The role of elevation, tree height, and crown position on the carbon balance of trees at the southern Andes treeline

Abstract Background and Aims Elevational treelines worldwide are thought to be temperature-driven. The most accepted hypothesis explaining the formation of treelines is that low temperature affects growth more than photosynthesis, rendering a surplus of carbon reserves in perennial tissues. This is the growth limitation hypothesis (GLH). However, support for this hypothesis is mostly biased toward evergreen conifers of the Northern Hemisphere. In this study, we tested the GLH in Nothofagus pumilio (Nothofagaceae) in Patagonia, Chile, across three different temperature gradients. Methods In two treeline sites, we measured non-structural carbohydrate concentrations (NSCs = starch + soluble sugars) in leaves, twig sapwood, and stem sapwood across 1) elevation, 2) conspecifics of individual trees of different tree heights at each elevation, and 3) intra-individual twigs at contrasting canopy heights. The expectation for the GLH to be true is that the colder the environment (i.e., the higher the elevation, the taller the tree, or the higher the twig position in the tree), the higher the tissue NSC concentrations. We also measured shoot elongation and basal area increment of the most recent 10 years (BAI10). Key Results BAI10, not shoot length, decreased with elevation and increased with relative tree height in both sites. NSC varied sometimes significantly with elevation and tree height but not with crown position. Stem’s NSC (and starch) concentrations decreased significantly with elevation in one site and increased in the other. NSC, starch, and soluble sugar concentrations decreased significantly in stem sapwood with relative tree height in both sites. Conclusions Our study gives mixed support to the GLH. First, variation in NSC with elevation was site-specific. Second, across all elevations, stem sapwood NSC decreased and BAI10 increased with tree height. One explanation for these ambiguous results is that carbon sinks, other than growth, blur the trend expected from the GLH in this deciduous treeline species.