Early impacts of fire suppression in Jeffrey pine – Mixed conifer forests in the Sierra San Pedro Martir, Mexico

Abstract

Fire exclusion was first implemented across large areas of California and other areas of western North America in the late 19th or early 20th centuries but few studies have investigated how forests in the early decades after this decision were impacted. Jeffrey pine-mixed conifer forests of northern Baja California, Mexico, offer an area where this can be examined since fire suppression did not begin until the early 1970’s. We use repeated forest and fuel measurements taken over a 25-year period (1998–2023) and found significant changes in forest structure and fuel loads (increased tree density, fine fuel loads, large fuel loads, snag density, snag basal area) that together clearly demonstrate the cumulative effects of fire removal. Interestingly, Jeffrey pine became more dominant despite the lack of recent fire, which is counter to the shift towards more shade-tolerant tree species observed in many other fire-suppressed, frequent-fire forests in the western US. Although these changes in Baja California forests point towards increased fire hazard, they are still in relatively low hazard conditions compared to long-fire suppressed forests in the western US. Prescribed fire or managed wildfire could easily be applied to counter the increased fuel loads and tree densities detected in this work, without the need for mechanical manipulation. In addition to maintaining resilient forests, using fire will reduce the risk of losing the large astronomical observatory at this site to wildfire. Restoration and stewardship of resilient forest structures similar to those in Baja California is the only way forward to conserve similar forests of the Sierra Nevada, southern California mountains, and elsewhere in the western US.