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ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

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68 INTRODUCTION/CLIMATE <strong>OF</strong> <strong>EAST</strong> <strong>TEXAS</strong><br />

late winter warmth followed by the movement of polar or arctic air into southern regions. A<br />

good example was the unseasonably late freeze on the night of 11–12 April 1997. Following<br />

a period of relatively warm weather, temperatures dropped to substantially below freezing<br />

over a large part of the northwestern part of East Texas and adjacent North Central Texas. For<br />

example, a low of 22° F (-55° C) was recorded for a native habitat (Garnett Preserve) in<br />

Montague County (H. Garnett, pers. comm.). The result was substantial damage to the young<br />

foliage of many native species and in some cases nearly complete defoliation. Some of the<br />

natives significantly damaged in Grayson County include Berchemia scandens (supple-jack),<br />

Cercis canadensis (redbud), Diospyros virginiana (common persimmon), Fraxinus americana<br />

(white ash), Gleditsia triacanthos (common honey-locust), Morus rubra (red mulberry),<br />

Platanus occidentalis (sycamore), Quercus macrocarpa (bur oak), Quercus marilandica (blackjack<br />

oak), Quercus muehlenbergii (chestnut oak), Quercus shumardii (Shumard’s red oak),<br />

Quercus stellata (post oak), and Rhus glabra (smooth sumac). Effects on native oaks at<br />

Hagerman National Wildlife Area (Grayson County) were serious enough that leaf damage<br />

was still obvious at a glance in late May (G. Diggs, pers. obs.). The intense subfreezing<br />

temperatures cause widespread damage to cultivated crops as well as native plant species<br />

which have begun spring growth prematurely triggered by the unusually mild winter<br />

temperatures. Forty-four major false spring episodes have been documented in Texas<br />

between 1650 and 1980 (Stahle 1990; Stahle & Cleaveland 1995). Such historical events can<br />

be studied because frost-damaged cambial tissues leave a permanent record in the annual<br />

growth rings of trees, and these can be dated dendrochronologically (i.e., via tree-ring dating)<br />

to the exact year of their formation. These distinctively damaged annual rings, known as<br />

“frost rings” have been studied in post oak and white oak trees and “can be microscopically<br />

identified by specific anatomical features only associated with freeze damage” (Stahle &<br />

FIG.37/MEAN ANNUAL TOTAL PRECIPITATION (IN INCHES) FOR <strong>TEXAS</strong> (ADAPTED FROM GRIFFITHS & ORTON [1968] BY HATCH ET AL.[1990]).

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