Long-term thinning alters ponderosa pine reproduction in northern Arizona

Abstract

The future of ponderosa pine (Pinus ponderosa var. scopulorum) forests in the southwestern United States is uncertain because climate-change-induced stresses are expected to increase tree mortality and place greater constraints on regeneration. Silvicultural treatments, which include thinning, are increasingly being used to address forest health concerns by restoring ponderosa pine forests to more open conditions representative of historical forest structure. In light of the greater use of thinning and mounting concerns about the future of the species at the southern edge of its range, further investigations about impacts of thinning on ponderosa pine regeneration and underlying mechanisms are needed. We used a long-term (>50years) experiment in northern Arizona to investigate impacts of repeated stand thinning that maintained different growing stock basal areas (0, 7, 14, 23, 34, 66m2ha−1) on early seedling survival, growth, and microenvironment. Seedling survival for the first two years after germination (2013–2015), which had above-average precipitation, was higher than reported in several earlier studies and ranged between 4 and 21% among all basal areas. Seedling density exhibited a negative quadratic relationship with basal area and was positively associated with litter cover. Growing stock levels that fostered the highest seedling survival and density were those with a low density of overstory trees, low canopy cover, high cone production, coverage of soil by a thin layer of litter, and high soil water content at a depth of 15–30cm. Overstory basal area was positively associated with seedling height but negatively associated with seedling diameter. During this relatively wet period, all basal area treatments supported higher average seedling densities than those previously recommended to produce a multi-aged stand or presettlement structure in the southwestern United States. Our results show that long-term maintenance of low to intermediate basal areas (7–23m2ha−1) by thinning over the last 50years led to a favorable microenvironment for early seedling establishment of ponderosa pine.