In tall conifers, leaf structure can vary dramatically with height due to decreasing water potential (Ψ) and increasing light availability. This variation in leaf structure can have physiological consequences such as increased respiratory costs (Rm), reduced internal CO2 conductance (gi), and reduced photosynthetic capacity (Amax). Compared to other tall conifers, Picea sitchensis (Bong.) Carrière leaf structure varies along the vertical gradient in ways that suggest compensatory changes to enhance photosynthesis with increasing height, and this variation is driven largely by light availability rather than by Ψ. These trends in leaf structure coupled with remarkably fast growth rates and dependence on moist environments inspire two important questions about P. sitchensis: 1) does foliar water uptake supplement soil-derived water to minimize the adverse effects of decreasing Ψ with height on leaf structure, and 2) do the observed trends in leaf structure work to increase photosynthetic rates despite increasing height? To answer these questions, we measured foliar water uptake capacity, predawn (Ψpd) and midday (Ψmd) water potential, and gas exchange rates (Rm, gi, and Amax) as they varied between 25 and 89 m heights in 300-year-old P. sitchensis trees in northwestern California.
Results/Conclusions
We report four major findings for P. sitchensis. 1) Foliar water uptake capacity was quite high relative to published values for other woody species, being roughly three times higher than co-occurring conifers and only exceeded by an evergreen fern. 2) Foliar water uptake capacity significantly increased with height, more than doubling between the crown base and treetop. 3) Wet season Ψpd was higher than predicted by the gravitational potential gradient throughout tree crowns, indicating widespread supplementation of soil-derived water via foliar water uptake. 4) Leaf-level physiology (Rm, gi, and Amax) varied little within tree crowns, presumably due to light-driven shifts in leaf structure between the crown base and treetop mitigating the height-related decreases in Amax observed in other conifers. Our Ψpd and foliar water uptake results highlight how foliar water uptake likely increases Ψ and stomatal conductance to further buffer treetop photosynthesis against height-related constraints in this species. These findings suggest that together, the use of foliar water uptake and shifts in leaf structure to conserve photosynthetic capacity with height likely contribute to the rapid growth rates observed in P. sitchensis.