Home About us Contact
|
Home About us Contact | ||
|
|
||
Evaporative Demand (evaporative + demand)
Selected AbstractsSoil water dynamics along a tree diversity gradient in a deciduous forest in Central GermanyECOHYDROLOGY, Issue 3 2010Inga Krämer Abstract This study aimed to investigate whether soil water dynamics differ along a tree species diversity gradient. The 12 study plots in the Hainich National Park, Germany, were composed of up to 11 tree species. Fagus sylvatica formed the monospecific plots. Mixed forest plots consisted of a variable admixture of other broad-leaved deciduous tree species such as Tilia spp., Fraxinus excelsior, Carpinus betulus, and Acer pseudoplatanus. Volumetric soil water content and soil water potential were measured for about two and a half years. Overall patterns of soil water dynamics were similar in all study plots. However, during a desiccation period in summer 2006, significant correlations between soil water in the upper soil and tree species diversity of the 12 study plots were observed. At the beginning of this period, soil water was extracted at higher rates in the species-rich plots than in the beech-dominated plots. However, later during the desiccation period, when atmospheric evaporative demand was higher, only the beech-dominated stands were able to increase soil water extraction. In plots of high tree species diversity, soil water reserves were already low and soil water extraction reduced. Possible explanations for high water extraction rates in mixed species plots at the beginning of the desiccation period include species-specific characteristics such as high maximum water use rate of some species, enhanced exploitation of soil water resources in mixed stands (complementarity effect), and additional water use of the herb layer, which increased along the tree species diversity gradient. Copyright © 2010 John Wiley & Sons, Ltd. [source] Ecophysiological significance of leaf size variation in Proteaceae from the Cape Floristic RegionFUNCTIONAL ECOLOGY, Issue 3 2010Megan J. Yates Summary 1.,Small leaves of species endemic to Mediterranean-type climate areas have been associated with both low rainfall and nutrient availability, but the physiological reasons for this association remain unknown. 2.,We postulated that small leaves have thin boundary layers that facilitate transpiration in winter and sensible heat loss in summer. High transpiration rates when water is available may facilitate nutrient acquisition in winter, whereas efficient sensible heat loss reduces the requirement for transpirational leaf cooling in summer. 3.,The consequences of varying leaf sizes for water and heat loss in Cape Proteaceae were examined at two scales. At the leaf level, gas exchange and thermoregulatory capacities of 15 Proteaceae species with varying leaf size were assessed under controlled conditions using phylogenetically independent contrasts. At an environmental level, leaf attributes of Proteaceae occurring in the winter-rainfall area of the Cape Floristic Region were correlated with climatic environments derived from distribution data for each species. 4.,Leaf temperature was positively correlated with leaf size when wind speed was negligible. However, transpiration decreased significantly with increasing leaf size when measured on individual leaves, detached branches and when expressed on a per stoma basis. 5.,From multiple stepwise regression analysis of climatic variables obtained from distribution data, leaf size was negatively correlated with A-Pan evaporation, mean annual temperatures and water stress in January. We conclude that leaf size is conservative for survival over relatively rare periods of hot dry conditions with low wind speeds. 6.,Narrow leaves enable plants to shed heat through sensible heat loss during summer droughts, without the need for transpirational cooling. Additionally, small leaf dimensions confer a capacity for high transpiration when evaporative demand is low and water is abundant (i.e. winter). This may be a particularly important strategy for driving nutrient mass-flow to the roots of plants that take up most of their nutrients in the wet winter/spring months from nutrient-poor soils. [source] Below-ground hydraulic conductance is a function of environmental conditions and tree size in Scots pineFUNCTIONAL ECOLOGY, Issue 6 2007J. MARTÍNEZ-VILALTA Summary 1Variations in water tension in a transpiring tree cause elastic changes in stem diameter. To better understand the dynamics of these variations, stem diameter changes and sap flow rates were monitored simultaneously in trees from two Scots pine chronosequences in Scotland. 2Tree below-ground hydraulic conductance (kbg) was estimated from the relationship between leaf-specific sap flow rates and the difference between stem and soil water potentials estimated from diameter variations in the stem. 3In a given tree, kbg varied both within and among days, with conductance increasing as a function of sap flow and evaporative demand. These patterns could be explained in terms of a composite model of root water transport and possible changes in the gating of aquaporins. 4We interpreted these trends of increasing kbg with evaporative demand as a mechanism to enhance the ability of trees to control leaf water potential and keep it within physiologically acceptable limits, with potential implications for our general understanding of plant water relations, and for the estimation and modelling of ecosystem water fluxes. 5Across trees, kbg declined with increasing tree age/size, but the proportional contribution of below-ground to whole-tree hydraulic resistance also declined. This is consistent with an increase in below-ground carbon allocation in old/tall trees and a partial acclimation of tall trees to hydraulic limitations. It is argued that these trends have to be considered when discussing the importance of tree height for water transport and growth. [source] Soil and atmospheric water deficits and the distribution of New Zealand's indigenous tree speciesFUNCTIONAL ECOLOGY, Issue 2 2001Leathwick J. R. Summary 1.,An extensive data set describing the composition of New Zealand's remaining indigenous forests was used to estimate the degree of correlation between measures of both soil and atmospheric water deficit and the distribution of common tree species. 2.,For most species, regression models incorporating measures of air saturation deficit in early autumn, as well as an annual integral of root zone water deficit, provided the best explanation of spatial distribution. This accords strongly with the mechanistic effects of air saturation deficits on transpiration from trees, and the hydraulic risks experienced by trees under high evaporative demand. 3.,Adjustment of root zone water deficits to account for reductions in rainfall in dry years substantially improved model predictions. This suggests that extreme climatic events, such as the El Niño phase of the Southern Oscillation, are likely to have strongly influenced the historic composition of forests in New Zealand's drier eastern lowlands. [source] Pan Evaporation Trends and the Terrestrial Water Balance.GEOGRAPHY COMPASS (ELECTRONIC), Issue 2 2009Pan evaporation is just that , it is the evaporation rate of water from a small dish located at the ground-surface. Pan evaporation is a measure of the evaporative demand over terrestrial surfaces. Declines in pan evaporation have now been reported in many regions of the world. The trends vary from one pan to the next, but when averaged over many pans, they are typically in the range of ,1 to ,4 mm a,2 (mm per annum per annum). In energetic terms, a trend of ,2 mm a,2 is equivalent to ,0.16 W m,2 a,1 and over 30 years this is a change of ,4.8 W m,2. For comparison, the top-of-atmosphere forcing due to doubled CO2 is estimated by the Intergovernmental Panel on Climate Change (IPCC) to be ~3.7 W m,2. Hence, the magnitude of the pan evaporation trend is large. What is of even greater interest is the direction , a decline , given the well-established warming of the last 30,50 years. In this article, the first in a two part series, we describe the underlying principles in using and interpreting pan evaporation data and then summarise the reported observations from different countries. In the second article, we describe the interpretation of the trends in terms of changes in the terrestrial water balance. [source] Last-century changes of alpine grassland water-use efficiency: a reconstruction through carbon isotope analysis of a time-series of Capra ibex hornsGLOBAL CHANGE BIOLOGY, Issue 4 2010INÊS C. R. BARBOSA Abstract The ecophysiological response of an alpine grassland to recent climate change and increasing atmospheric CO2 concentration was investigated with a new strategy to go back in time: using a time-series of Capra ibex horns as archives of the alpine grasslands' carbon isotope discrimination (13,). From the collection of the Natural History Museum of Bern, horns of 24 males from the population of the Augstmatthorn,Brienzer Rothorn mountains, Switzerland, were sampled covering the period from 1938 to 2006. Samples were taken from the beginning of each year-ring of the horns, representing the beginning of the horn growth period, the spring. The horns' carbon 13C content (,13C) declined together with that of atmospheric CO2 over the 69-year period, but 13, increased slightly (+0.4,), though significantly (P<0.05), over the observation period. Estimated intercellular CO2 concentration increased (+56 ,mol mol,1) less than the atmospheric CO2 concentration (+81 ,mol mol,1), so that intrinsic water-use efficiency increased by 17.8% during the 69-year period. However, the atmospheric evaporative demand at the site increased by approximately 0.1 kPa between 1955 and 2006, thus counteracting the improvement of intrinsic water-use efficiency. As a result, instantaneous water-use efficiency did not change. The observed changes in intrinsic water-use efficiency were in the same range as those of trees (as reported by others), indicating that leaf-level control of water-use efficiency of grassland and forests followed the same principles. This is the first reconstruction of the water-use efficiency response of a natural grassland ecosystem to last century CO2 and climatic changes. The results indicate that the alpine grassland community has responded to climate change by improving the physiological control of carbon gain to water loss, following the increases in atmospheric CO2 and evaporative demand. But, effective leaf-level water-use efficiency has remained unchanged. [source] Abiotic constraints on the establishment of Quercus seedlings in grasslandGLOBAL CHANGE BIOLOGY, Issue 2 2003Brett T. Danner Abstract High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first-year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland-forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long-term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late-season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present-day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue. [source] Rice leaf growth and water potential are resilient to evaporative demand and soil water deficit once the effects of root system are neutralizedPLANT CELL & ENVIRONMENT, Issue 8 2010BORIS PARENT ABSTRACT Rice is known to be sensitive to soil water deficit and evaporative demand, with a greatest sensitivity of lowland-adapted genotypes. We have analysed the responses of plant water relations and of leaf elongation rate (LER) to soil water status and evaporative demand in seven rice genotypes belonging to different species, subspecies, either upland- or lowland-adapted. In the considered range of soil water potential (0 to ,0.6 MPa), stomatal conductance was controlled in such a way that the daytime leaf water potential was similar in well-watered, droughted or flooded conditions (isohydric behaviour). A low sensitivity of LER to evaporative demand was observed in the same three conditions, with small differences between genotypes and lower sensitivity than in maize. The sensitivity of LER to soil water deficit was similar to that of maize. A tendency towards lower sensitivities was observed in upland than lowland genotypes but with smaller differences than expected. We conclude that leaf water status and leaf elongation of rice are not particularly sensitive to water deficit. The main origin of drought sensitivity in rice may be its poor root system, whose effect was alleviated in the study presented here by growing plants in pots whose soil was entirely colonized by roots of all genotypes. [source] An external heat pulse method for measurement of sap flow through fruit pedicels, leaf petioles and other small-diameter stemsPLANT CELL & ENVIRONMENT, Issue 12 2009MICHAEL J. CLEARWATER ABSTRACT The external heat ratio method is described for measurement of low rates of sap flow in both directions through stems and other plant organs, including fruit pedicels, with diameters up to 5 mm and flows less than 2 g h,1. Calibration was empirical, with heat pulse velocity (vh) compared to gravimetric measurements of sap flow. In the four stem types tested (Actinidia sp. fruit pedicels, Schefflera arboricola petioles, Pittosporum crassifolium stems and Fagus sylvatica stems), vh was linearly correlated with sap velocity (vs) up to a vs of approximately 0.007 cm s,1, equivalent to a flow of 1.8 g h,1 through a 3-mm-diameter stem. Minimum detectable vs was approximately 0.0001 cm s,1, equivalent to 0.025 g h,1 through a 3-mm-diameter stem. Sensitivity increased with bark removal. Girdling had no effect on short-term measurements of in vivo sap flow, suggesting that phloem flows were too low to be separated from xylem flows. Fluctuating ambient temperatures increased variability in outdoor sap flow measurements. However, a consistent diurnal time-course of fruit pedicel sap flow was obtained, with flows towards 75-day-old kiwifruit lagging behind evaporative demand and peaking at 0.3 g h,1 in the late afternoon. [source] Leaf growth and turgor in growing cells of maize (Zea mays L.) respond to evaporative demand under moderate irrigation but not in water-saturated soilPLANT CELL & ENVIRONMENT, Issue 6 2006OUMAYA BOUCHABKÉ ABSTRACT To test whether the inhibition of leaf expansion by high evaporative demand is a result of hydraulic processes, we have followed both leaf elongation rate (LER) and cell turgor in leaves of maize plants either normally watered or in water-saturated soil in which hydraulic resistance at the soil,root interface was abolished. Cell turgor was measured in situ with a pressure probe in the elongating zone of the first and sixth leaves, and LERs of the same leaves were measured continuously with transducers or by following displacements of marks along the growing leaves. Both variables displayed spatial variations along the leaf and positively correlated within the elongating zone. Values peaked at mid-distance of this zone, where the response of turgor to evaporative demand was further dissected. High evaporative demand decreased both LER and turgor for at least 5 h, with dose-effect linear relations. This was observed in five genotypes with appreciable differences in turgor maintenance among genotypes. In contrast, the depressing effects of evaporative demand on both turgor and LER disappeared when the soil was saturated, thereby opposing a negligible resistance to water flow at the soil,root interface. These results suggest that the response of LER to evaporative demand has a hydraulic origin, enhanced by the resistance to water flux at the soil,root interface. They also suggest that turgor is not completely maintained under high evaporative demand, and may therefore contribute to the reductions in LER observed in non-saturated soils. [source] Does engineering abscisic acid biosynthesis in Nicotiana plumbaginifolia modify stomatal response to drought?PLANT CELL & ENVIRONMENT, Issue 5 2001C. Borel ABSTRACT The consequences of manipulating abscisic acid (ABA) biosynthesis rates on stomatal response to drought were analysed in wild-type, a full-deficient mutant and four under-producing transgenic lines of N. plumbaginifolia. The roles of ABA, xylem sap pH and leaf water potential were investigated under four experimental conditions: feeding detached leaves with varying ABA concentration; injecting exogenous ABA into well-watered plants; and withholding irrigation on pot-grown plants, either intact or grafted onto tobacco. Changes in ABA synthesis abilities among lines did not affect stomatal sensitivity to ABA concentration in the leaf xylem sap ([ABA]xyl), as evidenced with exogenous ABA supplies and natural increases of [ABA]xyl in grafted plants subjected to drought. The ABA-deficient mutant, which is uncultivable under normal evaporative demand, was grafted onto tobacco stock and then presented the same stomatal response to [ABA]xyl as wild-type and other lines. This reinforces the dominant role of ABA in controlling stomatal response to drought in N. plumbaginifolia whereas roles of leaf water potential and xylem sap pH were excluded under all studied conditions. However, when plants were submitted to soil drying onto their own roots, stomatal response to [ABA]xyl slightly differed among lines. It is suggested, consistently with all the results, that an additional root signal of soil drying modulates stomatal response to [ABA]xyl. [source] Photosynthetic Acclimation to Simultaneous and Interacting Environmental Stresses Along Natural Light Gradients: Optimality and ConstraintsPLANT BIOLOGY, Issue 3 2004ü. Niinemets Abstract: There is a strong natural light gradient from the top to the bottom in plant canopies and along gap-understorey continua. Leaf structure and photosynthetic capacities change close to proportionally along these gradients, leading to maximisation of whole canopy photosynthesis. However, other environmental factors also vary within the light gradients in a correlative manner. Specifically, the leaves exposed to higher irradiance suffer from more severe heat, water, and photoinhibition stresses. Research in tree canopies and across gap-understorey gradients demonstrates that plants have a large potential to acclimate to interacting environmental limitations. The optimum temperature for photosynthetic electron transport increases with increasing growth irradiance in the canopy, improving the resistance of photosynthetic apparatus to heat stress. Stomatal constraints on photosynthesis are also larger at higher irradiance because the leaves at greater evaporative demands regulate water use more efficiently. Furthermore, upper canopy leaves are more rigid and have lower leaf osmotic potentials to improve water extraction from drying soil. The current review highlights that such an array of complex interactions significantly modifies the potential and realized whole canopy photosynthetic productivity, but also that the interactive effects cannot be simply predicted as composites of additive partial environmental stresses. We hypothesize that plant photosynthetic capacities deviate from the theoretical optimum values because of the interacting stresses in plant canopies and evolutionary trade-offs between leaf- and canopy-level plastic adjustments in light capture and use. [source] | ||||||||||