Water uptake and transport in lianas and co-occurring trees of a seasonally dry tropical forest

Water uptake and transport were studied in eight liana species in a seasonally dry tropical forest on Barro Colorado Island, Panama. Stable hydrogen isotope composition (δ D) of xylem and soil water, soil volumetric water content (θ(v)), and basal sap flow were measured during the 1997 and 1998 dry seasons. Sap flow of several neighboring trees was measured to assess differences between lianas and trees in magnitudes and patterns of daily sap flow. Little seasonal change in θ(v) was observed at 90-120 cm depth in both years. Mean soil water δ D during the dry season was -19&PTSTHOUSND; at 0-30 cm, -34&PTSTHOUSND; at 30-60 cm, and -50&PTSTHOUSND; at 90-120 cm. Average values of xylem δ D among the liana species ranged from -28&PTSTHOUSND; to -44&PTSTHOUSND; during the middle of the dry season, suggesting that water uptake was restricted to intermediate soil layers (30-60 cm). By the end of the dry season, all species exhibited more negative xylem δ D values (-41&PTSTHOUSND; to -62&PTSTHOUSND;), suggesting that they shifted to deeper water sources. Maximum sap flux density in co-occurring lianas and trees were comparable at similar stem diameter (DBH). Furthermore, lianas and trees conformed to the same linear relationship between daily sap flow and DBH. Our observations that lianas tap shallow sources of soil water at the beginning of the dry season and that sap flow is similar in lianas and trees of equivalent stem diameter do not support the common assumptions that lianas rely primarily on deep soil water and that they have higher rates of sap flow than co-occurring trees of similar stem size.