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The Tundra-taiga interface

The Tundra-taiga interface

The interface between the boreal forest and the arctic tundra is the Earth's greatest vegetation transition. It is over 13,000 km long, occupies around 5% of the vegetated surface of the Northern Hemisphere and represents major gradients in key climatological parameters such as carbon flux, water flux and albedo. The position of this interface region, and the species composition of the northern boreal forest, have undergone major shifts since the last glacial maximum. Modelling predicts northward shifts in boreal vegetation distributions in response to global warming, with roughly half to two thirds of the present tundra being displaced by forest by the end of the 21st century. Such changes would have major climatological implications through the probable increase in CO2 absorption and decrease in CH4 emission, decrease in regional albedo and alteration of the hydrological cycle. The processes that determine the northern limit of trees are, however, complex and not fully understood. Systematic monitoring data are scarce, and provide scant evidence for the northward shift predicted by models.

Circumarctic treelines
Circumarctic treelines as adopted by Hustich for three different tree species (red, green and blue lines), the Circumpolar Arctic Vegetation Map (white line), the WWF (black line), and the USGS/UNL/JRC global land cover database (boundary of green region).

Despite a century of research, we still lack consistent data on the location, nature and dynamics of the tundra-taiga interface (TTI) at all scales from global to landscape. The greatest uncertainties are in Siberia. The difficulties arise mainly from scarcity of data at the circumpolar scale, and in adapting traditional definitions of treeline, forest line etc, based on in situ measurements or high-resolution air photography, to encompass a wide range of spatial scales. Both of these difficulties can be addressed through application of remote sensing data and new quantitative methods of landscape analysis. It is obvious that only broad-swath satellite remote sensing has the capability of providing systematic and timely circumpolar-scale data. Historically, the principal source of such data has been the series of AVHRR (advanced very high resolution radiometer) instruments, operational since 1978 and offering spatial resolution of the order of 1 km and swath widths of around 2600 km. Major new opportunities are presented by the MODIS instrument (operational since 1998) which provides greater spectral and spatial resolution than AVHRR with similarly high temporal resolution. A number of high-level data products are now being generated from MODIS data with potential relevance to the discrimination of the TTI, but none of these is optimal for arctic/subarctic vegetation and none is yet capable of application to the task of treeline monitoring. It will therefore be necessary to develop new algorithms for the analysis of MODIS and similar data. The gap that has to be bridged is between techniques for describing landscape structure at the treeline from field-based and from higher-resolution satellite data and the use of these coarser-resolution datasets. It will be essential to take explicitly into account the spatial structure of the TTI itself, which has been characterised as a pattern of tundra islands within the forest, shading into a pattern of forest islands within the tundra. Three-dimensional structure of forest edge regions through analysis of airborne LiDAR data, recent research on forest edge structure and new spatiotemporal models, and application of fractal concepts all have significant potential to define new methods of upscaling data.

Forest (black) and non-forest (white) areas near Dividalen
Forest (black) and non-forest (white) areas near Dividalen, Norway, based on classification of airborne scanner imagery. The image covers an area 1.28 × 0.96 km at a resolution of 2 m. The linear non-forest feature towards the right-hand side of the image is a river.

An international, coordinated programme of research into the TTI has been developed under the auspices of the International Polar Year (IPY). This programme has the title Present-day processes, past changes and spatiotemporal variability of biotic, abiotic and socio-environmental conditions and resource components along and across the Arctic delimitation zone (PPS Arctic). PPS Arctic combines around 50 researchers from 13 countries, and will have 20 or more fieldwork sites across the circumarctic TTI region, chosen to provide adequate sampling of both latitudinal and altitudinal treelines, the oceanicity gradient, and the broad range of tree species composing the TTI. It is coordinated from Norway and the UK. Its work is organized into four modules:

  1. Global change effects on the arctic-boreal transition zone and modelling structural changes
  2. Past history and broad scale temporal variations of the transition zone
  3. Spatial variations in vegetation, land cover and land use, by remote sensing
  4. Land use and development of the tundra-taiga interface through the joint perspective of local traditional and scientific knowledge

We are currently developing projects within module 3 on forest structure, aimed at developing a characterisation of the structure of the treeline region that is sensitive to the scale of measurement. This work is being performed in collaboration with the Norwegian Institute for Nature Research (NINA). The study site for this work is located near Porsangmoen, in the county of Finnmark, Norway, centred around 69.9 ˚N, 25.1 ˚E. The area is forested at altitudes up to 200-400 m, with Scots Pine (Pinus silvestris) and Brown Birch (Betula pubescens).

Location of the Porsangmoen test site, Norway
Location of the Porsangmoen test site, Norway.

Airborne remote sensing data (scanner imagery and LiDAR) were collected in 2004 and 2005, with concurrent field data in 2004. In particular, the LiDAR data allows the 3-dimensional structure of the forest to be determined since the times of the first and last pulses are recorded

LiDAR interaction with Scots Pine

LiDAR interaction with Scots Pine. Background photo from Naturhistoriska Riksmuseet, Sweden

Three-dimensional forest structure from LiDAR data
Three-dimensional forest structure from LiDAR data

Results from the Porsangmoen test site are currently being used to assess the scale-dependence of forest edge parameters. Future work will extend this to other test sites in Scandinavia, and compare these very high-resolution datasets with kilometric-scale datasets such as from the MODIS satellite imager.

Publications

  • Callaghan, T.V., Crawford, R.M.M., Eronen, M., Hofgaard, A., Payette, S., Rees, W.G., Skre, O., Sveinbjrnsson, B., Vlassova, T.K. and Werkman, B.R., 2002. The dynamics of the tundra-taiga boundary: an overview and suggested coordinated and integrated approach to research. Ambio, Special Report 12: 3-5.
  • Crawford, R.M.M., Jeffree, C.E. and Rees, W.G., 2003. Paludification and forest retreat in Northern Oceanic environments. Annals of Botany, 91(213-226).
  • Rees, G., Brown, I., Mikkola, K., Virtanen, T. and Werkman, B., 2002. How can the dynamics of the tundra-taiga boundary be remotely monitored? Ambio, Special report 12: 56-62.