Green roofs have the potential to retain stormwater on the roof surface and lower the thermal loading on buildings. Because of this, the greatest environmental benefits from green roofs might be achieved in subtropical climates characterized by high temperatures and intense rain events. There is, however, little research to support this. In a replicated study in Texas, we compared the performance of six different extensive green roof designs vegetated with native species, to non-reflective (black) roofs, and reflective (white) roofs. Preliminary hydrologic and thermal profile data indicated not only differences between green and non-vegetated roofs, but also among green roof designs. Maximum green roof temperatures were cooler than conventional roofs by 38°C at the roof membrane and 18°C inside air temperature, with little variation among green roofs. Maximum run-off retention was 88% and 44% for medium and large rain events but some green roof types showed very limited retention characteristics. These data demonstrate indicate that: 1. Green roofs can greatly affect the roof temperature profile-cooling surface layers and internal space on warm days. 2. Green roofs can retain significant amounts of rainfall, this is dependent on the size of the rain event and design and can fail if not designed correctly. We suggest that as green roofs vary so much in their design and performance, they must be designed according to specific goals rather than relying on assumed intrinsic attributes.
Conservation of North American grasslands is hampered by the impact of invasive herbaceous species. Selective control of these plants, although desirable, is complicated by the shared physiology and phenology of the invader and the native components of the invaded plant community. Fortunately, there is evidence that some management practices, such as prescribed fire, herbicide, and mowing, can cause differential responses in native and invasive grassland species. However, timing of treatment is critical, and fire has been shown to increase rates of invasion when implemented during the dormant season. Bothriochloa ischaemum, an introduced C4 Eurasian grass is an increasing problem in grasslands, particularly in southern and central regions of North America. To date, there has been little success in effective selective control. Two invaded grassland sites representative of Blackland Prairie and Edwards Plateau ecoregions were subjected to two growing-season prescribed fire treatments, single and double herbicide applications, and single and double mowing treatments. Mowing had no effect on either B. ischaemum or other dominant species at either site one-year posttreatment. However, growing-season fire and herbicide were both effective at reducing the abundance of B. ischaemum, with other codominant species responding either negatively to herbicide or neutrally or positively to fire. The vulnerability of B. ischaemum to growing-season fire may be associated with the ecology of its native range. The negative growth response to growingseason fire, combined with its lower implementation costs, indicates that this method warrants further investigation as a selective management tool for other problematic species in invaded grasslands.
Revegetation is an essential component of roadside and building site construction and improvement. In the southern United States non-native grass species are frequently included in revegetation seed mixes used by highway authorities. Non-native species are frequently selected for aggressive growth characteristics, however these same traits also render them potentially invasive, and subsequently hazardous to, adjacent plant communities. Although the use of pure native seed mixes have been rejected in the past due to perceived inferior establishment characteristics, there have been few comparative quantitative field studies that justify this belief. The establishment characteristics of three seed mixes: one containing non-native species and two with native grass and forb species only, were compared in a randomized-block design along a Texas roadside following spring and summer sowing. After 60 days following the spring sowing, the two native-only seed mixes demonstrated 180% and 560% (F=10.18; P<0.0001) higher seed densities than the recommended native/non-native mix. The summer sowing results were similar with seedling densities 180% and 330% (F=9.20; P<0.01) greater than the standard non-native seeding. Although an aggressive colonizer from vegetative tissue such as stolons and rhizomes, the non-native Bermudagrass (Cynodon dactylon) has a lower than expected establishment rate thought to be due to high water demand during the first weeks following sowing. Given the invasive characteristics of this common component of many recommended revegetation seed mixes, these results call into question the widespread recommended use of Bermudagrass for such projects. These data indicate that examination of suites of early-and late-succesional native species can provide a highly effective mix for revegetation projects. Furthermore, this reduces the potential for negative ecological consequences and provides added benefits associated with wholly native plant communities.
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