Old growth forests: A blueprint for restoring mycorrhizal fungi in second-growth and thinned coast redwood (Sequoia sempervirens) stands

Globally, old-growth forests are a critical benchmark for restoring second-growth. In Northern California, the Redwood National and State Parks protects nearly 50 % of the remaining old-growth range of coast redwoods (Sequoia sempervirens). There, Pinaceae trees were aerially seeded in second-growth stands. While the deleterious effects of this approach in redwoods are well documented, the effects on mycorrhizal fungi have yet to be considered. This study aimed to determine how arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (EMF) are affected by the encroachment and subsequent thinning of Pinaceae. We expected the exclusive reliance of Sitka spruce (Picea sitchensis) and Douglas-fir (Pseudotsuga menziesii) on EMF to reduce the richness and relative abundance of AMF in second-growth stands compared to old-growth. Indeed, we found this to be the case (P < 0.05; Kruskal-Wallis). While we predicted the richness and relative abundance of EMF to be most prevalent in second-growth, there were no significant differences between old- and second-growth stands. Furthermore, we expected the soils of thinned forests to be intermediate, where the richness and relative abundance of AMF and EMF would fall between old- and second-growth. Instead, the richness and relative abundance of AMF in thinned forests was significantly lower compared to old-growth, and closer to that of unthinned second-growth stands. We only found marginal differences in the composition of the broader fungal community across old-growth, second-growth, and thinned redwood stands. Future efforts should aim to restore AMF in second-growth and thinned forests to resemble below-ground conditions of old-growth.