Unsupported inferences of high‐severity fire in historical dry forests of the western United States: response to Williams and Baker

Reconstructions of dry western US forests in the late 19th century in Arizona, Colorado and Oregon based on General Land Office records were used by Williams & Baker (2012; Global Ecology and Biogeography, 21 , 1042–1052; hereafter W&B) to infer past fire regimes with substantial moderate and high‐severity burning. The authors concluded that present‐day large, high‐severity fires are not distinguishable from historical patterns. We present evidence of important errors in their study. First, the use of tree size distributions to reconstruct past fire severity and extent is not supported by empirical age–size relationships nor by studies that directly quantified disturbance history in these forests. Second, the fire severity classification of W&B is qualitatively different from most modern classification schemes, and is based on different types of data, leading to an inappropriate comparison. Third, we note that while W&B asserted ‘surprising’ heterogeneity in their reconstructions of stand density and species composition, their data are not substantially different from many previous studies which reached very different conclusions about subsequent forest and fire behaviour changes. Contrary to the conclusions of W&B, the preponderance of scientific evidence indicates that conservation of dry forest ecosystems in the western United States and their ecological, social and economic value is not consistent with a present‐day disturbance regime of large, high‐severity fires, especially under changing climate.     

Soil functional responses to ecological restoration treatments in frequent‐fire forests of the western United States: a systematic review

We investigated general effects of ecological restoration treatments on soil function in frequent‐fire forests of the western United States using a systematic review methodology. We searched numerous publication databases for original research papers and used well‐defined criteria developed a priori to select papers for review. We used meta‐analysis and qualitative summaries to compare reported responses of macronutrients, nitrogen cycling, and soil respiration among tree thinning (thin), prescribed fire (burn), and thinning plus prescribed fire treatments (composite). Results of meta‐analysis showed that mean differences in macronutrients were consistently higher in composite treatments (standardized using controls) when compared to thin‐only and burn‐only treatments. Mean responses related to nitrogen cycling showed similar patterns, with significant increases detected in composite treatments for all nitrogen cycling variables (mineralization, ammonification, and nitrification) and insignificant responses for the majority of the burn‐only and thin‐only treatments. Mean difference in response for soil respiration following composite treatments showed increases as compared to the controls, and no significant differences were detected in response to burn‐ and thin‐only treatments. While soil function, nutrient cycling, and soil respiration differed among treatments, the most significant effects were observed for nitrogen and carbon responses, net mineralization and nitrification, ammonium availability, and soil respiration rate, which experienced the greatest increase following treatments that were both thinned and burned.     

Fire as a dimension of historical ecology: a response to Bowman et al. (2011)

Bowman et al. (Journal of Biogeography, 2011, 38 , 2223–2236) attempt a synthesis of the current status of study into human use of fire as an ecosystem management tool and provide a framework for guiding research on the human dimensions of global fire. While we applaud this ambitious effort, we believe the proposed ‘pyric phase and transition’ model to be too deterministic and simplistic to account for the complexity and diversity in human–fire relationships. After reviewing theoretical problems with the proposed framework, we question policy implications of their conclusions concerning tropical forest systems. We suggest that a theoretically informed perspective should build on an historical fire ecology framework for investigating social and ecological aspects of human–environment interactions.     

Influence of fire severity on stand development of Araucaria araucana–Nothofagus pumilio stands in the Andean cordillera of south‐central Chile

Fire is the prevalent disturbance in the Araucaria–Nothofagus forested landscape in south‐central Chile. Although both surface and stand‐replacing fires are known to characterize these ecosystems, the variability of fire severity in shaping forest structure has not previously been investigated in Araucaria–Nothofagus forests. Age structures of 16 stands, in which the ages of approximately 650 trees were determined, indicate that variability in fire severity and frequency is key to explaining the mosaic of forest patches across the Araucaria–Nothofagus landscape. High levels of tree mortality in moderate‐ to high‐severity fires followed by new establishment of Nothofagus pumilio typically result in stands characterized by one or two cohorts of this species. Large Araucaria trees are highly resistant to fire, and this species typically survives moderate‐ to high‐severity fires either as dispersed individuals or as small groups of multi‐aged trees. Small post‐fire cohorts of Araucaria may establish, depending on seed availability and the effects of subsequent fires. Araucaria's great longevity (often >700 years) and resistance to fire allow some individuals to survive fires that kill and then trigger new Nothofagus cohorts. Even in relatively mesic habitats, where fires are less frequent, the oldest Araucaria–Nothofagus pumilio stands originated after high‐severity fires. Overall, stand development patterns of subalpine Araucaria–N. pumilio forests are largely controlled by moderate‐ to high‐severity fires, and therefore tree regeneration dynamics is strongly dominated by a catastrophic regeneration mode.     

Spatial and temporal dimensions of fire activity in the fire‐prone eastern Canadian taiga

The forest age mosaic is a fundamental attribute of the North American boreal forest. Given that fires are generally lethal to trees, the time since last fire largely determines the composition and structure of forest stands and landscapes. Although the spatiotemporal dynamics of such mosaics has long been assumed to be random under the overwhelming influence of severe fire weather, no long‐term reconstruction of mosaic dynamics has been performed from direct field evidence. In this study, we use fire length as a proxy for fire extent across the fire‐prone eastern Canadian taiga and systematically reconstruct the spatiotemporal variability of fire extent and fire intervals, as well as the resulting forest age along a 340‐km transect for the 1840–2013 time period. Our results indicate an extremely active fire regime over the last two centuries, with an overall burn rate of 2.1% of the land area yr^?1, mainly triggered by seasonal anomalies of high temperature and severe drought. However, the rejuvenation of the age mosaic was strongly patterned in space and time due to the intrinsically lower burn rates in wetland‐dominated areas and, more importantly, to the much‐reduced likelihood of burning of stands up to 50 years postfire. An extremely high burn rate of ~5% yr^?1 would have characterized our study region during the last century in the absence of such fuel age effect. Although recent burn rates and fire sizes are within their range of variability of the last 175 years, a particularly severe weather event allowed a 2013 fire to spread across a large fire refuge, thus shifting the abundance of mature and old forest to a historic low. These results provide reference conditions to evaluate the significance and predict the spatiotemporal dynamics and impacts of the currently strengthening fire activity in the North American boreal forest.     

Historic variability in fire‐generated landscape heterogeneity of subalpine forests in the Canadian Rockies

Question: This study evaluates historical changes in landscape structure and heterogeneity in subalpine forests. We use response to severe fires in 2001 and 2003, along with historical reconstructions to examine crown‐fire effects on landscape heterogeneity and to assess, comparatively, effects of fire exclusion management in the 20th century. Location: Subalpine forests of Kootenay National Park (KNP), Canadian Rockies. Methods: Using a landscape‐level model based on a fire‐origin stand age map, we reconstructed decadal burned areas within the landscape for 1750‐2000 (forming reconstructed landscapes). Landscape pattern was analysed for each reconstructed landscape map, and we compared landscape pattern indices (total area, number of patches, mean patch area, patch area variation, largest patch index, edge density, perimeter–area ratio, landscape shape index) with those in 2005 after recent large fires. Results: After large fires in 1926, connectivity of the KNP landscape increased and its diversity was quite low. After 2001 and 2003 fires, the post‐fire landscape of 2005 was highly heterogeneous in terms of size, variation, edge density and perimeter–area ratio of the remnant forest patches. Since the decline in occurrence of large fires after 1926 reflected a period of wet weather, fuel build‐up resulting from landscape homogenization within the 20th century landscape could not be attributed solely to fire exclusion. This period without fires greatly enhanced connectivity of late‐successional forests that finally burned in 2001/2003, but connectivity was within the historical range for these forests. The gradual increase in stand connectivity before recent large fires may indicate that fire exclusion was less responsible than often believed for fuel build‐up in these fire‐susceptible older forests. Conclusions: The large fires at the beginning of the 21st century are within the natural range of disturbances for this landscape, and do not stand out as “human‐induced disasters” in their effects on landscape patterns. Such stochastic large disturbances contribute to maintenance of highly heterogeneous landscape structure, which is important for many taxa and natural ecological processes. Identifying future probability of such large disturbances and their ecological roles should be incorporated into management of these dynamic, disturbance‐prone systems.     

Effect of historical land‐use and climate change on tree‐climate relationships in the upper Midwestern United States

Contemporary forest inventory data are widely used to understand environmental controls on tree species distributions and to construct models to project forest responses to climate change, but the stability and representativeness of contemporary tree‐climate relationships are poorly understood. We show that tree‐climate relationships for 15 tree genera in the upper Midwestern US have significantly altered over the last two centuries due to historical land‐use and climate change. Realised niches have shifted towards higher minimum temperatures and higher rainfall. A new attribution method implicates both historical climate change and land‐use in these shifts, with the relative importance varying among genera and climate variables. Most climate/land‐use interactions are compounding, in which historical land‐use reinforces shifts in species‐climate relationships toward wetter distributions, or confounding, in which land‐use complicates shifts towards warmer distributions. Compounding interactions imply that contemporary‐based models of species distributions may underestimate species resilience to climate change.