Linking hydraulic conductivity and photosynthesis to water-source partitioning in trees versus seedlings.

The relationships between hydraulic and photosynthetic properties in plants have been widely studied, but much less is known about how these properties are linked to water-source partitioning, the spatial and temporal separation of water sources in ecosystems. Plant water-source partitioning is often influenced by the proximity of groundwater from the natural surface. We studied the water acquisition strategy and hydraulic and photosynthetic properties of Tuart (Eucalyptus gomphocephala D.C.), a large coastal tree species that occupies seasonally dry habitats underlain by superficial aquifers. Our goal was to quantify water-source partitioning as the proportion of xylem water derived from the vadose and saturated zones with respect to stage of development and proximity of groundwater. We then sought to associate the proportional contribution of a given water source with xylem hydraulic and photosynthetic properties, thus conferring a linkage. Seedlings were more inclined to use surface soil water when rainfall recharge of the upper profile occurred, suggesting that they maintained or rapidly developed a proportionally high amount of functional roots in the upper, seasonally dry, soil profile. This strategy was associated with a lower xylem-area-specific hydraulic conductivity (K(S)), leaf-area-specific hydraulic conductivity (K(L)) and maximum photon yield of photosystem II (F(V)/F(M)). In contrast, trees acquired water from a variety of sources in different seasons and had a higher K(S), K(L) and F(V)/F(M). Despite the higher K(S) and K(L) in trees, the midday hydrodynamic water potential gradient from soil to leaves, ΔΨ, was similar. We conclude that there was a linkage between hydraulic and photosynthetic properties with the partitioning of water sources and that this adaptation to long-term hydrological regimes accommodated the different hydraulic characteristics and hydrological environments of trees versus seedlings.