Assessing resilience/sensitivity of tropical mountain rainforests towards climate variability of the last 1500 years: The long-term perspective at Lake Kalimpaa (Sulawesi, Indonesia)

Abstract

The tropical montane rainforests of the Lore Lindu National Park in Sulawesi, Indonesia provide many ecosystem services for the population inhabiting the area and harbor unique biodiversity in a key area for phytogeography. The mountain regions of Central Sulawesi experience perhumid climate conditions with few seasonal changes in precipitation, making the vegetation a possible sensitive target for future changes of precipitation patterns. The ecological consequences are hard to predict due to the lack of knowledge of the dynamical processes that govern these tropical forests. This research aims to shed light on the long-term response of the montane vegetation of Lore Lindu National Park to stress caused by climate variability and human activities in the past. Palynological data are used to reconstruct forest vegetation dynamics and are compared to centennial time scale data of fire frequencies, palaeorainfall proxies and regional climate reconstructions to assess the drivers of these changes. Results reveal that the Fagaceae family dominates the entire recorded period, as they still do today. Fire episodes occurred locally only ten times in the last 1500 years but two periods were characterized by higher frequencies: between ca. AD 1070 and 1200 and between ca. AD 1450 and 1660. The regional correlation of these events with periods of drought registered in Java suggests that centennial-scale increases in fire frequencies at Lake Kalimpaa were consequences of the vegetation being more prone to fire, probably due to more frequent or more intense El Nino events. In both cases Fagaceae did not decrease, indicating resilience towards droughts and fires of at least one species of that family. Following the first period of increased fire frequencies, the vegetation went through a long secondary forest phase lasting about two and a half centuries (ca. AD 1200-1450). Weinmannia was co-dominant together with Lithocarpus/Castanopsis. The second period of increased fire frequencies corresponds to a phase when records across the tropics show that the Intertropical Convergence Zone (ITCZ) was displaced to the south. High effective rainfall enhanced the growing of swamp taxa like Pandanus around the lake. Human-landscape interactions are evident only starting from the 20th century (from ca. 1950 to present) with Weinmannia rising probably due to the logging of emergent Agathis trees and/or landslides caused by the construction of the road which today passes near the lake. In general, palynological diversity values indicate that within-landscape diversity (Whittaker's gamma diversity) decreased when fires increased. Palynological rate of change and compositional turnover indicate that the vegetation communities were more resilient to fire disturbance during period of high rainfall. A different trend is apparent starting from the second half of the 20th century, suggesting a change in the dynamical response of the vegetation communities to forest fires, possibly as a consequence of increasing human activities around the lake. The emergent tree Agathis, while being more responsive to long-term rainfall variability in the past, did not reestablish itself following the years of intensive selective logging in the second part of the last century. These findings improve our knowledge of the long-term ecology of Central Sulawesi, one of the world's hotspots of biodiversity. (C) 2014 Elsevier B.V. All rights reserved.