Montesano Paul M, Sun Guoqing, Dubayah Ralph O, Ranson K Jon
Science Systems and Applications, Inc., Lanham, MD, 20706, USA.
Biospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
Biogeosciences. 2016 Jul;13(13):3847-3861. doi: 10.5194/bg-13-3847-2016. Epub 2016 Jul 6.
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.
在泰加林-苔原交错带（TTE），与地点相关的森林结构特征会影响在广阔的环极范围内发生的细微且异质的结构变化。这些变化可能与交错带形态有关，交错带形态由TTE森林斑块内森林结构的水平和垂直模式（如树木覆盖、密度和高度）描述，受当地地点条件驱动，并与交错带动态相关。细微、多变且广泛的植被变化这一独特情况使得有必要应用包括高分辨率（<5米）星载图像（HRSI）在内的星载数据，在大尺度上研究TTE形态并预测动态。本研究分析了TTE斑块尺度的森林结构，以提供一种研究交错带形态垂直和水平组成部分的方法。我们通过以下方式展示了星载数据整合森林高度和密度以评估环极生物群落中森林斑块尺度TTE形态的潜力：（1）使用多分辨率星载数据集绘制西伯利亚北部沿TTE研究地点的森林斑块，（2）在主要为扩散型TTE形态的不同地点，研究该数据集对森林斑块高度的不确定性。结果表明，利用HRSI有机会改进斑块尺度星载森林高度估计，即TTE形态的垂直组成部分。基于预测区间的相对最大高度不确定性分布集中在约40%，这限制了利用高度来辨别森林斑块差异。我们根据一般交错带形态的概念模型讨论了这种不确定性，并强调星载高度估计的不确定性如何导致识别TTE形态时的不确定性。关注降低森林斑块高度估计的不确定性可能会改善TTE形态的描绘，这可能有助于解释TTE中森林对气候变化的可变响应以及TTE部分区域对森林结构变化的脆弱性。
