PARENT SESSION
Oral Session #17: Plant Ecology: Water Relations.
Presiding: W. Pockman
Monday, August 5. 1:00 PM to 3:45 PM. Coconino Meeting Room, TCC.
Does drought cause regeneration failure of Shorea robusta in the central Himalaya?
Garkoti, Satish1, Zobel, Donald*,2, 1 Kumaun University, Naini Tal, India2 Oregon State University, Corvallis, OR
ABSTRACT- Regeneration of Shorea robusta, sal, a major timber tree at the base of the Himalaya, is often ineffective; seedling death from drought is a possible cause. We studied the patterns with seedling size, season, and topographic location for predawn and midday water potential, leaf conductance, osmotic potential, and xylem conductance, in order to determine the likelihood that drought causes mortality and to identify processes of acclimation to drought. Measurements were made from late April, with new leaves enlarging, until the following early April, before leaf emergence. Soil water potential at -60 cm reached a low of -3.0 MPa in December. Predawn plant water potentials were lower for <20 cm tall seedlings than for taller plants, but even plants <20 cm tall were [:lt]-1 MPa only during late April (-1.3 MPa) and October (-1.1). Means were 0.4 MPa lower at 1300 m elevation than at the base of the mountains. The lowest midday mean water potential was -2.4 MPa for <20 cm seedlings in early April, with the lowest individual -3.0 MPa. Leaf diffusive conductance was high most of the year, highest in October and December; was usually lowest for the smallest seedlings; and declined with site elevation. Osmotic potential was lowest (-2.2 MPa at full turgor) during early April, highest during June before rains begin (-1.4 MPa), and intermediate during October and December; it did not differ with location or seedling size. Xylem conductance per unit active xylem area was less during early April than during June, but leaf specific conductance showed no difference; neither measure of xylem conductance changed with site or seedling size. In summary, water potential reached levels that may kill shoots only for the smallest seedlings, which compensate by having low leaf conductance. Given that sal seeds germinate early during the rainy season, young seedlings root deeply, and top-killed seedlings may sprout, drought seems unlikely to cause regeneration failure of the species.
KEY WORDS: water relations, seedling ecology, Shorea robusta, Himalaya