ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
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ORIGIN AND DIVERSITY <strong>OF</strong> <strong>EAST</strong> <strong>TEXAS</strong> <strong>FLORA</strong>/INTRODUCTION 223<br />
the spread in Texas of a number of introduced species can be obtained by comparing their<br />
distributions as recorded by early maps, early published reports (e.g., Cory 1940, 1950b;<br />
Shinners 1948), or herbarium records, etc., with their current day distributions (e.g., Turner<br />
et al. 2003).<br />
EXOTICS IN <strong>EAST</strong> <strong>TEXAS</strong><br />
Some exotic species are at the present time rapidly spreading in East Texas or have the<br />
potential to do so. For example, the offensive Carduus nutans subsp. macrocephalus, muskthistle<br />
or nodding-thistle, is each year becoming more abundant in the northwestern part of<br />
East Texas (e.g., Collin and Grayson counties). A possibly even more serious threat, Scabiosa<br />
atropurpurea, pincushions or sweet scabious, is currently taking over roadsides and adjacent<br />
areas in the northern part of East Texas (e.g., Collin and Dallas counties) and has the<br />
potential of becoming one of the most destructive invasive exotics in grassland habitats. A<br />
potentially economically devastating weed, Orobanche ramosa, branched broom-rape, is now<br />
spreading in the west central part of East Texas and is known from at least 22 counties<br />
(Texas Cooperative Extension 2003). Recently (2004), it was discovered as far north as Dallas<br />
County (J. Quayle, pers. comm.). It is apparently being spread widely by highway mowing<br />
equipment. This chlorophyll-less plant is a well known root parasite of agricultural crops and<br />
has the potential to have a significant economic impact in Texas. It is classified a federal noxious<br />
weed (USDA Natural Resources Conservation Service 2002). Texas cities, too, are under<br />
attack. Cuscuta japonica, Japanese dodder, is a federal noxious weed currently known only<br />
from Houston (Harris County), but there is concern about its possible spread (Huber 2002).<br />
It is an aggressive parasitic vine which attacks a variety of woody plants and has the potential<br />
to have serious ecological and economic consequences if not eradicated. Troubling also<br />
are observations that a number of previously known introductions such as Rapistrum rugosum,<br />
annual bastard-cabbage, and Silene gallica, windmill-pink, are at present becoming noticeably<br />
more common, and other species once found primarily in southeast Texas have now<br />
spread further north (e.g., Trifolium resupinatum, reversed clover, and Youngia japonica,<br />
Japanese hawkweed).<br />
Nor are aquatic habitats exempt from the threat of exotic species. Hydrilla verticillata,<br />
hydrilla, is a serious pest which can completely dominate aquatic habitats, eliminating native<br />
species, clogging waterways, and severely curtailing recreational use (Steward et al. 1984;<br />
Flack & Furlow 1996). This federal noxious weed is rapidly spreading at present in East<br />
Texas (M. Smart, pers. comm.), probably from lake to lake via boats or boat trailers. It may<br />
also be intentionally spread by fishermen (L. Hartman, pers. comm.) to “improve” the habitat,<br />
which is both illegal and ill-advised since it ultimately degrades the fishery. Likewise,<br />
Salvinia molesta, giant salvinia, considered by some to be “one of the world’s worst weeds”<br />
(Jacono 1999c), is at present spreading in the eastern part of the state and “all reservoirs in<br />
East Texas are imminently threatened” (R. Helton, pers. comm.). Plants used in aquaria or<br />
water gardens are among the likely sources for the escaped populations. This species, which<br />
has a very rapid growth rate and the ability to form thick mats on the water surface, can cover<br />
lakes and streams, crowd out native plants, and cause physical problems by impeding boats<br />
and clogging water intakes (Jacono 1999a, 1999b, 1999c; Wood et al. 2001). It is listed as a<br />
federal noxious weed (USDA Natural Resources Conservation Service 2002), and as such is<br />
prohibited in the U.S. by federal law. In fact, because of their potential as problematic invaders,<br />
seven aquatic species that occur in East Texas, Alternanthera philoxeroides, alligator-weed<br />
(Amaranthaceae); Eichhornia crassipes, common water-hyacinth (Pontederiaceae); Hydrilla<br />
verticillata (Hydrocharitaceae); Myriophyllum spicatum, Eurasian water-milfoil (Haloragaceae);<br />
Pistia stratiotes, water-lettuce (Araceae); Salvinia minima, common salvinia (Salviniaceae); and<br />
Salvinia molesta, giant salvinia, are considered “harmful or potentially harmful exotic plants,”<br />
and it is illegal to release, import, sell, purchase, propagate, or possess them in the state<br />
(Harvey 1998).