Leaf habit relates to herbivory tolerance in two Chilean Nothofagus species

Background/Question/Methods

The classical view of deciduous tree species having higher dependence on carbon reserves for spring leaf-out and growth than evergreen species has been challenged lately; just a small proportion of carbon reserves in deciduous trees is indeed used during the leaf-out period. Thus, an unsolved and relevant ecological question remains: what do deciduous species have higher levels of carbon storage for? One hypothesis is that natural selection could have favored higher levels of carbon reserves in deciduous species in order to replace leaves under eventual massive leaf removal. As deciduous species account for more palatable leaves than evergreens, they are more likely to experience massive defoliation. We tested this hypothesis by comparing carbon reserves and survival of saplings to complete experimental defoliation between two sympatric species, Nothofagus pumilio (deciduous) and Nothofagus betuloides (evergreen). We defoliated ten saplings of each species in November (onset of experiment) and February during three austral growing seasons and determined non-structural carbohydrates (NSC, carbon reserves) and survival at the end of the experiment. Ten saplings of each species were kept as controls. We expected that the deciduous species N. pumilio should exhibit higher survival and more carbon reserves than the evergreen N. betuloides, and carbon reserves should be used differently by both species after defoliation.

Results/Conclusions

In strong support to the hypothesis under study, we found significantly higher concentrations of NSC in the deciduous N. pumilio (root: 38.5% (±17); stem: 15.6% (±4); leaf: 21.2% (±5), concentrations in a dry matter basis) than in the evergreen N. betuloides (root: 15.8%(±14); stem: 11.9% (±7); leaf: 1.2% (±5)), along with lower resistance to defoliation in the evergreen species: all defoliated saplings of N. betuloides died one year after the defoliation started, while all saplings of the deciduous species were still alive in March 2012 and produced a complete leaf cohort each spring. In the evergreen species, defoliated saplings had 78 and 76% lower NSC concentrations in root and stem, respectively, than controls, while in the deciduous species, these values were 63 and 11%, respectively. We show here the first experimental evidence supporting the hypothesis that deciduous species have higher levels of carbon reserves to tolerate herbivory.