Due largely to an extended period of fire exclusion, forest structure, fuel characteristics (live and dead vegetation biomass), and fire regimes of mixed conifer forests in the western United States were dramatically altered during the 20th century (Parsons & DeBenedetti 1979; Skinner & Chang 1996; Stephens & Ruth 2005). A central feature of national policies and programs (reviewed in Stephens & Ruth 2005) addresses forest and fire management issues related to fuel reduction, including reducing density of small diameter trees (e.g. Abies concolor, Calocedrus decurrens). A number of methods can be used to reduce fuel loads, but prescribed burning alone is often considered the most appropriate and practical approach (Weatherspoon & Skinner 2002). The National Park Service (NPS) began using prescribed burns in the western United States to manage vegetation in the 1950’s, and since then has developed what is arguably the most sophisticated and comprehensive prescribed burning program in the United States (Parsons & van Wagtendonk 1996). Many NPS park units in forests of the western United States have a goal to restore forest conditions by reducing fuel loads of shade-tolerant species. An additional goal in some cases is to maintain or increase relative density of species in the genus Pinus (e.g., P. ponderosa, P. lambertiana) (Parsons & van Wagtendonk 1996). To meet these goals, the NPS conducts prescribed burns in forest ecosystems throughout the western United States, and has developed a comprehensive program to monitor outcomes of the prescribed fire programs (NPS 2003).
Recent studies indicate that historic fire regimes varied broadly between forest types, which has raised the question - Are fuel reduction policies being applied too broadly? (Schoennagel et al. 2004). If this is true, it implies that responses to prescribed burns by conifer species will likely vary between forest types and biogeographic regions.
Specifically, there is concern in the NPS Pacific West Region that some prescribed burns may be leading to higher than expected mortality of pines. While it is generally assumed that recruitment of pines will increase in stands with low to moderate severity burns, there is also a greater likelihood of high severity burns in stands with high stem density of shade-tolerant tree species. Therefore, mortality rates of adult, sapling, and seedling
Pinus in many prescribed burn stands could be quite high. High mortality rates could be offset by high recruitment rates, however neither mortality or recruitment of pines in NPS prescribed burn units have been quantified across all parks in the Pacific West Region. Consequently, it is unknown whether a general pattern of reduction in stand density of pines in prescribed burn units exists and, if it does, the degree to which the pattern varies spatially and temporally across the region.
This study consists of three parts:
(1) an analysis of spatial and temporal patterns of pine stand density, mortality, and recruitment in prescribed burn units across a dozen NPS units in Washington, Oregon, California, and Nevada;
(2) an analysis of factors correlated with pine mortality, recruitment and density;
(3) a meta analysis comparing pine recruitment and mortality rates in prescribed burns with other types of fire (wildfire, fire use burns) and other conditions (e.g., beetle outbreaks, drought).
USGS Contact For This Project
Simkin, SM, EB Allen, WD Bowman, CM Clark, J Belnap, ML Brooks, BS Cade, SL Collins, LH Geiser, FS Gilliam, SE Jovan, LH Pardo, BK Schulz, CJ Stevens, KN Suding, HL Throop, DM Waller. 2016. Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States. PNAS 113(15): 4086-4091. doi: 10.1073/pnas.1515241113