Soil drought increases leaf and whole-plant water use of Prunus dulcis grown in the Negev Desert

Water use, both at the level of a single leaf and the whole plant, was studied for 1- to 4-year-old almond trees (Prunus dulcis) under and conditions in the Negev Desert (Israel). By planting the trees into lysimeters of different volumes (7, 14 and 21 m(3)), the amount of water available to the plants was experimentally controlled. Each year, at the beginning of the growing season, the lysimeters, which had been filled with local homogenized loess, were watered to field capacity. The trees received different relative amounts of water in relation to their leaf area on the one hand and lysimeter volume on the other, which caused different rates of soil drying throughout the season. The following hypotheses were tested. (1) The amount of CO₂ assimilated per transpiration and (2) biomass production per unit of water used increases with (a) decreasing amount of soil water applied and (b) increasing leaf area, which should enhance growth in spring during periods of low evaporative demand. At the leaf level, the ratio of daily CO₂ assimilation (A) to daily transpiration (E) was independent of lysimeter size, leaf area and pre-dawn water potential, but decreased with increasing leaf-to-air vapour pressure difference (D-1). Consequently, during the course of the season, A/E decreased from spring to summer in accordance with rising D-1. However, when measured at a constant D-1, the seasonal course in A/E disappeared. At the whole plant level, the ratio of total lifetime biomass production (B) to the amount of water transpired (M increased with leaf area (i.e. demand for water), the increase being stronger with increasing water supply. We conclude that almond trees did not adapt physiologically to a limited water supply, but maximized their carbon gain for a given amount of water available by phenological processes such as high growth rate during periods of low evaporative demand of the atmosphere

Biomass partitioning in response to soil drought: A pot experiment with Prunus dulcis trees during four years

Growth and biomass partitioning of almond trees [Prunus dulcis (Miller) D.A.Webb] were investigated in a semi-controlled pot experiment under arid conditions in the Negev Desert (Israel). Single trees of two varieties grafted on a local rootstock were grown for 1 to 4 years in pots of 3 in diameter and 1, 2 and 3 m depth, which resulted in 7, 14 and 21 m(3) soil volume. The pots were watered to field capacity once per year at the beginning of each growing season. Thus the plants received different amounts of water in relation to their biomass and leaf area as a function of their age and pot volume. This procedure resulted in different rates of soil drying and different stages of water deficit throughout the season, which allowed an investigation of the interactive effect of both seasonally varying water status and atmospheric conditions for a woody species in large soil volumes. We primarily tested whether a progressive reduction in the amount of water applied to the trees in relation to their leaf area would increase (1) biomass partitioning in favour of roots, especially fine roots, (2) the ratio of fine root length to leaf area. Neither biomass partitioning into leaves nor fine roots was significantly influenced by the amount of water supplied. There was no effect of water supply on the relationship between fine root length and finch root biomass (specific root length) either. Consequently, the relationship between length of fine roots and leaf area was not dependent on the amount of available water. The constant relationship between most of the biomass partitioning parameters examined in spite of the great range in water availability and over several years of growth is discussed as the result of the seasonal variation in the interaction of water supply and demand on tree growth and biomass distribution. [References: 31

Xylem ABA controls the stomatal conductance of field-grown maize subjected to soil compaction and soil drying.

Stomatal conductance of individual leaves was measured in a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration in the same leaves. Stomatal conductance showed a tight relationship with xylem ABA, but not with the current leaf water status or with the concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non-compacted and irrigated soil, and to plant-to-plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions

Influences of changing land use and CO2 concentration on ecosystem and landscape level carbon and water balances in mountainous terrain of the Stubai Valley, Austria

A process-based spatial simulation model was used to estimate gross primary production, ecosystem respiration, net ecosystem CO₂ exchange and water use by the vegetation in Stubai Valley, Austria at landscape scale. The simulations were run for individual years from early spring to late fall, providing estimates in grasslands for carbon gain, biomass and leaf area development, allocation of photoproducts to the below ground ecosystem compartment, and water use. In the case of evergreen coniferous forests, gas exchange is estimated, but spatial simulation of growth over the single annual cycles is not included. Spatial parameterization of the model is derived for forest LAI based on remote sensing, for soil characteristics by generalization from spatial surveys and for climate drivers from observations at monitoring stations along the elevation gradient and from modelling of incident radiation in complex terrain. Validation of the model was carried out at point scale, and was based on comparison of model output at selected locations with observations along elevation gradients in Stubai Valley and Berchtesgaden National Park, Germany as well as with known trends in ecosystem response documented in the literature. The utility of the model for describing long-term changes in carbon and water balances at landscape scale is demonstrated in the context of land use change that occurred between 1861 and 2002 in Stubai Valley. During this period, coniferous forest increased in extent by ca. 11% of the vegetated area of 1861, primarily in the subalpine zone. Managed grassland decreased by 46%, while abandoned grassland and natural alpine mats increased by 14 and 11%, respectively. At point scale, the formulated model predicts higher canopy conductance in 1861 due to lower atmospheric CO₂ concentration which opens stomata. As a result, water use at point scale decreased by ca. 8% in 2002 in the valley bottoms versus 10% at tree line. At landscape level, the decrease in water use by vegetation in 2002 was predicted to be twice as high (ca. 17%) due to increase in subalpine forest, reduction of managed grassland in the valley and on slopes, as well as abandonment of grassland which results in natural succession. Net ecosystem CO₂ exchange (NEE) was predicted to increase (become more negative) at point scale depending on vegetation type by 10 to 20% in 2002 due to increasing atmospheric CO₂ concentration. However, due to the shift from grassland to forest and natural vegetation, landscape level CO₂ exchange did not change. As a result of land use change, the export of carbon in harvested biomass in 2002 was estimated at only 30% of that in 1861. While the need for further validation of model assumptions is recognized, especially changes in ecosystem behavior with changing atmospheric CO₂ concentration, the model analysis indicates a long-term reduction in water use by vegetation and a shift in ecosystem services. The results provide a case study, where land use change may compensate or override the influences of increasing atmospheric CO₂ concentration, maintaining a relatively constant NEE in present time period simulations as compared to 1861, as well as reducing export of carbon from the alpine landscape of Stubai Valley. Use of the model in evaluation of scenarios of future land use change and in relation to vulnerability of ecosystem services are discussed. (C) 2009 Elsevier B.V. All rights reserved. [References: 54