Ellis Q. Margolis

Historical fire-climate relationships of upper elevation fire regimes in the south-western United States

Understanding relationships between variability in historical fire occurrence and ocean-atmosphere oscilla- tionsprovidesopportunitiesforfireforecastingandprojectingchangesinfireregimesunderclimatechangescenarios.We analysedtree-ring reconstructed regional climate teleconnectionsand fire-climate relationships inupper elevation forests (.2700m) from 16 sites in eight mountain ranges in the south-western USA. Climate teleconnections were identified by testing for associations between regional Palmer Drought Severity Index (PDSI) and individual and combined phases of El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) indices for both the fire exclusion (1905-1978) and reconstructed fire periods (1700-1904). Fire-climate relationships wereidentifiedbycomparingreconstructedfires(84fireyears)inthreeclasses(all,synchronousandstand-replacingfires) with PDSI, precipitation, temperature, and individual and combined phases of ENSO, PDO and AMO indices. Individual and phase combinations of ENSO, PDO and AMO were associated with variability in regional PDSI. Upper elevation fire occurrence was related to variability in regional drought, ENSO phase and phase combinations of ENSO and PDO. We conclude that ENSO most consistently influenced variability in moisture and upper elevation fire occurrence, including stand-replacing fires, but this relationship was potentially modulated by phases of the PDO. Additional keywords: Arizona, Atlantic Multidecadal Oscillation, El Nino-Southern Oscillation, high-severity fire, New Mexico, Pacific Decadal Oscillation, stand-replacing fire, tree ring.

Fire regime shift linked to increased forest density in a piñon–juniper savanna landscape

Pinon-juniper (PJ) fire regimes are generally characterised as infrequent high-severity. However, PJ ecosystems vary across a large geographic and bio-climatic range and little is known about one of the principal PJ functional types, PJ savannas. It is logical that (1) grass in PJ savannas could support frequent, low-severity fire and (2) exclusion of frequent fire could explain increased tree density in PJ savannas. To assess these hypotheses I used dendroecological methods to reconstruct fire history and forest structure in a PJ-dominated savanna. Evidence of high-severity fire was not observed. From 112 fire-scarred trees I reconstructed 87 fire years (1547-1899). Mean fire interval was 7.8 years for fires recorded at ≥2 sites. Tree establishment was negatively correlated with fire frequency (r = -0.74) and peak PJ establishment was synchronous with dry (unfavourable) conditions and a regime shift (decline) in fire frequency in the late 1800s. The collapse of the grass-fuelled, frequent, surface fire regime in this PJ savanna was likely the primary driver of current high tree density (mean = 881 trees ha-1) that is >600% of the historical estimate. Variability in bio-climatic conditions likely drive variability in fire regimes across the wide range of PJ ecosystems.

Dendroecological Methods For Reconstructing High-Severity Fire In Pine-Oak Forests

ABSTRACT Recent high-severity fires in pine-oak forests of the southwestern United States are creating shrubfields that may persist for decades to centuries. Shrubfields embedded in conifer forests that pre-date documentary records are potential evidence of older high-severity fire patches, and may therefore provide insights into the occurrence and extent of past high-severity fires and vegetation type conversion dynamics. In this paper we test whether dendroecological evidence can be used to reconstruct a high-severity, type-changing fire of known date in a ponderosa pine-dominated (Pinus ponderosa var scopulorum Engelm.) forest. Dendroecological evidence included (1) Gambel oak (Quercus gambelii, Nutt.) regeneration dates, (2) fire scars, (3) death dates, and (4) tree-ring growth changes. We reconstructed the historical fire regime and fire-climate relationship to evaluate whether the recent high-severity fire was driven by climate or fuel build-up related to a fire regime disruption. The dendroecological evidence correctly dated the year (1993) and season (spring) of the documented fire, and synchronous oak re-sprouts provided a means to estimate the minimum high-severity patch size. The historical fire regime at the site (1625–1871) consisted of frequent, low-severity fires occurring in dry years preceded by wet years. Fires stopped in 1871, coincident with increased regional livestock grazing. The 1993 fire occurred under relatively cool and wet conditions, but followed a 122-year fire-free interval (four times the maximum historical interval). Multiple lines of evidence suggest that increased fuel loads from fire exclusion, combined with high winds, were primary drivers of the high-severity fire. The dendroecological approach we outline can be applied to reconstruct high-severity fire across a range of conifer-shrubland ecosystems.