Lower P contents and more widespread terpene presence in old Bornean than in young Hawaiian tropical plant species guilds

Leaf elemental and secondary metabolite contents and morphological traits are important measures of time-dependent ecosystem changes. We aimed to test whether plants from older tropical forests have lower nutrient contents and different elemental stoichiometry than plants from younger ecosystems (“soil age” hypothesis) and whether they had different contents of carbon based secondary compounds (CBSC) and morphological traits as a result of a longer evolution under tropical conditions. We conducted a phylogeny-independent study of the foliar chemical and structural traits in two sets of 86 species each measured in two different-aged tropical forests, a young soil forest in Hawaii and an old soil forest in Borneo. The leaf contents of nutrients and micronutrients tended to be higher in Hawaii than in Borneo but leaf N:P content ratio was not different. The “soil age” hypothesis was thus only partially supported by the results indicating that several other factors influence plant elemental content. Total phenolic content was twice larger in Hawaiian than in Bornean plant species. Terpene contents were not different in terpene-containing species but the percentage of species containing terpenes was much higher in Borneo (97%) than in Hawaii (34%) suggesting that the longer time of evolution in Borneo has allowed a more widespread development of very diverse defensive, allelopatic and information relationships of plants with specialist herbivores and other plants. Principal component analyses separated Hawaii and Borneo species on the basis of leaf elemental composition, total phenolics and terpene contents and leaf dry mass per area (LMA). The results collectively support the “leaf economic spectrum” and “carbon excess” paradigms because in both sets of species and also in the combined set of Borneo and Hawaiian species, there are negative relationships of N content with LMA and total phenolics. The results suggest thus that changes throughout time in N and P availability can be important but do not explain all the variability underlying the evolutionary changes in leaf chemistry and structure in these tropical forests. Other factors determining species biogeochemical niche such as K, Mg or S elemental stoichiometry, leaf economic traits and changes in plant defence and communication strategy are also likely to be involved.