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Forest health monitoring on the St. Croix National Scenic Riverway: Short-term change and longer-term projections1

Sanders, Suzanne, Kirschbaum, Jessica
The journal of the Torrey Botanical Society 2015 v.142 no.4 pp. 271-282
Abies balsamea, Acer rubrum, Acer saccharinum, Acer saccharum subsp. saccharum, Agrilus planipennis, Carya, Fraxinus nigra, Pinus, Quercus rubra, basal area, climate change, coniferous forests, evapotranspiration, fire suppression, forest health, habitats, hardwood, hardwood forests, highlands, juveniles, lowland forests, managers, monitoring, mussels, riparian forests, saturated conditions, seedbed preparation, soil, trees, wind speed
Long-term forest health monitoring programs are a valuable means of assessing the impacts of drivers and stressors and making projections about future structure and composition. A comprehensive, long-term forest monitoring program was established at the St. Croix National Scenic Riverway in 2007, and monitoring plots were revisited in 2013. Collectively, these plots spanned six forest types. Sugar maple forests, a climax type in the region, exhibited little change in the six-year interval between sampling periods. Substantial regeneration of Acer saccharum Marsh. and associated species suggests little change is expected in this type in the near future. In upland mixed hardwood-conifer forests, encroachment of Acer rubrum L. and Abies balsamea (L.) Mill. in the sapling and small tree layers was indicative of fire suppression and mesophication. Fire suppression was also evident in mixed conifer-red oak forests as plots in this forest type showed decreasing importance of Pinus spp. with increasing density and basal area of Quercus rubra L. These forests are in more well-drained soils than those of upland mixed hardwood/conifer forests; Q. rubra will likely become increasingly important in the coming decades. Mixed oak-aspen forests were in an earlier successional sere than the other five forest types, due to a high wind event in 2011; the high density of aspen observed here reflected clonal shoot growth in response to increased light availability. Both black ash-mixed hardwood forests and silver maple lowland forests supported a substantial amount of Fraxinus nigra Marsh., a species tolerant of saturated conditions. Major recruitment of several species occurred in black ash-mixed hardwood forests in the previous five to ten years and is likely a result of infrequent flooding and relatively shorter durations of inundation. The impending arrival of the emerald ash borer (Agrilis planipennis Fairmaire) poses one of the greatest threats in the St. Croix National Scenic Riverway, not only for the forests, but for the entire aquatic-terrestrial interface. As Fraxinus spp. trees begin to die, reduced evapotranspiration and increased flow velocity and water yield may result. Such changes may lead to increased sedimentation as well as decreased habitat quantity and quality for mussels and juvenile fish. Park managers should continue monitoring flow to help understand anticipated changes in riparian forests. In conifer forests, park staff should consider employing surrogate fire measures such as selective thinning and seedbed preparation, which can alter the successional pathways toward those of historic disturbance regimes. This strategy may facilitate northward migration of Quercus spp., Carya spp., and Pinus spp., as is anticipated as climate change continues.