Modulation of compound extremes of low soil moisture and high vapour pressure deficit by irrigation in India

ABSTRACTWe investigated the influence of a four-year-old forest edge near Manaus, Brazil, on soil moisture and vertical profiles of air vapour pressure deficit (VPD) within the forest. Soil moisture was measured (with a neutron probe) 0, 5, 10, 20, 40, 60, 80, 100, 150 and 200 m into the forest from the edge, in undisturbed control areas, and in the pasture. Control soil moisture was better explained by rainfall in the previous 2 or 10 days than by longer-term totals. Soil water potentials ≤ – 1.5 MPa occurred at some forest locations during the driest period. The variation in soil moisture with distance from the forest edge was complex, with higher values just inside the edge and depleted zones at the edge and 40–80 m inside it. At a given height, VPD (standardized relative to measurements in the open) was not related to distance from the edge, but VPD increased more with height near the edge than in control areas. The complexity of the edge's influence and the contrast with earlier data from the same edge can be explained by the changing vegetation structure near the edge. Regrowth ‘seals’ the edge with more leaves that transpire and deplete soil moisture, while protecting the understorey just inside the edge from desiccating conditions. A mosaic of gaps of differing ages develops behind the edge, increasing the variation in microclimatic conditions near the ground and consequently in evapotranspiration and soil moisture.

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Seasonal Patterns in Soil Moisture, Vapour Pressure Deficit, Tree Canopy Cover and Pre-dawn Water Potential in a Northern Australian Savanna

Australian Journal of Botany ◽

10.1071/bt96018 ◽

1997 ◽

Vol 45 (2) ◽

pp. 211 ◽

Cited By ~ 49

Author(s):

G. A. Duff ◽

B. A. Myers ◽

R. J. Williams ◽

D. Eamus ◽

A. O'Grady ◽

...

Keyword(s):

Soil Moisture ◽

Seasonal Variation ◽

Water Potential ◽

Vapour Pressure ◽

Canopy Cover ◽

Vapour Pressure Deficit ◽

Dry Season ◽

Tree Canopy ◽

Wet Season ◽

Tree Canopy Cover

The wet–dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour pressure deficit, although temperatures are relatively constant throughout the year.This seasonal variation is associated with marked changes in tree canopy cover, although the exact determinants of these changes are complex. This paper reports variation in microclimate (temperature, vapour pressure deficit (VPD)), rainfall, soil moisture, understorey light environment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northern Territory, Australia. Patterns of canopy cover are strongly influenced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture that occur with the onset of wet season rains. Decreases in canopy cover coincide with decreases in soil moisture following the cessation of wet season rains and associated increases in VPD. Patterns of pre-dawn water potential vary significantly between species and between leaf phenological guilds. Pre-dawn water potential increases with decreasing VPD towards the end of the dry season prior to any increases in soil moisture. Decline in pre-dawn water potential coincides with both decreasing soil moisture and increasing VPD at the end of the dry season. This study emphasises the importance of the annual transition between the dry season and the wet season, a period of 1–2 months of relatively low VPD but little or no effective rainfall, preceded by a 4–6 month dry season of no rainfall and high VPD. This period is accompanied by markedly increased canopy cover, and significant increases in pre-dawn water potential, which are demonstrably independent of rainfall. This finding emphasises the importance of VPD as a determinant of physiological and phenological processes in Australian savannas.

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Mechanisms linking plant productivity and water status for a temperate Eucalyptus forest flux site: analysis over wet and dry years with a simple model

Functional Plant Biology ◽

10.1071/fp08125 ◽

2008 ◽

Vol 35 (6) ◽

pp. 493 ◽

Cited By ~ 8

Author(s):

David A. Pepper ◽

Ross E. McMurtrie ◽

Belinda E. Medlyn ◽

Heather Keith ◽

Derek Eamus

Keyword(s):

Soil Moisture ◽

Water Availability ◽

Vapour Pressure ◽

Water Status ◽

Vapour Pressure Deficit ◽

Co2 Flux ◽

Lower Sensitivity ◽

Forest Site ◽

Eucalyptus Forest ◽

Dry Conditions

A simple process-based model was applied to a tall Eucalyptus forest site over consecutive wet and dry years to examine the importance of different mechanisms linking productivity and water availability. Measured soil moisture, gas flux (CO2, H2O) and meteorological records for the site were used. Similar levels of simulated H2O flux in ‘wet’ and ‘dry’ years were achieved when water availability was not confined to the first 1.20 m of the soil profile, but was allowed to exceed it. Although the simulated effects of low soil and atmospheric water content on CO2 flux, presumably via reduction in stomatal aperture, also acted on transpiration, they were offset in the dry year by a higher vapour-pressure deficit. A sensitivity analysis identified the processes that were important in wet versus dry years, and on an intra-annual timeframe. Light-limited productivity dominated in both years, except for the driest period in the dry year. Vapour-pressure deficit affected productivity across more of each year than soil moisture, but both effects were larger in the dry year. The introduction of a reduced leaf area tended to decrease sensitivity in the dry year. Plant hydraulic architecture that increases plant available water, maximises productivity per unit water use and achieves lower sensitivity to low soil moisture levels should minimise production losses during dry conditions.

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