A predictive equation for estimating fire frequency was developed from theories and data in physical chemistry, ecosystem ecology, and climatology. We refer to this equation as the Physical Chemistry Fire Frequency Model (PC2FM). The equation was calibrated and validated with North American fire data (170 sites) prior to widespread industrial influences (before $1850 CE) related to land use, fire suppression, and recent climate change to minimize non-climatic effects. We derived and validated the empirically based PC2FM for the purpose of estimating mean fire intervals (MFIs) from proxies of mean maximum temperature, precipitation, their interaction, and estimated reactant concentrations. Parameterization of the model uses reaction rate equations based on the concentration and physical chemistry of fuels and climate. The model was then calibrated and validated using centuries of empirical fire history data. An application of the PC2FM regression equation is presented and used to estimate historic MFI as controlled by climate. We discuss the effects of temperature, precipitation, and their interactions on fire frequency using the PC2FM concept and results. The exclusion of topographic, vegetation, and ignition variables from the PC2FM increased error at fine spatial scales, but allowed for the prediction of complex climate effects at broader temporal and spatial scales. The PC2FM equation is used to map coarse-scale historic fire frequency and assess climate impacts on landscapescale fire regimes.

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca  - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca  - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain.

Question: How frequent and variable were fire disturbances in longleaf pine ecosystems? Has the frequency and seasonality of fire events changed during the past few centuries? Location: Kisatchie National Forest, Western Gulf Coastal Plain, longleaf pine–bluestem ecosystem, in relatively rough topography adjacent to the Red River, Louisiana, USA. Methods: Cross‐sections of 19 remnant pines exhibiting 190 fire scars were collected from a 1.2‐km2 area. Tree‐rings and fire scars were precisely dated and analysed for the purpose of characterizing past changes in fire and tree growth. Temporal variability in fire occurrence and seasonality was described for the pre‐ and post‐European settlement periods. Seasonality of historic fires was determined by the scar position within the rings. The relationship between fire and drought was investigated using correlation and superposed epoch analysis. Results: The mean fire return interval for the period 1650‐1905 was 2.2 years (range 0.5 to 12 yr). Significant new findings include: evidence for years of biannual burning, temporal variability in fire seasonality, an increase in fire frequency and percentage of trees scarred circa 1790, and synchronous growth suppression and subsequent release of trees coinciding with land‐use changes near the turn of the 20th century. Drought conditions appeared unrelated to the occurrence of fire events or fire seasonality. Conclusions: Multi‐century fire history records from longleaf pine ecosystems are difficult to obtain due to historic land‐use practices and the species high resistance to scarring; however, our results indicate potential for reconstructing detailed fire histories in this ecosystem. Fire scars quantitatively documented one of the most frequent fire regimes known. Fire regime information, such as the temporal variability in fire intervals, prevalence of late‐growing season fire events and biannual burning, provide a new perspective on the dynamics of longleaf pine fire regimes.

Increasingly detailed records of long‐term fire regime characteristics are needed to test ecological concepts and inform natural resource management and policymaking. We reconstructed and analyzed twelve 350+ yr‐long fire scar records developed from 2612 tree‐ring dated fire scars on 432 living and dead pine (Pinus pungens, Pinus rigida, Pinus resinosa, Pinus echinata) trees from across central Pennsylvania. We used multiple spatial and time series analysis methods to quantify fire regime characteristics (frequency, seasonality, percentages of trees scarred, extent) and fire–climate–human associations. Prior to the 20th‐century fire suppression, fire regimes at the majority of sites consisted of frequent, low‐to‐moderate severity, dormant season fires. Fires were often regionally synchronous when preceded by significantly dry years. Using documentary archives, we provide the first description of a “wave of fire”—an anthropogenic signal in fire frequency that progressively moved across the region. This “wave of fire” reflects a changing progression of anthropogenic fire regimes from Native American occupation and depopulation, to Euro‐American settlement, to industrialization and declining fire use up to the 20th century era of fire suppression. The wave of fire provides a new perspective on historical and modern fire regime dynamics and identifies socio‐ecological impacts since North American colonization. Because the anthropogenic wave of fire exists at sites across North America, we emphasize the need for a broader determination of its geographic prevalence and variability as such determinations could influence historical ecology interpretations and perspectives on past and future roles of humans in managing ecosystems with fire.

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