1 This study compares the structural characteristics of 12 old‐growth and six postfire second‐growth hemlock–northern hardwood stands in north central Adirondack Park, New York, in order to test the null hypothesis that there are no differences in species composition, size structure, age structure and attributes such as dead wood and canopy gaps between old‐growth stands and this type of second‐growth forest. 2 The second‐growth forests of this study regenerated following widespread logging‐related fires in either 1903 or 1908; the old growth and second growth have similar environmental settings. 3 Estimates of stand ages, derived from an increment core of the oldest tree in each stand, range from 88 to 390 years. 4 Structural attributes are related to stand age (i.e. stage of development). In comparison with the second‐growth forests of this study, older stands are characterized as (a) a larger average diameter of canopy trees; (b) a greater basal area of trees; (c) a lower density of canopy trees and of all trees ≥ 10 cm d.b.h.; (d) a higher density of eastern hemlock (Tsuga canadensis (L.) Carrière) trees; (e) a higher density of large trees (≥ 50 cm d.b.h.); (f) larger canopy gaps; and (g) a greater volume of coarse woody debris (both logs ≥ 20 cm d.b.h. and snags ≥ 10 cm d.b.h.). 5 Despite differences between old growth and second growth, especially in species composition, it appears from observations of the 18 stands that second‐growth forests are developing some structural characteristics of old growth. 6 Structural attributes of the old‐growth forests are similar to characteristics of the same forest type in geographically distant areas in eastern USA.

Interactions among multiple causes of ecological perturbation, such as climate change and disturbance, can produce "ecological surprises." Here, we examine whether climate-fire-vegetation interactions can produce ecological changes that differ in direction from those expected from the effects of climate change alone. To do so, we focus on the "Big Woods" of central Minnesota, USA, which was shaped both by climate and fire. The deciduous Big Woods forest replaced regional woodlands and savannas after the severity of regional fire regimes declined at ca. AD 1300. A trend toward wet conditions has long been assumed to explain the forest expansion, but we show that water levels at two lakes within the region (Wolsfeld Lake and Bufflehead Pond) were low when open woodlands were transformed into the Big Woods. Water levels were high instead at ca. 2240-795 BC when regional fire regimes were most severe. Based on the correlation between water levels and fire-regime severity, we infer that prolonged or repeated droughts after ca. AD 1265 reduced the biomass and connectivity of fine fuels (grasses) within the woodlands. As a result, regional fire severity declined and allowed tree populations to expand. Tree-ring data from the region show a peak in the recruitment of key Big Woods tree species during the AD 1930s drought and suggest that low regional moisture balance need not have been a limiting factor for forest expansion. The regional history, thus, demonstrates the types of counterintuitive ecosystem changes that may arise as climate changes in the future.

Old-growth forests often have complex, uneven age structures reflecting both the long time elapsed since a major disturbance and the periodic formation of small canopy gaps. I established 12 plots of 0.1 ha in four areas of old growth to describe the stand-scale disturbance regime of forests dominated by eastern hemlock (Tsuga canadensis (L.) Carrière) in northern Adirondack Park, N.Y., U.S.A. I analyzed radial-increment patterns of cores from all canopy trees (398 trees in total) on each plot to determine the date of accession to canopy for each tree. Major growth releases indicated disturbance events that resulted in either gap origin (16% of events) or release from suppression (82% of events). The average decadal rate of disturbance for all plots and decades of the 130-year period from 1850 to 1979 is 4.8–5.4% of current exposed crown area. The average canopy-tree residence time is 184–211 years. The stand-scale disturbance regimes in these Adirondack forests are similar to those of hemlock–hardwood forests in Wisconsin, Michigan, Pennsylvania, and New York. These hemlock-dominated old-growth stands appear to be in quasi-equilibrium when viewed together over 13 decades.

Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression , 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index , 21), versus only 5 of 42 (12%) that established in wet years (PDSI . 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x 2 5 10.738, df 5 1, p-value 5 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.

Spruce budworm (Choristoneura fumiferana) is a native insect that defoliates needleleaf trees, especially balsam fir (Abies balsamea) and spruces (Picea spp.), in northern North America. Spruce budworm can defoliate millions of hectares of forest during an infestation, depressing regional economies that depend on the timber industry. Ecosystems, though, can benefit from spruce budworm because outbreaks rejuvenate the forest, maintaining optimal levels of primary production, and thereby carbon sequestration. Although many ecologists, entomologists, geographers, and resource managers have studied the effects of spruce budworm on spruce–fir forests throughout the region, no single explanation of what causes the number of insects in a forest to rise and fall is universally accepted. Spruce budworm populations can reach ‘outbreak’ levels, or densities high enough to defoliate and kill balsam fir and spruce on a landscape scale, on average every 30–40 years. We review the biology of spruce budworm, the processes that scientists follow to reconstruct spruce budworm outbreaks, the leading hypothesis to explain population dynamics and outbreak events, and the complexity of forecasting possible future trends of populations and distributions of spruce budworm. Reconstructions of past outbreaks help us understand their severity, frequency, and spatial synchrony, which might be linked in complex ways to climate, forest, and stand characteristics. Future spruce budworm dynamics are difficult to predict because the insect is part of a complex food web. In the coming decades, spruce budworm probably will survive climate change because it is adapted to a wide range of temperatures and precipitation amounts.