Functional Phenology of a Texas Post Oak Savanna from a CHRIS PROBA Time Series

Michael J Hill,Rebecca Lemons,Andrew Millington,Cherie New

doi:10.3390/rs11202388

Abstract

Remnant midwestern oak savannas in the USA have been altered by fire suppression and the encroachment of woody evergreen trees and shrubs. The Gus Engeling Wildlife Management Area (GEWMA) near Palestine, Texas represents a relatively intact southern example of thickening and evergreen encroachment in oak savannas. In this study, 18 images from the CHRIS/PROBA (Compact High-Resolution Imaging Spectrometer/Project for On-Board Autonomy) sensor were acquired between June 2009 and October 2010 and used to explore variation in canopy dynamics among deciduous and evergreen trees and shrubs, and savanna grassland in seasonal leaf-on and leaf-off conditions. Nadir CHRIS images from the 11 useable dates were processed to surface reflectance and a selection of vegetation indices (VIs) sensitive to pigments, photosynthetic efficiency, and canopy water content were calculated. An analysis of temporal VI phenology was undertaken using a fishnet polygon at 90 m resolution incorporating tree densities from a classified aerial photo and soil type polygons. The results showed that the major differences in spectral phenology were associated with deciduous tree density, the density of evergreen trees and shrubs—especially during deciduous leaf-off periods—broad vegetation types, and soil type interactions with elevation. The VIs were sensitive to high densities of evergreens during the leaf-off period and indicative of a photosynthetic advantage over deciduous trees. The largest differences in VI profiles were associated with high and low tree density, and soil types with the lowest and highest available soil water. The study showed how time series of hyperspectral data could be used to monitor the relative abundance and vigor of desirable and less desirable species in conservation lands.

Highlights

Midwestern oak savannas originally occupied the transition zone between the true prairie grasslands and the eastern deciduous forests in North America
This study on a remnant post oak woodland area has shown that the major differences in spectral phenology as defined by vegetation indices (VIs) are associated with deciduous tree density, the density of evergreen trees and shrubs—especially during deciduous leaf-off periods—broad vegetation types, and soil types
Since deciduous species are highly intermingled in both post oak savanna and bottomland hardwood forest

Summary

Introduction

Midwestern oak savannas originally occupied the transition zone between the true prairie grasslands and the eastern deciduous forests in North America. They stretched from eastern Texas in the south to northern Minnesota and occupied as much as 20 M ha in a continuous band [1]. In eastern Texas, Post Oak Motte and Woodland occupies about 38% and Savanna Grassland occupies about 58% of this vegetation type, with other minor types making up the remainder [3]

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