD, the lower the soil water content, and, the lower the predawn and midday water potentials were found. These results suggest the existence of an hydraulic lift from deep roots to shallow soil in this big tree. Further indication of this existence is provided by the improved water status of P. lentiscus (higher water potentials and δD, and lower δ¹³C and, therefore, lower water use efficiencies) when growing close to the big pine in comparison with the same shrub species growing close to small pines or just surrounded by other shrubs. Moreover, all these trends occurred in the dry summer season, but disappeared in the wet spring season. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance of European tree species to drought stress in mixed versus pure forests: evidence of stress release by inter‐specific facilitation

While previous studies focused on tree growth in pure stands, we reveal that tree resistance and resilience to drought stress can be modified distinctly through species mixing. Our study is based on tree ring measurement on cores from increment boring of 559 trees of Norway spruce (Picea abies [L.] Karst.), European beech (Fagus sylvatica [L.]) and sessile oak (Quercus petraea (Matt.) Liebl.) in South Germany, with half sampled in pure, respectively, mixed stands. Indices for resistance, recovery and resilience were applied for quantifying the tree growth reaction on the episodic drought stress in 1976 and 2003. The following general reaction patterns were found. (i) In pure stands, spruce has the lowest resistance, but the quickest recovery; oak and beech were more resistant, but recover was much slower and they are less resilient. (ii) In mixture, spruce and oak perform as in pure stands, but beech was significantly more resistant and resilient than in monoculture. (iii) Especially when mixed with oak, beech is facilitated. We hypothesise that the revealed water stress release of beech emerges in mixture because of the asynchronous stress reaction pattern of beech and oak and a facilitation of beech by hydraulic lift of water by oak. This facilitation of beech in mixture with oak means a contribution to the frequently reported overyield of beech in mixed versus pure stands. We discuss the far‐reaching implications that these differences in stress response under intra‐ and inter‐specific environments have for forest ecosystem dynamics and management under climate change.     

Carbon isotope discrimination and wood anatomy variations in mixed stands of Quercus robur and Quercus petraea

The two most common oak species in western Europe, Quercus robur and Quercus petraea, display different ecological behaviours, particularly with respect to their responses to drought. The ecophysiological basis of this niche difference is not understood well. Here we test the hypothesis that these two species present distinct water use efficiencies (WUEs), using the carbon isotope discrimination approach. Leaves and 13 dated ring sequences were sampled in 10 pairs of adult trees growing side by side. Carbon isotope composition was measured on cellulose extracts. In addition, relationships between carbon isotope discrimination and wood anatomy were assessed at the tree level. Quercus robur displayed a 1·0‰ larger isotopic discrimination than Q. petraea, and therefore a lower intrinsic WUE (?13%). This interspecific difference of isotopic discrimination was quite stable with time and independent of tree radial growth and climate fluctuations. A strong positive correlation was observed between average tree values of earlywood vessel surface area and ¹³C isotopic discrimination. This correlation was even higher with ¹³C of the 1976 dry year (r = 0·86). These observations led to the hypothesis that hydraulic properties of xylem could exert a constraint on leaf gas exchange, resulting in a larger WUE for individuals with smaller vessel cross‐section area.     

Below-ground competition between trees and grasses may overwhelm the facilitative effects of hydraulic lift

Under large East African Acacia trees, which were known to show hydraulic lift, we experimentally tested whether tree roots facilitate grass production or compete with grasses for below‐ground resources. Prevention of tree–grass interactions through root trenching led to increased soil water content indicating that trees took up more water from the topsoil than they exuded via hydraulic lift. Biomass was higher in trenched plots compared to controls probably because of reduced competition for water. Stable isotope analyses of plant and source water showed that grasses which competed with trees used a greater proportion of deep water compared with grasses in trenched plots. Grasses therefore used hydraulically lifted water provided by trees, or took up deep soil water directly by growing deeper roots when competition with trees occurred. We conclude that any facilitative effect of hydraulic lift for neighbouring species may easily be overwhelmed by water competition in (semi‐) arid regions.     

Estimating the relative nutrient uptake from different soil depths in Quercus robur, Fagus sylvatica and Picea abies

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb⁺ and NH₄⁺ by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [⁸⁶Rb⁺] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths.     

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradientsin water potential, either between soil and atmosphere (transpiration)or soil layers of different moisture content (termed hydraulicredistribution). The latter has the potential to influence waterbudgets and species interactions, but we lack information forall but a few plant communities. We combined heat pulse measurementsof sap flow with dye and isotope tracing techniques to gaugethe movement of xylem sap within, and exudation from, rootsof Banksia prionotes (Lindley). We demonstrated ‘ hydrauliclift’ during the dry season and provide some evidencethat extremely dry soils limit hydraulic lift. In addition wereport difficulties posed by spiralled xylem tissue in rootsfor the application of heat pulse techniques. Copyright 2000Annals of Botany Company Banksia prionotes, sap flow, hydraulic lift, heat ratio method, deuterium, stable isotopes, root architecture.     

Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

The impact of the 2003 summer drought on the intra-annual growth pattern of beech (Fagus sylvatica L.) and oak (Quercus robur L.) on a dry site in the Netherlands

Climate change is expected to result in more extreme weather conditions over large parts of Europe, such as the prolonged drought of 2003. As water supply is critical for tree growth on many sites in North-Western Europe, such droughts will affect growth, species competition, and forest dynamics. To be able to assess the susceptibility of tree species to climate change, it is necessary to understand growth responses to climate, at a high temporal resolution. We therefore studied the intra-annual growth dynamics of three beech trees (Fagus sylvatica L.) and five oak trees (Quercus robur L.) growing on a sandy site in the east of the Netherlands for 2 years: 2003 (oak and beech) and 2004 (oak). Microcores were taken at 2-week intervals from the end of April until the end of October. Intra-annual tree-ring formation was compared with prior and contemporary records of precipitation and temperature from a nearby weather station.The results indicate that oak and beech reacted differently to the summer drought in 2003. During the drought, wood formation in both species ceased, but in beech, it recovered after the drought. The causes of species-specific differences in intra-annual wood formation are discussed in the context of susceptibility to drought.     

Hydraulic redistribution in three Amazonian trees

About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.     

Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry.     

Spring Sap of Trees

In the spring, sap in perennial plants becomes available after winter dormancy. As is well known, substances including minerals and organic nutrients are transported from storage compartments in roots and stems to growth regions. For this solute distribution, xylem as well as phloem are used and probably also the cambial region and the ray system. In the present study, a puncture method has been used to record sap availability in six north European tree species during the spring. Attempts were made to withdraw sap from the trees at points 0.5 m, 1.3 m and 3.0 m above ground with “phloem needles” (constructed according to Hammel 1968), and a syringe. Sap could be withdrawn for the longest period for the sycamore (71 days). Birch (47 days) gave an intermediate period, beech (12 days), oak (12 days) and alder (13 days) gave short periods, and sap could not be withdrawn from ash. Effects of two environmental factors, temperature and light intensity on sap availability were examined. Temperature dependence of sap availability, already observed in the genus Acer, was confirmed in sycamore maple (Acer pseudoplatanus). There also seemed to be a correlation between sap availability and light intensity. Analysis of sugar in the tree sap revealed that sucrose was the only sugar in the sap of sycamore but hexoses (glucose and fructose) were preponderant in the sap of the other trees. The pH of sycamore sap (pH 6.9–5.4) fell as the season advanced. However, the pH-values of sap from the other trees did not vary significantly.     

Seasonal courses of key parameters of nitrogen,carbon and water balance in European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) grown on four different study sites along a European North–South climate gradient during the 2003 drought

During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were foliar C contents, δ¹³C and δ¹⁸O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when compared to literature data (c. 1 μmol N mL⁻¹). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time. Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN profiles at all study sites, however, did not indicate drought stress. Foliar δ¹³C, but not foliar C and δ¹⁸O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ¹³C and δ¹⁸O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and δ¹⁸O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar.     

不同水分条件下两个树种木质部栓塞对P素添加的响应

 在两种水分供给(干旱胁迫和适宜水分,土壤含水量分别为田间持水量的30%～40%和70%～80%)下,研究了耐旱树种元宝枫(Acer truncatum)和 中生树种女贞(Ligustrum lucidum )木质部栓塞(以导水率(Percentage loss of hydraulic conductivity, PLC)损失程度衡量)对P素添加的 响应。结果发现,两个树种PLC的日变化均呈现出先上升后降低的规律,表明木质部栓塞的形成与恢复是植物体的一种平常事件;除适宜水分条 件的女贞外,P素可以显著提高元宝枫和遭受干旱胁迫时女贞的PLC;两种水分条件下,干旱胁迫时元宝枫木质部栓塞明显高于适宜水分供给时 。女贞的PLC在两种水分状况下无显著差异;树种间,干旱胁迫促进了元宝枫木质部的栓塞形成,明显高于同等水分条件下的女贞。该研究结果 证实了“木质部限流耐旱假设”。     

Stomatal conductance and intrinsic water use efficiency in the drought year 2003: a case study of European beech

Key message

Beech trees were able to cope with the drought of 2003. Harmful water shortage has been avoided by an effective stomatal closure while use of carbon storage pools may have prevented carbon starvation and growth reduction.

Abstract

We applied hydrodynamic modeling together with a tree ring stable isotope approach to identify the physiological responses of beech trees to changing environmental conditions. The drought conditions of the extreme hot and dry summer in 2003 were hypothesized to significantly influence the radial growth of European beech mainly triggered by the stomatal response towards water scarcity leading, in turn, to a decline in carbon assimilation. The functional–structural single tree modeling approach applied, revealed in fact a strong limitation of water use and carbon gain during drought. However, tree ring width data did not show a clear drought response and no differentiation in radial growth during six subsequent years examined (2002–2007) has been observed. Using integrated results from mechanistic carbon–water balance simulations, tree ring carbon and oxygen isotope analysis and tree ring width measurements we postulate that the suggested drought-induced growth decline has been prevented by the remobilization of stored carbohydrates, an early onset in growth and the relatively late occurrence of the severe drought in 2003. Furthermore, we demonstrate that the stomatal response played a significant role in avoiding harmful water tension that would have caused xylem dysfunction. As a result of the combined investigation with physiological measurements (stable isotope approach) and hydrodynamic modeling of stomatal aperture, we could give insights into the physiological control of mature beech tree functioning under drought. We conclude that beech trees have been operating at their hydraulic limits and that the longer or repeated drought periods would have affected the growth considerably.

     

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quereus gambelii Nutt)

Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (D) and oxygen (¹⁸O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem D and ¹⁸O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.     

Respiration activity of ectomycorrhizas from Cenococcum geophilum and Lactarius sp. in relation to soil water potential in five beech forests

Forest trees are involved in root symbioses with hundreds of species of ectomycorrhizal fungi which constitute functional guilds able to improve the water and mineral nutrition of host trees. In temperate ecosystems, water shortage is a main factor limiting tree vitality. To assess how soil water conditions affected the physiological state of beech (Fagus silvatica L.) ectomycorrhizal roots, we monitored glucose respiration of two ectomycorrhizal types (Lactarius sp. and Cenococcum geophilum) during two complete growing seasons. Five stands of contrasting soil conditions were chosen in north-eastern France. The top soil horizons were equipped with micropsychrometers for measuring water potential and temperature. Glucose respiration on individual ectomycorrhizas was measured in vitro by trapping [¹⁴C]-CO₂ from radiolabelled glucose. For soil water potential <-0.2 MPa, the potential respiration activity of C. geophilumectomycorrhizas was significantly less altered than that of Lactariussp. ectomycorrhizas, indicating that C. geophilumis more likely than Lactariussp. to maintain the physiological integrity of beech roots facing drought stress.     

Rooting depth,water relations and non‐structural carbohydrate dynamics in three woody angiosperms differentially affected by an extreme summer drought

In 2012, an extreme summer drought induced species‐specific die‐back in woody species in Northeastern Italy. Quercus pubescens and Ostrya carpinifolia were heavily impacted, while Prunus mahaleb was largely unaffected. By comparing seasonal changes in isotopic composition of xylem sap, rainfall and deep soil samples, we show that P. mahaleb has a deeper root system than the other two species. This morphological trait allowed P mahaleb to maintain higher water potential (Ψ), gas exchange rates and non‐structural carbohydrates content (NSC) throughout the summer, when compared with the other species. More favourable water and carbon states allowed relatively stable maintenance of stem hydraulic conductivity (k) throughout the growing season. In contrast, in Quercus pubescens and Ostrya carpinifolia, decreasing Ψ and NSC were associated with significant hydraulic failure, with spring‐to‐summer k loss averaging 60%. Our data support the hypothesis that drought‐induced tree decline is a complex phenomenon that cannot be modelled on the basis of single predictors of tree status like hydraulic efficiency, vulnerability and carbohydrate content. Our data highlight the role of rooting depth in seasonal progression of water status, gas exchange and NSC, with possible consequences for energy‐demanding mechanisms involved in the maintenance of vascular integrity.