Abstract
In the taiga-tundra ecotone (TTE), site-dependent forest structure characteristics can influence the subtle and heterogeneous structural changes that occur across the broad circumpolar extent. Such changes may be related to ecotone form, described by the horizontal and vertical patterns of forest structure (e.g., tree cover, density and height) within TTE forest patches, driven by local site conditions, and linked to ecotone dynamics. The unique circumstance of subtle, variable and widespread vegetation change warrants the application of spaceborne data including high-resolution (< 5m) spaceborne imagery (HRSI) across broad scales for examining TTE form and predicting dynamics. This study analyzes forest structure at the patch-scale in the TTE to provide a means to examine both vertical and horizontal components of ecotone form. We demonstrate the potential of spaceborne data for integrating forest height and density to assess TTE form at the scale of forest patches across the circumpolar biome by (1) mapping forest patches in study sites along the TTE in northern Siberia with a multi-resolution suite of spaceborne data, and (2) examining the uncertainty of forest patch height from this suite of data across sites of primarily diffuse TTE forms. Results demonstrate the opportunities for improving patch-scale spaceborne estimates of forest height, the vertical component of TTE form, with HRSI. The distribution of relative maximum height uncertainty based on prediction intervals is centered at ~40%, constraining the use of height for discerning differences in forest patches. We discuss this uncertainty in light of a conceptual model of general ecotone forms, and highlight how the uncertainty of spaceborne estimates of height can contribute to the uncertainty in identifying TTE forms. A focus on reducing the uncertainty of height estimates in forest patches may improve depiction of TTE form, which may help explain variable forest responses in the TTE to climate change and the vulnerability of portions of the TTE to forest structure change.
Highlights
1.1 TTE vegetation structure and processesThe circumpolar biome boundary between the boreal forest and Arctic tundra – known as the tree line, the forest– tundra ecotone, or the taiga–tundra ecotone (TTE) – is an ecological transition zone covering > 1.9 million km2 across North America and Eurasia (Payette et al, 2001; Ranson et al, 2011)
Our work demonstrates the potential from spaceborne remote sensing for depicting a key structural characteristic of TTE form and suggests where improvements are needed in order to identify portions of the TTE vulnerable to warming-induced structural changes
Height is a key attribute of the diffuse TTE forms, which generally feature a gradual decrease of height and tree density across the ecotone and are the most likely form to demonstrate tree line advance
Summary
1.1 TTE vegetation structure and processesThe circumpolar biome boundary between the boreal forest and Arctic tundra – known as the tree line, the forest– tundra ecotone, or the taiga–tundra ecotone (TTE) – is an ecological transition zone covering > 1.9 million km across North America and Eurasia (Payette et al, 2001; Ranson et al, 2011). The location, extent, structure, and pattern of vegetation in the TTE influences interactions between the biosphere and the atmosphere through changes to the surface energy balance and distribution of carbon (Bonan, 2008; Callaghan et al, 2002a). These TTE vegetation characteristics affect local and regional Arctic and subarctic biodiversity (Hofgaard et al, 2012) and are controlled by a variety of factors that are scale-dependent (Holtmeier and Broll, 2005). At local scales the spatial configuration of trees is determined largely by site-level heterogeneity in hydrology, permafrost, disturbance, topography (aspect, slope, elevation), land use, and the geomorphologic conditions associated with each (Dalen and Hofgaard, 2005; Danby and Hik, 2007; Frost et al, 2014; Haugo et al, 2011; Holtmeier and Broll, 2010; Lloyd et al, 2003).
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