Tree investigations in the Noatak National Preserve, Alaska, 2011?2022: Old-growth and new forests

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Tree rings in the Noatak National Preserve provide information about the growth of trees at the cold limit of tree survival in northwestern North America. The present study was based on cores and other tree measurements (tree basal area, height, and number per unit area) of white spruce (Picea glauca) and balsam poplar (Populus balsamifera) trees taken from 39 permanent monitoring plots (34 with coreable trees) at three locations in the Preserve. The tree rings widths were measured and then normalized using a 50-year smoothing spline to remove the effects of growth variations through the life cycles of the trees. Old-growth white spruce forests, which here include numerous trees over 200 years old and some that are more than 300 years old, form open stands on well-drained slopes. Stands of younger trees that became established in the 1900s are present near elevational tree-line, and in small groves on tussock tundra. These younger stands are interpreted primarily as the result of forest expansion due to climate warming, though re-establishment of trees after wildfire is also possible in the tussock tundra. On river floodplains and terraces, stands of both white spruce and balsam poplar were also initiated in the 1900s, but here the youth of the trees is probably due to colonization of new areas exposed by river channel migration. Both the old-growth and younger forests showed continuing growth (as expressed by an increase in stand basal area) between our initial visit in 2011 and re-visit in 2021 or 2022, with the greatest increases occurring on floodplains. Tree rings showed much year-to-year variation in width, but the effect of individual cold summers was surprisingly weak. Some of the major global climate perturbations due to volcanic eruptions were visible in the tree ring record, but the resulting ring growth was generally no worse than other bad growth years within a few decades of the volcanic event. Tree ring width was statistically correlated more closely with the average warmth of several preceding growing seasons (as expressed by the annual sum of thaw degree-days) than with the current year?s or the previous year?s warmth alone. This is probably due to the cumulative effect of several years? warmth (or cold) on the conditions in the tree rooting zone, on the amount of foliage available for photosynthesis, and the level of stored reserves in the tree.