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Aim Studies of fire activity along environmental gradients have been undertaken, but the results of such studies have yet to be integrated with fire-regime analysis. We characterize fire-regime components along climate gradients and a gradient of human influence. Location We focus on a climatically diverse region of north-western North America extending from northern British Columbia, Canada, to northern Utah and Colorado, USA. Methods We used a multivariate framework to collapse 12 climatic variables into two major climate gradients and binned them into 73 discrete climate domains. We examined variation in fire-regime components (frequency, size, severity, seasonality and cause) across climate domains. Fire-regime attributes were compiled from existing databases and Landsat imagery for 1897 large fires. Relationships among the fire-regime components, climate gradients and human influence were examined through bivariate regressions. The unique contribution of human influence was also assessed. Results A primary climate gradient of temperature and summer precipitation and a secondary gradient of continentality and winter precipitation in the study area were identified. Fire occupied a distinct central region of such climate space, within which fire-regime components varied considerably. We identified significant interrelations between fire-regime components of fire size, frequency, burn severity and cause. The influence of humans was apparent in patterns of burn severity and ignition cause. Main conclusions Wildfire activity is highest where thermal and moisture gradients converge to promote fuel production, flammability and ignitions. Having linked fire-regime components to large-scale climate gradients, we show that fire regimes – like the climate that controls them – are a part of a continuum, expanding on models of varying constraints on fire activity. The observed relationships between fire-regime components, together with the distinct role of climatic and human influences, generate variation in biotic communities. Thus, future changes to climate may lead to ecological changes through altered fire regimes.

Aim Recent forest encroachment into montane and subalpine grasslands has occurred in the Rocky Mountains and many other mountain ranges globally. The timing, rate, and extent of tree invasion can depend on interactions among topography, positive spatial feedbacks, and temporally variable factors (especially climate, grazing, and fire). Here we examine spatial and temporal patterns of tree invasion in the Valles Caldera of the Jemez Mountains. Location This study was conducted in the Valles Caldera (35°50′–36°00′ N; 106°24′–106°37′ W), a 24-km-wide volcanic basin in northern New Mexico, USA. Grasslands in this otherwise forested region occur in broad valley bottoms of the caldera floor between 2575 and 2700 m, and on south-facing slopes and mountain tops up to 3300 m. Methods We used a GIS analysis of orthorectified aerial photos taken in 1935 and 1996, covering a 40,000-ha study area, to quantify the extent of tree invasion and to assess its relationship to spatial factors. We obtained dates of establishment from 299 increment cores and basal disks from 50 sites in the Valles Caldera National Preserve (VCNP) to reconstruct temporal patterns of tree invasion. Results The area of grasslands in our study area declined from 11,747 to 9336 ha (nearly 18%) between 1935 and 1996. Tree invasion increased with slope, elevation, and proximity to the previous tree line, but showed no relationship to aspect. Tree invasion was more rapid and continuous on upper mountain slopes, while the invasion of valley-bottom grasslands below reversed tree lines was more episodic, and appeared to track mean summer minimum temperatures. Main conclusions The rapid and continuous invasion of steep, high-elevation slopes suggests that frequent fire was the single most important factor in maintaining grassy communities in these sites. The slower, episodic invasion of valley-bottom grasslands, and the apparent relationship between increased invasion and years of higher summer minimum temperatures are consistent with the hypothesis that these grasslands have been maintained by low temperatures or frosts damaging to tree seedlings. We encourage prescribed fire to restore and maintain grasslands in the VCNP, especially small patches on steep, high-elevation slopes.