Climate influences forest structure through effects on both species demography (recruitment and mortality) and disturbance regimes. Here, I compare multi‐century chronologies of regional fire years and tree recruitment from ponderosa pine forests in the Black Hills of southwestern South Dakota and northeastern Wyoming to reconstructions of precipitation and global circulation indices. Regional fire years were affected by droughts and variations in both Pacific and Atlantic sea surface temperatures. Fires were synchronous with La Niñas, cool phases of the Pacific Decadal Oscillation (PDO), and warm phases of the Atlantic Multidecadal Oscillation (AMO). These quasi‐periodic circulation features are associated with drought conditions over much of the western United States. The opposite pattern (El Niño, warm PDO, cool AMO) was associated with fewer fires than expected. Regional tree recruitment largely occurred during wet periods in precipitation reconstructions, with the most abundant recruitment coeval with an extended pluvial from the late 1700s to early 1800s. Widespread even‐aged cohorts likely were not the result of large crown fires causing overstory mortality, but rather were caused by optimal climate conditions that contributed to synchronous regional recruitment and longer intervals between surface fires. Synchronous recruitment driven by climate is an example of the Moran effect. The presence of abundant fire‐scarred trees in multi‐aged stands supports a prevailing historical model for ponderosa pine forests in which recurrent surface fires affected heterogenous forest structure, although the Black Hills apparently had a greater range of fire behavior and resulting forest structure over multi‐decadal time scales than ponderosa pine forests of the Southwest that burned more often.

Mount Rushmore National Memorial in the Black Hills of South Dakota is known worldwide for its massive sculpture of four of the United States' most respected presidents. The Memorial landscape also is covered by extensive ponderosa pine (Pinus ponderosa) forest that has not burned in over a century. We compiled dendroecological and forest structural data from 29 plots across the 517‐ha Memorial and used fire behavior modeling to reconstruct the historical fire regime and forest structure and compare them to current conditions. The historical fire regime is best characterized as one of low‐severity surface fires with occasional (>100 years) patches (<100 ha) of passive crown fire. We estimate that only ∼3.3% of the landscape burned as crown fire during 22 landscape fire years (recorded at ≥25% of plots) between 1529 and 1893. The last landscape fire was in 1893. Mean fire intervals before 1893 varied depending on spatial scale, from 34 years based on scar‐to‐scar intervals on individual trees to 16 years between landscape fire years. Modal fire intervals were 11–15 years and did not vary with scale. Fire rotation (the time to burn an area the size of the study area) was estimated to be 30 years for surface fire and 800+ years for crown fire. The current forest is denser and contains more small trees, fewer large trees, lower canopy base heights, and greater canopy bulk density than a reconstructed historical (1870) forest. Fire behavior modeling using the NEXUS program suggests that surface fires would have dominated fire behavior in the 1870 forest during both moderate and severe weather conditions, while crown fire would dominate in the current forest especially under severe weather. Changes in the fire regime and forest structure at Mount Rushmore parallel those seen in ponderosa pine forests from the southwestern United States. Shifts from historical to current forest structure and the increased likelihood of crown fire justify the need for forest restoration before a catastrophic wildfire occurs and adversely impacts the ecological and aesthetic setting of the Mount Rushmore sculpture.

Ecotones are boundaries between plant assemblages that can represent a physiological or competitive limit of species’ local distributions, usually through one or more biotic or abiotic constraints on species’ resource requirements. However, ecotones also result from the effects of chronic or episodic disturbances, and changes in disturbance regimes may have profound effects on vegetation patterns in transitional areas. In this study, centuries-long chronologies of surface fire events were reconstructed from fire-scarred ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees in three sites at the ecotone between ponderosa pine forest and Northern Great Plains mixed-grass prairie in the southeastern Black Hills of South Dakota. The fire chronologies provide baseline data to assess the possible role of fire in this transitional area and to document historical variability in fire regimes in this region of the Northern Great Plains. Regular fire events were recorded at all three sites from the beginning of the fire chronologies in the 1500s up to the late 1800s or early 1900s, at which time spreading fires ceased. Fire frequencies derived from the fire chronologies were compared to each other and to four sites from interior ponderosa pine forest in the south-central Black Hills. Mean fire intervals at the savanna sites were between 10 to 12 years, whereas Weibull median probability intervals were one year shorter. Fire frequency at the savanna sites was twice as high as at the interior forest sites, and most likely was due to spatial extent of fires on the mixed-grass prairie coupled with warmer and drier climate regime. Post-settlement shifts in the ponderosa pine savanna during the twentieth century in this area may be largely attributed to lack of fire occurrences, although grazing and other factors also likely contributed to observed changes in forest and grassland margins.

We inferred climate effects on fire occurrence from 1630 to 1900 for a new set of crossdated fire-scar chronologies from 18 forested sites in Utah and one site in eastern Nevada. Years with regionally synchronous fires (31 years with fire at ≥20% of sites) occurred during drier than average summers and years with no fires at any site (100 years) were wetter than average. Antecedent wet summers were associated with regional-fire years in mixed-conifer and ponderosa pine forest types, possibly by affecting fine fuel amount and continuity. NINO3 (an index of the El Niño–Southern Oscillation, ENSO) was significantly low during regional-fire years (La Niñas) and significantly high during non-fire years (El Niños). NINO3 also was high during years before regional-fire years. Although regional fire years occurred nearly twice as often as expected when NINO3 and the Pacific Decadal Oscillation were both in their cool (negative) phases, this pattern was not statistically significant. Palmer Drought Severity Index was important for fire occurrence in ponderosa pine and mixed-conifer forests across the study area but ENSO forcing was seen only in south-eastern sites. Results support findings from previous fire and climate studies, including a possible geographic pivot point in Pacific basin teleconnections at ~40°N.

Patterning in fire regimes occurs at multiple spatiotemporal scales owing to differences in scaling of local and regional influences. Local fire occurrence and behavior may be controlled largely by site factors, while regional climate and changes in human land use can synchronize fire timing across large areas. We examined historical patterns in fires during the past five centuries across gradients in forest types and physiography and in relation to regional climate variability and land use change in the Sacramento Mountains in southern New Mexico. Forest stand-level chronologies of fires were reconstructed for 19 pinyon-juniper, ponderosa pine, and mixed-conifer stands using fire-scar records in crossdated tree-ring series. The fire history documents both local and regional factors effected fire occurrences in stands. Lower-elevation stands recorded more frequent fire than higher-elevation stands, although there were not significant differences between means of fire frequencies from clusters of ponderosa pine and mixed-conifer stands. Mean fire intervals ranged from approximately 3 to 11 years in ponderosa pine sites to 4 to 14 years in mixed-conifer sites. Sites on the steeper west side of the range, where fire spread more readily between forest types, recorded significantly more frequent fire than sites on the more physiographically heterogeneous east side. Fires were also synchronized by regional factors. Fire occurrences and fire-free years are related to variability in both annual Palmer Drought Severity Indices and El Niño-Southern Oscillation events. Fire regimes in the stands were also profoundly effected by changes in human land use patterns, with fire cessation in all sites following intensive Euro-American settlement beginning in the 1880s.