Hydraulic properties of Mexican fan palms (Washingtonia robusta) varying in height with implications for hydraulic limitation and embolism refilling

Background/Question/Methods

As trees grow taller, the path length for water flow increases and could begin to limit stomatal conductance, subsequent carbon gain and further height growth. This study seeks to determine how hydraulic properties of fronds from Washingtonia robusta palms change along a height gradient. Fronds were measured from palms varying in height from 1m to 16m tall growing on the campus of the University of Western Sydney in NSW, Australia. Leaf and frond area, conductivity, vascular anatomy, gas exchange properties and leaf water potential were measured and vulnerability curves were made in order to determine how these properties change with height and if these hydraulic properties limit stomatal conductance. Additionally, double-dye staining of fronds from small palms was performed to determine if embolism and night-time refilling was occurring on a daily basis.

Results/Conclusions

Stomatal conductances, photosynthetic assimilation and minimum leaf water potentials did not differ significantly among palms of differing heights. Leaf areas, frond cross-sectional areas, frond specific conductivity (K_S), maximum vessel length and vessel diameters initially increased with height, reached a maximum in palms around 9m tall, then decreased in taller palms. Huber values of fronds were found to remain constant with height, while leaf specific conductivities (K_L) decreased slightly. Vascular bundle densities and stomatal densities increased with height. Additionally, the water potential at which 50% of conductivity is lost, P₅₀, became more negative with increased height. Based on the double-dye staining, there is evidence that vessels embolize on a daily basis in the fronds of small palms and refill overnight. These data suggest that while W. robusta palms are able to maintain stomatal conductances, photosynthetic rates and leaf water potentials across a range of heights, alterations in frond vascular anatomy, conductivity, and vulnerability to embolism occur in these fronds in order to facilitate the maintenance of gas exchange irrespective of height. Finally, comparison of P₅₀ values with minimum leaf water potentials and double-dye staining suggest that vessels, at least in fronds from small palms, embolize and refill on a regular basis.  This work leaves open the question of what factors are proximally and ultimately driving height growth reductions in these palms. We are now further investigating growth controls like changes in frond phenology with age and the role it might play in slowing height growth in tall palms.