Leaf hydraulic conductance, measured in situ, declines and recovers daily: leaf hydraulics, water potential and stomatal conductance in four temperate and three tropical tree species

Adequate leaf hydraulic conductance (K(leaf)) is critical for preventing transpiration-induced desiccation and subsequent stomatal closure that would restrict carbon gain. A few studies have reported midday depression of K(leaf) (or petiole conductivity) and its subsequent recovery in situ, but the extent to which this phenomenon is universal is not known. The objectives of this study were to measure K(leaf), using a rehydration kinetics method, (1) in the laboratory (under controlled conditions) across a range of water potentials to construct vulnerability curves (VC) and (2) over the course of the day in the field along with leaf water potential and stomatal conductance (g(s)). Two broadleaf (one evergreen, Arbutus menziesii Pursh., and one deciduous, Quercus garryana Dougl.) and two coniferous species (Pinus ponderosa Dougl. and Pseudotsuga menziesii [Mirbel]) were chosen as representative of different plant types. In addition, K(leaf) in the laboratory and leaf water potential in the field were measured for three tropical evergreen species (Protium panamense (Rose), Tachigalia versicolor Standley and L.O. Williams and Vochysia ferruginea Mart) to predict their daily changes in field K(leaf) in situ. It was hypothesized that in the field, leaves would close their stomata at water potential thresholds at which K(leaf) begins to decline sharply in laboratory-generated VC, thus preventing substantial losses of K(leaf). The temperate species showed a 1566% decline in K(leaf) by midday, before stomatal closure. Although there were substantial midday declines in K(leaf), recovery was nearly complete by late afternoon. Stomatal conductance began to decrease in Pseudotsuga, Pinus and Quercus once K(leaf) began to decline; however, there was no detectable reduction in g(s) in Arbutus. Predicted K(leaf) in the tropical species, based on laboratory-generated VC, decreased by 74% of maximum K(leaf) in Tachigalia, but only 2232% in Vochysia and Protium. The results presented here, from the previous work of the authors and from other published studies, were consistent with two different strategies regarding daily maintenance of K(leaf): (1) substantial loss and subsequent recovery or (2) a more conservative strategy of loss avoidance.