Tree planting outcomes after severe wildfire depend on climate, competition, and priority

Abstract

With dramatic increases in both area burned and fire severity in Western North American conifer forests, the demand for postfire replanting greatly exceeds land managers’ capacity. Despite the importance of tree planting for forest recovery in many areas, it remains unclear how environmental variation and planting timing affect tree planting success relative to passive natural tree regeneration, or how to optimize limited planting resources by focusing on the right places at the right time. To address this gap, we surveyed replanting success across five fires in California Sierra Nevada mixed conifer forest that had been partially replanted with conifer species after intense wildfire. We selected these fires to contain substantial variation in environmental conditions (temperature, elevation, other topographic contrasts) and planting timing, while being as consistent as possible in postfire management. At each fire, we surveyed randomly located 400 m2 circular plots in planted and nearby unplanted areas (total 182 plots), counting seedlings by species as well as shrub cover and other environmental variables. Using mixed models to analyze the data, we found that passive natural regeneration is weaker (<50 seedlings/ha) in hotter, drier sites and that active tree planting can provide a boost (up to 200 %) to forest recovery in these sites. We also found that the timing of tree planting matters, but that the importance of timing depends on the level of competition from shrubs. In places where shrub competition is intense, tree planting is much more successful if planting occurs the year immediately following a fire, the soonest that it is usually practical to plant. In contrast, in places where shrub competition is weaker, delaying tree planting until some shrubs establish can facilitate tree seedling survival, perhaps because shrubs provide shelter from harsh conditions. We also found that tree planting was strongly associated with a higher proportion of pine seedlings, although this positive planting effect was weaker when planting happened later after fire, and was cancelled out at higher levels of shrub cover and shrub height.