Leaf Water Potential Of Trees Research Articles

L-band vegetation optical depth as an indicator of plant water potential in a temperate deciduous forest stand

Abstract. Vegetation optical depth (VOD) retrieved from microwave radiometry correlates with the total amount of water in vegetation, based on theoretical and empirical evidence. Because the total amount of water in vegetation varies with relative water content (as well as with biomass), this correlation further suggests a possible relationship between VOD and plant water potential, a quantity that drives plant hydraulic behavior. Previous studies have found evidence for that relationship on the scale of satellite pixels tens of kilometers across, but these comparisons suffer from significant scaling error. Here we used small-scale remote sensing to test the link between remotely sensed VOD and plant water potential. We placed an L-band radiometer on a tower above the canopy looking down at red oak forest stand during the 2019 growing season in central Massachusetts, United States. We measured stem xylem and leaf water potentials of trees within the stand and retrieved VOD with a single-channel algorithm based on continuous radiometer measurements and measured soil moisture. VOD exhibited a diurnal cycle similar to that of leaf and stem water potential, with a peak at approximately 05:00 eastern daylight time (UTC−4). VOD was also positively correlated with both the measured dielectric constant and water potentials of stem xylem over the growing season. The presence of moisture on the leaves did not affect the observed relationship between VOD and stem water potential. We used our observed VOD–water-potential relationship to estimate stand-level values for a radiative transfer parameter and a plant hydraulic parameter, which compared well with the published literature. Our findings support the use of VOD for plant hydraulic studies in temperate forests.

Application of Paclobutrazol to Mitigate Environmental Stress of Urban Street Trees

Paclobutrazol is a tree growth regulator that is frequently applied by arborists to control tree growth in utility rights of way. Paclobutrazol is also marketed to mitigate tree stresses associated with urban environments. In this study we applied paclobutrazol as a soil drench to honeylocust (Gleditsia triacanthos L var. inermis (L.) Zab.) and Callery pear (Pyrus calleryiana Decne.) trees planted as street trees on two sites in Lansing, Michigan USA. We evaluated physiological and morphological responses for two years after treatment. Application of paclobutrazol increased SPAD chlorophyll index of trees of both species in both years, compared to untreated control trees. Application of paclobutrazol increased leaf water potential of trees on one study site (Downtown) but not the other (Old Town). Paclobutrazol increased gas exchange (net photosynthesis and stomatal conductance) of Callery pear trees on one of four measurement dates (gas exchange was not measured on honeylocust trees). Leaf size of Callery pear trees was reduced following paclobutrazol application whereas leaf size of honeylocust trees was unaffected by paclobutrazol. These results indicate that paclobutrazol can help to reduce stress of trees and improve physiological function under urban conditions. However, paclobutrazol should be viewed as part of a suite of options for arborists and landscapers to manage trees on stressful sites, rather than as a replacement for proper overall care.

Managing high quality timber plantations as silvopastoral systems: tree growth, soil water dynamics and nitrate leaching risk

The management of high-quality timber plantations prioritizes obtaining high productions with short rotation cycles, which results in high economic and environmental costs, similar to intensive agricultural systems. The suppression of the herbaceous understorey by ploughing, and the use of fertilizers to boost tree growth potentially pose high environmental risks. Managing these plantations as silvopastoral systems (e.g. by introducing livestock and sowing legume-rich pastures) could reduce environmental risks. However, this management shift can only be effective if it is accompanied by an increased complementarity in the use of soil resources to maintain production. We evaluated rooting systems, tree water status and growth, pasture production, soil moisture and nitrate content in the soil solution in a 13-year-old hybrid walnut (Juglans major × regia) plantation under alternative management schemes. The management aimed at reducing herbaceous understorey cover (grazing, ploughing and mowing) and fertilizer usage (no fertilization, inorganic fertilization and legume sowing). Grazed and mowed plots had higher root surface density than ploughed plots. Ploughed plots also showed lower values of soil water (in summer) and tree leaf water potential than grazed and mowed plots. Tree growth and nitrate content in the soil solution was higher in ploughed than grazed or mowed plots. Fertilized and sown plots had higher understorey biomass and lower soil water (in summer) than unfertilized plots. Tree growth was higher in fertilized than unfertilized plots, with intermediate values in sown plots. Managing high-quality timber plantations under silvopastoral schemes with improved legume-rich forages might be a feasible way to reduce the high ecological maintenance costs of these plantations, without compromising their productivity.

Effect of Pit Design and Soil Composition on Performance of Pyrus calleryana Street Trees in the Establishment Period

Evapotranspirational cooling from urban trees is an effective way of reducing the urban heat island. However, the appropriate planting design to maximize the cooling benefit of street trees has not been widely examined. The current study investigated the growth and physiology of a commonly planted urban tree, Pyrus calleryana, in Manchester, UK. Trees were planted in April 2010 using three standard planting techniques: in a small open pit, and in small or large closed pits with non-compacted load-bearing soils and sealed with permeable paving slabs. The growth rate, leaf area index, and stomatal conductance were monitored over the next three growing seasons, together with chlorophyll analysis and fluorescence and leaf water potential, allowing researchers to determine tree health, water status, and evapotranspirational cooling. Trees in the open pits grew twice as fast as those in small covered pits and 1.5 times as fast as trees in large covered pits. Having significantly higher canopy density, canopy spread, and stomatal conductivity, the trees in the open pits provided up to 1 kW of cooling, compared to around 350 and 650 W by the small and large covered pits, respectively. Phenological observations, chlorophyll fluorescence, total chlorophyll, and foliar nutrient content confirmed that the trees in open pits were healthier. However, the leaf water potential of trees in the covered pits was less negative, showing that they were not suffering from water stress. Instead, limited aeration probably affected their root respiration and nutrient uptake, impairing their growth and physiological performance.

干热河谷主要造林树种气体交换特性的坡位效应

植物光合与水分生理特征是植物逆境适应能力评价的重要参考指标。极端生境下,坡位的选择有时成为造林成败的关键。探讨了元谋金沙江干热河谷9个主要造林树种光合与蒸腾作用在不同坡位间的空间差异及干热季向湿润季转换的时序差异。结果表明,低的坡位有助于树种维持相对高的净光合速率,且树种不同,由坡位引起的光合增益效应亦具有较大差别;低坡位的光合增益效应在干热季更为明显,而在湿润季,干热胁迫解除,低坡位的光合增益效应具有较大程度的降低;在不同坡位间发生的光合限制主要受非气孔因素主导。无论在干热季或湿润季,与净光合速率的变化情形一致,低的坡位均促进了树种的蒸腾速率。水分利用效率受坡位的影响较为复杂,因树种、季节而异。;Plant photosynthesis and transpiration are important reference indices for evaluating adaptable ability in adverse habits. In extreme growth environment, the selection of slope position is often becoming the key factor determining the success or failure of reforestation. Valley-type savanna is a unique type of savanna that develops in deep-incised river valleys, such as those of the dry-hot valleys in the Hengduan Mountains, southwest China, and the total area of this type in China is approximately 3.0×10⁶ hm². In the dry-hot valleys of China, the dry-hot valley of Jingsha River is typical, where the environment shows the changes from dry-hot to wet weather in one year, and because of its long dry-hot season, tree species often are difficult to survive and grow well. In this study, based on the consideration that different slope positions have different soil and atmosphere condition in the extreme habitat, the effects of slope position on photosynthesis and transpiration of nine main tree species for vegetation restoration in dry-hot season and wet season of the dry-hot river valleys were analyzed. The trial tree species include nine species, there are <em>Azadirachta indica</em> A.Juss, <em>Leucaena glauca</em> L. Benth., <em>Tephrosia candida</em> DC., <em>Eucalyptus camaldolensia</em> Dehn., <em>Acacia auriculiformis </em>A. Cunn., <em>Melia toosenden</em> Sieb.et Zucc., <em>Dodonaea viscosa</em> L. Jacq., <em>Phyllanthus emblica</em> L. and <em>Jatropha curcas</em> L. These tree species all grow in fields. The results showed that: comparing with upper slope position, lower slope position was in favor of keeping leaf net photosynthetic rate (<em>Pn</em>) a relative high level, but this favorable effect of lower slope position on <em>Pn</em> had obvious difference among different tree species. The effect of slope position on <em>Pn</em> was more obvious in dry-hot season than in wet season, while in wet season the favorable effect of lower slope position on <em>Pn</em> decreased at great extent because the dry-hot stress got relieved. Photosynthesis restriction, induced by raise of slope position, was mainly controlled by non-stomatal factor. Whether in dry-hot season or wet season, lower slope position both promoted transpiration rate (<em>Tr</em>) of the tested tree species, which was accordant with the change of <em>Pn</em>. The change of water use efficiency(WUE) with different slope positions was relatively complicated, the effects were different among different tree species and between the two seasons. It is obvious that the lower slope position benefits the photosynthesis of the tested tree species, and which helps trees to spend the dry-hot season. Because water deficit is one of the main factors that restrict the growth of tree species for vegetation restoration in the dry-hot valley of Jingsha River, and the leaf water potential of trees in lower slope position is more higher than in upper slope position, it can be knowed that the difference of water condition may be the main factor that cause the difference of photosynthesis in different slope positions.

착과량 수준 및 생육성기 토양수분 함량 변화에 따른 '후지'/M.9 품종의 수액이동 특성

착과량을 달리한 '후지'/M.9 품종에서 생육 최성기중 토양수분 변화에 따른 수액 이동 특성과 수체 생리반응을 조사하였다. 적정 착과는 적습 조건인 -50 kPa에서 VPD 및 최대증발산량과 비슷한 양상의 수액이동량을 보였으나 과다 착과는 -50 kPa 및 -80 kPa 조건에서 최대증발산량보다 낮은 수액흡수량을 보였다. 적정 착과는 모든 토양수분 조건에서 최대증발산량보다 일중 <TEX>$1.06{\sim}3.93L$</TEX> 많은 수액이 이동되는 특성을 보였고 과다 착과는 적정 착과보다 토양수분 -50 kPa 조건에서 21%, -20 kPa과 -80 kPa에서 <TEX>$31{\sim}36%$</TEX> 정도 낮은 수액 이동량을 보였다. 착과 처리에 따른 신초생육과 엽면적은 적정 착과 처리구가 다른 처리구보다 유의성 있게 높았으나 과다 착과 처리구는 가장 낮았다. 적정 착과 처리구의 엽수분포텐셜은 다른 처리구에 비해 모든 토양수분 조건에서 가장 낮아서 수분소모율이 가장 높았다. 따라서 '후지'/M.9 품종은 적정 착과량 유지와 더불어 생육성기 중 적습조건을 유지하는 것이 수액이동을 고려할 경우, 증산효율을 높이는데 바람직할 것으로 판단되었다. This study was conducted for 10 days from 17 July to 26 July in 2005 to measure the amount of xylem sap flow under short-term variation of soil moisture regimes at -20 kPa, -50 kPa and -80 kPa in eight-year-old 'Fuji'/M.9 apple trees with different fruit loads. Fruit load was adjusted as three different treatments with standard (100%), 1/2 times (50%) and 2 times (200%) on the basis of optimum fruiting number per tree as the standard fruit load of Fuji cultivar. Trees with standard fruit load during the experimental period showed higher xylem sap flow at -50 kPa of soil moisture regimes than those of trees with 1/2 times and 2 times fruit load. Trees with 1/2 times and 2 times fruit load had similar patterns of the diurnal changes of xylem sap flow, vapor pressure deficit (VPD), and maximum evapotranspiration (ETm). However, trees with 2 times fruit load at -50 kPa and -80 kPa of soil moisture regimes produced lower amount of xylem sap flow than ETm. Trees with standard fruit load produced <TEX>$1.06{\sim}3.93$</TEX> L/tree more amount of xylem sap flow than ETm at all soil moisture regimes. But xylem sap flow of tees with 2 times fruit load had 21% lower at -50 kPa and <TEX>$31{\sim}36%$</TEX> lower at -20 kPa and -80 kPa of soil moisture regimes, respectively than that of trees with standard fruit load. Shoot growth and leaf area were significantly the highest in trees with standard fruit load while those of trees with 2 times fruit load recorded significantly lowest. Leaf water potential of trees with standard fruit load was lower than that of trees with 1/2 times and 2 times fruit load. It indicated that tees with standard fruit load had higher water use for transpiration than other treatments and tees with 2 times fruit load received more stress for the transpiration process under low soil moisture regimes. Consequently, 'Fuji'/M.9 apple trees, the fruit load and soil moisture should be maintained optimum to increase xylem sap flow and transpiration during higher growth period.

Growth CO2 concentration modifies the transpiration response of Populus deltoides to drought and vapor pressure deficit.

Cottonwood (Populus deltoides Bartr. ex Marsh.) trees grown for 9 months in elevated carbon dioxide concentration ([CO2]) showed significant increases in height, leaf area and basal diameter relative to trees in a near-ambient [CO2] control treatment. Sample trees in the CO2 treatments were subjected to high and low atmospheric vapor pressure deficits (VPD) over a 5-week period at both high and low soil water contents (SWC). During these periods, transpiration rates at both the leaf and canopy levels were calculated based on sap flow measurements and leaf-to-sapwood area ratios. Leaf-level transpiration rates were approximately equivalent across [CO2] treatments when soil water was not limiting. In contrast, during drought stress, canopy-level transpiration rates were approximately equivalent across [CO2] treatments, indicating that leaf-level fluxes during drought stress were reduced in elevated [CO2] by a factor equal to the leaf area ratio of the two canopies. The shift from equivalent leaf-level transpiration to equivalent canopy-level transpiration with increasing drought stress suggests maximum water use rates were controlled primarily by atmospheric demand at high SWC and by soil water availability at low SWC. Changes in VPD had less effect on transpiration than changes in SWC for trees in both CO2 treatments. Transpiration rates of trees in both CO2 treatments reached maximum values at a VPD of about 2.0 kPa at high SWC, but leveled off and decreased slightly in both canopies as VPD increased above this value. At low SWC, increasing VPD from approximately 1.4 to 2.5 kPa caused transpiration rates to decline slightly in the canopies of trees in both treatments, with significant (P = 0.004) decreases occurring in trees in the near-ambient [CO2] treatment. The transpiration responses at high VPD in the presence of high SWC and throughout the low SWC treatment suggest some hydraulic limitations to water use occurred. Comparisons of midday leaf water potentials of trees in both CO2 treatments support this conclusion.

Adjustment of tree structure in response to the environment under hydraulic constraints

Summary 1. The hypothesis is tested that functional limitations, possibly related to the risk of disruptive xylem embolism, maintain the leaf water potential of trees within a safety range. This strategy would particularly apply to conifers. 2. It is also hypothesized that homeostasis in water transport is achieved by a combination of short‐term stomatal regulation and the optimal allocation of resources between transpiring foliage, conductive sapwood and absorbing roots. 3. The implications of this hypothesis for responses of plant structure to the environment are explored from a theoretical viewpoint, focusing in particular on the effects of temperature, vapour pressure deficit and soil water availability and texture. 4. A comparison with published data sets confirms model predictions of greater allocation to transpiring foliage with increasing temperature, and lower allocation with increasing vapour pressure deficit and soil drought. Allocation to fine roots would respond more strongly to drought, and the effect would be magnified by coarse soil textures. 5. The applicability of the model to other taxa remains to be determined, and the extent to which the observed structural adjustment is the result of phenotypic plasticity or long‐term adaptation is still unclear.