CARBON AND WATER-BALANCE IN POLYLEPIS-SERICEA, A TROPICAL TREELINE SPECIES

Polylepis sericea trees grow well above the continuous forest line in the Venezuelan Andes. In these environments, extreme daily temperature ranges can occur at any time of the year and trees experience a 4 mon th dry period. The purpose of this work was to study carbon and water relations of this species in the field during wet and dry seasons in o rder to understand this species' success at such high altitudes. Leaf gas exchange (portable system in open mode) and leaf water potential ( pressure chamber) were measured at 1-2 h intervals during several dail y courses at 4000 m elevation in the Paramo de Piedras Blancas. CO2 as similation versus leaf temperature curves were also obtained for this species in the laboratory. Clear differences in the measured parameter s were observed between seasons. For a wet season day, maximum CO2 ass imilation rate was 7.4 mu mol m(-2) s(-1) and leaf conductance was rel atively constant (approximately 100 mmol m(-2) s(-1)). In the dry seas on day, maximum CO2 assimilation rate was 5.8 mu mol m(-2) s(-1) and l eaf conductance was close to 60 mmol m(-2) s(-1). Minimum leaf water p otentials measured were -1.3 MPa for the wet and -2.2 MPa for the dry season. The CO2 assimilation-leaf temperature relationship showed a 13 .4 degrees C leaf temperature optimum for photosynthesis with maximum and minimum compensation points of 29.5 and -2.8 degrees C, respective ly. Maximum night-time respiration was relatively high (2.7 mu mol m(- 2) s(-1)). Our results show that P. sericea maintains a highly positiv e carbon balance through all daily courses, even though there is a sli ght water stress effect during the dry season; this suggests that its carbon assimilation machinery is well adapted to the low temperatures and seasonal water stress found in the high tropical mountains.