Forest adaptation strategies aimed at climate change: Assessing the performance of future climate-adapted tree species in a northern Minnesota pine ecosystem

Climate change is expected to impact the function, health, and productivity of many northern latitude forests, including North American mixed-pine ecosystems. Additionally, forest managers face increasing challenges to sustaining forests in the face of high uncertainty associated with response to climate change. The Adaptive Silviculture for Climate Change (ASCC) project was developed to provide operational-scale research opportunities to assess and demonstrate various adaptation approaches to forest management in regionally important forest types. The ASCC project framework includes three treatments (resistance, resilience, and transition) representing a gradient of silvicultural approaches aimed at climate change. The first of five ASCC installations is located on the Cutfoot Experimental Forest-Chippewa National Forest (CEF), Minnesota USA, in a mixed-species northern pine forest. Using habitat suitability models under projected future climates, as well as expert opinion, we chose eight future climate-adapted species for planting as part of the transition treatment, consisting of four native species to the CEF: eastern white pine (Aims strobus L.), northern red oak (Quercus rubra L.), bur oak (Quercus macroccupa Michx.), and red maple (Ater rubrwn L.); and four novel species to the CEF: white oak (Quercus alba L.), bitternut hickory (Carya cordiformis Wangenh.), black cherry (Prunus serotina Ehrh.), and ponderosa pine (Pinus ponderosa C. Lawson). The seedlings were planted under two canopy conditions, a thinned matrix (14-18 m(2) ha(-1) residual basal area) and 0.2-ha gap openings (through harvesting). We measured how overstory canopy conditions (gap openings versus a thinned matrix) and understory shrub/herbaceous cover affect species performance. We measured 45 plots over the course of three growing seasons (Spring 2016-Fall 2018), taking measurements of seedling basal diameter and survival, as well as estimates of understory vegetation density. Our findings highlight strong variations in species performance across treatment conditions. While overstory canopy condition was found to be a weak predictor of seedling survival and growth, our results show post-treatment colonization of understory shrub/herbaceous cover being a strong predictor of seedling survival (p < 0.001), with 86.67% (+/- 8.04; 1 standard error) of seedlings surviving in the lowest understory cover class, and 74.23% (+/- 3.04) surviving in the highest understory cover class. Seedling growth did not appear affected by understory shrub/herbaceous cover. Additionally, growth and survival varied significantly among native and novel seedlings, with ponderosa pine exhibiting the highest growth rates at 0.31 cm cm(-1) year(-1) (+/- 0.004), while also having the lowest survival of any species at 45.66% (+/- 1.2). Results from this study can be directly used by managers to inform planting decisions and species selections that align with management planning to promote forest health and sustainably in the face of climate change.