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

Piñon–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 highseverity 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 (r1⁄4 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 savannawas likely the primary driver of current high tree density (mean1⁄4 881 trees ha ) 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. Additional keywords: fire scar, Juniperus monosperma, Juniperus scopulorum, Pinus edulis, PJ savanna, tree ring. Received 31 March 2013, accepted 31 July 2013, published online 28 January 2014