Fire, Soil Heating, and the Formation of Vegetation Patterns in Chaparral

Abstract

We documented patterns of surface heating associated with chaparral fire to characterize fundamental scale variation in the intensity of this stand‐replacing disturbance. To test how this variation may influence community structure, we studied its effect on the soil seed bank and the distribution of seedlings and resprouts that emerged after fire. To evaluate the long‐term significance of initial patterns, we monitored vegetation development for 4–5 yr, thereby encompassing the dynamic portion of the chaparral fire cycle. We studied two stands on level uniform terrain before, during, and after fall fires. Stands were dominated by chamise (Adenostoma fasciculatum), a postfire seeder/sprouter. Nonsprouting Arctostaphylos and Ceanothus spp. were also present. Preburn vegetation, seed populations, soil heating, and postburn plant growth were analyzed along transects of contiguous 1‐m2 plots, so that we could block them together incrementally to identify scale dependence of patterns. In addition, we directly compared heating effects under the fuel array with those just outside by establishing plots in canopy gaps, under the adjacent canopy, and in gaps created and eliminated by reciprocally translocating fuel. Pre‐ and postburn seed populations were estimated in soil samples collected from all plots. The proportion of seed that survived above and below 2.5 cm in the soil was determined in a subset of plots. The amount and distribution of canopy fuel that collapsed during fire and smoldered on the ground caused pronounced spatial variation in total surface heating. The strength of relationships among patterns of soil heating, preburn canopy, surviving seeds, and seedlings and herbaceous resprouts was consistently most pronounced in blocks 3–5 m long. At this scale, postburn patterns were strongly negatively associated with the amount of preburn canopy and the pattern of soil heating this fuel created. Seedlings or herbaceous resprouts of numerous species were abundant where soil heating was relatively low, most notably in natural and created canopy gaps. Conversely, areas where dense canopy occurred before fire, especially gaps displaced by fuel addition, were barren except for occasional Arctostaphylos and Ceanothus seedlings. These obligate postfire seeders, along with the subshrub Helianthemum scoparium, had more deeply buried seeds, and some of them were able to survive where soil heating was prolonged. However, Helianthemum did not emerge from depth. Seedlings of Arctostaphylos and Ceanothus nearest Adenostoma burls survived significantly better when Adenostoma failed to resprout. This was common in one burn where heating was relatively high and burl size was small. Seed mortality prevented Adenostoma seedling emergence from occurring where its seeds were most abundant prior to fire, which was in proximity to its burls. Adenostoma seedlings did emerge in areas of lower soil heating, but their survival was inversely related to the density of Helianthemum seedlings. No shrub seedlings emerged after the first year fol...

Fire Severity and Ecosytem Responses Following Crown Fires in California Shrublands

“…14. This should not be surprising since diversity is commonly related to disturbance severity (Huston 1994), and in chaparral these effects are evident at different scales, from microhabitats (Odion and Davis 2000) to landscapes (Keeley et al 2005a). good predictor of vegetative regeneration after fire, either in the immediate postfire years (Fig.…”

“…Even so, there is substantial variation between and within chaparral fires determined by species-specific differences in fuel structure and chemistry as well as wind speed and topography (Rundel 1981). Although there are a few experimental studies in chaparral that have related measures of fire intensity to changes in ecosystem properties (Borchert and Odion 1995, Tyler 1995, Odion and Davis 2000, most of what we know about fire intensity effects is based on surrogate measures known as fire severity metrics. Operationally, fire severity measures the aboveground and/or belowground organic matter loss (Keeley, in press), and these metrics are correlated with measures of fire intensity (McCaw et al 1997, Perez andMoreno 1998).…”

Peak Fire Temperatures and Effects on Annual Plants in the Mojave Desert

“…Desert fires can kill a high proportion of the seedbank which is concentrated beneath shrubs (Young and Evans 1978, Hassan and West 1986, Price and Joyner 1997), and may ameliorate undercanopy and drip line microhabitats that are favored by many native annual plants (Shreve 1931, Went 1942, Muller 1953, Halvorson and Patten 1975, Nelson and Chew 1977, Shmida and Whittaker 1981), thereby decreasing species richness. In contrast, chaparral fires kill relatively few seeds since relatively few are present beneath shrubs (Davis et al 1989), and expose undercanopy soil creating a regeneration niche for opportunistic species and obligate seeders (Keeley and Zedler 1978, Zammit and Zedler 1994, Odion and Davis 2000, thereby enhancing species richness. At the small scale represented by the shrub-intershrub gradient, fire can therefore decrease plant diversity in desert shrubland and increase plant diversity in chaparral shrubland.…”

“…Woody and fine fuels are highest beneath creosote bushes, intermediate at the canopy drip line, and lowest in the interspaces between shrubs. Heterogeneous fuel distributions often create variable peak fire temperatures (Davis et al 1989, Odion andDavis 2000), and spatially and seasonally heterogeneous peak fire temperatures should produce variable effects across the landscape.…”

High-Severity Fire in Chaparral

“…In fact, long fire-free periods are required for many species to properly regenerate (Odion and Tyler, 2002, Odion and Davis, 2000, Keeley, 1992.…”