22 - Water relations of Canarian laurel forest trees

Summary

SUMMARY

Water supply capacity of the xylem, water loss avoidance and drought tolerance of leaves of Canarian laurel forest trees were investigated and compared with corresponding data for Mediterranean sclerophylls. Generally, the lauriphyllous species are unable to control their canopy water relations if they are affected by arid conditions. This may be one reason why the Macaronesian laurel forests are restricted to perhumid sites.

INTRODUCTION

The laurel forest of the Macaronesian archipelagos (Canary Islands, Madeira, Azores) is restricted to moist and humid mountain slopes where humidity brought by the trade winds condenses to form clouds (Ceballos & Ortuno, 1976). The tree species of these forests are obviously not competitive at low humidity. This inability to withstand arid conditions may arise from (i) an insufficient xylem capacity for water transport to the crowns, (ii) an ineffective stomatal regulation of transpiration, (iii) a low drought tolerance of the leaves. These functional attributes of the laurel forest tree species were investigated comparatively and compared with pertinent data of Mediterranean species.

MATERIALS AND METHODS

The species are named in the legend of Fig.l. Xylem anatomy and drought tolerance of the leaves were studied with plant material collected in the Anaga Mountains of Tenerife, Canary Islands (28.5° N, 15.9° W). Transpirational water loss was assessed with leaves from greenhousegrown plants. No differences in structure and function could be recognized between field-collected and cultivated plants. The theoretical water supply capacity of the twigs was calculated according to the Hagen- Poiseuille formula (Huber, 1956) using data from microscopic determinations of vessel numbers and diameters.