| Summary: | Forest managers need site productivity estimates for currently grown tree species as well as those that might be considered for future management on a given site. Existing models are generally for pure stands and don't factor in the effects of climate change on growth. Therefore, site index (SI) conversion equations were developed for jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana Mill. B.S.P.) trees grown in mixed natural stands. For this, 180 trees (90 jack pine and black spruce each) were sampled from 30 even-aged natural origin mixed stands (sites) (3 per species per site) across Ontario, Canada. Stem analysis data collected from these trees were used to develop stand height growth models that incorporate climate variables during growth for each species. A mixed effects modeling approach was used to fit the models. The SI conversion equations were then developed by regressing SI of one species against that of the other species and climate variables. To evaluate climate effects on site productivity, stand heights were predicted for 4 geographic areas of Ontario for the period 2021 to 2080 under 3 emissions scenarios reflecting different amounts of heat at the end of the century (i.e., 2.6, 4.5, and 8.5 W m −2) using the stand height growth models developed here. Climate effects were more pronounced for black spruce than for jack pine and varied among locations. The models developed here can be used to estimate stand height and SI of jack pine and black spruce trees grown in natural origin mixed stands in a changing climate. Using SI conversion equations, the SI of one species can be estimated in terms of the SI of the other species and climate variables. In the absence of climate data, models can be used to estimate SI of both species. © 2021
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