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 these systems. Despite these issues, the majority of studies of fire effects in Artemisia tridentata wyomingensis-dominated systems 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 burnt the majority of it in 2000 and 2007. We analysed data from permanent vegetation transects established in 1996 and resampled in 2002 and 2009. Our objective was to describe how the fires, and subsequent post-fire 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 farthest 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, though there was some recovery where communities were dominated by re-sprouters. Bromus tectorum dominance was reduced by herbicide application in areas where it was previously abundant but increased significantly in untreated areas. Several re-sprouting 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), though 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

Holocene tree line changes in the Canadian Cordillera are controlled by climate and topography

Aim Although ongoing climate change is expected to lead to an upward shift of tree lines in mountain areas, evidence for widespread tree line advances remains scarce, implying secondary controls on tree line dynamics at the local scale. We aim to determine if vegetation change in response to past warm periods was regionally synchronous or if local factors such as topography, geomorphology or fire caused divergent local responses. Location The Canadian Cordillera in south-eastern British Columbia (Canada). Methods We analysed post-glacial sediments from three lakes at or just below the present tree line for macrofossils, pollen and charcoal to infer past local forest composition, density, dynamics and fire disturbance. Results At two lakes (Windy and Redmountain), tree macrofossil concentrations were highest in the warmer-than-present Early Holocene (11,700–7000 cal. bp), indicating higher forest density and tree line position during this time period. At the third lake (Thunder), macrofossil concentrations were low during the Early Holocene and reached maximum values in the mid-Holocene (7000–3000 cal. bp). The divergent vegetation dynamics and species composition at Thunder Lake suggest that moisture availability may have limited the establishment of closed forests on steep south-facing slopes or shallow soils in the Early Holocene. Main conclusions Summer temperature was the main driver of tree line dynamics over millennial to decadal time-scales. Closed forests, however, occurred only in areas of adequate moisture availability, which is controlled by topography and geomorphology. We therefore expect a rapid upward shift of tree lines during the 21st century in response to warmer temperatures, but only where deep soils or favourable aspects provide sufficient moisture for tree growth. Upward forest expansion will therefore be patchy and occur first in favourable microsites