Fire severity and vegetation response in the boreal Swedish forest

Ecology, July, 1996 by Johnny Schimmel, Anders Granstrom

INTRODUCTION

Several characteristics of fire are of potential importance for vegetation, but fire return interval and fire intensity are frequently stated to be the most significant variables (Heinselman 1981, Hobbs and Gimingham 1987, Moreno and Oechel 1991, Payette 1992). Fire return interval determines the time available for successional processes, and is crucial for tree species that take many years to reach reproductive stage or to attain a size and morphology enabling them to survive a fire (Noble and Slatyer 1981, Johnson 1992). Fire intensity, defined as energy output rate per unit length of fire front (Byram 1959), determines the survival of plant parts above ground and is of critical importance, particularly for tree stands. Vegetation structures above ground in the understory and forest floor are usually killed, even in low-intensity fires, due to their low stature and lack of protective structures such as thick bark. A temperature of only [approximately equal to]60 [degrees] C during 1 min is, in most cases, sufficient to coagulate proteins and cause lethal damage to plant tissue (Precht et al. 1973). Survival of understory species during a fire event, therefore, usually depends on meristems (seeds, rhizomes, root corms) in the soil, and should be relatively indifferent to variations in fire intensity, since fire intensity has little direct influence on heat transport into the soil (Alexander 1982, Johnson 1992, Bradstock and Auld 1995).

In boreal forest ecosystems, there is often an accumulation of a distinct organic soil layer, which has particular effects vis-a-vis fire. Firstly, a thick organic soil layer may insulate against heat penetration during fire (Uggla 1957, Mallik and Gimingham 1985, Vasander and Lindholm 1985). Although these studies were done in connection with management fires and under relatively moist soil conditions, it appears that heat is effectively dampened by the organic soil layer. Secondly, the organic soil layer may itself be consumed by fire to a variable degree, with potentially great consequences (Wein 1983).

Rowe (1983) suggested the term fire severity to signify the degree of removal of organic material and soil heating. The term severity implies a rating of the effects of fire rather than a rating of the behavior of the fire itself. It would be preferable if the fire behavior that is important for the belowground part of the ecosystem could be quantified. However, the amount of energy delivered to the soil depends on the duration of burning and the rate of energy output (from flaming and smoldering combustion), and on factors that affect emissivity of the flames (Van Wagner 1972, Drysdale 1985). Further, the penetration of heat and the resulting heat pulse are determined by the heat capacity and thermal conductivity of the soil (Aston and Gill 1976). It is, therefore, difficult to define a single fire behavior variable that would be relevant for the reaction of plant parts below the soil surface, that could be measured readily, and to which particular effects could be related. However, the degree of consumption of the organic soil layer, i.e., depth of burn, could be a good field indicator of belowground biological impact (Van Wagner 1983), provided that it is well correlated with soil heating.

Several authors have recognized that fires in boreal forests can vary considerably in their belowground effects, and that this is important for the colonization process after fire (Kujala 1926, Rowe 1983, Johnson 1992). Nevertheless, only a few studies give quantitative data on these relations and most deal with tree colonization, which in boreal areas nearly always depends on seeds dispersing onto the burnt soil (Zasada et al. 1983, Thomas and Wein 1984, Weber et al. 1987). For plants regenerating either from buried seeds or from soil rhizomes, the depth distribution of buds and seeds in the soil should determine their ability to survive deep-burning fires. Quantitative information on depth distribution of these structures in boreal forest soils is at present available mainly for seeds (Moore and Wein 1977, Granstrom 1982, Petrov 1984). As for the bud bank in the soil, much less is known. McLean (1969) grouped a number of North American boreal plants according to the position of their belowground shoot systems, and Flinn and Wein (1977) measured the maximum depth of rhizomes, but there was no field verification of the response to fire in these investigations.

A different initial response to variations in fire impact on the soil would be expected for plants with different modes of colonization. For sprouters, increasing heat penetration should eliminate progressively more of the bud bank, but for seed bank species, fire may both kill seeds and affect germination of the surviving seeds. For species dispersing seeds onto the ground after fire, the quality of the seed bed will be linked to the amount of residual organic material.

To understand the mechanisms of postfire colonization, both the distribution of regenerative structures and the response to variations in fire behavior need to be quantified. The aim of this study was to analyze the effect of variation in soil heating by fire on plant survival and colonization in typical boreal forest vegetation. We arranged an experimental gradient by adding different amounts of fuel to small plots, where we studied heat distribution during fire, survival of plant parts in the soil, and the regeneration and colonization of vegetation over a 5-yr period after fire. Tree colonization on these plots has been reported elsewhere (Schimmel 1993). We also examined the depth distribution of viable plant rhizomes and seeds in soil samples from burned and unburned parts of a recent wildfire site. At this site, the initial colonization and regrowth were quantified after 1 yr.