BHASKAR, RADIKA* and DAVID ACKERLY. Dept. of Biological Sciences, Stanford University, Stanford, CA 94305. - Responses of hydraulic traits to light and water availability in a California chaparral shrub.
Water transport to the leaves is influenced in part by leaf-specific
hydraulic conductance (LSC). Studies have found that LSC may be
sensitive to environmental factors such as the supply of water in the
soil or the evaporative demand. Among individuals LSC can vary due to
changes in biomass allocation such as differences in the ratio of leaf
area/sapwood area (AL/AS). Increased sapwood
specific hydraulic conductivity (KS) also affects LSC but
leads to increased xylem vulnerability to embolism. These two aspects
of LSC were examined separately in seedlings of Heteromeles
arbutifolia, a native California chaparral species. H.
arbutifolia is present in shade as well as full sun, where summer
drought conditions resulting in water stress are intensified by the
combination of incoming radiation and high air temperatures. In order
to examine the effects of light and water on hydraulic architecture,
H. arbutifolia seedlings were grown in a common outdoor garden
under a factorial design with sun and shade conditions and high and
low soil water availability. Physiological measurements made on the
seedlings at the end of the growing season, including stomatal
conductance and maximum transpiration rates, indicated significant
differences in water use in the four treatments. Aspects of canopy
architecture were also measured before seedlings were harvested to
quantify biomass allocation patterns. Hydraulic conductance was
determined using a bench-top conductance set-up in order to examine
variation in LSC. In particular plasticity in different aspects of LSC
was examined to determine whether AL/AS varied
in response to higher transpiration rates while KS
responded to variation in soil water supply.
Key words: chaparral, Heteromeles arbutifolia, hydraulic architecture, leaf-specific conductance, plasticity