Assessing Annual Forest Ecological Change in Western Canada Using Temporal Mixture Analysis of Regional Scale AVHRR Imagery Over a 14 Year Period Joseph M. Piwowar Waterloo Laboratory for Earth Observations University of Waterloo Waterloo, Ontario, Canada Derek R. Peddle Diedre P. Davidson Department of Geography University of Lethbridge Lethbridge, Alberta, Canada

Objectives n to model the inter-annual characteristics of peak vegetation vigour in Central North America n to assess the effectiveness of Temporal Mixture Analysis for identifying and modelling the temporal characteristics of vegetation vigour

Rationale n the North American boreal forest is an important component in the global carbon cycle – it has been the focus of recent international attention through large global climate change research projects such as the Boreal Ecosystem Atmosphere Study (BOREAS) in western Canada. n the spatial extent of this vast tract of land coupled with an emphasis on assessing regional scale environmental change has created a distinct need for comprehensive temporal analyses of ecological change over various time periods using regional scale remote sensing imagery

Data n 14 years ( ) of NOAA AVHRR imagery from the NOAA/NASA Pathfinder program were used n coincident subscenes of the central Canadian boreal forest region acquired in late July, near the peak of the growing season, were extracted from each of the 14 years n NDVI images were used as estimates of plant vigour – different cover types have been shown to have characteristic profiles corresponding with their phenology on NDVI sequences

Temporal Mixture Analysis (TMA) n based on Spectral Mixture Analysis (SMA) – a procedure that attempts to extract the fractional radiance components from the pixels in an image endmembers are defined which characterize the most extreme, or "pure" spectra present in the data endmembers are then entered into a mixing model which defines the spectral mixing of the scene components from which a set of fraction images is produced n in this study, the SMA technique is extended to analyze temporal spectra of NDVI values of the boreal forest

Endmember Definition n spectral endmembers are defined as spectrally "pure" features (e.g. vegetation, soil, etc.). n pure spectral endmembers are usually defined under idealized in situ or laboratory conditions where reflectance spectra are acquired using a portable spectroradiometer focussed only on a single surface (e.g. a single leaf from a maple tree). n when in situ measurements are not possible, spectral endmembers can also be derived from "pure" features in the imagery

Purification Formula n since idealized in situ measurements are impractical in TMA, we used image-based endmembers for our analyses n sampling spectrally pure pixels cannot always be guaranteed when using image-based endmembers n we have devised a purification formula to "purify" a sample of image-based spectra:

Procedure Êstratify the study region into its 2 principal cover types: prairie grassland and boreal forest Ësample temporal spectra from locations within these two land covers and purify them for the creation of the first 2 endmembers Ìdefine additional temporal endmembers at locations of high residuals, using the resulting Error Image as a guide Ícontinue this process until there is appropriate endmember coverage of the study region

Results a 3 endmember model was defined: n Prairie n Boreal 1 n Boreal 2

Fraction Prairie Endmember

n characterized by consistently low to moderate NDVI values n the temporal signature shows higher NDVI values between and , and a lower regime during the mid- to late-1980s; coincident with increased drought conditions n regions with the highest endmember fractions are in good spatial agreement with the known extents of the grassland prairies of Central North America

Boreal 1 Endmember Fraction

n elevated NDVI values suggest that this is an area of consistently high plant vigour n lower NDVI values are at the beginning and end of the temporal record, and higher readings occur during the middle years n spatially, highest fractions from this endmember follow a line extending southeast from the northern Alberta border to Lake Winnipegosis n Environment Canada identifies this region as the Boreal Plains Ecozone, characterized by mixed deciduous- coniferous forests

Boreal 2 Endmember Fraction

n moderately high NDVI values indicate good plant vigour n this region is defined as the Boreal Shield Ecozone; it is over 80% forested with dense coniferous stands n highest fractions are in the northern sections of the study area n a temporal anomaly is evident in 1983 where some NDVI values dip below +0.1 –there was a particularly strong El Niño event in 1983 which brought hot, dry weather over this region which may have resulted in a dramatic decline in plant vigour

Example Interpretation n The NDVI values for a Boreal Shield location can be separated into a temporal mixture of 76% Boreal 2 and 15% Boreal 1, with 14% of the temporal signal being consistent with the Prairie temporal. This suggests that much of the biomass at this location can be expected to remain moderate from year to year (at least during the last weeks of July), except under more severe drought conditions. There are portions of this location whose annual NDVI values more closely resemble those of prairie grasslands, while other fractions appear as denser forest stands similar to those represented by the first boreal endmember. Thus, based on the temporal characteristics of its annual July vegetation index we can deduce that the forest canopy is more variable here. Some of this variability may also be attributed to variations in timing of the peak of a particular growing season, which may be affected by, for example, a late winter thaw or different precipitation and temperature regimes in spring and early summer as a result, the peak of the growing season will not always occur at the same time, and this "peak" therefore may not necessarily be captured consistently in the timing of these images we analysed. This is a key point in the seasonal temporal interpretation that may explain some of the seasonal variability noted

Conclusions n an annual sequence of NDVI images of Central North America can be temporally unmixed into three characteristic temporal patterns: one representing the prairie grasslands, one from the boreal plains ecozone, and a third from the boreal shield ecozone n TMA is an effective technique for identifying and modelling the various temporal characteristics of spatial features. TMA encourages both temporal and spatial interpretations through the use of graphical and image displays, hence it is also a useful tool for exploratory image analysis