Mechanical fuel treatments are a primary pre-fire strategy for potentially mitigating the threat of wildland fire, yet there is limited information on how they impact shrubland ecosystems. This publication contains data related to vegetation structure and composition in mechanically masticated chaparral communities used to assess the impact of these fuel treatments on shrubland vegetation and to determine the extent to which they emulate postfire succession. Data were collected from within chaparral dominated communities on the Angeles, Cleveland, Los Padres, and San Bernardino national forests of southern California. The climate of the region is Mediterranean with mild, wet winters and hot, dry summers and the topography is rugged and steep with elevations from near sea level to over 3500 m in the Transverse and Peninsular ranges. The rocky and shallow soils of the area are predominantly granitic and support a wide range of shrubland communities that include stands dominated by a single species (>50% cover) such as Adenostoma fasciculatum (chamise), Arctostaphylos spp. (manzanita), Ceanothus spp. (wild lilac) and Quercus spp. (oak) and mixed stands without a single dominant.
The mechanically masticated fuel treatments utilized for this study were identified using the USGS Southern California Fuel Treatment Data Set (http://www.calfiresci.org) and were limited to single-entry mastication treatments with no follow-up treatment of burning or re-mastication. The size and shape of available treatments were highly variable and thus a random sampling design was used to maximize the number of study sites. This was accomplished by selecting sites from within treatment boundaries using the random-point generator in ArcGIS and a buffer of at least 400 m between points. The final sample size of accessible locations included 149 mechanically masticated study sites, each with a treatment plot and a control. All treatments were completed between 2004 and 2011 using a variety of masticating equipment and ranged in size from less than a hectare to large-scale treatments spanning thousands of hectares across entire ridgelines. The timing of mastication treatments extended across all seasons and ranged in completion time from several days to several years depending on their size.
In order to evaluate the differences between mechanically masticated and early successional postfire vegetation two comparisons were made. The first was a single site case study on the Cleveland National Forest where a spark from a masticator ignited the 39 acre Long Canyon Wildfire on September 23rd, 2010 that burned next to the mechanical treatment being implemented and comprised similar pre-disturbance vegetation. This comparison consisted of four study plots in the masticated treatment and four study plots in the adjacent burned area that were monitored for the first two years following the disturbances. The second was a regionally broad comparison of two-year old mechanically masticated study plots from this fuel treatment study (n = 25) to a subset of two-year-old postfire plots (n = 56) from a regional study of early postfire succession in southern California chaparral published in an earlier paper (Keeley et al. 2008). This study investigated factors determining fire severity and ecosystem responses in 250 randomly selected study plots within the 2003 Cedar, Grand Prix, Old, and Paradise fire perimeters. The subset of 56 plots chosen from the original 250 plots were based on the criteria that the site was located within one of the four southern California national forests, was in chaparral vegetation, and had a pre-disturbance stand age and elevation within the same range as the two-year-old masticated sites used in the regional comparison.
These data support the following publication:
Brennan, T.J. and Keeley, J.E., 2017. Impacts of mastication fuel treatments on California, USA, chaparral vegetation structure and composition. Fire Ecology, 13(3), pp.120-138. https://doi.org/10.4996/fireecology.130312013.
Regional postfire data were extracted from this publication:
Keeley J.E., T.J. Brennan, and A.H. Pfaff. 2008. Fire severity and ecosystem responses following crown fires in California shrublands. Ecological Adaptations 18: 1530-1546. https://doi.org/10.1890/07-0836.1.