Thematic Mapper characterization of lodgepole pine seral stages in Yellowstone National Park, USA

Abstract

Landsat Thematic Mapper multispectral data were used to identify the spectral reflectance characteristics of Yellowstone lodgepole pine ( Pinus contorta var latifolia) successional stages, and to examine the relationships between spectral and biophysical factors. Ten spectrally defined forest cover types were created from unsupervised classification of the Landsat TM data, using a geographic information system to restrict data analysis to areas of similar slope, elevation, and surficial geology within the Central Plateau region of the park. Biotic data on forest overstory and understory conditions were collected from 69 sample sites within the 10 spectral cover classes. Field data were used to regroup the 69 sites into six biotically and spectrally distinct cover types, ranging from early postfire regeneration (LPO) to late-stage (LP3) subalpine fir succession. Increased absorption in the visible (TM 1, 2, and 3) and middle-infrared (TM 5 and 7) bands were related to the age and development of a stand. Changes in absorption were rapid during the initial stages of stand regeneration, but the rate of change slowed as stands progressed into later successional stages. Biotic factors relating to the physical structure of the forest canopy (height, basal area, biomass, and LAI) are correlated with the visible and middle-infrared bands of the Thematic Mapper. Understory factors were poorly correlated with spectral response, except soil and fireweed, which are dominant early in succession, but rapidly decrease in later stages. The spectral reflectance of a successional forest stand over time is a function of the combined effects of the overstory canopy, the amount of shadow within a canopy, and the condition of the forest understory. As a forest develops from a disturbance to old-growth, the spectral response of a stand progresses along a vector or vectors linking the three factors. Spectral response changes are nonlinear with respect to time, as large-magnitude changes are observed in the first 20–30 years following a disturbance, and the rate of change lessens as forests develop into old-growth.