Vegetation Composition & Structure
Resource Brief - March 2009
Importance/Issues
Vegetation is integral to ecosystem function, and both influences and responds to environmental drivers. See the protocol description.
Remotely-sensed data can effectively address many monitoring needs at the landscape scale, providing wall-to-wall coverage at a relatively low cost. Ground-based monitoring is desirable for validation of remotely-sensed imagery, and for addressing questions of structure and composition at a finer resolution (e.g., community-level) in sensitive plant communities. Both remote sensing and ground-based monitoring approaches will be used to track Vegetation Composition & Structure and Sensitive Plant Communities. Land Cover/Land Use will be monitored by remote sensing only.Monitoring Objectives
- Map and quantify long-term, landscape-level changes in the distribution and extent of major land cover classes using satellite imagery and/or aerial photographs.
- Document changes in land use patterns in and adjacent to SWAN parks.
- Estimate long-term changes in species richness, cover and diversity in focal ecosystems.
- Estimate long-term changes in vegetation structure (physiognomy), including changes in the demography and density of woody species.
Sampling Design and Objectives
In 2006, the SWAN established an Interagency Agreement with the USFS-Pacific Northwest Research Station to adapt an existing protocol developed for the North Coast and Cascades Network (NCCN) for use in the landscapes of the SWAN. SWAN monitoring objectives include detection of changes in shrub cover (e.g., shrub expansion and infilling); changes in coniferous and lichen cover; establishment of vegetation following glacial retreat and/or channel abandonment; and changes in surface hydrology, including infilling of ponds by vegetation and creation of new water bodies. The protocol uses paired Landsat TM/ETM+ images to identify changes through time, using a tasseled-cap transformation to partition out major physiognomic types.
Current and Future Work Efforts
Remote Sensing
Landsat TM/ETM+ data are being used to monitor landscape-scale changes, including vegetation establishment and loss, changes in surface hydrology, and glacier extent (Fig. 1). A draft protocol and SOPs for acquisition, processing, and interpretation of Landsat TM/ETM+ images has been developed following methods established by the North Coast and Cascades Network (NCCN). Continuing work will focus on protocol testing and revision, including methods to increase the certainty of the change models. Mechanisms of change are identified by comparing change vectors relative to reference data, or by using one of several other validation techniques. Time series of aerial photography are also being used to look at finer-scale changes, such as incursion of woody species or loss of small ponds (Fig. 2).
Figure 1. Difference images are derived from two Landsat TM/ETM+ images taken from different years (left, center).
Changes on the landscape, such as these fire scars of different ages, appear in the difference image (right).
Stony River drainage (1991-2002), Lake Clark NPP.
Figure 2. A time series of aerial photographs
(1955-2005) shows spruce expansion onto
beach terraces in Lake Clark NPP.Larger image.
Ground-based Monitoring
Sites for ground-based monitoring will be selected using a spatially balanced probabilistic sampling framework (GRTS design), weighted by accessibility and landscape attributes (e.g., elevation, aspect classes). The sampling design is a modification of methods developed by the NCCN, with a revisit interval of 10 years. Plot attributes are comparable to those used by the Central Alaska Network (CAKN) . Field testing (Fig. 3) will continue in 2008.
Figure 3. Field testing of ground-based
monitoring protocol, Lake Clark NPP, 2007.
Management Applications
Landscape-level monitoring through remote sensing provides the context for understanding plot-level monitoring results, enabling inference to park and regional scales. Landscape-level changes can be considered both system drivers (e.g., changes in surface hydrology, glacier extent, or adjacent land use/management) and response variables (e.g., changes in forest structure due to insect outbreaks). Ground-based monitoring provides information about species composition (e.g., presence and abundance of rare or exotic species) and structure (e.g., changes in vegetation height and density) that cannot be detected through imagery alone.
