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

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 following fire because their seed banks were exhausted by fire-induced mortality and/or germination. After 4–5 yr, few young Adenostoma remained. The combination of seedling and resprout regeneration allowed this shrub to maintain dominance, but to a lesser extent in the older stand. Our results support a vegetation pattern–process model in which local species distributions after fire in Adenostoma chaparral are antecedently linked to the physical and chemical properties of the canopy. These control the nature of combustion, the soil heating that results, and the distribution of seeds and resprout tissues that survive. The vegetation develops entirely from these sources, so fire-induced patterns are manifest in the long-term structure of this vegetation.