The Holocene treeline in the northern Andes (Ecuador): First evidence from soil charcoal

摘要

Indications for the speed and timing of past altitudinal treeline shifts are often contradictory. Partly, this may be due to interpretation difficulties of pollen records, which are generally regional rather than local proxies. We used pedoanthracology, the identification and dating of macroscopic soil charcoal, to study vegetation history around the treeline in the northern Ecuadorian Andes. Pecloanthracology offers a complementary method to pollen-based vegetation reconstructions by providing records with high spatial detail on a local scale. The modem vegetation is tussock grass paramo (tropical alpine vegetation) and upper montane cloud forest, and the treeline is located at ca. 3600 m. Charcoal was collected from soils in the paramo (at 3890 and 3810 m) and in the forest (at 3540 m), and represents a sequence for the entire Holocene. The presence of paramo taxa throughout all three soil profiles, especially in combination with the absence of forest taxa, shows that the treeline in the study area has moved up to its present position only late in the Holocene (after ca. 5850 ca] years BP). The treeline may have been situated between 3600 in and 3800 m at some time after ca. 4900 cal years BP, or it may never have been higher than it is today. The presence of charcoal throughout the profiles also shows that fires have occurred in this area at least since the beginning of the Holocene. These results contradict interpretations of palacological data from Colombia, which suggest a rapid treeline rise at the Pleistocene-Holocene transition. They also contradict the hypothesis that man-made fires have destroyed large extents of forest above the modem treeline. Instead, paramo fires have probably contributed to the slowness of treeline rise during the Holocene. (c) 2007 Elsevier B.V. All rights reserved.