Old-growth characteristics 20 years after thinning and repeated fertilization of lodgepole pine forest: Tree growth, structural attributes, and red-backed voles

- Thinning and fertilization can accelerate future stand growth and structure.
- Heavily thinned (≤1000 stems/ha) and fertilized stands yield several old-growth attributes.
- Structural developments achieved in lodgepole pine stands at 20 years post-treatment.
- Red-backed voles use young stands with appropriate old-growth structures.

Restoration of old forests by developing late seral structural attributes in young forests may utilize pre-commercial thinning (PCT) and fertilization to accelerate stand growth and development. These silvicultural interventions are very likely required to develop complex stand structure in young forests, both vertically and horizontally, and late-seral structural features over decades, rather than centuries. We evaluated hypotheses (H) that large-scale PCT of young lodgepole pine (Pinus contorta) stands to a wide range of densities, and repeated fertilization, at 20-years after initiation of treatments would enhance (H1) size (diameter and height growth), structural features, and merchantable volume of crop trees; (H2) development of old-growth structural attributes; and (H3) abundance, reproduction, and survival of the southern red-backed vole (Myodes gapperi). Study areas were located near Summerland and Kelowna in south-central British Columbia, Canada. Each study area had ten treatments: four pairs of stands thinned to densities of ∼250 (very low), ∼500 (low), ∼1000 (medium), and ∼2000 (high) stems/ha with one stand of each pair fertilized five times at 2-year intervals, an unthinned stand, and an old-growth stand.Mean 20-year diameter of lodgepole pine crop trees was similar (19-20 cm) in the heavily thinned (≤1000 stems/ha) fertilized stands with that (19-22 cm) in the old-growth stands. Mean height of all managed stands (9-12 m) was lower than that (18-20 m) of the old-growth stands. Mean crown volume of crop trees was highest in the very low-, low, and fertilized medium-density stands (3.5-5.9 times higher, on average) than those in the unthinned and old-growth stands. Large tree diameters and crowns occurred in the heavily thinned and fertilized stands, but mean stand merchantable volume was much reduced, primarily because of the lower densities of trees, and hence provided partial support for H1. Mean species diversity of conifers was highest in the very low-density stands and old-growth stands. Mean structural diversity of conifers was generally similar among stands. Old-growth structural attributes: large dominant trees with substantial crowns, multi-layered canopies of conifers, some canopy openings, and understory abundance and diversity of herbs and shrubs appeared in the heavily thinned and fertilized stands, and hence partially supported H2. M. gapperi populations in 5 of 8 managed stands at Summerland were at the same or higher abundance (particularly the very low-density) than in the old-growth stand, with resident breeding animals and recruitment of both adult and juvenile voles, and hence H3 seemed to be supported. Heavy thinning to ≤1000 stems/ha combined with fertilization seemed necessary to achieve these structural developments in our 33-year-old second-growth stands.