Long-Term Persistence and Fire Resilience of Oak Shrubfields in Dry Conifer Forests of Northern New Mexico
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Extensive high-severity fires are creating large shrubfields in many dry conifer forests of the interior western USA, raising concerns about forest-to-shrub conversion. This study evaluates the role of disturbance in shrubfield formation, maintenance and succession in the Jemez Mountains, New Mexico. We compared the environmental conditions of extant Gambel oak (Quercus gambelii) shrubfields with adjoining dry conifer forests and used dendroecological methods to determine the multi-century fire history and successional dynamics of five of the largest shrubfields (76–340 ha). Across the study area, 349 shrubfields (5–368 ha) occur in similar topographic and climate settings as dry conifer forests. This suggests disturbance, rather than other biophysical factors, may explain their origins and persistence. Gambel oak ages and tree-ring fire scars in our sampled shrubfields indicate they historically (1664–1899) burned concurrently with adjoining conifer forests and have persisted for over 115 years in the absence of fire. Aerial imagery from 1935 confirmed almost no change in sampled shrubfield patch sizes or boundaries over the twentieth century. The largest shrubfield we identified is less than 4% the size of the largest conifer-depleted and substantially shrub-dominated area recently formed in the Jemez following extensive high-severity wildfires, indicating considerable departure from historical patterns and processes. Projected hotter droughts and increasingly large high-severity fires could trigger more forest-to-shrub transitions and maintain existing shrubfields, inhibiting conifer forest recovery. Restoration of surface fire regimes and associated historical forest structures likely could reduce the rate and patch size of dry conifer forests being converted to shrubfields.
Keywordsfire-origin shrubfields high-severity fire gambel oak alternative stable states metastability tree rings dry conifer ecosystems
This study was funded by the Western Mountains Initiative and Bandelier National Monument (CESU Agreement #UADS-402), the US Geological Survey (Cooperative Agreement G13AC00247), and an EPA STAR Fellowship (#F13F51318 to C.H.G.). Additional support was provided by Bandelier National Monument, the USGS New Mexico Landscapes Field Station, and the Laboratory of Tree-Ring Research. We were aided in the field and lab by Eze Ahanonu, Chris Baisan, Andrew Bieg, John Danloe, Galen Gudenkauf, Collin Haffey, Nick Kessler, and Ben Olimpio. We thank Chris Baisan, Kay Beeley, Erica Bigio, and Collin Haffey for helpful discussions. Comments by Brandon Bestelmeyer, Jonathan Coop, and two anonymous reviewers helped to improve this manuscript. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.
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