Document: KEI-3-32-38

Transpiration, root architecture, and biomass allocation in cottonwood (Populus fremontii) and mesquite (Prosopis velutina): Responses to experimental manipulations.

SNYDER, K.A.* and D.G.WILLIAMS

University of Arizona, Tucson, AZ 85721 USA. ¹

Abstract:
We conducted a greenhouse experiment with cottonwood (Populus fremontii) and mesquite (Prosopis velutina) to determine if allocation to fine and coarse roots, and rates of water absorption from hydraulically separated upper and lower soil compartments, would be altered by available soil moisture and defoliation. The lower soil compartments were irrigated to maintain high ("wet") or low ("dry") moisture availability. Half of the plants in each watering treatment were defoliated at regular intervals. Isotopically labeled water was supplied in pulses to upper soil compartments to determine the fraction of total transpiration water derived from each compartment. Transpirational water loss was measured with stem heat balance sensors. Transpiration per unit sapwood area was reduced in dry treatments relative to wet treatments for cottonwood, yet the effect of defoliation on transpiration was minimal. Water absorption from shallow soil layers appeared to increase in dry and defoliated treatments for cottonwood, whereas trees in the non-defoliated wet treatment used a lower proportion of shallow soil water. Dry treatments reduced total plant biomass and root-mass ratio relative to wet treatments in cottonwood. Within the wet treatment, root-mass ratio was lower for defoliated cottonwoods than non-defoliated controls. The ratio of fine to coarse roots in upper soil compartments increased in dry and defoliated treatments. In contrast, watering treatments did not produce a consistent effect on the growth or biomass partitioning of mesquite. However, defoliation treatments were associated with greater allocation to shoots than roots, regardless of watering treatment. There was no consistent treatment effect on fine to coarse root ratios for mesquite. It appears that above and belowground resource limitations will differentially alter the water absorption and root proliferation of these two species. Cottonwood preferentially allocated to fine roots in upper soil layers when water and/or carbon was limited, and this appeared to be associated with a greater proportion of shallow water use. In contrast, mesquite altered total abovegound to belowground allocation in response to defoliation treatment, yet there were no consistent effects of water or carbon limitations on root architecture.

Keywords: transpiration, water-use, root architecture, biomass allocation

This abstract is being presented at: 1:45 PM in session:
Oral Session #34: Water Relations in Trees.