Canopy transpiration of a Pinus canariensis forest at the tree line: implications for its distribution under predicted climate warming

Canopy transpiration (E _c) of a 50-year-old Pinus canariensis Chr. Sm. Ex DC. stand at tree line in Tenerife, Canary Islands, was estimated continuously throughout a year from March 1, 2008, to February 28, 2009, by means of xylem sap flow measurements. E _c varied markedly throughout the entire year generally following the seasonal trends in soil water availability and varied between 0.89 mm day^?1 under the conditions of non-limiting soil water availability and close to zero under soil drought. This is because canopy conductance declined significantly with increasing evaporative demand and thus significantly reduced tree water loss, and this decrease was more pronounced during the soil drought. Total annual E _c was 79.6 mm, which is significantly below the values estimated for other Mediterranean forest ecosystems and even 70 % lower than the value estimated for a P. canariensis forest at 1,650 m a.s.l. where the soil water content was higher than at the tree line site. Therefore, these results highlighted the importance of drought stress in tree line ecotone and should be taken more into account in semiarid tree lines.     

Effect of increased rainfall on water dynamics of larch (<Emphasis Type="Italic">Larix cajanderi</Emphasis>) forest in permafrost regions,Russia: an irrigation experiment

Sap flow measurements, from July to August 2004, were coupled with micrometeorological, soil moisture, and soil temperature measurements to analyze forest water dynamics in irrigated and undisturbed (control) larch (Larix cajanderi) forest plots in eastern Siberia. Plots were irrigated with 120 mm (20 mm day⁻¹) of water from 17 to 22 July. Sap flow measurements of ten trees at each plot were scaled up to daily stand canopy transpiration (E _c ). Canopy transpiration at the irrigation and control plots was similar before irrigation. Forest evapotranspiration (E _a ) was obtained from Ohta et al. (Agric For Meteorol 148:1941–1953, 2008) while E _a in the irrigation plot was estimated based on the E _{c_irrig}/E _{c_cont} ratio. Rainfall during July–August was 63.4 mm but, after including water from thawing soil layers, the actual water input was 109.9 and 218.5 mm in the control and irrigation plots, respectively. Despite this large difference, a corresponding difference in E _c (and E _a ) was not observed [42.6 (61.5) mm and 46.4 (71.8) mm in control and irrigation plots, respectively]. Daily canopy conductance (g _c ) increased as long as moisture was well supplied in the upper soil layers and evaporative demand was high. Soil moisture and rainfall contribution to E _a was 36.9 and 24.6 mm in the control plot and 34.5 and 37.3 mm in the irrigation plot, respectively. Water supply from soil thawing layers in the control plot and high runoff (105.6 mm) rates in the irrigation plot accounted for the similarity in water dynamics. Under increased precipitation, the forest used less soil water stored from previous growing seasons.     

Seasonal evolution of water status after outplanting of two provenances of Holm oak nursery seedlings

Forest restoration programs using Holm oak (Quercus ilex ssp. ballota [Desf.] Samp.) have had limited success. The effect of plant provenance on plantation success is uncertain, although some previous studies suggest that some provenances may be better able to tolerate stress. We studied the tolerance to drought in seedlings from two Spanish provenances of Holm oak before and after outplanting. One provenance was from a continental climate with cold winters (GR) and the other was from a xeric climate (HU). Seedlings were subjected to a water stress test in the nursery during the summer and survival was visually assessed after 2?weeks. In addition, 35 healthy seedlings of each provenance that were not subjected to the water stress tests were used for outplanting experiment. In these plants the seasonal changes in water potential at dawn (Ψ), specific leaf area (SLA), cuticular transpiration (E_c), and loss of xylem hydraulic conductance of twigs (PLC) were measured over 18?months. After the water stress test in summer, mortality was 44.3?% for GR seedlings and 12.6?% for HU seedlings. In addition there were differences between the two provenances in plant water status after planting. The HU provenance had a better water status and was more water conservative in the summer (higher Ψ, lower E_c, lower PLC), but not in the winter. The different drought tolerance and water relations parameters of these two provenances indicate that provenance should be considered in forest restoration and conservation programs involving Holm oak.     

Physiological response of irrigated and non-irrigated Norway spruce trees as a consequence of drought in field conditions

Physiological reactions of 25-year-old Norway spruce (Picea abies (L.) Karst.) trees to drought were examined during 2009 vegetation period. During the second half of summer, the decrease in soil water content was observed and irrigation was applied to a group of spruce trees, while the second group was treated under natural soil drought. The response to water deficit was recorded at the level of leaf water potential (Ψ_L). However, it appears that Ψ_L plays minor role in early stomata regulation of Norway spruce as CO₂ assimilation rate (P _N) and stomatal conductance (g _S) were reduced already before water potential decrease. Leaf water potential decreased significantly only in case when soil water content was low in the long run and when transpiration losses were simultaneously relatively high. Almost complete stomatal closure even of the irrigated trees was caused by the increase in the vapour pressure deficit of the air (D) above the value of approximately 1.5?kPa. Low values of D were accompanied by partial stomata opening of drought-treated trees. In non-irrigated spruce trees, the values of P _N decreased by 35–55% in comparison with irrigated trees. No drought-induced significant changes were found either in chlorophyll and carotenoid concentration (chl a?+?b, car) or in maximal photochemical efficiency of photosystem II (F _v/F _m). High rates of sap flow (F) did not always lead to stomatal closure during midday. It?appears that high transpiration rates do not control stomatal response to D.     

Drought effects on water use efficiency, freezing tolerance and survival of Eucalyptus globulus and Eucalyptus globulus × nitens cuttings

Genetic improvement of Eucalyptus genotypes for drought and frost resistance is essential for successful intensive management of commercial plantations. Understanding the physiological mechanisms that relate water use and frost resistance for highly deployed genotypes may allow for better prediction of their future performance, genetic selection and seedling management for site specific purposes. We studied whether instantaneous water use efficiency (WUE _i ) may serve as drought, freezing and photoinhibition tolerance predictor by studying its response on six E. globulus clones (Eg1–Eg6) and four E. globulus × E. nitens hybrid seedlings (Egn1–Egn4) under drought and irrigated (control) treatments. Net photosynthesis (A) and transpiration (E) were studied using a gas exchange system in order to calculate WUE _i (A/E). Simultaneous chlorophyll a fluorescence measurements were performed to assess the non photochemical quenching components. Frost tolerance of plants under control and drought treatments were evaluated by measuring temperatures that exert 50% photoinactivation of photosystem II. Finally, drought tolerance was evaluated by plant survival within each genotype after rehydration. Our results showed significant genotype variability in the rate of soil and xylem water potential decrease during drought. While most of the genotypes reached ?4.0 MPa in about 35 days of drought, genotypes Eg6 and Egn4 required 56 days of drought to reach this xylem water potential. WUE _i exhibited significant differences among genotypes and irrigation treatments. Genotypes Eg5 and Egn4 increased their WUE _i between 70 and 80% after drought. This was associated with a more conservative control of water loss at the stomatal level combined with maintenance of relatively higher rates of net photosynthesis than the other genotypes under drought conditions. Plants exposed to drought were more freezing tolerant than control plants, having in average 3°C lower LT₅₀ than well irrigated ones. There was no a clear correlation between WUE _i and drought tolerance or drought-induced photoinhibition, however WUE _i was inversely correlated with LT₅₀. Our results suggest that WUE _i is not suitable by itself to select drought tolerant genotypes, but may provide evidence for discarding drought sensitive genotypes. In addition, it could provide valuable information to select for freezing tolerance.     

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.

     

Evapotranspiration and deep percolation of a small catchment with a mature Japanese cypress plantation

Quantification of hydrological components was conducted in order to clarify the water budget, especially deep percolation (D) into underlying rock, of a small headwater catchment covered by an old Japanese cypress plantation. Precipitation (P), canopy interception (E _i), soil evaporation (E _s) transpiration (E _t), and runoff (R) were observed for 1 year in a 0.41-ha catchment in southern Kyushu, Japan. E _i was calculated from P, throughfall and stemflow measurements, and E _s and E _t were measured using evaporimeters and heat pulse sensors, respectively. From these observations, deep percolation (D) under steady state condition into underlying sedimentary rock was estimated. As for the annual water budget, almost half of P was R and one-third was evapotranspiration (E), which was the sum of the E _i, E _s and E _t. E _i was almost the same magnitude as predicted for an old conifer plantation modeled by Komatsu et al. (J Hydrol 336:361–375, 2007b), but E _t was lower compared to the model. E was not comparable to water loss in the catchment (P minus R). Therefore, D was estimated as a quarter of P and was slightly higher compared to previously reported values for sedimentary rock catchments.     

Estimating water use of a black locust plantation by the thermal dissipation probe method in the semiarid region of Loess Plateau, China

Black locust (Robinia pseudoacacia) is a major reforestation species in the semiarid region in the Loess Plateau of China. There has been increasing concern about the sustainability of the plantations because of their possible high water-use. This study was, accordingly, undertaken to quantify the stand-scale water use of a middle-aged black locust plantation in the region. The thermal dissipation probe method was applied to 27 trees to measure sap flux densities in an experimental plot during the growing season of 2008. The monoculture stand has a basal area of 23.3 m² ha^?1 and a maximum plant area index (PAI) of 2.89. Sapwood areas were estimated by use of a regressive relationship with the diameter at breast height (DBH) for scaling up of stand transpiration. The results showed that DBH could be a good predictor of sapwood area of individual trees. The diurnal cycles of average sap flux densities differed among DBH classes. Daily transpiration can be predicted from mean daily daytime vapor pressure deficit (VPD_m) using a fitted exponential saturation model. Model variables were different among seasons, probably owing to different soil water conditions and leaf phenology. By using the derived model for each month, stand canopy transpiration over the growing season was estimated to be 73.8 mm, with an average daily value of 0.41 mm day^?1 and a maximum of 0.89 mm day^?1. The relatively small estimates of stand transpiration might be attributed to low PAI and sap wood area of the middle-aged stand.     

Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought

In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.     

Nitrogen promotes water consumption in seedlings of Cryptomeria japonica but not in Chamaecyparis obtusa

To evaluate the influence of excessive N deposition on the water consumption of a Japanese plantation forest, 1-year-old seedlings of major plantation trees, Cryptomeria japonica (Japanese cedar) and Chamaecyparis obtusa (Hinoki cypress), were treated with combinations of two N levels (Moderate N and High N) and two soil water conditions (Dry and Moist) for 4 months. The High N treatment received five times as much N as in the Moderate N treatment; the total amount of N added in the High N treatment was roughly 25 times the annual N deposition in precipitation. An increase in soil N availability increased the needle transpiration rate, needle biomass, and needle N content of C. japonica under the Moist treatment, whereas those of C. obtusa were not significantly affected by soil N treatment at either soil water level. Needle N content in C. japonica was positively related to needle photosynthetic rate and transpiration rate. Our results suggested that excessive N deposition has the potential to enhance water consumption in C. japonica stands on moist soils. However, the effects of increased N deposition would be insignificant for C. japonica grown on dry sites. Unlike in C. japonica, water consumption in C. obtusa would be unlikely to respond to excess N deposition, regardless of the soil moisture level. Moreover, the significant reduction in the fine root to needle ratio observed with excessive N application in C. japonica under both Dry and Moist treatments suggests that excessive N deposition is likely to cancel out the tree's morphological adaptation to drought.     

Impact of time lags on diurnal estimates of canopy transpiration and canopy conductance from sap-flow measurements of <Emphasis Type="Italic">Populus cathayana</Emphasis> in the Qinghai–Tibetan Plateau

Recently, canopy transpiration (E_c) has been often estimated by xylem sap-flow measurements. However, there is a significant time lag between sap flow measured at the base of the stem and canopy transpiration due to the capacitive exchange between the transpiration stream and stem water storage. Significant errors will be introduced in canopy conductance (g_c) and canopy transpiration estimation if the time lag is neglected. In this study, a cross-correlation analysis was used to quantify the time lag, and the sap flow-based transpiration was measured to parameterize Jarvis-type models of g_c and thus to simulate E_c of Populus cathayana using the Penman–Monteith equation. The results indicate that solar radiation (R_s) and vapor pressure deficit (VPD) are not fully coincident with sap flow and have an obvious lag effect; the sap flow lags behind R_s and precedes VPD, and there is a 1-h time shift between E_c and sap flow in the 30-min interval data set. A parameterized Jarvis-type g_c model is suitable to predict P. cathayana transpiration and explains more than 80% of the variation observed in g_c, and the relative error was less than 25%, which shows a preferable simulation effect. The root mean square error (RMSEs) between the predicted and measured E_c were 1.91 × 10^?3 (with the time lag) and 3.12 × 10^?3 cm h^?1 (without the time lag). More importantly, E_c simulation precision that incorporates time lag is improved by 6% compared to the results without the time lag, with the mean relative error (MRE) of only 8.32% and the mean absolute error (MAE) of 1.48 × 10^?3 cm h^?1.     

Species selection in hardwoods research: variations in baseline physiological responses of select temperate hardwood tree species

Drought periods are becoming more extreme worldwide and the ability of plants to contribute towards atmospheric flux is being compromised. Properly functioning stomata provide an exit for water that has been absorbed by the roots, funneled into various cell parts, and eventually released into the atmosphere via transpiration. By observing the effects that weather conditions such as climate change may have on stomatal density, distribution, and functioning, it may be possible to elucidate a portion of the mechanisms trees use to survive longer periods of water stress. This study analyzed stomatal density (SD), stomatal conductance (gs ), CO2 assimilation (A), instantaneous water-use efficiency (WUEi ), and transpiration (E) rates in six native tree species in the Midwestern USA and showed that trees within the same ecotype followed similar trends, but that trees within the same family did not when exposed to identical greenhouse conditions. Naturally drought tolerant tree species demonstrated lower g s and higher WUEi , while intolerant species had higher SD. This study showed negative or no correlation between SD and g s , A, E, and WUEi and positive correlations between E and A and gs and E.     

Impact of soil drought on sap flow and water status of evergreen trees in a tropical monsoon forest in northern Thailand

Hill evergreen forest is the dominant vegetation type in northern Thailand. In this region, there is higher atmospheric evaporative demand and lower soil moisture during the 5- to 7-month dry season than in the rainy season under influences from Asian monsoons. In an earlier study we revealed that canopy-scale transpiration is actively maintained even during the latter part of the dry season in hill evergreen forest. However, the impact of soil drought on tree water use was not investigated. To clarify the ecohydrological processes at this site, we used individual tree-scale measurements during a 2-year period to base our examination of whether limited water use in individual trees is caused by soil drought in the latter part of the dry season. Sap flow and water potential measurements were conducted in four evergreen trees, two large emergent trees 29.8 and 25.4 m high, and two smaller understory trees 4.8 and 1.4 m high.The amount of rainfall preceding the late dry season of 2004 was significantly less than that preceding the late dry season of 2003. Although a distinct decrease in sap-flow velocities in individual trees due to soil water stress was not found in the late dry season of 2003, it did become comparatively apparent in the late dry season of 2004; ranging from 10 to 40% for a given atmospheric evaporative demand. Furthermore, the reductions in sap-flow velocities and predawn stem-water potential were most significant in the smallest tree. The recovery of sap-flow velocities and water potential in the smallest tree after irrigation confirmed that the reductions in sap-flow velocity and predawn stem-water potential in the smallest tree were caused by soil drought. These results suggest that shallower roots could be reason for the significant decrease in water use in the smallest trees. The deeper roots of larger trees could be the reason for the reduced impact of soil drought on water use in larger trees, and canopy-scale transpiration might be maintained by larger trees, even in an unusually severe drought. These possibilities provide a new insight for management of evergreen forests under Asian monsoon influences.     

Mechanisms of drought response in Thuja occidentalis L. II. Post-conditioning water stress and stress relief

We examined the extent of osmotic adjustment and the changes in relative water content (RWC) and transpiration rate (i.e., relative stomatal function) that occur in water-deficit-conditioned 6-year-old Thuja occidentalis L. (eastern white cedar) trees in response to a severe drought. Trees conditioned by successive cycles of mild or moderate nonlethal water stress (conditioning) and nonconditioned trees were exposed to drought (i.e., -2.0 MPa predawn water potential) to determine if water deficit conditioning enhanced tolerance to further drought stress. Following drought, all trees were well watered for 11 days to evaluate how quickly osmotic potential, RWC and transpiration rate returned to preconditioning values. Both nonconditioned trees and mildly conditioned trees exhibited similar responses to drought, whereas moderately conditioned trees maintained higher water potentials and transpiration rates were 38% lower. Both conditioned and nonconditioned trees exhibited a similar degree of osmotic adjustment (-0.39 MPa) in response to drought relative to the well-watered control trees. The well-watered control trees, nonconditioned trees and mildly conditioned trees had similar leaf RWCs that were about 3% lower than those of the moderately conditioned trees. Following the 11-day stress relief, there were no significant differences in osmotic potential between the well-watered control trees and any of the drought-treated trees. Daily transpiration rates and water potential integrals (WPI) of all drought-treated trees approached those of the well-watered control trees during the stress relief period. However, the relationship between cumulative transpiration and WPI showed that previous exposure to drought stress reduced transpiration rates. Leaf RWC of the moderately conditioned trees remained slightly higher than that of the nonconditioned and mildly conditioned trees.     

Estimating water use of sclerophyllous species under East-Mediterranean climate: III. Tabor oak forest sap flow distribution and transpiration

The annual course of daily transpiration and the hydrological balance of a Tabor oak forest were determined. The study was done in a representative forest within the natural geographical range of the species in the lower Galilee region of Israel. The climate is sub-humid with a rainless dry season from May to October. A partial water balance of a 0.1 ha area supporting an average of 14 trees was calculated from: (a) soil water content (SWC) measured by a Neutron Probe at depths of from 0.2 to 8 m, and (b) daylight transpiration rate measured with sap flow sensors by the heat pulse technique.Soil–bedrock complex water content (%) in the first 2 m of the profile fluctuated strongly between 5 and 20% depending on the season. The water content increased with depth from about 10% at 2.0 m depth to more than 20% at 5.0 m depth. For depths exceeding 5.5 m seasonal fluctuations in water content were negligible and water content ranged from 30 to 35%. After a dry winter, water content generally decreased within the main root zone down to about 2.0 m depth. Monthly changes in water content (mm) were greatest at depths of 0.35–1.0 m. Only minor changes in the soil–bedrock complex water content were recorded at greater depths. After a very rainy winter (2002/2003), decreases in soil–bedrock complex water content in the upper 2 m were much larger than after a dry winter. Fluctuations of soil–bedrock complex water content in deeper regions were larger in the wetter year, probably the result of drainage.Sap velocity was measured at six depths in the sapwood, from 4 to 44 mm, at 8 mm intervals. Sap velocity declined with depth, hence, sap flux density too.Based on sap velocity measurements performed during 4 years, the annual average daily transpiration (T) was 0.796 mm/day. This sums up to 239 mm during ∼300 days of leaf carriage, i.e. 41.3% of the 578 mm average annual rainfall for the area in the last 50 years. In a relatively dry year (rainfall of 432.7 mm) total water withdrawal from the 8 m soil–bedrock profile was 81% of the annual rainfall; of this amount 69% were transpired by the oak trees (239.0 mm), or 55% of the annual rainfall. In a relatively wet year (annual rainfall 801.4 mm) total water withdrawal was 67%; of this amount 45% would be transpired by the oak trees, or 30% of the annual rainfall.     

Interspecific variation in leaf water use associated with drought tolerance in four emergent dipterocarp species of a tropical rain forest in Borneo

By use of tree-tower and canopy-crane systems we studied variations in the water use, including transpiration, stomatal conductance, and leaf water potential, of the uppermost sun-exposed canopy leaves of four emergent dipterocarp species in an aseasonal tropical rain forest in Sarawak, Malaysia. Midday depression in stomatal conductance and leaf water potential was observed in all the species studied. Interspecific differences were clearly observed in the maxima of transpiration rates and stomatal conductance and the minima of leaf water potential among the four dipterocarp species. These interspecific variations were closely related to wood density and to factors affecting ecological patterns of distribution. Specifically, Shorea parvifolia and S. smithiana, both of which have a relatively low wood density for Dipterocarpaceae and are found on clay-rich soil, had a high transpiration rate in the daytime but had a large midday depression and a low leaf water potential. In contrast, Dryobalanops aromatica, which has a high wood density and is found in sandy soil areas, consumed less water even during the daytime. Dipterocarpus pachyphyllus, which has a high wood density and is found on clay-rich soil, stood intermediate between Shorea and D. aromatica in leaf water use. The two Shorea species had higher mortality than the others during the severe drought associated with El Ni?o in 1998, so daily pattern of leaf water use in each dipterocarp species might be correlated with its susceptibility to unusual drought events.     

Tolerance of Japanese larch to drought is modified by nitrogen and water regimes during cultivation of container seedlings

Improving drought tolerance of container seedlings of Japanese larch is of high importance to afforestation. We hypothesized that adequate nitrogen (N) and limited water supply would increase the tolerance of container seedlings to water-deficit stress, circumventing photoinhibition, by means of (i) enhanced photosynthetic capacity with higher leaf N and (ii) decreased water loss from leaves with lower biomass allocation into aboveground parts. Container seedlings of Japanese larch were grown under the treatment combinations of adequate (+?N: 300 mg N container^?1) or limited (??N: 150 mg N container^?1) N and adequate (+?W: daily irrigation) or limited (??W: twice-a-week irrigation) water. Then, seedlings were subjected to a progressive drought treatment. Higher leaf N was observed in container seedlings grown under?+?N and???W. During progressive drought, lower stomatal conductance and net photosynthetic rate were observed in leaves with higher leaf N at a given predawn leaf water potential. Furthermore, the maximum efficiency of PSII photochemistry (F_v/F_m) was lower in leaves with higher leaf N, suggesting that higher leaf N might impair intrinsic tolerance to drought at the leaf level contrary to expectations. Conversely,???N and???W seedlings with lower shoot biomass delayed soil drying as a whole-plant response via a reduction in leaf transpiration, leading to delayed photoinhibition as indicated by a decline in F_v/F_m. To circumvent stress at the initial stage of water deficit, lower leaf N via limited N regime and smaller shoot biomass driven by limited N and water regimes would be important.

     

Effects of extreme soil water stress on photosynthetic efficiency and water consumption characteristics of <Emphasis Type="Italic">Tamarix chinensis</Emphasis> in China’s Yellow River Delta

Soil moisture is a major limiting factor for plant growth on shell ridge islands in the Yellow River Delta. However, it is difficult to carry out situ experiment to study dominant plant photosynthesis physiological on the shell ridge islands under extreme soil water stress. To evaluate the adaptability of plants to light and moisture variations under extreme soil moisture conditions present on these islands, we measured photosynthetic gas exchange process, chlorophyll fluorescence, and stem sap flow variables for 3-year-old trees of Tamarix chinensis Lour, a restoration species on these islands, subjected to three types of soil water levels: waterlogging stress (WS), alternating dry–wet (WD), and severe drought stress (SS) to inform decisions on its planting and management on shell ridge islands. Gas exchange, chlorophyll fluorescence, and stem sap flow in T. chinensis were then measured. Net photosynthetic rate (P _N), transpiration rate (E), and water use efficiency (W _UE) were similar under WS and alternating dry–wet conditions, but their mean E and W _UE differed significantly (P < 0.05). Under SS, the P _N, E and W _UE of T. chinensis leaves varied slightly, and mean P _N, E and W _UE were all low. Apparent quantum efficiency (A _QY), light compensation point (L _CP), light saturation point (L _SP), and maximum net photosynthetic rate (P _Nmax) of leaves were not significantly different (P > 0.05) under WS and dry–wet conditions; however, under extreme drought stress, compared with the dry–wet conditions, L _CP was higher, L _SP was lower, and A _QY and P _Nmax were both at the lowest level. Therefore, drought stress weakened light adaptability of leaves, and the efficiency of light transformation was poorer. (3) Maximum photochemical efficiency (F _v/F _m) and the actual photochemical efficiency (Φ _PSII) were similar under waterlogged stress and dry–wet conditions, indicating a similar healthy photosynthetic apparatus and photosynthetic reaction center activity, respectively. Under SS, F _v/F _m was 0.631, and the coefficient of non-photochemical quenching (N _PQ) was 0.814, which indicated that while the photosynthetic mechanism was damaged, the absorbed light energy was mainly dissipated in the form of heat, and the potential photosynthetic productivity was significantly reduced. The daily cumulants of sap flow of T. chinensis under dry–wet alternation and severe drought stress were 22.25 and 63.97% higher, respectively, than under waterlogging stress. Daily changes in sap flow velocity for T. chinensis differed under the three soil water levels. Stem sap flow was weak at night under severe drought stress. Under dry–wet alternation, daytime average stem sap flow velocity was the highest, and night stem flow accounted for 10.26% of the day cumulants, while under waterlogged stress, the average nightly stem flow velocity was the highest, accounting for 31.82% of the day cumulants. These results provide important information for regional vegetation restoration and ecological reconstruction.     

Trees improve water storage and reduce soil evaporation in agroforestry systems on bench terraces in SW Uganda

The success of agroforestry in semi-arid areas depends on efficient use of available water and effective strategies to limit tree/crop competition and maximise productivity. On hillsides, planting improved tree fallows on the degraded upper section of bench terraces is a recommended practice to improve soil fertility while cropping continues on the lower terrace to maintain food production. This study examined the influence of tree fallows on soil water content (θ _w ) and evaporation (E _s ). Alnus acuminata Kunth (alnus), Calliandra calothyrsus Meissner (calliandra), Sesbania sesban L. (sesbania), a mixture of all three species, or sole crops (beans (Phaseolus vulgaris L.) or maize (Zea mays L.)) were grown on the upper terrace. The same sole crops were grown on the lower terrace. Four management regimes (unpruned, root, shoot and root + shoot pruned) were applied to the tree rows adjacent to the cropping area. Neutron probe and microlysimeter approaches were used to determine θ _w and E _s when the trees were c. 3.5 years old. Sesbania and alnus increased θ _w by 9–18 % in the cropping area on the lower terrace but calliandra reduced θ _w by 3–15 %. After heavy rain, E _s comprised 29–38 % of precipitation in the tree-based treatments and 53 % under sole crops. Absolute values declined as rainfall decreased, but E _s as a proportion of rainfall increased to 39–45 % in the tree-based treatments and 62 % for sole crops. Root + shoot pruning of alnus and the tree mixture increased θ _w in the cropping area but had no significant effect in the other tree-based treatments. The results suggest that sesbania and alnus can be planted on smallholdings without compromising water supply to adjacent crops, whereas calliandra decreased water availability despite reducing E _s . These results provide a mechanistic understanding of reported effects on crop yield in the same site.     

Water availability preceding long-term drought defines the tolerance of <Emphasis Type="Italic">Eucalyptus</Emphasis> to water restriction

Plantations of Eucalyptus species are expanding across South America into regions where drought conditions can reduce growth rate and result in substantial commercial loss. Understanding the mechanisms of drought tolerance in Eucalyptus is essential for the successful production in drought-regions. The main objectives of this study were to evaluate how water availability preceding a long-term drought period affects morphological, physiological and molecular traits of four Eucalyptus clones grown under field conditions. The study areas are located in north-eastern Brazil with an average rainfall of 800 and 1500 mm per year. At each rainfall regime, the following clones were evaluated: 1404 (Eucalyptus urophylla), 1407 (E. urophylla × E. camaldulensis), 1296 and 6500 (E. grandis × E. urophylla). Our results indicate that trees growing in the area with higher annual precipitation were more stressed after long-term drought, compared to those stands previously exposed to mild water-restriction period. The genetic materials showed distinct responses to drought, which allowed their separation in two groups: drought-tolerant (1404 and 1407) and drought sensitive (6500 and 1296). The former group shows some important adaptations to drought, such as decreased leaf area (avoiding excessive transpiration rates), higher antioxidant activity and carotenoid concentration (leading to lower lipid peroxidation). In conclusion, previous exposure to water deficit may provide the benefit of increased defense protection during future water deficit. From all measured variables, the leaf area, antioxidant compounds and changes in ¹³C and ¹⁸O isotope abundance reflect some of the most important morphological and physiological alterations in order to mitigate the water stress damage in drought-tolerant genotypes.