| Palms are an attractive group for physiological research because their columnar trunks and simple leaf habit allow for estimation of variables important for hydraulic functioning more easily than in complex arborescent dicotyledons. I made measurements of hydraulic functioning across palm size in order to characterize hydraulic limitations and/or compensations found in trees that grow taller, and therefore, move water further distances. I focused on species from a variety of moisture gradients from the very dry (Washingtonia robusta) to tropical rainforests ( Iriartea deltoidea and Mauritia flexuosa). While both rainforest species exhibited differing patterns in height growth rate along boles, rates ultimately decreased in the tallest palms. Sap flux per unit leaf area was constant across palm height in all three species, suggesting that taller palms were not showing evidence of hydraulic limitation. However, in Iriartea deltoidea, stomata restricted transpiration more in the dry season compared to the wet season. The two tropical species exhibited opposing trends with height in both live frond number and frond leaf area, but showed convergence in the trend of increasing total leaf area with height. However, in sub-tropical Washingtonia robusta, total leaf area decreased in taller palms. Both Mauritia flexuosa and Washingtonia robusta increased their reliance on stored water with height; consistent with a hydraulic limitation. Regarding petiole conductivities, neither leaf specific conductivity (KL) nor P50 values (point at which 50% of hydraulic conductivity is lost) changed with height in Iriartea deltoidea, while in Washington a robusta, P50 values increased with height which is consistent with hydraulic compensation. Comparing P 50 values with measurements of midday leaf water potentials, as well as a double-dye staining experiment, suggested that palms experience and repair embolisms on a daily basis instead of avoiding embolisms through tight stomatal control. Taken together, these results demonstrate that palm species exhibit a variety of hydraulic alterations with height that can compensate for hydraulic limitation in taller palms. These alterations are not unique to palms and can, therefore, provide insights on the hydraulic functioning of all woody species as they grow taller. |
