Carbon storage does not promote shade tolerance in southern South American trees

Background/Question/Methods

Herbivory is an important constraint for seedling survival in shade, because low-light availability impedes a fast leaf reposition (i.e. re-foliation) after herbivory damage. It is thought that the difficulty in recovering from biotic or abiotic defoliation in low-light environments, and the low opportunity cost of storage in low light, had selected for carbohydrate storage in roots and stems of seedlings and saplings. The expectation of shade-tolerant species accounting for higher levels of carbon storage than shade-intolerant counterparts has been met in tropical forests, but results for temperate forests are mixed. I examined the relationship between C storage and shade-tolerance in the Chilean temperate rainforest by comparing the concentration and content (i.e. total amount) of non-structural carbohydrates (soluble sugars and starch) in roots, stems, and leaves of seedlings naturally established in the understory, for seven evergreen species with contrasting light requirements. Additionally, I conducted a similar research at a Mediterranean forest of Central Chile, where light levels in the understory are higher than in the rainforest.

Results/Conclusions

In the temperate rainforest, species differed significantly in the concentration and total amount of non-structural carbohydrates in roots, stems, and leaves. However, in contrast to the main expectation, shade-tolerant species tended to have lower carbohydrate concentrations across tissues than shade-intolerant counterparts. This trend was driven by both soluble sugars and starch. In Myrceugenia planipes (Myrtaceae), the most shade-tolerant species examined, root and stem carbohydrate concentrations were c. 40% lower than those found in Aristotelia chilensis (Elaeocarpaceae), the less shade-tolerant species examined. In this site, carbohydrate content did not relate to shade tolerance. In the Mediterranean forest, although significant interspecific differences were found for both carbohydrate concentration and content, no clear association between carbon storage and shade tolerance was observed. These results do not support the hypothesis of shade tolerance mediated by carbon storage in southern-South American temperate species. I suggest that low herbivory pressure in the understory may account for these results.