Trajectories of change in sagebrush steppe vegetation communities in relation to multiple wildfires.

Repeated perturbations, both biotic and abiotic, can lead to fundamental changes in the nature of ecosystems, including changes in state. Sagebrush steppe communities provide important habitat for wildlife and grazing for livestock. Fire is an integral part of these systems, but there is concern that increased ignition frequencies and invasive species are fundamentally altering them. Despite these issues, the majority of studies of fire effects in systems dominated by Artemisia tridentata wyomingensis have focused on the effects of single burns. The Arid Lands Ecology Reserve (ALE), in south-central Washington (U.S.A.), was one of the largest contiguous areas of sagebrush steppe habitat in the state until large wildfires burned the majority of it in 2000 and 2007. We analyzed data from permanent vegetation transects established in 1996 and resampled in 2002 and 2009. Our objective was to describe how the fires, and subsequent postfire restoration efforts, affected communities' successional pathways. Plant communities differed in response to repeated fire and restoration; these differences could largely be ascribed to the functional traits of the dominant species. Low-elevation communities, previously dominated by obligate seeders, moved furthest from their initial composition and were dominated by weedy, early-successional species in 2009. Higher-elevation sites with resprouting shrubs, native bunchgrasses, and few invasive species were generally more resilient to the effects of repeated disturbances. Shrub cover has been almost entirely removed from ALE, although there was some recovery where communities were dominated by resprouters. Bromus tectorum dominance was reduced by herbicide application in areas where it was previously abundant, but it increased significantly in untreated areas. Several resprouting species, notably Phlox longifolia and Poa secunda, expanded remarkably following competitive release from shrub canopies and/or abundant B. tectorum. Our results suggest that community dynamics can be understood through a state and transition model with two axes (shrub/grass and native/invasive abundance), although such models also need to account for differences in plant functional traits and disturbance regimes. We use our results to develop a conceptual model that will be validated with further research.

Finite element analysis of wall stress in the equine pulmonary artery.

REASONS FOR PERFORMING STUDY: Arterial calcification is found frequently in the pulmonary artery of racehorses, but the aetiology is unknown. Calcification might be associated with increased wall stress due to arterial geometry (shape) and exercise-induced hypertension. HYPOTHESIS: High wall stress levels are found in the regions associated with calcified lesion formation, exacerbated as transluminal pressure increases to levels associated with exercise. METHODS: The pulmonary arteries of 5 horses, unaffected by calcification, were dissected and pressurised to resting and exercising physiological transluminal pressures and scanned with MRI. Arterial geometries were reconstructed to form 3D computer models and finite element analyses performed. Wall stress levels were measured in 4 regions of interest: the arterial trunk and bifurcation, the wall ipsilateral and contralateral to the bifurcation. Measurements were made for arterial transluminal pressures of 25, 50 and 100 mmHg. RESULTS: High wall stress levels were consistently found at the pulmonary artery bifurcation and wall ipsilateral to the bifurcation, where calcified lesions typically form. Lower wall stress levels were found along the trunk and the wall contralateral to the bifurcation where lesions are less frequently found. Wall stress levels increased 5-fold over a 4-fold increase in pressure. The wall stress levels ranged 10 kPa in the wall of the branch contralateral to the bifurcation at 25 mmHg to 400 kPa in the bifurcation at 100 mmHg. CONCLUSIONS: Wall stress from arterial geometry and increased pulmonary artery transluminal pressure are factors that may be associated with calcification of the equine pulmonary artery. POTENTIAL RELEVANCE: Arterial calcification may increase the risk of arterial wall failure in racing horses.

Unrelated donor marrow transplantation in children: transplant policy and outcome in Leiden Paediatrics SCT-Centre.

The transplant policy for unrelated donor (UD) BMT at Leiden Paediatrics' SCT-Centre consisted of the use of (1) fully HLA-matched donors or, if not available, HLA-class I matched and/or cytotoxic T-lymphocyte precursor (CTLp)-negative donors and (2) protective isolation of the recipient and antimicrobial suppression of his/her gut microflora to prevent infections and acute GVHD. Engraftment, GVHD, relapse in the case of malignancy and survival were studied retrospectively in 126 evaluable children, transplanted between 1988 and 2005. In addition to the effect of HLA-matching, that of other transplant-relevant variables on the outcome was also studied. Actuarial OS was 65% and the EFS was 59%, 13% graft failures occurred and 7.5% > or =grade II acute GVHD. HLA-class II mismatches combined with HLA-class I matches resulted in a superior OS of 92%, as did a negative vs positive CTLp test, that is, 65 vs 33%. Analysis of other variables showed a poorer OS in patients > or =10 yrs vs <10 yrs, that is, 54 vs 73%, and in male recipients of a female donor graft, that is, 53 vs 69% for other combinations. UD-BMT can be optimized by permitting HLA-class I-matched and/or CTLp-negative donors, and probably by choosing male donors for male recipients.