Widespread herbivory cost in tropical nitrogen-fixing tree species.

Recent observations suggest that the large carbon sink in mature and recovering forests may be strongly limited by nitrogen1-3. Nitrogen-fixing trees (fixers) in symbiosis with bacteria provide the main natural source of new nitrogen to tropical forests3,4. However, abundances of fixers are tightly constrained5-7, highlighting the fundamental unanswered question of what limits new nitrogen entering tropical ecosystems. Here we examine whether herbivory by animals is responsible for limiting symbiotic nitrogen fixation in tropical forests. We evaluate whether nitrogen-fixing trees experience more herbivory than other trees, whether herbivory carries a substantial carbon cost, and whether high herbivory is a result of herbivores targeting the nitrogen-rich leaves of fixers8,9. We analysed 1,626 leaves from 350 seedlings of 43 tropical tree species in Panama and found that: (1) although herbivory reduces the growth and survival of all seedlings, nitrogen-fixing trees undergo 26% more herbivory than non-fixers; (2) fixers have 34% higher carbon opportunity costs owing to herbivory than non-fixers, exceeding the metabolic cost of fixing nitrogen; and (3) the high herbivory of fixers is not driven by high leaf nitrogen. Our findings reveal that herbivory may be sufficient to limit tropical symbiotic nitrogen fixation and could constrain its role in alleviating nitrogen limitation on the tropical carbon sink.

Surviving in a Cosexual World: A Cost-Benefit Analysis of Dioecy in Tropical Trees.

Dioecy has a demographic disadvantage compared with hermaphroditism: only about half of reproductive adults produce seeds. Dioecious species must therefore have fitness advantages to compensate for this cost through increased survival, growth, and/or reproduction. We used a full life cycle approach to quantify the demographic costs and benefits associated with dioecy while controlling for demographic differences between dioecious and hermaphroditic species related to other functional traits. The advantage of this novel approach is that we can focus on the effect of breeding system across a diverse tree community. We built a composite integral projection model for hermaphroditic and dioecious tree populations from Barro Colorado Island, Panama, using long-term demographic and newly collected reproductive data. Integration of all costs and benefits showed that compensation was realized through increased seed production, resulting in no net costs of dioecy. Compensation was also facilitated by the low contribution of reproduction to population growth. Estimated positive effects of dioecy on tree growth and survival were small and insignificant for population growth rates. Our model revealed that, for long-lived organisms, the cost of having males is smaller than generally expected. Hence, little compensation is required for dioecious species to maintain population growth rates similar to those of hermaphroditic species.

Root and leaf traits reflect distinct resource acquisition strategies in tropical lianas and trees.

In Neotropical forests, lianas are increasing in abundance relative to trees. This increased species richness may reflect a positive response to global change factors including increased temperature, atmospheric CO2, habitat fragmentation, and drought severity; however, questions remain as to the specific mechanisms facilitating the response. Previous work suggests that lianas may gain an ecological advantage over trees through leaf functional traits that offer a quick return on investment of resources, although it is unknown whether this pattern extends to root traits and relationships with fungal or bacterial symbionts belowground. We sampled confamilial pairs of liana and tree species and quantified morphological and chemical traits of leaves and fine roots, as well as root symbiont abundance, to determine whether functional traits associated with resource acquisition differed between the two. Compared to trees, lianas possessed higher specific leaf area, specific root length, root branching intensity, and root nitrogen (N) and phosphorus (P) concentrations, and lower leaf and root tissue density, leaf and root carbon (C), root diameter, root C:P and N:P, and mycorrhizal colonization. Our study provides new evidence that liana leaf and root traits are characterized by a rapid resource acquisition strategy relative to trees. These liana functional traits may facilitate their response to global change, raising questions about how increased liana dominance might affect ecosystem processes of Neotropical forests.