AbstractPolylepis 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 month 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 order 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 12 h intervals during several daily courses at 4000 m elevation in the Pramo de Piedras Blancas. CO2 assimilation versus leaf temperature curves were also obtained for this species in the laboratory. Clear differences in the measured parameters were observed between seasons. For a wet season day, maximum CO2 assimilation rate was 7.4 mol m2 s1 and leaf conductance was relatively constant (approximately 100 mmol m2 s1). In the dry season day, maximum CO2 assimilation rate was 5.8 molm2s1 and leaf conductance was close to 60 mmolm2s1. Minimum leaf water potentials measured were 1.3 MPa for the wet and 2.2 MPa for the dry season. The CO2 assimilation-leaf temperature relationship showed a 13.4C leaf temperature optimum for photosynthesis with maximum and minimum compensation points of 29.5 and 2.8C, respectively. Maximum night-time respiration was relatively high (2.7 mol m2 s1). Our results show that P. sericea maintains a highly positive carbon balance through all daily courses, even though there is a slight 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.