Ischaemum

Abstract: Atmospheric CO2 (Ca) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4-year study, we examined leaf gas exchange and photosynthetic acclimation in C3 and C4 plants using unique chambers that maintained a continuous Ca gradient from 200 to 550 mmol mol in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing Ca from past to future Ca levels. Photosynthesis (A), stomatal conductance (gs), leaf water-use ef®ciency (A/gs) and leaf N content were measured in three common species: Bothriochloa ischaemum, a C4 perennial grass, Bromus japonicus, a C3 annual grass, and Solanum dimidiatum, a C3 perennial forb. Assimilation responses to internal CO2 concentrations (A/Ci curves) and photosynthetically active radiation (A/PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with Ca in all species (P < 0.05). S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient Ca (P < 0.05). To our knowledge, this is the ®rst published evidence of A up-regulation at subambient Ca in the ®eld. No species showed downregulation at superambient Ca. Stomatal conductance generally showed curvilinear decreases with Ca in the perennial species (P < 0.05), with steeper declines over subambient Ca than superambient, suggesting that plant water relations have already changed signi®cantly with past Ca increases. Resource-use ef®ciency (A/gs and A/leaf N) in all species increased linearly with Ca. As both C3 and C4 plants had signi®cant responses in A, gs, A/gs and A/leaf N to Ca enrichment, future Ca increases in this grassland may not favour C3 species as much as originally thought. Non-linear responses and acclimation to low Ca should be incorporated into mechanistic models to better predict the effects of past and present rising Ca on grassland ecosystems.

Abstract: Chromosome counts are presented for 217 species of 103 genera of grasses from Costa Rica. Four of the genera and 90 of the species are reported for the first time. In addition, 26 counts differ from previous records for the same taxon. The generaGynerium, Lithachne, Luziola, andThrasya were heretofore unknown cytologically. First counts are also given for the following species:Agrostis bacillata, A. pittieri, A. sub patens, Andropogon angustatus, A. bicornis, A. virgatus, Aristida capillacea, A. jorullensis, Arundinella berteroniana, A. deppeana, Axonopus aureus, A. capillaris, A. chrysoblepharis, Bouteloua americana, B. disticha, Bromus exaltatus, Calamagrostis intermedia, C. nuda, C. pittieri, Chusquea lehmannii, C. tonduzii, Cinna poaeformis, Deschampsia pringlei, Digitaria panicea, Echinochloa polystachya, Echinolaena gracilis, Eragrostis simplicijlora, Eriochloa distachya, E. polystachya, Eriochrysis cayennensis, Festuca dolichophylla, Gymnopogon fastigiatus, Gynerium sagittatum, Hymenachne amplexicaulis, Imperata contracta, Isachne polygonoides, Ischaemum latijolium, Lithachne pauciflora, Luziola fragilis, Manisuris aurita, Panicum aquaticum, P. arundinariae, P. boliviense, P. cordovense, P. grande, P. haenkeanum, P. milleflorum, P. parviglume, P. pilosum, P. polygonatum, P. rudgei, P. stenodes, P. trichanthum, P. trichoides, P. viscidellum, Paspalum candidum, P. centrale, P. decumbens, P. fasciculatum, P. jimenezii, P. microstachyum, P. multicaule, P. nutans, P. parviflorum, P. pilosum, P. prostratum, P. pumilum, P. reclinatum, P. saccharoides, P. scabrum, P. serratum, P. squamulatum, P. standleyi, Pennisetum bambusiforme, P. distachyum, Pereilema beyrichianum, Pharus parvifolius, Sacciolepis myuros, Schizachyrium condensatum, Sorghastrum incompletum, Sporobolus ciliatus, S. purpurascens, Streptochaeta sodiroana, Thrasya gracilis, T. petrosa, T. robusta, Trisetum pringlei, T. tonduzii, Uniola pittieri, and Zeugites pittieri. In addition, the following counts are different from previous records for the same taxon:Aegopogon cenchroides, Axonopus poiophyllus, Bouteloua media, Brachiaria plantaginea, Chusquea subtessellata, Digitaria adscendens, Homolepis aturensis, Hymenachne donacifolia, Ichnanthus axillaris, I. pallens, I. tenuis, Ischaemum ciliare, Ixophorus unisetus, Oplismenus burmannii, O. hirtellus, Panicum geminatum, P. glutinosum, P. mertensii, P. parvifolium, P. sellowii, Paspalum convexum, Pennisetum nervosum, P. setosum, Polypogon elongalus, Raddia costaricensis, Schizachyrium hirtijlorum.

Abstract: 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.

Abstract: The goals of this study were to identify habitats occupied by the Eurasian grass Bothrio- chloa ischaemum (King Ranch bluestem) on the eastern Edwards Plateau of central Texas, USA, and to measure the effects of this invasive species on plant diversity. A set of descriptive field studies were conducted to determine the rela- tionships between habitat characteristics (woody cover, slope, presence of roads and trails, history of prescribed burning, and presence of grazing) and the presence and abundance of this species. Species richness and diversity of perennial her- baceous species were compared between plots in which B. ischaemum was absent and plots that it dominated.The only habitat in which B. ischae- mum was never found was under the canopies of woody plants. B. ischaemum grew in plots of all inclinations (flat sites to steep hillsides), with little evidence of habitat preference. B. ischaemum was more often, although not exclusively, found in plots near roads, probably because roads facilitate seed dispersal. There was no significant relation- ship between either grazing or fire history and the distribution of this species. B. ischaemum- dominated plots had lower species diversity and species richness than plots in which no B. ischaemum was found. The results of this study confirm that B. ischaemum is very common on the eastern Edwards Plateau and suggest that it will continue to spread throughout this region and to reduce native herbaceous plant diversity there.