Atmospheric and hydraulic limitations on transpiration in Brazilian cerrado woody species

Abstract

1. Seasonal regulation of transpiration was studied in four evergreen woody species growing in a Brazilian cerrado (savanna) site with a 5 month dry season. 2. Both seasonal and interspecifc variation in leaf water potential (Ψ(L)) were relatively small. Predawn Ψ(L) was about 0 · 1-0 · 25 MPa more negative in the dry season than in the wet season. 3. The dependence of maximum whole-plant sap flow rates on sapwood area was similar among all four species during both the wet and dry seasons. When total daily sap flow on a leaf area basis was normalized by the daily average air saturation deficit (ASD), only one of the four species showed significantly-greater water use during the wet season. 4. Although seasonal differences in regulation of transpiration were not pronounced, strong stomatal limitation of both maximum daily transpiration rates and total daily transpiration was evident during both the wet and dry seasons. Sap flow typically increased sharply in the morning; briefly attained a maximum value by about 09.30-10.30 h, then decreased sharply, despite steadily increasing solar radiation and atmospheric evaporative demand. 5. The total leaf area-specific apparent hydraulic Conductance of the soil/leaf pathway (G(t)) varied among plants and diurnally. The identical linear dependence of transpiration and stomatal conductance (g(s)) on G(t) among the four study species suggested that stomatal adjustment to variation in G(t) limited transpiration over the entire range of G(t) observed. 6. When g(s) was normalized for daily variation in G(t), about 80% of the remaining variation in g(s) was associated with variation in ASD. The results suggested that transpiration in these species was not limited by Soil water availability per se, but by high atmospheric evaporative demand and hydraulic constraints possibly arising from their deep rooting habit.