Lianas and trees exhibit divergent intrinsic water-use efficiency along elevational gradients in South American and African tropical forests

Elevational gradients provide excellent opportunities to explore long-term morphological and physiological responses of plants to environmental change. We determined the difference in the elevational pattern of foliar carbon isotope composition (δ¹³C) between lianas and trees, and assessed whether this difference arises from changes in photosynthesis or stomatal conductance. We also explored the pattern of nutrient limitations with the elevation of these two growth forms. We conducted inventories of lianas and trees using standardized techniques along elevational gradients in Ecuador and Rwanda. We determined the values of several foliar traits including δ¹³C and chemical traits in dominant liana and tree species. We set up Bayesian linear mixed-effect models to quantify the effects of elevation and these two growth forms, and the difference of the effect of elevation between the two growth forms on each of the foliar traits. We found consistent growth form specific divergences in foliar δ¹³C and carbon to nitrogen ratio (C:N) responses to elevation. While we noted a meaningful increase in foliar δ¹³C and C:N with elevation for trees, lianas did not exhibit such a trend. Foliar δ¹³C and C:N remained relatively constant for lianas along the transects. The physiological processes at the basis of foliar carbon isotope fractionation shift differently in lianas and trees along elevation. Lianas operate at relatively constant intrinsic water- and nitrogen- use efficiencies with elevation as opposed to trees. Altogether, the study suggests the existence of a functional divergence of water and nutrient use strategies between lianas and trees along tropical elevational transects.