Predawn Water Potential Research Articles

Long-term impacts of stand management on ponderosa pine physiology and bark beetle abundance in northern Arizona: A replicated landscape study

Ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) forests in northern Arizona have degraded due to overgrazing, logging, and fire suppression that accompanied Euro-American settlement in the late 1800s. Overstocked stands of suppressed trees with low structural diversity dominate the landscape. These conditions create high risk of catastrophic fires and insect outbreaks. We investigated long-term effects (8–16 years post-treatment) of thinning and thinning + prescribed burning on ponderosa pine water stress, leaf carbon isotope discrimination and nitrogen concentration, oleoresin exudation flow, phloem thickness, radial growth, and bark beetle abundance relative to unmanaged control stands over 2 years of measurement in 12 stands replicated across the landscape. Predawn water potential in late June, phloem thickness, and basal area increment were lower in unmanaged than managed stands. Oleoresin exudation flow in July was greater in unmanaged and thinned + burned stands than thinned stands, and greater in a warm year than a cooler year. Leaf nitrogen concentration differed between years, but not among treatments. Tree competition and water stress were positively correlated, and tree competition was negatively correlated with radial growth and phloem thickness. Pheromone-baited trap catches of Dendroctonus spp. ( D. brevicomis Leconte pooled with D. frontalis Zimmerman) were higher in unmanaged than managed stands, whereas catches of Ips spp. did not differ among treatments. We conclude that thinning with and without prescribed burning can have long-term effects on ponderosa pine water stress, growth, phloem thickness, resin flow, and bark beetle abundance. Low levels of tree mortality from bark beetles at our study sites suggest remarkable resistance of ponderosa pine in mid-elevation forests in northern Arizona, even at high tree densities.

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψpd) and midday water potential were measured in transpiring (Ψleaf) and non-transpiring leaves (Ψxyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (gs) and sap flow (Fd) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (Kh) and whole tree hydraulic conductance (Khsf) were calculated by using the simple analogy of the Ohm’s law. Kh was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψlxyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψuxyl) at the uppermost canopy. Predawn water potential (Ψpd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (Kh) decreased progressively as water stress increases. Minimum values of Kh and Khsf were recorded when Ψpd was lower. However, Q. petraea showed higher Kh than F. sylvatica for the same Ψpd. The decrease in Kh with water stress was mainly linked to its fall from the soil to the lowermost canopy (Ksrs). Nevertheless, a significant resistance in the petiole–leaf lamina (Kpl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in Kh was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (gs) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.

(452) Growth Responses of Salvia greggii and Dalea frutescens to Drought Stress

Salvia greggii (salvia) and Dalea frutescens (dalea) are two popular shrubs. However, little information is available on their drought tolerance. The objectives of this study were to investigate the effect of various degrees of water stress on growth and to characterize the dynamics of water relations to root substrate water content for developing best irrigation management. Salvia and dalea plants in 12-L plastic containers were grown in a greenhouse and pruned to one node at the base of the soft shoots for salvia or at the same height for dalea prior to the start of the experiment. There were three irrigation regimens: plants were irrigated daily (control), or irrigation was withheld until moderate or severe water stress signs exhibited. After several weeks of intermittent cyclic dry-down irrigation regimens, total shoot number per container was reduced by 40% to 50% for salvia and 35% to 40% for dalea. Average shoot length was reduced by 35% to 45% for salvia and 50% to 65% for dalea in moderate and severe stressed treatments compared to the control. Drought stress resulted in less shoot elongation and fewer new shoots in both species. To examine the relationship between plant water status and substrate water content, a dry down test was performed on five well-watered plants by withholding irrigation until midday water potential dropped to below –4 MPa. As substrate water contents in both species reached 8%, the predawn water potentials did not recover from the midday water potential of the previous day, indicating there was no available water in the substrate for roots to take up. The drought tolerance of these two species needs further study using various growing media.

(451) Acclimation to Long-term Water Deficit Stress in Four Birch Taxa (Betula L.): Water Relations and Gas Exchange

Four birch taxa, Betulaalleghaniensis, B. davurica, B. nigra `Heritage', and B. papyrifera, were grown in a greenhouse and exposed to water deficit stress for 35 days. Daily water was withheld from the plants until the combined weight of each container and the plant was reduced to 40% of the control. Supplemental water was supplied to each container to maintain 40% of the initial combined weight. Predawn water potential, gas exchange, and abscised leaf area were measured daily during the first week and at 3-day intervals after the first week. Predawn water potential dropped to lowest value before the containers were rehydrated and remained low throughout the treatment period. Net CO2 assimilation (A), stomatal conductance (gs) and evapotranspiration (Et) of all taxa decreased significantly during the first week of water deficits. Changes in A and Et varied among taxa after the first week of water deficits. Water use efficiency (WUE = A/Et) of four birch taxa decreased to significant lower values than control in the first week that water was withheld, and then increased to significantly higher values than control. Gas exchange data were fit into 2- or 3-segmented linear model. The type and shape of the models, and the joint of each segment gave an indication of how the gas exchange responded to the long-term water deficit stress. At the end of the water deficit treatment, B. papyrifera had abscised most of its leaves and maintained a high A, while B. alleghaniensis maintained more leaves on the tree, but had a relatively low A.

The significance of episodic rains for reproductive phenology and productivity of trees in semiarid regions of northwestern Venezuela

The semiarid regions of northwestern Venezuela have extremely low and highly unpredictable precipitation, yet these conditions support species with contrasting phenology and leaf longevity. Episodic rains significantly increased leaf water potential (from −5 to −2.5 MPa) in several species and, in some cases, triggered flowering, leading us to hypothesize that the coexistence of species with contrasting phenology is due to differences in their ability to utilize small rainfall events. Irrigation treatments were used to simulate brief rainfall events, and the response of three species (Erythrina velutina [deciduous], Croton heliaster [semideciduous], and Capparis odoratissima [evergreen]) was monitored over a period of 14 months. To partition the effects of water reaching the canopy versus the soil, irrigation was supplied either in the form of mist to the canopy or by minisprinklers near the base of the trees. Nonirrigated trees were used as controls. Productivity (estimated as aboveground litter production) and water potential were enhanced by soil irrigation in two species. However, in the evergreen species canopy irrigation had a greater effect on water relations and productivity than soil irrigation, as indicated by higher predawn water potential, higher total annual flower (40 g m−2 year−1) and fruit (5 g m−2 year−1) production, and longer leaf longevity (410 days in control trees versus 520 days in canopy-irrigated trees). Canopy irrigation augmented flower and fruit production in all three species. Our findings suggest that reproductive phenology in these species is driven by episodic rains and that evergreen species may sustain productivity by their ability to make use of water deposited on leaf surfaces.

Effetti di carenze idriche prolungate su pinete mediterranee: insegnamenti da due esperimenti in Italia meridionale

A synthesis is presented of two studies carried out on the effects of imposed water shortage in two Mediterranean pine stands growing in Southern Italy, highlighting how physiological responses and structural acclimation can combine in the overall response of mature pine trees to long-term drought. The impact of a severe reduction in water availability on the water relations, transpiration and growth of 50-year-old Pinus halepensis trees (growing along the Ionian coast, Puglia, experiment 1) and 35-year-old Pinus laricio trees (growing in Sila, Calabria, experiment 2) was studied in two replicated experiments, in which the water available to trees was restricted by suspending a `roof’ made up by transparent polyethylene sheet. The experimental effects were followed for 12 months in experiment 1 and for 36 months in experiment 2, comparing “covered” plots and “uncovered” control plots. In experiment 1 trees experienced a severe water stress during the experimental period; a direct and rapid impact of drought was evident on the water relations and gas exchanges; there was an evident decrease of predawn water potential and transpiration was strongly reduced by stomatal closure (short-term physiological adjustment). A few months after the removal of the covering, no difference in whole plant hydraulic resistance was observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees, but no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. In experiment 2, albeit rather extreme at the first sight, the treatment resulted in a realistic simulations of water availability as predicted by climate change scenarios; a 35% reduction in terms of timber yield and a 30% reduction of leaf area was estimated over the experiment in covered plots; however, the reduction of radial growth over the three years translated into a mere 2% reduction in sapwood area, so that a shift of resources from leaves to sapwood was apparent after three years (structural adjustment). As for the effect of treatment on transpiration, a 43% reduction of transpiration was observed in covered trees over the summer, but recovering to the same control values was measured over the autumn when, due to differences in leaf area, leaf specific transpiration was greater in covered trees; no significant differences was found for leaf hydraulic resistance and minimum daily values of water potential between covered and control plots; this suggests that stomatal closure was effective in counterbalancing the effects of soil dryness, maintaining minimum water potential values rather constant (homeostasis of water potential), largely decoupled from predawn water potential (isohydric behaviour).

The use of plant water relations to characterize tree species and sites in the drylands of northern Ethiopia

Water relations of four tree species were studied in four sites in the drylands of northern Ethiopia. Predawn water potentials were significantly higher at all sites as compared to midday measurements for Acacia etbaica and Boswellia papyrifera. In contrast, Lannea fruticosa revealed this significant difference only once, while Terminalia brownii at only two of the four sites. An analysis of variance showed that A. etbaica has a wider tolerance range as compared to the other species. There was no significant difference among the four sites in terms of predawn, midday and diurnal ranges of water potential. The study revealed that A. etbaica and B. papyrifera are performing better as compared to T. brownii and L. fruticosa under the current climatic conditions. Understanding the physiological basis of survival, productivity, ability to cope with low moisture availability, and growth potential of indigenous species in the drylands is essential for the utilization and promotion of these and other species.

Seasonal variations in monoterpene profiles and ecophysiological traits in Mediterranean pine species of group “halepensis”

Abstract: Foliar and cortical terpene profile, and needle gas exchange and water potential of P. halepensis, P. brutia and P. eldarica were compared over three consecutive seasons (1996-1998) in an experimental plantation nearby Firenze (Italy). Terpene percentages in mature tissue (cortex and needle) did not change in response to water stress during summer period and remained stable through seasons and years. Terpene profiles were not affected by seasonal drought, and are thus valuable to characterize Mediterranean pine species of the group “halepensis”. There was a threshold-type response of maximum daily gas exchange to decreasing predawn water potential in all pines. Net photosynthesis and needle conductance were linearly related, regardless of the species.

Osmotic adjustment in two temperate oak species [Quercus pyrenaica Willd and Quercus petraea (Matt.) Liebl] of the Iberian Peninsula in response to drought

One-year-old seedlings of Quercus pyrenaica and Quercus petraea were subjected in a greenhouse to three consecutive water stress cycles. Watering was withdrawn in water stressed seedlings until 5% volumetric soil water content in the two first cycles and up to 3.5% in the third one. At the end of each cycle, predawn water potential (Ψpd) was measured by a pressure bomb. Furthermore, other water parameters: osmotic potential at full (Ψπ100) and at turgor loss point (Ψπ0) as well as bulk modulus of tissue elasticity at maximum turgor (Emax), were derived from the establishment of pressure-volume curves at the end of the second and third water stress cycles. In the three drought periods there was a significant decrease of Ψpd in both species as compared to the controls which were maintained at field capacity. This decrease progressed from the first to the following cycles in Quercus pyrenaica up to –1.7 ± 0.31 MPa minimum value. In Quercus petraea the Ψpd value reached at the end of the first cycle (±1.8 ± 0.52 MPa), was no significantly different from those reached afterwards. A large decrease in Ψπ100 and Ψπ0 together with an increase of Emax were recorded in both species after the second water stress cycle, without any new significant variation in the third one. When well-watered and water-stressed seedlings were compared, Quercus pyrenaica showed a decrease in Ψπ100 of 0.8 MPa with regard to 0.3 MPa in Quercus petraea. These results show the occurrence of osmotic adjustment in both species. However, the osmoregulation capacity was higher in Quercus pyrenaica, which pointed out to a higher ability to develop mechanisms of water stress tolerance. This could be recognized as one of the physiological traits that explain why Quercus pyrenaica is better adapted to xeric conditions than Quercus petraea.

Effects of genetic variability and habitat of Qualea parviflora (Vochysiaceae) on herbivory by free-feeding and gall-forming insects.

Differences in the chemical and physical traits of plants caused by both genetic and habitat characteristics may influence attack by herbivores. Leaves of Qualea parviflora (Vochysiaceae), a common tree of different habitats in the Brazilian Neotropical savannas (cerrado), are susceptible to severe attack by herbivorous free-living and gall-forming insects. Attack by free-living and gall-forming insects within and between populations of Q. parviflora were examined and it was determined to what extent genetic variability (detected by RAPD markers), phenotypic characteristics of the plants and habit traits influence the number of free-living and gall-forming insect species and individuals attacking the plants, and the intensity of attack. On four occasions in 2000, leaves were sampled from ten individual trees in each of three types of vegetation in the cerrado: campo sujo, cerrado sensu stricto and cerradão at the Ecological Station of Pirapitinga (ESP), in Três Marias, north-western Minas Gerais, Brazil. Genetic variability was detected by RAPD markers, and concentrations of nutrients, phenols and tannins, sclerophylly and pre-dawn water potential of leaves were measured. Water and nutrient contents in the soil below each tree characterized the habitat. The free-living and gall-forming herbivorous insects were determined. Of 69 RAPD markers analysed, 41 were polymorphic and were used for analyses of genetic variability of Q. parviflora. Most of the variability occurred within habitats, accounting for 97.65 % of the genetic variability. Plants in the cerrado sensu stricto and campo sujo were the most similar. There were no significant associations between genetic similarity and the chemical and physical traits of Q. parviflora, or with habitat, nor was there significant correlation between phenotypic and habitat traits. Increasing concentrations of tannins and sulphur, C : N ratio and sclerophylly correlated with increasing percentage of leaf area damaged by herbivores. Decreased sclerophylly, concentration of tannins and C : N ratio, and increased concentration of nutrients in leaves correlated with increased severity of attack by gall-forming insects. Nutrient concentration in the soil had more influence, indirectly, on free-feeding insects than did composition of Q. parviflora leaves. However, gall-forming insects are affected more by leaf quality, attacking fewer sclerophyllous leaves, with larger nutrient but smaller tannin concentrations.

Differential seed mortality among habitats limits the distribution of the invasive non-native shrub Ardisia elliptica

During the early phases of biological invasions, both limited dispersal and habitat preference may contribute to observed patterns of distribution for non-native species. In a disturbed area of Everglades National Park, the non-native Ardisia elliptica (Myrsinaceae) does not occur in all habitat types nor is it evenly distributed among habitats where it does occur. One goal of this study was to determine whether the patchy distribution of A. elliptica may be due to differential effects of habitat on seed fates and germination. Screen mesh bags containing seeds of A. elliptica were buried in the surface litter of five habitats and were later sampled periodically over a three month period to measure seed germination, survival and death. Seed mortality and mean germination speed was higher in open exposed habitats relative to closed canopied forests. During the study period, which coincided with the dry season, there was no germination of seeds that had been placed in pine forests or in Schinus thickets that were highly disturbed pine forests. Because hydroperiod is a critical determinant of habitat type in the Everglades ecosystem, a second goal of this study was to determine whether water availability was correlated with seed germination and seedling survival in five sites where A. elliptica has invaded. Seeds were sown under mesh boxes and monitored for germination and seedling survival for ten months. Estimates of water availability taken as predawn water potentials on A. elliptica were positively correlated with percent seed germination but not with percent seedling survival. Additionally, pre-dawn water potential of A. elliptica differed among sites. This study suggests that differential water availability among habitats may have influenced the patchy distribution of A. elliptica in Everglades National Park. Lack of sufficient moisture availability for seeds in exposed habitats such as pine forests and wetland prairies during the dry season has contributed to its distribution. Because some of the habitats where A. elliptica currently occurs were pine forests and wetland prairies prior to disturbance, this study suggests that substrate disturbance and canopy formation have altered the soil microenvironment, favoring the establishment of A. elliptica.

Selection of species and provenances for low-rainfall areas: physiological responses of Eucalyptus cloeziana and Eucalyptus argophloia to seasonal conditions in subtropical Queensland

Responses of stomatal conductance ( g s) and net photosynthesis ( A) to changes in soil water availability, photosynthetic photon flux density ( Q), air temperature ( T) and leaf-to-air vapour pressure deficit ( D) were investigated in 4-year-old trees of a dry inland provenance of Eucalyptus argophloia Blakely, and two dry inland provenances (Coominglah and Hungry Hills) and a humid coastal provenance (Wolvi) of Eucalyptus cloeziana F. Muell. between April 2001 and April 2002 in southeast Queensland, Australia. There were minimal differences in A, g s and water relations variables among the coastal and inland provenances of E. cloeziana but large differences between E. argophloia and E. cloeziana. E. argophloia and to a lesser extent the Hungry Hills (inland) provenance of E. cloeziana maintained relatively higher pre-dawn water potential ( ψ pd) during the dry season suggesting possible access to water at depth. Simple phenomenological models of stomatal conductance as a function of Q, T and D explained 60% of variation in g s in E. cloeziana and more than 75% in E. argophloia, when seasonal effect was incorporated in the model. A Ball–Berry model for net photosynthesis explained between 70 and 80% of observed variation in A in both species. These results have implications in matching the dry and humid provenances of E. cloeziana and E. argophloia to suitable sites in subtropical environments.

Effects of salt stress on plant growth, stomatal response and solute accumulation of different maize genotypes

Seeds from eight different maize genotypes (BR3123, BR5004, BR5011, BR5026, BR5033, CMS50, D766 and ICI8447) were sown in vermiculite, and after germination they were transplanted into nutrient solution or nutrient solution containing 100 mmol.L-1 of NaCl and placed in a greenhouse. During the experimental period plant growth (dry matter, shoot to root dry mass ratio, leaf area, relative growth rate, and net assimilation rate), leaf temperature, stomatal conductance, transpiration, predawn water potential, sodium, potassium, soluble amino acids and soluble carbohydrate contents were determined in both control and salt stressed plants of all genotypes studied. Salt stress reduced plant growth of all genotypes but the genotypes BR5033 and BR5011 were characterized as the most salt-tolerant and salt-sensitive, respectively. Stomatal response of the salt-tolerant genotype was not affected by salinity. Among the studied parameters, shoot to root dry mass ratio, leaf sodium content and leaf soluble organic solute content showed no relation with salt tolerance, i.e., they could not be considered as good morpho-physiological markers for maize salt tolerance. In contrast, sodium and soluble organic solutes accumulation in the roots as a result of salt stress appeared to play an important role in the acclimation to salt stress of the maize genotypes studied, suggesting that they could be used as physiological markers during the screening for salt tolerance.

Growth and water relation parameters in drought-stressed Coriaria nepalensis seedlings

In the present study, growth and water relation parameters were analysed in drought-stressed Coriaria nepalensis Wall. seedlings. C. nepalensis seedlings were subjected to four drought cycles of 7, 14, 21, and 28-days, and to one control level (watered on alternate days) in a glasshouse. The seedlings failed to survive a 28-days drought during summer. Osmotic adjustment (defined as the decrease in osmotic potential at zero or full turgor in response to water deficit) was measured as the difference between the osmotic potential of seedlings watered on alternate days (control) and those subjected to 21-days drought cycle. Seedlings subjected to 21-days drought had a predawn water potential of −2.60 MPa, and showed an osmotic adjustment of −1.95 MPa at full turgor and −2.17 MPa at zero turgor. The growth of seedlings was positively related to moisture and with water potential. With decline in soil moisture the root:shoot ratio increased while leaf weight ratio decreased. Leaf characteristics, such as leaf number, leaf area, leaf area ratio, specific leaf area and leaf drop, were also affected by moisture stress. This study has indicated that osmotic adjustment is a major adaptive mechanism of C. nepalensis that aids successful regeneration of seedlings in degraded sites with inhospitable soil conditions.

Synergistic responses of oak, pine and shrub seedlings to edge environments and drought in a fragmented tropical highland oak forest, Oaxaca, Mexico

Processes of natural regeneration in highly fragmented landscapes are strongly influenced by edge environments due to the effects of vegetation cover on microclimate conditions. These effects may be diminished or exacerbated through synergistic interactions with extreme climatic events such as drought. This study assessed the growth and physiological responses of five native tree and shrub species ( Quercus acutifolia Neé, Q. castanea Neé, Pinus oaxacana (Mart.) Mirov, Dodonea viscosa (L.) Jacq., Rhus virens Adams) to microsites along edge gradients occurring between forest remnants and openings of different sizes (<0.1 and >1 ha) in both a highly fragmented and a relatively undisturbed adjacent landscape over a 2-year period (1 year with normal rainfall and one with extreme drought). Of the species studied, the shrub seedlings had significantly lower mortality (3–13%) than the oak (37–53%) and pine (35%). Seedling survival for all species was greatest in the edge and forest understory environments, suggesting that vegetation cover may have a facilitative effect on seedling regeneration. Greater survivorship and biomass of the shrub seedlings, especially in the open microsites, was attributed to their root system having a sufficient biomass and depth to more effectively access deeper water supplies. Interactions between edge effects and an extreme drought event were found to have a more pronounced impact on the performance of oak and pine seedlings compared to shrub seedlings. Patterns of pre-dawn water potential ( ψ pd<−3.8) and leaf phenology (slow turnover) indicated that oak species experienced high water stress that prevented their successful establishment in open environments, especially during drought years. In contrast, shrub seedlings, and to a lesser extent pine seedlings, maintained higher pre-dawn moisture potentials ( ψ pd>−2.9) and greater phenological plasticity (fast turnover) in response to drought, contributing to greater growth and survival success. To effectively manage fragmented tropical highland oak forest landscapes for reestablishment of the native oak ecosystem, there is a need to understand the complex and often synergistic interactions between different plant species, the edge environment, and climatic variability that strongly influence regeneration processes.

Tree Size and Drought Affect Ponderosa Pine Physiological Response to Thinning and Burning Treatments

Abstract Thinning and burning treatments based on presettlement (prior to Euro-American settlement) stand conditions have been proposed for improving the vigor and growth of Pinus ponderosa. No study has examined effects of different levels of such thinning treatments on tree water, carbon, and nitrogen relations, or compared effects between postsettlement (trees established after Euro-American settlement) and presettlement (established before Euro-American settlement) trees. We investigated responses of presettlement and postsettlement trees to three levels of thinning and burning (unthinned/unburned control, light thinning/burning, heavy thinning/burning) over 2 yr that differed in precipitation in northern Arizona. Both thinning treatments consistently increased predawn water potential of both tree sizes compared with the control. Effects of thinning on leaf gas exchange varied between tree sizes and measurement times. Thinning increased net photosynthetic rate and stomatal conductance only when soil water availability was lowest, and increases were greater for postsettlement than presettlement trees. In contrast, thinning had no effect on foliar nitrogen concentration.Our results suggest greater positive effects of restoration thinning on tree water and carbon relations for postsettlement versus presettlement trees, and under drought versus nondrought conditions. Photosynthetic response to thinning in old trees may be constrained by physiological factors associated with large size such as low soil-to-leaf hydraulic conductance. FOR. SCI. 50(1):81–91.

Rooting depth and plant water relations explain species distribution patterns within a sandplain landscape.

Tree and shrub species of the Banksia woodlands on the sandplains of northern Swan Coastal Plain, Western Australia possess a range of strategies to avoid or tolerate soil water deficits during the annual summer drought. Shallow-rooted shrub species (< 1 m rooting depth) inhabit a range of locations in the landscape, from top of dune crests to wetland embankments. These are the most drought-tolerant of all sandplain species, surviving extremely low summer soil water potentials (< -7 MPa) and tissue water deficits by significantly reducing their transpirational water loss (< 0.2 mmol m-2 s-1). This is in contrast to the few shallow-rooted species restricted to low-lying or seasonally waterlogged areas which are reliant on subsurface soil moisture or groundwater to maintain their relatively high summer water use. Recent studies of water source usage of selected Banksia tree species have shown that these deep-rooted species access groundwater up to a maximum depth of 9 m depth during the summer months, or soil moisture at depth when groundwater was greater than maximum rooting depths, depending on the species. Medium- and deep-rooted (1-2 m and > 2 m, respectively) shrub species cope with the summer soil drying phase and related decrease in groundwater levels by conserving leaf water loss and incurring predawn water potentials between -1 and -4 MPa, enabling them to occur over a range of topographic positions within the sandplain landscape.

WATER RELATIONS AND STOMATAL CONDUCTANCE IN NINE TREE SPECIES DURING A DRY PERIOD GROWN IN A HOT AND DRY VALLEY

The valleys along the rivers in southwestern China are hot and dry because of the rain shadow effect. The primary vegetation on the slopes in these valleys is savanna. However, due to heavy human disturbance, the vegetation on these slopes has been devastated or severely degraded, resulting in severe water and soil erosion. Rehabilitation or revegetation of these slopes is urgently needed. Some eucalypt and legume trees have been introduced for the purpose of revegetation, and these have shown fast growth during the first seven years. However, it is not known how these trees cope with the hot and dry stresses during the dry period. In the present study, we monitored the water status, stomatal conductance and osmotic regulation, and their seasonal changes of 9 tree species in a hot and dry valley. The height and diameter of these trees attained over 5 years of growth also were compared. The mean height of the 9 species ranged from 0.7 to 7.6 m, the diameter from 2.6 to 6.7 cm. Four species ( Eucalyptus dulebsis, Acacia leptocarpa, A. auriculiformis and A. cunninghamii ) showed fast growth, two species ( E. citriodora, Leucaena leucocephala ) had intermediate growth and the remaining species ( A. mangium and Robinia pseudoacacia ) had slow growth. The physiological measurements showed that as drought conditions progressed, all species decreased their leaf water potentials and stomatal conductance. However, the decrease was more rapid in A. leptocarpa, A. auriculiformis, A. cunninghamii and Eucalyptus dulebis. During the peak drought period (March and April), these species also reached the lowest water potentials (predawn water potential lower than -1.0 MPa) and nearly closed their stomata ( A. leptocarpa, A. auriculiformis, A. Cunninghamii ) or remained at low stomatal conductance ( Eucalyptus dulebsis, A. holosericea and Dodonaea wiscosa ) while the remaining species maintained relatively high water potentials and a low stomatal conductance ( Eucalyptus citriodora and Leucaena leucocephala ) or nearly closed stomata ( A. mangium). Nevertheless, during the drought period, no species showed leaf wilting, except R. pseudoacacia and the native species Vitex negundo and Terminalia franchetii, which were deciduous. All species increased osmoregulation capacity, as indicated by the decrease of osmotic potentials at full saturation, and the increase of osmoregulation substances. Osmoregulatory substances increased over time in the following sequence: K +, free Pro, other free amino acids and soluble sugars. It appeared that K + was important in osmoregulation at the beginning of the drought, while soluble sugars were important in the late drought period. E. dulebsis, A. auriculiformis and A. cunninghamii exhibited the highest osmoregulation capacities. Our results show that most of the introduced species are suitable to grow in these hot and dry habitats because of, among other things, they have strong capacities of water_absorption and water conservation.

Water status and leaf elongation of C3- and C4 grasses of Flooding Pampa grassland

Abstract Question: Do warm season (i.e. mainly C4) grass species exhibit a higher water status than winter-growing competitors, (mainly C3 plants) and is this part of their competition ability? Location: Flooding Pampa grassland at Balcarce, eastern Argentina. Methods: Predawn leaf water potential was measured in five species: (1) three cool-season species: Festuca arundinacea, Lolium multiflorum and Stipa neesiana, and (2) two warm-season species: Sporobolus indicus and Paspalum dilatatum, throughout a summer season. The sensitivity of leaf growth to low water status was investigated in a greenhouse experiment where plants were grown in pots at three levels of water deficit. Results: The field study revealed strong differences of water status between the five species likely indicating large contrasts in soil water availability, irrespective of their photosynthetic metabolism. By contrast, the response of leaf elongation rate to predawn water potential was similar in all species studied. Species to which ...

Afforestation with Pinus halepensis reduces native shrub performance in a Mediterranean semiarid area

It has been often suggested that the introduction of Pinus halepensis with afforestation could facilitate long-term colonisation and development of late-successional species in Mediterranean dry and semiarid areas, but this affirmation has rarely been experimentally tested in the field. In this study, we analyse the effects of planted P. halepensis density (0, 1500 and 3470 trees·ha –1 ) on soil moisture at two depths (0–10 and 10–30 cm), and on the physiological status of three shrub species (Quercus coccifera, Erica multiflora and Rhamnus lycioides )i n a semiarid area of SE Spain. Afforestation with P. halepensis had a negative effect on soil moisture, an effect that increased with tree density. This effect was more pronounced at 0–10 cm depth than at 10–30 cm depth, and was especially evident after the spring and late summer rainfalls. During spring and summer, mean predawn water potentials in the area without P. halepensis were –1.5, –2.4 and –2.2 MPa for Q. coccifera, E. multiflora, and R. lycioides, respectively. Mean net photosynthesis values in the same area and period were 1.2, 1.0 and 1.5 µmol CO2 m –2 s –1 for Q. coccifera, E. multiflora, and R. lycioides, respectively. Similarly, mean stomatal conductance was 15.1, 16.4 and 21.7 mmol H2 Om –2 s –1 for Q. coccifera, E. multiflora, and R. lycioides, respectively. Values were substantially reduced (up to 56%) with increasing P. halepensis density, suggesting the presence of a strong competition for water. Our results provide evidence that the introduction of P. halepensis with afforestation has a negative effect on existing late-successional shrubs, and suggest that this introduction does not stimulate successional processes in Mediterranean semiarid areas.