Fire suppression has altered composition, structure, fire behavior, and carbon dynamics in dry forests throughout western North America. Thinning and prescribed fire have been widely used in these forests to reduce fuels and restore ecosystem dynamics. Tree regeneration is important to future forest composition and structure, but few studies have examined long-term regeneration responses to fuel reduction treatments. We examined tree regeneration, its heterogeneity, and the influence of planted seedlings prior to, one, four, ten, and fifteen years after prescribed fire and thinning treatments at Teakettle Experimental Forest, a mixed-conifer forest in the southern Sierra Nevada, California, USA. We specifically asked: 1) What combination of treatments promoted regeneration of sugar and Jeffrey pine (Pinus lambertiana, P. jeffreyi) while reducing white fir (Abies concolor) and incense cedar (Calocedrus decurrens)? That have become more abundant with fire exclusion 2) What treatment combinations increased heterogeneity of regeneration? and 3) what role has planted (versus natural regeneration) played in changing the composition, abundance, and heterogeneity of regeneration?
Results/Conclusions
Prescribed burning alone resulted in a small increase in sugar pine after 15 years. Understory thinning (individually or in combination with prescribed burning), resulted in high regeneration density of shade-tolerant species and small increases of more fire resistant pine species. Overstory thinning (individually or in combination with prescribed burning) increased natural regeneration of pine species while also reducing density of shade-tolerant species. No combination of fire and/or thinning increased long-term regeneration heterogeneity, while overstory thinning combined with burning reduced regeneration heterogeneity below pre-treatment levels. Planting seedlings increased pine regeneration, but also reduced regeneration heterogeneity. Our findings suggest no combination of prescribed fire and thinning achieved desired regeneration composition and heterogeneity. Restoration treatments in these forests should to incorporate ecological information about forest regeneration spatial heterogeneity and pre-treatment vegetation to be successful, especially if planting seedlings is to be incorporated into restoration activities.