The Mathematical Simplicity of Impenetrable Tropical Peat Swamp Forests

The Mathematical Simplicity of Impenetrable Tropical Peat Swamp Forests

Tropical peat forests have grown for millennia in the low-elevation regions of Southeast Asia, Peninsular Malaysia, Sumatra, and Borneo.  A positive feedback between peat accumulation and a rising watertable promoted accumulation of peat in ten-meter thick domes, kilometers across, that sequester tens of gigatons of organic carbon. These tropical peat swamp forests are modern analogues of the forests in the Carboniferous Period that produced coal deposits.  In the last twenty years, almost all these tropical forest have been cut and drained and now tropical peatlands emit enormous fluxes of CO2 to the atmosphere and create plumes of toxic smoke that sometimes blanket Southeast Asia as they subside towards sea level.

I will describe one of the world’s last remaining undisturbed peat forests in the county of Brunei on the island of Borneo. By combining hydrologic, carbon-flux, and LIDAR data gathered in this forest, we developed a mathematical framework for the coupled hydrologic and ecological processes that shape tropical peatlands. Model simulations and data agree that the peat ecosystem has approached an equilibrium over five millennia such that: (1) Carbon emissions and photosynthetic uptake balance throughout the landscape; (2) The watertable responds uniformly to rainstorms across the landscape, and; (3) The topographic curvature of the land surface is described by a spatially uniform Laplacian value that is predicted by rainfall statistics.  We test this model by reconstructing paleo-surfaces from peat radiocarbon dates and simulating the development of these surfaces with a numerical model of peat forest dynamics driven by reconstructed paleo-rainfall.  

We find that peatlands reach their ultimate shape first at the edges of peat domes where they are bounded by rivers, so that the rate of carbon uptake accompanying their growth is proportional to the area of the still-growing dome interior. We use this model to study how tropical peatland carbon storage and fluxes are controlled by changes in climate, sea level, and drainage networks. We find that fluctuations in net precipitation on timescales from hours to years can reduce long-term peat accumulation. Finally, we apply our mathematical model to propose infrastructure design (canals and roads) that would reduce drainage and hence the rate of peatland loss.