Disturbances in European beech water relation during an extreme drought

Context

In the context of a probable increase in intensity and frequency of extreme summer drought events, a better understanding of the key processes involved in water relations is needed to improve the theoretical foundations of predictive process-based models.

Aims

This paper aims to analyse how temperate deciduous trees cope with water shortage.

Methods

The exceptional summer drought of 2003 in Europe provided an opportunity to monitor stomatal conductance and twig water potential in European beech (Fagus sylvatica L.) at predawn and midday and to analyse variations with respect to leaf height within the canopy. By comparing our field measurements of twig water potential to values found in the literature, we confirmed the strong impact of soil water shortage on crown water relations.

Results

This paper shows that (1) the vertical gradient of stomatal conductance within the crown disappeared under extreme soil water depletion; (2) at maximum drought intensity, predawn twig water potential (ψ _pd) reached ?2.3 MPa at a height of 14 m in the crown and ?2.0 MPa at a height of 10 m. The significant differences in ψ _pd between the two measurement heights in the canopy may be due to night transpiration; (3) there was a close relationship between predawn twig water potential and relative extractable soil water; (4) as drought conditions intensified, there was a close relationship between canopy radiation interception and predawn water potential, as estimated daily from relative extractable soil water.     

Differential response to soil drought among co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand

? Introduction

In the context of global environmental changes, better understanding of tree response to soil drought in young mixed species stands is needed to anticipate forest adaptation and management practices for the future.

? Materials and methods

We investigated the functional response of five co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand in northeastern France during the 2006 summer drought. We measured functional traits related to water acquisition (phenology, rooting pattern and vulnerability of xylem to cavitation) and the ecophysiological response (sap flux density, leaf water potential) of these species to soil water shortage.

? Results

Our study highlights contrasted drought response strategies among these species and a trade-off between leaf phenology, resistance of xylem to cavitation and root system depth.

? Conclusion

At this site, a deep root system seemed to be a key functional trait for the species to cope with drought. Quercus robur and Salix capreae can be characterised as drought-avoidance species as they possess a deep root system and therefore did not strongly experience soil drought. Despite deep rooting capacity, Betula pendula did not really avoid soil drought and strongly regulated transpiration during dry periods. Nevertheless, the earliness of budburst of this species contributes to high annual growth rate. In contrast, Carpinus betulus and Fagus sylvatica both displayed typical characteristics of drought-sensitive species.     

Ectomycorrhizal communities associated with silver fir seedlings (Abies alba Mill.) differ largely in mature silver fir stands and in Scots pine forecrops

Context

The requirement for rebuilding forecrop stands besides replacement of meadow vegetation with forest plants and formation of soil humus is the presence of a compatible ectomycorrhizal (ECM) fungal community.

Aims

This study aims to assess ectomycorrhizal fungi diversity associated with silver fir (Abies alba Mill.) seedlings regenerating in silver fir stands and Scots pine forecrops.

Methods

One-year-old seedlings were sampled in six study sites: three mature fir forests and three pine forests. ECM fungi were identified by polymerase chain reaction amplification and sequencing of the internal transcribed spacer of rDNA.

Results

The mean mycorrhizal colonization exceeded 90 %. Thirty-six ectomycorrhizal taxa were identified in fir stands and 23 in pine forecrops; ten out of these species were common to both stands. The fungal communities were different between study sites (R?=?0.1721, p?=?0.0001). Tomentella stuposa was the only species present at all sites.

Conclusion

Silver fir seedlings in Scots pine forecrops supported smaller ECM fungal communities than communities identified in mature silver fir stands. Nevertheless, fungal colonization of seedling roots was similar in both cases. This suggests that pine stands afforested on formerly arable land bear enough ECM species to allow survival and growth of silver fir seedlings.     

Drought sensitivity of Norway spruce is higher than that of silver fir along an altitudinal gradient in southwestern Germany

Context

For Central Europe, climate projections foresee an increase in temperature combined with decreasing summer precipitation, resulting in drier conditions during the growing season. This might negatively affect forest growth, especially at sites that are already water-limited, i.e., at low elevation. At higher altitudes trees might profit from increasing temperatures.

Aims

We analyzed variations in radial growth of silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst.) along an altitudinal gradient from 400 until 1,140 m a.s.l. in the Black Forest, to assess climate responses with increasing elevation.

Methods

Climate–growth relationships were analyzed retrospectively using tree-ring and climate data. In total, we sampled stem discs of 135 trees to build 27 species- and site-specific chronologies (n _fir?=?13, n _spruce?=?14).

Results

Our results indicate distinct differences in climate–growth relations between fir and spruce along the gradient. Growth of high-altitude fir was positively related to temperature from January till March. Growth of low-altitude fir and spruce at all elevations was positively related to precipitation and negatively to temperature during the growing season, particularly in July. A self-calibrating Palmer drought severity index underlined summer drought sensitivity of these trees.

Conclusion

Overall, we found that climatic control of tree growth changes over altitude for fir. For spruce, a remarkable synchrony in growth variation and climate response was shown, which indicates that this species is drought sensitive at all studied elevations. In a future warmer climate, the growth of low-altitude fir and spruce along the entire studied gradient may be negatively affected in the Black Forest, if an increased evaporative demand cannot be compensated by increased water supply.     

Individual vulnerability factors of Silver fir (Abies alba Mill.) to parasitism by two contrasting biotic agents: mistletoe (Viscum album L. ssp. abietis) and bark beetles (Coleoptera: Curculionidae: Scolytinae) during a decline process

? Context

In recent decades, there have been increasing reports of forest decline, especially in Mediterranean forest ecosystems. Decline in tree vitality is usually due to complex interactions between abiotic factors and biotic agents that attack weakened trees.

? Aims and methods

Estimating dendrometrical characteristics [basal area increment (BAI), age at DBH from tree ring counting, social status, height, and diameter], tree health status, and a competition index, we investigated the individual vulnerability of a French declining silver fir forest to both mistletoe (Viscum album L. ssp. abietis) and bark beetles (Pityophthorus pityographus Ratz., Pityokteines vorontzovi Jac., and Pityokteines spinidens Reitt.).

? Results

BAI was negatively correlated with both mistletoe infection (via mistletoe biomass) and bark beetle attack (number of insects per square meter), but there was evidence of divergence in tree choice between two groups of parasites. Mistletoe preferentially infected isolated and dominant trees that showed higher past growth rates than non-infected ones. Conversely, bark beetles mainly attacked defoliated and preferably declining trees with diameter (DBH) lower than 44.5 cm and slower past growth.

? Conclusion

While successive severe drought periods are thought to greatly weaken southern silver fir populations, mistletoe and bark beetles may contribute actively to their decline processes as inciting and contributing factors, respectively.     

Douglas-fir is vulnerable to exceptional and recurrent drought episodes and recovers less well on less fertile sites

Context

Since the 2003 drought and heat wave, there have been many reports of Douglas-fir decline and dieback in France. Given the climate change that is predicted, more frequent drought episodes could induce recurrent decline processes. The nature and background of this threat requires a careful assessment.

Aims

The objectives of this study were: (1) to test the hypothesis that the major climate hazard that accounts for the decline in Douglas-fir growth is drought, and (2) to identify the main vulnerability factors involved.

Methods

Decline and recovery of radial growth were quantified of Douglas-fir in two particularly affected regions with a dendroecological approach; 899 trees were sampled on 58 plots. Mean climate data, ecological and dendrometric stand characteristics were tested in order to identify potential vulnerability factors

Results

A clear relationship was evidenced between growth and soil water deficit. A severe decline of radial growth was induced by the 2003 drought and was maintained during subsequent years due to recurrent drought episodes. Growth recovery at the stand level was enhanced by soil nutrient fertility.

Conclusion

Radial growth of Douglas-fir is clearly driven by the intensity of drought. In a context of climate change, an adaptive silvicultural strategy is required in order to cope with more frequent drought events. To mitigate the frequency and the intensity of water shortage episodes, site selection must take into account not only local climate characteristic but also soil properties like maximum extractable water content. Soil nitrogen fertility was found to play a key role for an efficient recovery of radial growth after drought episodes and thus must be preserved.     

Physiological variation among western redcedar (Thuja plicata Donn ex D. Don) populations in response to short-term drought

? Variation in the ability of western redcedar (Thuja plicata Donn ex D. Don) populations to withstand water stress may exist because this species is found in coastal and interior biogeoclimatic subzones representing the full range of precipitation regimes in British Columbia, Canada.

? Seven western redcedar populations from locations in British Columbia, representing a wide range of habitat types, were assessed for their gas exchange and water relations response to controlled drought.

? Before drought, population variation occurred in stomatal conductance, net CO₂ assimilation rate and intrinsic water use efficiency and the relative water content at turgor loss point. During drought, populations had different responses of net CO₂ assimilation to decreasing predawn shoot water potential. After drought, populations differed in stomatal conductance and intrinsic water use efficiency, plus osmotic potential at turgor loss point, osmotic potential at saturation and apparent cuticular transpiration. Western redcedar populations from drier-inland habitats had a lower osmotic potential at turgor loss point, lower relative water content at turgor loss point and lower apparent cuticular transpiration in response to drought than populations from coastal origin with temperate maritime habitat.

? Reduction of cuticular water loss and adjustments of cellular water relations in response to drought was found to occur among seven western redcedar populations originating along a precipitation gradient while; there were minimal population differences in the gas exchange response to drought.

     

Recruitment of tree species in mixed selection and irregular shelterwood forest stands

Context

Recruitment is an important process in forest stand dynamics, especially in uneven-aged stands. Continuous recruitment is a prerequisite for diverse, uneven-aged silvicultural systems, but patterns may vary significantly.

Aims

The main goals of the study were to examine the recruitment of the main tree species in selection and irregular shelterwood stands in silver fir?CEuropean beech?CNorway spruce forests and to determine the main predictors of the recruitment occurrence.

Methods

Data from 5,486 permanent inventory plots were used to study recruitment of saplings into the tree layer (diameter at breast height ??10?cm).

Results

Recruitment rate differed significantly between selection (7.6?trees?ha^?1?year^?1) and irregular shelterwood (26.1?trees?ha^?1?year^?1) stands. Shade-tolerant fir and beech recruited with higher probability in selection stands, while light-dependent sycamore recruited with higher probability in irregular stands. In addition, forest types, soil pH, stand basal area, mean diameter, and the basal area of the same tree species with respect to recruitment were found to be important predictors of recruitment occurrence.

Conclusions

The application of different uneven-aged silvicultural systems and their forms makes it possible to considerably influence the future tree species composition of uneven-aged forests.     

Retrospective analysis of tree architecture in silver fir (Abies alba Mill.): ontogenetic trends and responses to environmental variability

Context

Understanding the effects of exogenous factors on tree development is of major importance in the current context of global change. Assessing the structure development of trees is difficult given that they are large and complex organisms with lifespan of several decades.

Aims

We used a retrospective analysis to derive the ontogenetic trends in silver fir development and assess the effects of climate or light environment on tree architecture.

Methods

Thanks to the identification of relevant growth markers (bud cataphylls and pseudo-whorl branches), a retrospective analysis allowed to record annual shoot extension and to date them on silver firs of various sizes under different environmental conditions.

Results

The length of successive annual shoots located on different axes clearly show gradual trends related to the physiological age of meristems. Within- and between-tree variations are noted due to the plasticity of development and growth induced by light environment and climate.

Conclusion

Retrospective analysis is an efficient method for getting information on the history of trees architecture and subsequently to relate it to environmental factors.     

Coastal and interior Douglas-fir provenances differ in growth performance and response to drought episodes at adult age

Context

Since the 2003 drought and heat wave, Douglas-fir dieback has been reported in France in trees older than 30 years. Consequently, it is questioned whether selected Douglas-fir provenances are suited to the frequent and severe drought events which are forecast due to climate change.

Aims

Our objective was to contribute to the screening of variability in productivity and growth response to soil-water deficit of mature trees from provenances not currently used for plantation in France.

Methods

We sampled 22 provenances, including coastal and interior Douglas-fir, covering a wide part of its natural distribution, from Oregon to California for coastal provenances and from British Columbia to New Mexico for interior provenances. These provenances were planted at the mid 1970s in two provenance trials located in the south-west area of France. Variability of productivity, of wood density, and of radial growth in response to drought episodes among provenances was quantified and related to soil-water deficit computed by daily water balance calculations.

Results

Whatever the provenance, annual radial growth is highly dependent on local soil-water deficit (Felines R ² = 0.57, Le Treps R² = 0.49). Radial growth and wood properties exhibit large differences between provenances at 30 years old. Variability between provenances for all wood characteristics studied is mainly structured geographically. Coastal provenances perform best for productivity at 30 years old (619 cm²?±?59), and exhibit a small growth reduction in 2004, the second successive year of drought (?10.7 %?±?3.8). Surprisingly, the southern interior provenances from the driest environments in the natural range show a large growth reduction in 2004 (?30.5 %?±?5.2).

Conclusions

The provenances tested exhibited significant differences in growth performance and growth reduction induced by the soil-water deficit. The approach coupling retrospective analysis of radial growth on mature trees and water balance modelling is relevant for evaluating provenance adaptation to more frequent or severe drought episodes.     

Chronic cork oak decline and water status: new insights

Chronic decline and Sudden death are two syndromes of cork oak (Quercus suber) dieback. Mortality is associated with water stress, but underlying physiological mechanisms are poorly understood. Here, we investigated the physiological performance of declining trees during the summer drought. Leaf water potential, gas-exchange, fluorescence of photosystem II and leaf and root starch concentration were compared in healthy (asymptomatic) and declining trees. Low annual cork increment in declining trees indicated tree decline for several years. All trees showed similar water status in spring. In summer, declining trees showed lower predawn leaf water potential (?2.0 vs. ?0.8 MPa), but unexpectedly higher midday leaf water potential than healthy trees (?2.8 vs. ?3.3 MPa). The higher midday water potential was linked to by means of strongly reduced stomatal conductance and, consequently, transpiration. This study is pioneer showing that declining trees had high midday water potential. A tendency for lower sap flow driving force (the difference between predawn and midday water potential) in declining trees was also associated with reduced photosynthesis, suggesting that chronic dieback may be associated with low carbon uptake. However, starch in roots and leaves was very low and not correlated to the health status of trees. Declining trees showed lower water-use efficiency and non-photochemical quenching in summer, indicating less resistance to drought. Contrarily to chronic decline, one tree that underwent sudden death presented predawn leaf water potential below the cavitation threshold.     

Impact of local slope and aspect assessed from LiDAR records on tree diameter in radiata pine (Pinus radiata D. Don) plantations

Context

Reliable information on tree stem diameter variation at local spatial scales and on the factors controlling it could potentially lead to improved biomass estimation over pine plantations.

Aims

This study addressed the relationship between local topography and tree diameter at breast height (DBH) within two even-aged radiata pine plantation sites in New South Wales, Australia.

Methods

A total of 85 plots were established, and 1,302 trees were sampled from the two sites. Airborne light detection and ranging (LiDAR) was used to derive slope and aspect and to link them to each individual tree.

Results

The results showed a significant relationship between DBH and local topography factors. At both sites, trees on slopes below 20° and on southerly aspects displayed significantly larger DBHs than trees on steeper slopes and northerly aspects. Older trees with similar heights also exhibited a significant relationship between DBH and aspect factor, where greater DBHs were found on southerly aspects.

Conclusions

The observed correlation between tree DBH and LiDAR-derived slope and aspect could contribute to the development of improved biomass estimation approaches in pine plantations. These topographical variables are easily attained with airborne LiDAR, and they could potentially improve DBH predictions in resource inventories (e.g. stand volume or biomass) and support field sampling design.     

Fir expansion not controlled by moderate densities of large herbivores: a Mediterranean mountain grassland conservation issue

Key message

Whilst livestock has maintained grasslands over centuries in south European mountains, current ungulate densities are insufficient to control tree and forest expansion effectively. The present results based on fir population dynamics and radial/height growth raise questions about the sustainable management of new forests that are former mountain grasslands. This empirical case study contributes to debate about methods for long-term management of grasslands in Mediterranean mountains.

Context

Forest expansion following the cessation of grazing is a threat to biodiversity in mountain grasslands that are components of Mediterranean cultural landscapes.

Aims

We hypothesised that ungulates could mitigate tree recruitment, thus conserving mountain grasslands. We tested the efficiency of grazing and browsing by domestic and wild ungulates (sheep, roe and red deer) at inhibiting silver fir (Abies alba Mill.) encroachment in a conservation area, one of the main browsed trees.

Methods

The fir trees’ age structure was analysed by dendrochronology, and the individual growth patterns and scars were recorded in their tree-ring series.

Results

Fir density has increased since the 1960s, simultaneously with the increase in ungulate populations. The scar occurrences only appeared during the 1970s. Most scars were observed on seedlings and small saplings. The impacts of ungulates are observed on the radial and height growth of trees, mostly on trees <?100 cm, but it did not however result in controlling the recruitment of fir.

Conclusion

The hypothesis that a moderate density of ungulates might inhibit tree encroachment is therefore rejected, even in the case of a highly palatable species such as silver fir. Ungulates, solely, would not be an appropriate policy for grassland conservation in Mediterranean mountains.

     

Thinning has a positive effect on growth dynamics and growth–climate relationships in Aleppo pine (Pinus halepensis) trees of different crown classes

? Context

Modification of stand density by thinning may buffer the response of tree growth and vigor to changes in climate by enhancing soil water availability.

? Aims

We tested the impact of thinning intensity on cambial growth of Aleppo pine (Pinus halepensis L.) under semi-arid, Mediterranean conditions.

? Methods

A multiple thinning experiment was established on an Aleppo pine plantation in Spain. We analysed the stem growth dynamics of two different crown classes under four different thinning intensities (15 %, 30 %, and 45 % removal of the basal area) for 2 years, based on biweekly band dendrometer recordings. Local relative extractable soil water was derived from the use of a water balance model Biljou© (available at https://appgeodb.nancy.inra.fr/biljou/) and used as an explanatory variable.

? Results

Radial growth was mainly controlled by soil water availability during the growing season, and differed by crown class. The growth rates of dominant trees were significantly higher than the growth rates of suppressed trees. Removal of 30 % and 45 % of the initial basal area produced a growth release in both dominant and suppressed trees that did not occur under less intense thinning treatments.

? Conclusions

Soil water availability was the main driver of radial growth during the growing season. Forest management confirmed its value for ameliorating the effects of water limitations on individual tree growth. These results may help managers understand how altering stand density will differentially affect diameter growth responses of Aleppo pine to short-term climatic fluctuations, promoting forests that are resilient to future climatic conditions.     

Grazing affects shoot growth,nutrient and water status of Quercus ilex L. in Mediterranean open woodlands

Context

Mediterranean open woodlands (dehesa) have faced a dual process of intensification and abandonment of grazing which has resulted in alteration of the understory vegetation.

Aims

We analysed the effects of land use changes on the physiological status of holm oak in different open woodlands (dehesa) in southern Iberian Peninsula.

Methods

In an area of extensive grazing, we selected six paired plots (one grazed, one abandoned) and grouped them by habitat types according to understory composition (nearly all monospecific Cistus ladanifer L. shrub or mixed shrub). Six plots of moderate and heavy grazing intensity were chosen within a settled area of livestock use. Shoot growth, macronutrient concentrations and water content were assessed in samples of holm oak leaves.

Results

Abandonment of grazing affected some nutrient concentrations and water content of holm oak leaves, but the effects were different according to habitat type. C. ladanifer shrub reduced N and P concentrations and water content while mixed shrub increased P concentration and water content. High grazing intensity improved shoot growth and leaf N and Mg concentrations.

Conclusion

Extensive grazing could be a useful management tool to enhance growth, nutritional and water status of holm oak in a habitat with limited resources such as Mediterranean open woodlands (dehesa).     

Uneven-aged management options to promote forest resilience for climate change adaptation: effects of group selection and harvesting intensity

Context

Climate change is expected to increase forest vulnerability through disturbances such as windstorms and droughts. Forest managers are therefore investigating strategies to increase forest resistance and resilience, especially by promoting uneven-aged and mixed forests through group selection, and by reducing stand stocking and large trees proportion. However, there is little information on the long-term impacts of these two practices.

Aims

The objectives of this study were (1) to develop an original silviculture algorithm designed for uneven-aged management and (2) to use it to assess the effects of the above-mentioned management methods in long-term simulations.

Methods

We simulated individual and group selection techniques in order to study the effects of group size, harvesting intensity and their interactions on wood production, stand heterogeneity, and regeneration in mountain spruce–fir forests. We used the spatially explicit individual-based forest model Samsara2 to simulate forest dynamics.

Results

Our simulation results confirmed the positive effect of group selection practices on structure diversity and regeneration but not on spruce maintenance. Increasing harvesting intensity enabled forest destocking but decreased structure diversity and led to non-sustained yields for the most intensive scenarios.

Conclusion

As adaptation measure, we thus recommend moderate group selection harvesting creating 500 m² gaps.     

Weak competitive ability may explain decline of Taxus baccata

Context

Taxus woodlands suffer from a lack of natural regeneration across its whole distribution range.

Aims

In Knyazhdvir Reserve in Ukraine Taxus baccata primarily competes with Abies alba. It has been assumed that in the conditions of deep shade, yews have a greater competitive advantage than firs.

Methods

To verify this hypothesis, a plot with both species growing under the canopy was established. Wood samples were taken from 20 trees of T. baccata and 20 trees of A. alba.

Results

There were no differences in the mean tree-ring width, height and age between both species. Due to a lack of spatial separation of both species, yews and firs inhabited the same area in the analysed plot. Fir seedlings (up to 0.5?m) were recorded to grow under yew saplings (above 0.5?m), but the inverse situation of yew seedlings growing under fir saplings was not seen.

Conclusion

Seedling distribution, tree-ring growth rate, possibility of growth of fir seedlings under yew saplings and greater potential growth suggested that fir was predisposed to having the competitive edge over yew. Our results may confirm the hypothesis that T. baccata has a restricted distribution because of weak competitive abilities and can exist only in habitats where faster growing species are absent. The results call into question the prevailing opinion that human influences are the principal cause for the decline of T. baccata.     

Improving the estimation of landscape scale seed dispersal by integrating seedling recruitment

? Context

Dispersal and recruitment play a critical role in determining the abundance and the spatial structure of regeneration.

? Aim

In this study, we estimated landscape scale seed dispersal using the inverse modeling method by fitting seedling counts to seed tree location and fertility in the European silver fir Abies alba Miller.

? Methods

Seed trees and seedlings from three age cohorts were counted in 30 plots on Mont Ventoux (southeastern France) at elevations from 995 to 1,605?m, where the following ecological variables were measured: elevation, slope, aspect, light availability, and tree density. We developed and fitted a dispersal model, including a new parameter, recruitment rate, which depended both on the ecological characteristics of the plots and on seed production.

? Results

Elevation was the factor that affected seedling recruitment the most, with an optimum value at around 1,200?m. Estimated A. alba seed dispersal distances were short (median values for the three cohorts, respectively, 16.4?C13.2?C19.6?m).

? Discussion

We demonstrated that including the ecological characteristics of plots and post-dispersal recruitment processes realistically reduced estimates of seed dispersal distances which are otherwise grossly over-estimated using inverse modeling procedures.     

Assessing temporal variation of primary and ecosystem production in two Mediterranean forests using a modified 3-PG model

Context

Forest ecosystem carbon uptake is heavily affected by increasing drought in the Mediterranean region.

Aims

The objectives of this study were to assess the capacity of a modified 3-PG model to capture temporal variation in gross primary productivity (GPP), and ecosystem net carbon uptake (NEE) in two Mediterranean forest types.

Methods

The model was upgraded from a monthly (3-PG) to a daily time step (3-PG_day), and a soil water balance routine was included to better represent soil water availability. The model was evaluated against seasonal GPP and NEE dynamics from eddy covariance measurements.

Results

Simulated and measured soil water content values were congruent throughout the study period for both forest types. 3-PG_day effectively described the following: GPP and NEE seasonal patterns; the transition of forest ecosystems from carbon sink to carbon source; however, the model overestimated diurnal ecosystem respiration values and failed to predict ecosystem respiration peaks.

Conclusions

The model served as a rather effective tool to represent seasonal variation in gross primary productivity, and ecosystem net carbon uptake under Mediterranean drought-prone conditions. However, its semi-empirical nature and the simplicity inherent in the original model formulation are obstacles preventing the model working well for short-term daily predictions.     

Drought susceptibility and recovery of transplanted Quercus rubra seedlings in relation to root system morphology

? Transplant shock, implicated by depressed seedling physiological status associated with moisture stress immediately following planting, limits early plantation establishment. Large root volume (Rv) has potential to alleviate transplant shock because of higher root growth potential and greater access to soil water.

? We investigated impacts of drought and transplant Rv on photosynthetic assimilation (A), transpiration (E), stomatal conductance (g _s ), predawn leaf xylem water potential (Ψ_L), and growth of northern red oak (Quercus rubra L.) seedlings to explain mechanisms associated with susceptibility to transplant shock. One year-old barerooot seedlings were graded into four Rv categories and either well watered or subjected to drought consisting of low, medium, or high moisture stress by discontinuing irrigation at 22-day intervals for 3 months. Thereafter, all treatments were re-watered to examine recovery.

? Transplant shock was signified by reduced A, E, g _s, and Ψ_L, which generally increased with increasing moisture stress and Rv. Physiological status improved during recovery, though stress was still evident in seedlings exposed to medium or high moisture stress and in larger Rv seedlings. Growth declined with increasing moisture stress but was generally similar among Rv treatments, likely reflecting greater A at the whole plant level and/or reliance upon stored reserves in large Rv seedlings.

? The most effective drought avoidance mechanisms were root growth, stomatal regulation, reduced leaf area, and higher growth allocation to roots relative to shoots. Our results suggest that large initial Rv does not enhance drought avoidance during the first season after transplant in northern red oak seedlings.