Fire in the Rainforest: Fire History and Carbon Pools in Southwestern Borneo's Tropical Rainforest

Tropical rainforests are globally important resources for carbon storage as well as biodiversity. Despite high precipitation, fires now occur on an annual basis in Southeast Asia’s forests. Fire in the rainforest is not totally unprecedented; many paleorecords show evidence of fire throughout the Holocene. Humans have been using fire in these environments for millennia, but some fires cannot be clearly attributed to human influence. Furthermore, we do not have a good understanding of the prevalence or spatial pattern of these fires earlier in the Holocene, nor how they have altered terrestrial carbon stocks. Here, I investigate the late Holocene fire history in a primary rainforest located in southwestern Borneo and its imprint in carbon pools. This work begins with the task of understanding the pattern of fire through the last 3,200 years at Cabang Panti Research Station (CPRS; West Kalimantan, Indonesia). I use 14C dating of soil charcoal and its relative abundance across forest types to assess the role that humans played in fire occurrence in this primary rainforest. Using a natural gradient of fire susceptibility, I find that fire activity across the site peaked ca. 1500 CE. This coincides with both regional drought and increased human pressure in Southeast Asia. I conclude that humans likely supplied ignitions, but drought allowed fire to spread throughout the study area. In the third and fourth chapters I quantify soil carbon pools at the study site with a particular focus on the contribution of pyrogenic carbon (PyC). I show that a simple, inexpensive chemical oxidation method using only standard laboratory equipment and reagents consistently and accurately quantifies PyC in soil samples when the pyrogenic carbon is produced in fires >400°C. Using this method, I find that PyC makes up a non-negligible portion of carbon pools at Gunung Palung National Park (approximately 5%), and thus should be considered separately in future models of carbon stocks and cycling at the site due to its potential to persist for hundreds to thousands of years. This work expands our understanding of fire in tropical rainforest and the role humans have played throughout history. This dissertation includes previously unpublished co-authored material.2022-03-1