Trees and lianas are growth forms that account for the vast majority of the plant biomass in tropical forests. There is now compelling evidence that lianas are increasing in abundance relative to trees, particularly throughout Neotropical forests. The mechanisms that underlie this increase in lianas remain poorly understood. To address this knowledge gap, I asked: 1) To what degree is the physiological performance of tree seedlings limited by soil macronutrients (N,P,K) within the deeply shaded habitats that characterizes tropical forest understories; 2) Are seedlings of lianas more limited by these macronutrients than seedlings of trees; and 3) Are photosynthetic and leaf trait differences that exist in trees versus lianas caused by inherent growth-form differences or are they due to phylogenetic relatedness? This research demonstrated that the addition of two key soil resources, nitrogen and potassium, increased the photosynthetic performance of seedlings of an abundant tree species, Alseis blackiana, even in deep shade. Surprisingly, while seedlings of lianas and trees were limited to a similar degree by soil nutrients, lianas always out-performed trees in terms of their photosynthetic performance, regardless of nutrient addition. Moreover, my analyses demonstrated that lianas, as a distinct growth form, had increased rates of photosynthesis and respiration versus trees. Nonetheless, a suite of other traits that were thought to clearly distinguish the liana growth-form from trees (low LMA and high foliar nitrogen), were determined to be due to phylogeny. Together, these findings provide a potential mechanistic basis for increases in liana abundance in the Neotropics.