LEAF BOUNDARY-LAYER CONDUCTANCE OF 2 NATIVE SPECIES IN SOUTHEAST SPAIN

Leaf boundary layer conductance was measured in the field using heated leaf replicas placed in the canopies of two of the three main semi-ar id species found in the Rambla Honda (Almeria, South East of Spain), R etama sphaerocarpa (L.) Boiss, and Stipa tenacissima L. R. sphaerocarp a is a woody perennial bush with approximately cylindrical cladodes ar ranged on randomly orientated stems. In contrast S. tenacissima is a p erennial tussock grass with leaves that can roll during periods of wat er stress. Cylindrical leaf replicas were constructed with dimensions similar to the leaves of S. tenacissima and cladodes of R. sphaerocarp a. The relationship between wind speed (u) measured outside the plant canopy and boundary layer conductance (g(a)) was analysed on the basis of an equation, derived from engineering formulae describing forced c onvection for a cylinder in laminar flow. The coefficients a and b wer e derived empirically from experimental data for replicas placed withi n and outside the canopies of the two species. Measured boundary layer conductance in the open tended to be higher than that estimated from engineering equations resulting in higher estimates of a. There was no significant difference in the estimate of b between replicas in the o pen and the engineering value. In the R. sphaerocarpa canopy the estim ate of a tended to decrease with depth into the canopy but there was n o significant difference in estimates of b between replicas inside and outside the canopy. Inside the S. tenacissima canopy both coefficient s a and b tended to be significantly lower than those derived from eng ineering equations. Significant differences between the parameters of the relationship inside and outside the canopy showed the difficulty i n deriving a unique equation to calculate the boundary layer conductan ce of a whole plant from wind speed measured outside the canopy. This work confirms that serious errors can be made if engineering equations are applied without a better knowledge of the relationship between bo undary layer conductance and wind speed. Tile results suggest that red uction of average wind speed by shelter within the canopy and increase d turbulence are important in influencing leaf boundary layer. The hea ted leaf replica method used in this work provides a simple and effect ive system for measuring leaf boundary layer conductance for plants in the field, and is preferable to the use of uncalibrated empirical equ ations. These results can be related to the importance of leaf shape a nd canopy structure in determining water use of natural vegetation in semi-arid regions. R. sphaerocarpa has an open canopy allowing a bette r penetration of air flow. S. tenacissima has a dense canopy producing shelter decreasing wind speed rather than increasing turbulence. Whil e both species have similar shaped transpiring plant parts, the strate gy of S. tenacissima to maintain dead materials as part of the plant c anopy could be important in control of water loss.