Defoliation, leaf area index, and the water use of four temperate pasture species under irrigated and dryland conditions

Abstract

Measurements were made over a 12-month period of the water use and leaf area index (LAI) of both dryland and irrigated monoculture swards of four temperate pasture species under two defoliation regimes.

All four species used similar quantities of water on the dryland plots despite large differences in their ability to grow under such conditions. Even though very dry conditions prevailed during part of the study, the dryland swards generally failed to exploit reserves of soil moisture below a depth of c. 120 cm.

The water use of the irrigated swards was sensitive to the manipulation of LAI by defoliation, while in contrast, dryland water use was not. On the irrigated swards, at an LAI of 1, a 1% decrease in LAI was associated with a 1% decrease in water use. This sensitivity of water use decreased as LAI increased until, at an LAI of 3 and above, water use appeared to be insensitive to charges in LAI.

During the late spring to early autumn period both irrigated and dryland water use were significantly related to LAI. In this period, those irrigated and dryland swards which had common values of LAI generally used similar quantities of water. This finding indicated that stomatal control was ineffective in reducing water use per unit of leaf area. The quantity of dead herbage present in the swards suggests that pronounced leaf senescence (and hence reduction of leaf area) may have been a consequence of ineffective stomatal control of transpiration.