Microenvironmental heterogeneity and space utilization by desert vines within their host trees

The three-dimensional biomass distribution and the microenvironments experienced by several desert Vine species growing within the canopy of host trees were studied at the Centro Ecologico de Sonora in Mexico. The light environment within the crown of the host tree Cercidium microphyllum showed a horizontal and vertical gradient from the base of the trunk to the edges of the canopy. Within this gradient total daily photosynthetically active radiation (PAR) varied from 478 mol m(-2) outside the crown to 4.6 mol m(-2) at the centre of the crown and close to the ground. Maximum air temperature was 3 degrees C lower beneath the crown than outside. Within the canopy, most vines experienced less than 50 % of the daily available PAR outside the canopy. For most of the day, leaves of vines received 15 % or less of the maximum available PAR. Our study shows that vines do not grow towards full sunlight but rather they exploit different habitat possibilities within their host tree crown. Leaves along the stems of vines experienced a wide range of light environments, showing coefficients of variation (CV) in total daily PAR from 36.4 to 94.6 %. Daily courses of PAR also showed that leaves within the canopy experienced short-term temporal variation in the light environment. Differences in CV of daily PAR values and preferences in heterogeneous light microenvironments among species suggested that different vine species might be spatially separated in the canopy. We suggest that in desert habitats, conditions within the crown of host trees result in an important microhabitat that vines can exploit, allowing them to avoid the high light, temperature and water deficits found in the surrounding environment. (C) 1999 Annals of Botany Company.