Maximizing Vegetation Response on Management Burns by Identifying Fire Regimes1

Maintenance of vegetation is a central goal of watershed management. When prescribed burning of chaparral is included in management practice, then it is important for managers to understand and use the natural chaparral fire regime to maximize vegetation response. Variations from the natural fire regime in intensity, frequency, season, and environmental conditions at the time of burning can all have substantial effects. These factors interact differently with the species that comprise chaparral. This paper focusses on the variation in responses of different groups of chaparral species to changes in fire regime. Prescribed burning often has been used to reduce fuel loads to meet fire safety objectives. An assumption inherent in this type of management is that prescribed burning reduces the likelihood of a wildfire yet has little net effect on the vegetation, which is basically true for many species and communities. One exception, however, is California chaparral, widely recognized as a fire-type vegetation. Chaparral tolerates burning only under certain conditions at limited times of the year. Under other conditions or times, the recovery of chaparral following prescribed burning can be limited. Particular types of species are most sensitive and several environmental conditions appear to exert the most influence on recovery. My objective in this paper is to illustrate these vegetation and environmental characteristics. Only after a careful consideration of these factors can managers hope to maximize the response of their vegetation. Overall watershed management involves not only short-term objectives like fuel reduction, but also, the long-term objective of maintaining the health of the vegetation. The health of the vegetation depends upon species diversity as well as ensuring vegetation recovery. Many chaparral dominants in the genera Arctostaphylos and Ceanothus, for example, are usually killed in fires and are greatly reduced in regeneration following most prescribed burns (Parker 1987b). Twenty species of these two genera, furthermore, Presented at the Symposium on Fire and Watershed Management, October 26-28, 1988, Sacramento, California. Professor of Biology, San Francisco State University, San Francisco, Calif. are listed rare and endangered species or under consideration. Chaparral contains a number of additional sensitive species. Most of these rare and endangered chaparral species are vulnerable to management practices like prescribed burning. Protection of rare and endangered species is an issue that will continue to increase in importance. INFLUENCES ON RECOVERY OF CHAPARRAL Vegetation Characteristics The diversity of species in chaparral is reflected in the variation in plant response to burning. This diversity can be grouped according to population changes and methods of surviving fire. In this way, four regeneration syndromes can be distinguished. Many chaparral dominant species, for example, are obligate seeders with respect to fire. This means that their populations are killed by fire and require regeneration from dormant seed stored in the soil seed banks. Other dominant species also have soil seed banks, but can also resprout and are termed facultative sprouters. Populations of another group of woody species are called obligate sprouters; they resprout after fire and have no soil seed reserves. A fourth important group of species are post-fire annuals and shortlived perennials that are present only as dormant soil seed banks before a fire. Several recent reviews of these regeneration syndromes exist and should be consulted for more information (Christensen 1985, Keeley and Keeley 1988, Parker and Kelly, in press). What is apparent is a spectrum of species, some of which sprout and some of which maintain seed banks in the soil. The various combinations establish a spectrum of vulnerability for management practice. Some species are extremely resilient, while others are readily eliminated. To maximize the diversity and rate of vegetation response and to know how careful one must be requires knowledge of what combination of species exists at the site, at least in terms of their regeneration responses. The rate of chaparral post-fire recovery and the resilience of the vegetation depend in part, therefore, on the combination of species present at a site. If all the woody species are obligate sprouters and a large and diverse seed bank of temporary species exists, then the site will appear to recover rather rapidly. If all the woody species are obligate seeders and few temporary species are in the seed bank, then the USDA Forest Service Gen. Tech. Rep. PSW-109. 1989 87 vegetation remains open and appears to recover rather slowly. Environmental Variables Not only are vegetation characteristics important to understand, but so too are environmental characteristics. For example, in Marin County, California, serpentine soil and sandstone soil chaparral occur side by side in many areas, but these two chaparral vegetations respond very differently to fire at any given season or condition. In part the response reflects species differences, but the species in common also respond uniquely, indicating that different phenologies result from soil-influenced moisture and nutrition environments (Parker 1987b). The result is that timing for a prescribed burn that would be effective in one stand would be disastrous in the other. While soil type is a demonstrably important influence, so too are other environmental conditions. A large proportion of chaparral plant species depend upon soil seed banks for regeneration (Parker and Kelly, in press). To survive the high soil temperatures during fires, many seeds must be dry, while other seeds require relatively high temperatures to break open their seed coats so that germination is stimulated. Soil moisture conditions vary greatly in prescribed burns and will influence survival of certain species whose seed imbibe water, while reducing germination rates of species whose seed are stimulated by higher temperatures. These types of variation in influence on recovery, and their interaction with other vegetation characteristics will be more fully described with reference to the concept of fire regime.