At a Glance
Simulations reveal how nitrogen fixation determines the rate of tropical forest regrowth and amount of carbon uptake in a recovering tropical forest.
The premise of this research project is that nutrient dynamics strongly regulate the ability of tropical forests to sequester atmospheric carbon dioxide1-3. Recent findings imply the existence of an ecosystem-level carbon-nitrogen feedback mechanism (symbiotic with certain types of bacteria) in which dinitrogen fixing trees can provide the nitrogen needed to maintain high forest growth rates, following any type of forest disturbance that reduces tree population (see Figure 1.2.1)4. However, field-based evaluation of this feedback has been difficult because
- the carbon pools of a forest equilibrate slowly, over decades to centuries of ecological succession;
- experimental inhibition of nitrogen fixation is not possible in real-world forests; and
- nitrogen fixers may influence forest succession, but this interaction is difficult to isolate.
To address these problems, the Medvigy and Hedin groups examined carbon-nitrogen feedbacks by applying a simulation model to 64 large-scale plots distributed across 300 years of forest succession at the Agua Salud Project, Panama (see Figure 1.2.2). Field observations of plant traits were used to develop a representation of plant diversity in the model. This model representation consisted of different plant functional types (PFTs), where each PFT was endowed with a characteristic combination of traits seen in the observations. The model represented the large-scale plots by assigning the observed trees to PFTs according to their traits. This trait-based approach contrasts with the conventional approach of treating all tropical forest trees identically. The model can thus resolve PFT-nutrient feedbacks by evaluating nutrient dynamics within ensembles of spatially-linked individual trees of differing PFTs. The results showed that nitrogen fixation accelerates forest carbon accumulation, doubling the accumulation rate in early succession (0-30 years following disturbance) and increasing carbon storage in old-growth forests by 10%. An indirect effect on carbon accumulation was also found, showing how fixation interacted with the abundance of different PFTs. These results helped the Medvigy and Hedin groups to infer that nitrogen fixation can support the sequestration of a substantial quantity of carbon in the land biosphere (~24 petagrams of carbon) if extended to tropical forests worldwide.
Regrowing tropical forests currently contribute to over 40% of terrestrial carbon uptake5. These forests will remain a critically important element of the terrestrial carbon cycle as tropical deforestation continues in the coming decades6. This initiative has thus identified nitrogen fixation as essential for rapid tropical forest regrowth. This result runs counter to the conventional interpretation that tropical forests are nitrogen-rich7.
This work is expected to be of broad interest to climate change scientists, ecologists, earth-system modelers, policy-makers, and practitioners conserving and restoring tropical forests. Modelers may improve the representation of biological nitrogen fixation in the next generation of earth-system models. Practitioners may ensure biodiversity and functional diversity are present in reforested landscapes. Policy-makers may use this information to make decisions about how to use tropical lands (i.e., agriculture versus reforestation).
This project is now expanding to include investigation of nitrogen-phosphorus feedbacks on ecosystem carbon accumulation. Phosphorus (P) availability is particularly low in lowland tropical forests because there is little parent material P available to provide fresh input of P through weathering. This lack of P results in P-limitation in tropical forests. Despite the importance of P, none of the climatecarbon cycle models participating in the 5th Assessment Report of the Intergovernmental Panel on Climate Change included P dynamics. This project will address this deficiency with the following objectives: (1) develop a model for P cycling in terrestrial ecosystems, including the interactions of P with nitrogen (N); (2) constrain critical model uncertainties using new and existing field and LIDAR remote-sensing measurements; and (3) incorporating P and P-N dynamics into terrestrial biosphere models and carrying out simulations to assess the impacts of coupled P-N dynamics on the simulated terrestrial carbon sink.
- Davidson, E.A., C.J.R. de Carvalho, A.M. Figueira, F.Y. Ishida, J.P.H.B. Ometto, G.B. Nardoto, R.T. Saba, S.N. Hayashi, E.C. Leal, I.C.G. Vieira, and L.A. Martinelli, 2007. Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment. Nature, 447: 995-998. doi:10.1038/nature05900.
- Davidson, E.A., C.J.R. de Carvalho, I.C.G. Vieira, R.O. Figueiredo, P. Moutinho, F.Y. Ishida, M.T.P. dos Santos, J.B. Guerrero, K. Kalif, and R.T. Saba, 2004. Nitrogen and phosphorus limitation of biomass growth in a tropical secondary forest. Ecological Applications, 14: S150-S163. doi:10.1890/01- 6006.
- Wright, S.J., J.B. Yavitt, N. Wurzburger, B.L. Turner, E.V.J. Tanner, E.J. Sayer, L.S. Santiago, M. Kaspari, L.O. Hedin, K.E. Harms, M.N. Garcia, and M.D. Corre, 2011. Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest. Ecology, 92(8): 1616-1625. doi:10.1890/10-1558.1.
- Batterman, S.A., L.O. Hedin, M. van Breugel, J. Ransijn, D.J. Craven, and J.S. Hall, 2013. Key role of symbiotic dinitrogen fixation in tropical forest secondary succession. Nature, 502: 224-227. doi:10.1038/nature12525.
- Pan, Y., R.A. Birdsey, J. Fang, et al., 2011. A large and persistent sink in the world’s forests. Science, 333: 988-993. doi:10.1126/science.1201609.
- Soares-Filho, B.S., D.C. Nepstad, L.M. Curran, G.C. Cerqueira, R.A. Garcia, C.A. Ramos, E.Voll, A. McDonald, P. Lefebvre, and P. Schlesinger, 2006. Modeling conservation in the Amazon basin. Nature, 440: 520-523. doi:10.1038/nature04389.
- Hedin, L.O., E.N.J. Brookshire, D.N.L. Menge, and A.R. Barron, 2009. The Nitrogen Paradox in Tropical Forest Ecosystems. Annual Review of Ecology, Evolution, and Systematics, 40: 613-635. doi:10.1146/annurev.ecolsys.37.091305.110246.