A comparative study of the conducting tissue of evergreen and deciduous trees of the Northern Territory

The climate of the top end of the Northern Territory is dominated by alternating wet and dry seasons. Trees of the Northern Territory have developed a variety of responses to this seasonal climate, including a range of leaf phenologies (evergreen, semi-deciduous, brevi-deciduous and deciduous). These phenological groups differ in their anatomical and physiological characteristics. This study investigated the relationship between vessel anatomy and susceptibility to drought induced xylem cavitations. Xylem vessel characteristics were compared among deciduous and evergreen species growing in seasonally dry forests near Darwin. Vessel diameter, vessel length, vessel element length, vessel density and pit membrane pore dimeters were measured in six evergreen and six deciduous species and one coniferous species. This study also examined the linkage between hydraulic conductance, vulnerability to xylem embolism and vessel diameter in deciduous and evergreen species. Deciduous species had a greater proportion of vessels with large diameters than did evergreen species. The mean vessel diameters in (branches, petioles and mid-ribs) were larger in deciduous species (50 μm ±1.7, 28 μm ± 1.56, 28 μm ± 2.1) than evergreen species (32 μm ± 0.6, 18 μm ± 1.2, 18 μm ± 1.1). The mean pit membrane pores diameter in deciduous species (0.49 μm± 0.34) was larger than in evergreen species (0.24.μm ± 0.15). The average hydraulic conductivity in branch of deciduous species (0.03 g m s-1 MPa-1) was larger than in evergreen species (0.01 g m s-1 MPa-1). These results support the conclusion that since deciduous species only have to conduct water during the wet season, their large xylem vessels with large pit membrane pores, which confer relatively larger hydraulic conductivity do not expose the canopy to an unacceptably high risk of xylem cavitation. By contrast, evergreen trees must transport water during more extreme dry season conditions, when they would potentially be more susceptible to xylem embolism. To protect against this, they have developed vessels with generally smaller diameters and with small pit membrane pores or vestured pits.