Woody vegetation encroachment has been assumed to occur uniformly over the past century across the southern Great Plains of the United States. To assess this assumption, we evaluated changes in pixels classified as woody vegetation from aerial photographs from 1937 to 2004 that were acquired approximately every 13 yr for a 7 109-ha wildlife refuge located in the eastern Edwards Plateau region of central Texas. We found that the percentage of total area classified as woody vegetation changed minimally (62.0–64.2%) from 1937 to 2004. However, on average, 32% of the study area changed classification between woody vegetation and nonwoody vegetation pixels between each photograph date. To assess potential causes of woody vegetation change, we analyzed contiguous pixels with area ≤ 1 ha and area > 1 ha representing small- and large-scale disturbances, respectively. Small-scale woody vegetation pixel loss was found to be moderately low, ranging between 11.1% and 12.6% of the study area for the period of analysis, indicating relatively constant levels of canopy-level disturbance. Large-scale woody vegetation pixel loss peaked in the 1951–1964 time interval, where we identified 98 patches averaging 8.1 ha and covering 7.6% of the study area. The timing and area of these potential disturbances were correlated with drought and increased fire frequency within our study area. This methodology, which includes careful georectification and radiometric standardization of the historical photographs, can be used to detect interdecadal variability related to changes in types of disturbance over longer periods of time. This study also shows that repeated observations, such as those from aerial photographs, may be required to adequately characterize woody plant encroachment, particularly in subhumid grasslands, where disturbance and regrowth of woody plants may occur at decade time scales.
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Vol. 66 • No. 3