Latitudinal forest advance in northernmost Norway since the early 20th century

Abstract

Aim The arctic forest–tundra boundary is expected to advance rapidly northwards in response to climate warming. The purpose of this study was: (1) to analyse region‐wide changes in the positions of the latitudinal forest line and tree line since the early 20th century; (2) to calculate rates of northward movement and to estimate uncertainties from different sources of data; (3) to analyse how rates of advance differ between two common tree species, Betula pubescens (downy birch) and Pinus sylvestris (Scots pine); and (4) to place the results in the context of dynamic global vegetation models. Location The study covers Finnmark county in northern Norway, which has warmed by 1–2 °C since the early 20th century. Methods Historical forest maps, topographic maps, aerial photographs and satellite imagery were used to delineate region‐wide, species‐specific and time‐specific subarctic forest lines and tree lines. Rates of advance were calculated using two geometrical methods, placing different emphases on the shape and mean position of the lines, but yielding similar results. Results The analyses revealed an average northward advance of the birch and pine forest lines of 156 and 71 m year−1, respectively. Both showed considerable spatial and temporal variation. The birch tree line showed the most pronounced advance (340 m year−1), whereas the pine tree line showed very limited advance (10 m year−1). Data sources contributed an uncertainty of around 10 m year−1 to these rates. Main conclusions The analysis of historical and recent forest delineation data showed a very restricted advance rate compared to the predictions of dynamic global vegetation models. If these results are representative of the behaviour of the entire circumarctic forest–tundra zone over the present century, they suggest that the modelled prediction of the loss of 40% of the current tundra is a serious overestimate. A stronger focus on factors limiting the response of the forest–tundra ecotone to climate change is needed to refine the output from dynamic global vegetation models.

Multisensor NDVI-Based Monitoring of the Tundra-Taiga Interface (Mealy Mountains, Labrador, Canada)

“…This is consistent with the advances of 3 to 11 m per year for the period of 1980-2000 in northern Russia [32], but it is much lower than the mean advance of 378 m per year (2.5 km from 2000 to 2009) reported for the pine forest line in northern Norway [77]. The variations observed for the Mealy Mountains study are attributed to site conditions, as seedling in the TTI in Labrador can be affected by the soil wetness, wind exposure, temperature [78], seedbed composition [79] and the topography [80].…”

Infrastructure Development Accelerates Range Expansion of Trembling Aspen (<i>Populus tremuloides</i>, Salicaceae) into the Arctic

“…This warming coincides with increased woody shrub cover (Tape et al, 2006;Myers-Smith et al, 2015). The latitudinal treeline is also predicted to advance northward (Everett and Fitzharris, 1998;Pearson et al, 2013); however, observed rates of treeline advance have been slower than those predicted (Skre et al, 2002;Harsch et al, 2009;Hofgaard et al, 2013). In addition to warming, disturbance may be necessary to facilitate treeline advance by exposing new seedbeds, degrading shallow permafrost, and improving drainage, thereby creating opportunities for establishment in an ecosystem where recruitment is otherwise rare (Landhäusser and Wein, 1993;Lloyd et al, 2002;Chapin et al, 2014).…”

Treelines—Approaches at Different Scales

“…According to a model presented by Paulsen and Körner [82], for example, treeline would be located at an elevation where the minimum length of the growing season is 94 days, with a daily minimum temperature just above zero (0.9 • C) and a mean of 6.4 • C during all these days. The extent of treeline advancement to greater elevations and more northerly latitudes, however, varies regionally, locally, and temporally as well as by tree species (e.g., [8,29,43,63,[83][84][85][86][87][88][89]). The direct influence of heat deficiency on tree establishment is often overruled by other abiotic and biotic factors (e.g., permanent strong winds and recurrent periods of drought).…”

“…are of primary importance. Thus, the distance from the seed source, the way of seed dispersal (anemochoric, zoochoric), the overlying of seeds, the availability of viable seeds, seed predation, and the availability of favorable seed beds (e.g., [5,85,87,88,[90][91][92][93]) are relevant factors that need further examination. However, discussion of whether reduced availability of viable seeds are playing a major role or not is controversial (e.g., [3,5,17,19,45,87,90,[93][94][95][96][97]).…”