Gas exchange characteristics of a Canarian laurel forest tree species (Laurus azorica) in relation to environmental conditions and leaf canopy position

Diurnal courses of gas exchange were measured over a 1-year period in fully expanded current-year leaves in the upper (sun-exposed, 18 m above ground) and the lower (shaded, 12 m above ground) canopy of Laurus azorica (Seub.) Franco, a major canopy species of the Canarian laurel forest in Tenerife, Canary Islands, Spain. Laurus azorica exhibited high leaf plasticity in gas exchange characteristics, with a maximum carbon assimilation rate (A(max)) of shade leaves about 50% that of sun leaves. This difference reflects the high leaf area index (LAI) of the stand and the correspondingly sharp ligh t attenuation with increasing canopy depth. In sun leaves, A(max) peaked at about 11 mu mol m(-2) s(-1) and maximum transpiration (E) was about 8 mmol m(-2) s(-1), which corresponded with a maximum stomatal conductance (g(s)) of about 650 mmol m(-2) s(-1). Mean maximum instantaneous water-use effici ency (WUE) was 1.5 mmol mol(-1) and the mean maximum A/g(s) was 20-35 mu mo l mol(-1). Mean minimum internal CO2 concentration (C-i) was 225 mu mol mol (-1). Although high air vapor pressure deficit (VPD) caused a small decreas e in g(s), it remained high enough to maintain relatively high A and E. The se gas exchange characteristics indicate a non-conservative use of water, w hich is appropriate for a species subject to droughts that are mild or of s hort duration. In this respect, Laurus azorica differs from its congener, L . nobilis L., of the Mediterranean region and other shrubs growing in Medit erranean-type climates in California and Chile that have to withstand more severe or more prolonged droughts.