Lianas are particularly abundant in seasonally dry tropical forests in contrast to trees that increase in abundance with an increase in rainfall. The liana-rainfall relationship has been attributed to lianas being deep-rooted, permitting them to access deep sources of water and perform better in forests that experience periods of seasonal drought; however, this had not been directly tested. It was also unclear whether co-occurring lianas, deciduous trees, and evergreen trees had different levels of drought resistance. We used whole-plant harvests of mature tropical dry forest deciduous trees, evergreen trees, and lianas, hydraulic trait measurements, and model simulations to address the following knowledge gaps: (1) Is either leaf phenology or life form a predictor of rooting depth? (2) Do deciduous trees, evergreen trees, and lianas differ in drought resistance? In a Costa Rican tropical dry forest, we harvested a total of 33 mature individuals—15 lianas, 12 deciduous-trees, and 6 evergreen-trees. At the same location, we also measured leaf wilting point (TLP—turgor loss point), loss of 50% water-conducting capacity (P50), and drought resistance or hydraulic safety margins (TLP-P50) of 32 species of deciduous trees, evergreen trees, and lianas. Simulations with the Ecosystem Demography 2 (ED2) model were used to investigate the necessity of developing deep roots for evergreen species.
Results/Conclusions
Lianas had the shallowest coarse roots, and evergreen trees had the deepest (χ2=34.33, P<0.001; deciduous– evergreen t=-3.205, P= 0.026; deciduous–liana t=3.14, P= 0.034; evergreen – liana t=5.70, P<0.001). Lianas had greater resistance to the loss of water-conducting capacity than evergreen trees (F2,32=3.89, P=0.031; t=-2.72, P=0.028); however, there was no difference between lianas and deciduous trees (t=-1.83, P=0.176), nor deciduous and evergreen trees (t=-1.44, P=0.334). Hydraulic safety margins did not differ across leaf phenology and life form (F2,31=2.45, P=0.102). Our main conclusions are: (1) lianas were not deeper-rooted than trees as had been previously proposed—they actually have shallower roots; (2) evergreen trees had the deepest roots, which is necessary to maintain canopy during simulated dry seasons with the ED2 model; (3) lianas had the greatest resistance to loss of water-conducting capacity, which may be necessary because they do not have deep roots, whereas evergreen trees were the least resistance, presumably because they rely on deep roots to access deep sources of water and incur lower risks of losing water-conducting capacity; and (4) all species operated with similar hydraulic safety margins meaning that they all have similar levels of drought resistance.