Radiocarbon-dated peat and wood remains from the Finnish Subarctic: evidence of treeline and landscape history

摘要

Seven peat and 40 dead wood remains (mountain birch) were sampled within and above the present treeline ecotone on two mountains (Rodjanoaivi, Koahppeloaivi/Staloskaidi) along the Tenojoki in northernmost Finnish Lapland. The oldest peat samples ('summit peats') date back to about 2000 yr BP. They accumulated during cool and increasingly humid climatic conditions. The other peat samples taken from wind-eroded peat remains on convex topography 60-100 m above the present tree limit are about 700-1300 years old. These peat layers developed during a phase of increasing Sphagnum peat formation. An eroded peat-covered Podzol that had developed under former tree stands with dwarfshrub vegetation in the understorey also reflects this climatic change. The oldest birch wood samples date from the same period. These and the many younger samples are evidence for a general treeline decline since the Holocene climatic optimum until present. As a result of the decline of the upper forest stands, the alpine zone has extended downslope and wind erosion has increased considerably in the former and present treeline ecotone, removing most of the peaty layers and top soils from wind-exposed topography. Whilst the cooling climate was the main factor triggering treeline retreat, episodic mass-outbreaks of the autumnal moth (Epirrila autumnata), particularly when coincident with cold summers, must also be taken into consideration as a factor that enhanced or accelerated treeline decline during this period. As is evidenced by the wood samples, forest decline continued into modern times when overgrazing by reindeer (Rangifer tarandus) became an additional factor enhancing wind erosion and affecting the regeneration of birch in the present treeline ecotone.