The long-term effects of low intensity fires in a mature red pine (Pinus resinosa Ait.) plantation

This study focuses on the long-term effects of one- to two-decade-old prescribed low-intensity fires at varying intervals on the vegetation dynamics of a red pine plantation in northern Lower Michigan. One-burn, two-burns at a five-year interval, four-burns at two-year intervals, and unburned control plots were organized in a randomized complete block design during 1985 to 1991 and measured in the summers of 2001 and 2003. Overall, the effects of fire were apparent two decades after cessation of burning.;The fires had no effect on basal area, volume and mean annual increment (MAI) of the overstory (>10 cm dbh); species richness, diversity and density of the woody understory (>1.4m tall and < 10cm dbh); and percent cover of ground vegetation (< 1.4m tall). However, when compared to unburned controls, burnings decreased overstory tree density. The dominant overstory (>42cm dbh) and small understory (0-2 cm dbh) woody density were higher in burned plots, especially those repeatedly burned; whereas, sub-canopy (10-32 cm dbh) and large understory (6.1 to 10 cm dbh) woody densities were lowest in four-burn treatments. The more frequent the fires, the smaller were the diameters of understory woody plants.;Ground vegetation (<1.4 m tall) species diversity was highest in repeatedly burned plots. Bracken fern, Rubus spp., wild lily-of-the-valley, and red pine seedlings had highest coverage. The consistent increase in importance values of overstory and the understory red pine in burned plots as well as suppression of red maple and other hardwoods indicated a shift in composition towards highly fire-adapted red pine, which in the long run will dominate the future canopy. Red maple, beech and white ash will also persist through new recruits.;The above data were projected 100 years into the future with and without a series of thinnings via the Forest Vegetation Simulator (FVS). Compared to unburned controls the key features of projected burned stands were extremely dense post-fire regeneration, higher inequality in size-classes, higher mortality/self thinning, and a sharper decline in density. At the end of no-management, unburned controls had grown slower and retained the densest understory and lowest overstory density. High red pine sub-canopy density produced by one- and two-burns indicates that low-intensity burns were sufficient to kill competing low vegetation and stimulate red pine reproduction, which grew into the mid-canopy layer of four-burn plots.;Burning, especially if repeated and followed by thinning, slowed down mortality, enhanced recruitment, stimulated growth of larger trees, extended the growth culmination age, and increased MAI and cumulative volume production. Four-burn treatments, particularly after the second thinning, produced the highest cumulative volume and the most uniform stand structure across tree size classes. Thus, frequent burning in mid-rotation, especially if combined with subsequent thinning, could be the basis of long-term sustainability, high productivity, and increases in biodiversity of red pine stands.