Variations in stream water uptake by Eucalyptus camaldulensis with differing access to stream water

The stable isotopes ²H and ¹⁸O were used to determine the water sources of Eucalyptus camaldulensis at three sites with varying exposure to stream water, all underlain by moderately saline groundwater. Water uptake patterns were a function of the long-term availability of surface water. Trees with permanent access to a stream used some stream water at all times. However, water from soils or the water table commonly made up 50% of these trees' water. Trees beside an ephemeral stream had access to the stream 40–50% of the time (depending on the level of the stream). No more than 30% of the water they used was stream water when it was available. However, stream water use did not vary greatly whether the trees had access to the stream for 2 weeks or 10 months prior to sampling. Trees at the third site only had access to surface water during a flood. These trees did not change their uptake patterns during 2 months inundation compared with dry times, so were not utilising the low-salinity flood water. Pre-dawn leaf water potentials and leaf ¹³C measurements showed that the trees with permanent access to the stream experienced lower water stress and had lower water use efficiencies than trees at the least frequently flooded site. The trees beside the ephemeral stream appeared to change their water use efficiency in response to the availability of surface water; it was similar to the perennial-stream trees when stream water was available and higher at other times. Despite causing water stress, uptake of soil water and groundwater would be advantageous to E. camaldulensis in this semi-arid area, as it would provide the trees with a supply of nutrients and a reliable source of water. E. camaldulensis at the study site may not be as vulnerable to changes in stream flow and water quality as previously thought.     

Sources of water used by riparian Eucalyptus camaldulensis overlying highly saline groundwater

Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm^–1, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm^–1. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05–0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm^–1 (equivalent to approximately –1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.     

Water sources accessed by arid zone riparian trees in highly saline environments,Australia

The flow regimes of arid zone rivers are often highly variable, and shallow groundwater in the alluvial aquifers can be very saline, thus constraining the availability and quality of the major water sources available to riparian trees—soil water, shallow groundwater and stream water. We have identified water sources and strategies used by riparian trees in more highly saline and arid conditions than previously studied for riparian trees of arid zone rivers. Our research focused on the riparian species Eucalyptus coolabah, one of the major riparian trees of ephemeral arid zone rivers in Australia. The water sources available to this riparian tree were examined using δ¹⁸O isotope data from xylem, soil water, groundwater and surface water. Additionally, soil chloride and matric potential data were used to infer zones of water availability for root uptake. Despite the saline conditions, the trees used a mixture of soil water and groundwater sources, but they did not use surface water directly. The study identified three strategies used to cope with typically high groundwater and soil water salinities. Firstly, the trees preferentially grow in zones of most frequent flushing by infiltrating streamflow, such as the bank-tops of channels. Secondly, the trees limit water use by having low transpiration rates. Thirdly, the trees are able to extract water at very low osmotic potentials, with water uptake continuing at chloride concentrations of at least 20,000–30,000 mg L⁻¹.     

Forest-scale sap flux responses to rainfall in a dryland eucalyptus plantation

Dryland salinity is caused by rising saline water tables, the result of relatively recent landscape-scale clearance of deep-rooted vegetation. One obvious solution to this problem is the reintroduction of deep-rooted vegetation into these landscapes, most likely non-deciduous trees. Ideally, continually-transpiring deep-rooted trees would remove moisture from throughout the soil profile, increasing the capacity of the soil to store water, thus lowering water tables by effectively reducing the number of rainfall events that contribute to groundwater recharge. In this study, we examined how water use by a Eucalyptus sideroxylon A. Cunn. ex Woolls plantation, growing in a salinity-prone landscape, varied in response to rainfall events across four years of sap flux monitoring. Responses of the plantation were observed across multiple seasons, from above average to well below average rainfall. We observed that the plantation forest, while capable of continuous water use during drought, was also quite responsive to rainfall events. During the driest periods, during which shallow soil moisture was reduced to a stable minimum, the forest continued using water at around 1 mm/day. Generally we observed increases in forest water use following only 5 mm of rainfall, in contrast to 20 mm for neighbouring native vegetation. We compared a range of plausible empirical models for describing forest water use responses to rainfall. The best model demonstrated that rainfall size, post-rainfall PET and the interaction between rainfall size and antecedent soil moisture made significant contributions to variation in forest water use across rainfall events. Interestingly, the model showed that all else equal, higher antecedent soil moisture tended to reduce potential increases in forest water use in response to rainfall.     

锐齿栎水力结构和生长对降雨减少的响应

为探究气候变化背景下降雨减少对森林的影响,2013年在宝天曼锐齿栎天然次生林原位建立了3块降水减少(截雨)样地,研究降雨减少对锐齿栎水碳关系和生长的影响。结果表明:降雨减少后锐齿栎枝条水势显著低于对照,最低水势为(-1.36±0.11)MPa,但锐齿栎木质部栓塞88%的水势值为-3.19MPa,叶片气孔关闭时的水势值为-2.5MPa,故降雨减少在这一地区没有对锐齿栎水力结构造成严重的干扰。降雨减少后,锐齿栎的叶片、韧皮部和木质部的总非结构性碳浓度与对照没有显著差异。木质部导管密度和叶片气孔密度变大,而导管直径和气孔长度变小。在天气较为干旱时,降雨减少处理的锐齿栎气孔导度日变化呈"双峰"曲线,而在湿润天气时呈"单峰"曲线且中午峰值显著大于对照。降雨减少处理的锐齿栎木材密度、Huber值、比叶面积和胸径生长与对照没有显著差异。降雨减少后锐齿栎树木没有遭受水力失衡或碳饥饿的危害,生长也没有受到显著影响,但是水力输导系统发生了适应性调节。     

Stable isotopic observation of water use sources of Pinus sylvestris var. mongolica in Horqin Sandy Land, China

Since the late 1950s, Mongolian pine (Pinus sylvestris var. mongolica) has been widely planted for vegetation restoration in arid and semi-arid areas in North China. We used stable isotope signals from precipitation, soil water, and xylem water of Mongolian pine trees, which were planted in early 1980s on sand dunes in the east-southern margin of Horqin Sandy Land, to identify water uptake sources of this tree. (1) Stable ¹⁸O isotope composition of the xylem water exhibited little seasonality, suggesting that the trees use a relatively stable water source; (2) the water source of the pine trees primarily came from a soil depth of 20–60 cm (sampling depth up to 60 cm in this study) and the trees might use groundwater when soil moisture became extremely low; and (3) there was not much difference in water sources used by the pine trees grown at the top of the fixed dune and in the inter dune lowland, although these two sites had 3–8 m elevation difference. This study suggests that it is critical for Mongolian pine trees to access relatively reliable and stable water sources to grow in sandy land habitats, and timely recharging of rainwater to the trees’ rooting depth is requisite for avoiding and/or reducing their degradation caused by water shortage.     

青海湖流域具鳞水柏枝植物水分利用氢同位素示踪研究

具鳞水柏枝(Myricaria squamosa)是我国高寒地区广泛分布的优势河谷灌木, 具有维持河谷湿地系统稳定的功能。然而, 目前国内外有关具鳞水柏枝水分利用来源的定量研究很少。该文运用氢稳定同位素示踪方法, 分析了青海湖流域具鳞水柏枝茎(木质部)水和潜在水源(地下水、河水和土壤水)的氢稳定同位素比率(δD)的季节变化, 发现具鳞水柏枝在不同水文环境下的植物水分利用来源有明显差异。研究结果表明, 生长在河岸边的具鳞水柏枝在6、7月主要利用地下水与河水, 分别占其所利用水分的89%、86%和55%、65%, 8月主要利用0-20 cm土层的土壤水, 9月水源不详。生长在离河岸约100 m处的具鳞水柏枝在6月主要利用地下水与河水(91%、70%), 在7-9月以0-60 cm土层的土壤水为主要水源。这表明生长在河岸边的具鳞水柏枝对地下水和河水的依赖程度较高, 而距离河岸约100 m时对土壤水的利用量较多, 反映出生长在不同生境中的具鳞水柏枝对特定水分条件的特殊适应结果。     

鼎湖山锥栗木质部形成及其对气候的响应

为了解鼎湖山锥栗(Castaneahenryi)木质部的年内生长动态及其对气候因子的响应,利用微树芯采样技术对锥栗木质部进行连续监测,采用广义可加模型拟合生长过程,并利用混合效应模型分析木质部形成与气候因子的关系。结果表明,鼎湖山锥栗木质部在2015年几乎保持全年生长,呈现显著的季节性差异。在doy200 (200 days of year)之前木质部快速生长(生长季前期),doy300之后(生长季后期)生长速率出现下降趋势。生长季前期的木质部生长速率与温度、光合有效辐射存在显著正相关关系,生长季后期木质部生长速率与气候因子之间的相关关系并不显著。这说明中国南亚热带阔叶树种锥栗木质部形成及其调节机制具有季节性差异,反映了木质部形成对环境的适应性。     

Elephant utilization of Colophospermum mopane: possible benefits of hedging

The pattern of utilization of Colophospermum mopane by elephant was investigated in a semi‐arid savanna, the Venetia‐Limpopo Nature Reserve, South Africa, using an animal‐based approach. C. mopane is a staple food item in the diet of elephant, with most individual trees being utilized lightly (< 10% of biomass removed). Utilization of branch, foliage and main stems accounted for most of the utilization of trees < 4 m high whilst utilization of bark accounted for the majority of utilization of trees > 4 m high. The most preferred height of utilization of branches was < 1 m above ground level, reflecting the short stature of C. mopane in the study area. Elephant selected C. mopane trees < 2 m in height that had been previously utilized by them; specifically trees where the terminal part of the main stem had been previously broken and had been coppiced (hedging). The preferred size range of branches utilized by elephant was 0.946–1.718 cm in diameter; hedged C. mopane trees in the 1–2 m category had, on average, significantly more branches within this size range than non‐hedged trees. This hedging of C. mopane appears to have increased the availability of a preferred food item.     

Water sources of Populus euphratica and Tamarix ramosissima in Ejina Delta,the lower reaches of the Heihe River,China

Aims We aim to evaluate the water sources of typical riparian arbor species (Populus euphratica) and shrub species (Tamarix ramosissima), and analyze the spatial and temporal dynamics of plant water source in Ejina Delta, the lower reaches of the Heihe River, China.Methods Eight sampling sites were selected in the riparian zones along the East River and West River in Ejina. The plant xylem water, soil moisture, rainwater, stream water and groundwater were taken and pretreated during the growing season in 2015-2016, and the stable oxygen isotope ratio (δ¹⁸O) for each water sample was measured. The δ¹⁸O of plant xylem water and soil moisture were compared to estimate the dominant depth of root water uptake, and the linear-mixed model called “IsoSource” were applied to determine plant water sources and quantify their proportions.Important findings This study indicated that the main recharge sources for P. euphratica and T. ramosissima were stream water and groundwater. The contributions of rain water to them was negligible due to the limited amount and the shallow infiltration depth of local rainfall. As affected by groundwater level fluctuation, soil physical properties, as well as lateral and vertical recharge of stream water on soil moisture, the dominant depth of root water uptake spatially varied. However, the relative contributions of stream water or groundwater to plant water sources did not change significantly across space. Populus euphratica used more stream water (68%), while T. ramosissima used more groundwater (65%). Plant water sources were sensitive to environmental flow controls. The contributions of stream water to the water sources of the two species went up to 84% and 48% for P. euphratica and T. ramosissima respectively during the discharge period, but dropped to 63% and 30% during the non-discharge period. On the other hand, the contributions of groundwater decreased to 16% and 52% during the discharge period, but increased to 37% and 70% during non-discharge period. It is noteworthy that the high similarity of δ¹⁸O between stream water and groundwater due to extensive water exchange in the riparian zone made increase the uncertain in quantifying plant water sources.     

The relationship between rainfall,water source and growth for an endangered tree

Abstract It is now reasonably well understood that the human impact on the environment since industrialization has led to significant changes in climate. Here we attempt to develop a predictive understanding of the effects that future changes in climate may have on vegetation structure and species diversity. We do this through a determination of the relationship between radial growth and water source for Widdringtonia cedarbergensis Marsh. Our results show that there was no significant relationship between monthly radial growth, as determined using dendrometer bands, and rainfall. There is, however, a significant relationship between the δ¹⁸O composition of the water extracted from the trees and the rain δ¹⁸O values. We speculate that W. cedarbergensis exploits water derived from rain that flows off the rocky substrate of the study area into sumps between the bedding planes of the rocks on which they grow. This runoff occurs rapidly during rain events resulting in δ¹⁸O values for the trees sourcing this water not to be significantly different from that of the rain. Rainfall therefore has to be sufficient to refill these sumps on which the trees are dependent. The dendrometer bands reflect a slow but steady growth of the trees at the study site. While this growth is not dependent on rainfall, it is dependent on reliable access to available water. If climate change predictions for the region are realized and rainfall is reduced then this species will be affected. W. cedarbergensis is endemic to only a very small area within the Cedarberg Mountains in South Africa and is also one of the few trees growing in the fynbos. The extinction of this species in the wild will fundamentally affect both the vegetation structure and species composition of the region.     

Drought stress and recovery of riparian cottonwoods due to water table alteration along Willow Creek,Alberta

A 5-m-deep gravel pit was excavated from 1996 to 1998 in the floodplain between Willow Creek, Alberta, and a grove of balsam poplars ('cottonwoods', Populus balsamifera L.) and water level at the pit was lowered 2.5 m through pumping. This interrupted the infiltration of stream water into the riparian groundwater and imposed drought stress on the cottonwoods. Trees in the drought-affected grove displayed extensive leaf senescence and abscission in late August 1998, while trees in nearby control groves remained green until autumnal senescence in late September. The precocious senescence was accompanied by a two-thirds reduction in leaf stomatal conductance (g _s) but mid-day leaf xylem water potentials (ψ_l) were only slightly reduced (?1.55 vs 1.42 MPa). Pumping ceased in 1999, the pit was partially refilled, and the hydraulic linkage between the stream and the riparian zone recovered. Subsequently in August 1999, g _s and ψ_l were similar for trees in the affected and control groves and senescence phenologies were similar in 1999 and 2000. Annual branch growth increments varied 3-fold across years between 1994 and 1999, but there was no reduction in these growth increments in the drought-affected trees in 1998 or 1999. This study supports the hydraulic linkage between a stream and the adjacent riparian zone in a semi-arid region and demonstrates the vulnerability of riparian cottonwoods to drought due to water table depletion. It also indicates rapid physiological recovery of cottonwoods following restoration of water availability.     

Radial distribution of Ni in stemwood of Quercus ilex L. trees grown on serpentine and sandy loam (umbric leptosol) soils of NE-Portugal

Concentrations of Ni were determined in xylem and phloem of Quercus ilex trees growing on serpentine and sandy loam soils of northeast Portugal. Radial distribution patterns of Ni in stemwood were compared to variations in annual growth increments of the trees.Concentrations of Ni in xylem and phloem were higher in Q. ilex from serpentine soil, when compared with Q. ilex of a sandy loam soil.The radial distributions of Ni showed large variations among the trees, although they all grew in the same area within a short distance from each other. These differences can be caused by small-scale spatial variations in the soil. Therefore, the use of such radial Ni distributions for a retrospective biomonitoring of the Ni pollution of larger areas appears questionable.     

Savanna trees in Namibia—Factors controlling their distribution at the arid end of the spectrum

This study reviewed the distribution of ten common savanna trees in Namibia. Tree distributions were investigated in relation to bioclimatic, topographic and edaphic variables at a national scale. The factors of importance at these broad geographic scales appeared to be rainfall, substrate and, likely, the incidence of frost. Baikiaea plurijuga, Burkea africana, Guibourtia coleosperma and Pterocarpus angolensis seem to reach their bioclimatic limits in Namibia.At the local level, plant traits become important and contribute to explaining distribution patterns. High water and/or nitrogen use efficiency (Acacia erioloba, Colophospermum mopane), dual water obtaining strategies (Faidherbia albida), fire tolerance (e.g. Acacia species, Burkea africana and Pterocarpus angolensis) and drought tolerance (Boscia albitrunca) are some key attributes providing additional explanations for current distributions.Amongst the selected trees and at broad geographic scales, below-ground adaptations are governed by rainfall regime in combination with coarse-textured soils, whereby shallow-rooted trees prevail in the Kalahari sandveld. Deep-rooted species are found largely on non-sandy soils. Physiological performance of many trees appears to be directly linked to rainfall regime and trees may hence show varying performance throughout their distribution range. Insight into plant functional attributes of trees in Namibia is required to develop appropriate management strategies in the light of climate change. Modelling climate change impacts should consider the relative contribution of bioclimatic versus local environmental factors that explain the current distribution patterns of the selected trees.     

Determinants of savanna vegetation structure: Insights from Colophospermum mopane

Abstract Savannas are structurally heterogeneous at the local, community‐level scale due to fine‐scale floristic heterogeneity as well as the responses of individual species to underlying environmental variation. The structure of mopane woodland, an arid savanna of southern Africa, is dictated largely by local variation in the relative dominance of tall, single‐stemmed and shorter, multi‐stemmed forms of the dominant tree species, Colophospermum mopane (Kirk ex Benth) Léonhard. Here we evaluate the hypothesis that the existence of these alternative forms of C. mopane, as well as the factors that dictate their distribution at a local scale, are driven by fine‐scale environmental variability in available water. We surveyed trees at four sites in the Kruger National Park of South Africa, in each instance surveying both forms of the species, from both riparian and non‐riparian zones. A survey of genetic variation across our sample (n = 80 individuals), using inter‐simple sequence repeat (ISSR) amplification profiles, indicates that the two forms are not genetically distinct, instead being environmentally determined. While measurements of xylem pressure potentials, determined using a Scholander pressure chamber, show a significant difference between riparian and non‐riparian zones, there is no significant difference between the two growth forms. Although this seems paradoxical in view of the prevalence of tree and shrub form mopane at riparian and non‐riparian sites, respectively, we speculate that such a pattern may emerge through the interaction of moisture stress and top‐down controls, such as those imposed by large mammal browsing and fire.     

Linking water uptake with rooting patterns in grassland species

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004–2005) in which we measured the isotopic signature of plant xylem water from seven species (including C₃ forbs and shrubs and C₄ grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (δ¹⁸O) values were compared to soil water δ¹⁸O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C₃ species used proportionally more water from deeper portions of the soil profile relative to the C₄ grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.     

Water status,drought responses,and growth of <Emphasis Type="Italic">Prosopis flexuosa</Emphasis> trees with different access to the water table in a warm South American desert

Prosopis flexuosa trees dominate woodlands in the Central Monte Desert (Mendoza, Argentina), with <200 mm rainfall, exploiting the water table recharged by Andean rivers, and also growing in dunes with no access to the water table. Prosopis woodlands were extensively logged during development of the agricultural oasis, and surface and groundwater irrigation could lower the depth of the water table in the future. We evaluated tree populations with decreasing access to the water table: valley adult trees, valley saplings, and dune adult trees, in order to assess their ecophysiological response to water table accessibility. High and seasonally stable pre-dawn leaf water potentials (−2.2 ± 0.2 to −1.2 ± 0.07 MPa) indicated that valley adults utilize larger and more stable water reservoirs than valley saplings and dune adults (−3.8 ± 0.3 to −1.3 ± 0.07 MPa), with higher midday leaf conductance to water vapor (valley adults ~250; dune adults <60 mmol m⁻² s⁻¹), potentially higher CO₂ uptake, and increased radial growth rate (valley adults 4.1 ± 0.07; dune adults 2.9 ± 0.02 mm year⁻¹). Trees with poor access to the water table exhibited drought tolerance responses such as midday stomata closure, leaflet closure, and osmotic adjustment. Stomata density decreased in response to drought when leaf expansion was restricted. The combination of phreatophytism and drought tolerance would enlarge P. flexuosa habitats and buffer populations against changes in rainfall dynamics and water table depth.     

Intrinsic effects of species on leaf litter and root decomposition: a comparison of temperate grasses from North and South America

A simple model of water uptake by vegetation is used to aid the discrimination of plant water sources determined with isotope data. In the model, water extracted from different soil depths depends on the leaf–soil potential difference, a root distribution function and a lumped hydraulic conductance parameter. Measurements of plant transpiration rate, and soil and leaf water potentials are used to estimate the value of the conductance parameter. Isotopic ratios in soil water and xylem are then used to constrain the root distribution. The model is applied to field measurements of transpiration, leaf water potential and ¹⁸O composition of xylem water on Corymbia clarksoniana, Lophostemon suaveolens, Eucalpytus platyphylla and Melaleuca viridiflora, and soil water potential and ¹⁸O composition of soil water to 8.5 m depth, in an open woodland community, Pioneer Valley, North Queensland. Estimates of the water uptake from various depths below the surface are determined for each species. At the time of sampling, the proportion of groundwater extracted by the trees ranged from 100% for C. clarksoniana to <15% for L. suaveolens and E. platyphylla. The advantages of the model over the traditional approach to determining sources of water used by plants using isotope methods are that it: (1) permits more quantitative assessments of the proportion of water sourced from different depths, (2) can deal with gradational soil water isotope profiles (rather than requiring distinct values for end-members), and (3) incorporates additional data on plant water potentials and is based on simple plant physiological processes.     

Common trade‐offs between xylem resistance to cavitation and other physiological traits do not hold among unrelated Populus deltoides ×Populus nigra hybrids

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides×Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Ψ₅₀) varied from ?1.60 to ?2.40 MPa. Drought‐acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Ψ₅₀ being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Ψ₅₀ was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water‐use efficiency. At the whole‐plant level, increased safety was associated with higher shoot growth potential under well‐irrigated regime only. We conclude that common trade‐offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.     

Effects of a 20-day-long dry period on cambial and apical meristem growth in Abies balsamea seedlings

There is general agreement that in many regions additional precipitation with climate change will not be able to balance the increased evaporation rate induced by higher air temperatures, causing periods of intense drought. Although seedlings of Abies balsamea growing in the boreal forest are known for their resistance to harsh environmental conditions, the impact of water stress on their growth still remains largely unexamined. The aim of this study was to investigate growth responses of this species during and after a dry period by monitoring cambial and apical meristem activity at short time scale. Meristem growth was studied from May to October 2005 on seedlings of A. balsamea submitted to a 20-day-long dry period in June–July. Lower rates of shoot lengthening were observed in non-irrigated seedlings only in the first part of the growing season. Irrigated and non-irrigated trees showed the same trend of xylem formation and timings of cell differentiation. Cell production during cambial activity was estimated at about one xylem cell per day thus achieving in 100 days 108 tracheids in the tree ring and a width of 2 mm, with thinner tree rings observed in non-irrigated plants. A reduction of up to 50% in lumen area and cell diameter was observed for the cells produced during the dry period. Response of A. balsamea seedlings to a 20-day-long dry period consisted of good resistance of the cambial meristems during and after water stress, high sensitivity and rapid recovery of cell sizes during water depletion and slow but effective recovery of shoot growth after treatment.