Over a century of fire exclusion has altered composition, structure, and function across millions of acres of frequent fire forests throughout western North America. Thinning and prescribed fire have been widely applied in these forests to reduce fuels and restore ecosystem patterns and functions. Many studies have focused on the effects of single applications of thinning and prescribed fire, yet few have examined how repeated prescribed burning that approximates historic fire return intervals will effect tree regeneration and understory vegetation. We examined tree regeneration and understory vegetation for over fifteen years after prescribed burning and thinning treatments, and one year after second entry prescribed burning at Teakettle Experimental Forest, a mixed conifer forest in the southern Sierra Nevada, California, USA. We specifically asked: 1) What was the long-term effect of initial prescribed burning and thinning treatments on regeneration and understory vegetation composition and abundance? 2) What was the short-term effect of second entry prescribed burning on regeneration and understory vegetation composition and abundance? and 3) Did second entry prescribed burning alter regeneration and understory composition and abundance in ways the initial entry prescribed burning did not?
Results/Conclusions
Initial prescribed burning alone resulted in small increases in shade-intolerant sugar pine and Jeffrey pine regeneration, initial prescribed burning combined with understory thinning dramatically increased regeneration of shade-tolerant white fir and incense-cedar, while initial prescribed burning combined with overstory thinning increased regeneration of pine species and reduced the density of shade-tolerant species. Second entry prescribed burning had little impact on regeneration composition, but reduced its overall density, with the greatest density reductions occurring in treatments that were previously thinning. Initial prescribed burning in combination with thinning at first promoted herbaceous plant cover, but these effects were short lived. Second entry prescribed burning resulted in increases in herb cover and slight decreases in shrub cover, however these changes were less substantial than those seen after the initial prescribed burns. It is sometimes assumed that restoring the ecological process of fire in frequent fire forests will result in restoration of ecological patterns. However, our findings indicate application of a second entry prescribed burn that closely matched the historic fire return interval did not significantly alter the trajectory of tree regeneration or understory vegetation.