Two tropical lianas and their host trees have contrasting hydraulic strategies

Background/Question/Methods

Tropical liana abundance has been increasing over the past 40 years, which has been associated with longer dry seasons and reduced rainfall. The mechanism that has been proposed to allow lianas to thrive in dry conditions is access to deeper pools of water than co-occurring trees via deep root systems.

To test the hypothesis that two abundant liana species, Prionostemma aspera and Trichostigma octandrum, incorporate characteristics that allow them to withstand drought better than their host trees,  Anacardium excelsum, we measured rooting depth, leaf, stem and root hydraulic properties, plant water status, and leaf level gas exchange. The research was conducted during the dry season of 2011 in a seasonally-dry tropical forest in the Republic of Panama. We also used a model to compare water use during drought in Trichostigma and Anacardium.

Results/Conclusions

The two species of lianas and their host trees had few roots below 60 cm. The liana species were more vulnerable to embolism than host trees and experienced water potentials that corresponded to 74 to 78 % loss of hydraulic conductance in leaves and 19 to 49 % loss of hydraulic conductivity in stems. Water potentials in Anacardium leaves and stems were not negative enough to result in significant hydraulic losses. Although stem hydraulic conductivity was not different between Anacardium and Trichostigma, root conductivity was 2.2 times greater in Trichostigma than in Anacardium. Based on a water transport model, Trichostigma was predicted to have greater gas exchange than Anacardium both when soil water was abundant and during drought.  Additionally, Trichostigma was predicted to use 2.7 times more water, per leaf area, than Anacardium.

In the current study, Anacardium had a more conservative strategy for hydraulic maintenance than did Trichostigma and Prionostemma.  The two liana species experienced embolism in stems and leaves and these emboli were presumably repaired prior to the next morning. However, Anacardium reduced stomatal conductance to prevent leaf or stem embolism. The ability to rapidly repair embolisms may allow these lianas to thrive during the dry season.