In the present study, carried out from 2004 to 2006, leaf and stem water potential and stem water content were measured in Quercus pyrenaica Willd. individuals in an experimental forested catchment located in Central Western Spain under Mediterranean subhumid conditions. These indicators of tree water status were compared with soil moisture contents measured in the same area from 0 to 1 m depth and from 0 to 2.5 m depth during the last year of the study. The objectives were to clarify the seasonal and year-to-year variations in tree water status, to examine applicability of stem water content as useful water stress indicator and to discuss how deep soil water and root uptake contribute to survival during the long dry summer. Seasonal variations in the tree variables measured revealed a typical pattern, with maximum values at the end of spring followed by a progressive decline during the summer drought in response to the decrease in soil water content (almost exhausted at 0-100 cm depth). The relatively high values and the non-signiWcant variation in predawn leaf water potentials (except for 2005, which was exceptionally dry) indicate that no clear water stress situations occurred. This may be explained in terms of a progressive absorption of water from the deeper layers. The results also suggest that the stem water content is a more sensitive indicator of long-term water limitation than the other variables measured.

Abstract:Soil-tree water relationships were studied using time domain reflectometry (TDR) in two small forested basins in Spain. The stem water content of two Mediterranean Quercus species (Quercus pyrenaica and Quercus rotundifolia) was measured using previously constructed species-specific equations. To monitor soil moisture, a TDR station network was used in both cases. Sixteen Q. pyrenaica and six Q. rotundifolia individuals were selected to install two TDR probes in their trunks (at 20 and 120 cm above the ground) to monitor stem water content. Stem and soil water contents were measured fortnightly. The stem water content of both species showed a similar temporal trend for the period studied. A spring maximum (0Ð654 cm 3 cm 3 for Q. pyrenaica and 0Ð568 cm 3 cm 3 for Q. rotundifolia) was found to be associated with high transpiration and no soil moisture deficit, and a late-summer minimum (0Ð520 cm 3 cm 3 for Q. pyrenaica and 0Ð426 cm 3 cm 3 for Q. rotundifolia) was associated with the end of the dry season. This drop in stem water content occurs when the available water in the soil decreases. This seasonal difference presumably reflects water withdrawn from stem storage to support the transpirational demands of the tree. Since plant water stress results in reduced stem water content and since this drop can be measured by TDR, it may be concluded that this technology offers a suitable tool for detecting plant water stress.

No abstract