Palaeoecological Evidence of Ecosystem Dynamics in Sumatra, Indonesia. Case Studies of Tropical Submountains and Mangroves

The mountain and mangrove ecosystems are an important component of Sumatra’s landscape. These highly diverse ecosystems provide numerous ecological services as well as socio-economic function, but they are also sensitive to disturbance such as climate and human activities. I use the results of my research as a point of discussion for what are the most important drivers of ecosystem change as well as ecosystem resilience and recovery. As part of the Pacific ‘ring of fire’, volcanism is one of the most important element in Sumatra and volcanic processes play an important role in the history of vegetation. A 5000 years old record from Danau Njalau in the Kerinci Seblat National Park (KSNP) provides first evidence on how much volcanism can impact the vegetation composition in the tropical mountain of Sumatra. Our palynological record show that the formation of the modern forest took several centuries after the volcanic deposition in the soil ended. The results also show no evidence for prehistoric human-landscape interactions in the area despite the close proximity to known megalith sites. The local fire reconstruction indicates that fire was rare for the last 5000 years and the phases of increased fire frequency could not be linked to either any of the vegetation phases or regional climatic changes. Our results overall suggest the effect of volcanism on the western side of Sumatra might be spatially and temporally more important than expected in shaping the composition and structure of these diverse rainforests. A second record from the Kerinci valley, provides a better understanding of the history of human activities, another important driver of change in Sumatra. While today rice is the main subsidence crop in Indonesia, historical evidences of its cultivation are still scarce and there is no clear understanding to when rice was first introduced. The water buffalo (Bubalus sp.) was used in wet-rice cultivation to plough wet-rice field before the introduction of machineries. Therefore, finding evidence of the presence of swamp buffaloes can hint to important information regarding rice cultivation and its introduction in Sumatra. I used palaeoecological proxies like pollen and non-pollen palynomorphs to reconstruct the history of rice and buffaloes grazing in order to improve our understanding on when human activities intensified in Sumatra. Our results from Danau Bento are in agreement with the archaeological and linguistic records of the introduction of rice and the water buffaloes husbandry with the arrival of Austronesian migrant to Sumatra about 4000 years ago. Pollen results show an intensification of rice cultivation in the Danau Bento, at about 2500 years ago. A similar age was found in the Toba Plateau, in the north of Sumatra. Both these findings might suggest an increase in population and/or change in technology corresponding to the Bronze Age period in Sumatra. However, this phase lasted only a couple of centuries until ca. 2100 cal yr BP. Following the decline of rice cultivation, the swamp forest never recovered and human continued to maintain the swamp as a grassland for buffalo’s grazing. Overall the results indicate that men have shaped this ecosystems for thousands of years continuously. Mangrove forests in Sumatra are widely distributed along the east coast of the island and provide important ecological services as well as socio-economic function. However, present development of mangroves is under threat due to human activities and possible effect of future sea-level rise (SLR). Our palynological record from a secondary forest in the Mendahara Ilir (MI), Jambi Province suggests that mangroves in the southeast coast of Sumatra could adapt to the changes in sea-level which have occurred in the past 2300 years. The pollen record and the sediment accretion suggests that mangroves in MI could keep up with a SLR of about 2.2 mm/yr (average), however the current estimate is above both the sediment accretion estimate and the rate of sea-level change experienced in the past 2300 years (0.68 mm/yr for the past 1000 years’ transgression phase; 5 mm/yr current estimate for the last century; 7 mm/yr future estimate by 2100). Our pollen results show that even under a lower than current and projected SLR, mangroves migrated seaward (regression phase) and landward (transgression phase). If sea-level will rise at the pace predicted, mangroves will need a migratory path inland. This escape is however impeded, due to intensive human agriculture and urbanization in the coastal areas in Sumatra. With the application of palaeoecology, I could reconstruct thousands of years of vegetation history. Overall the results are important evidence of the role of human activities and volcanism in the highlands of Sumatra and the sensitivity of mangroves to future sea-level change.