Forest Structure and Light Regimes Following Moderate Wind Storms: Implications for Multi-Cohort Management

Abstract

Moderate-severity disturbances appear to be common throughout much of North America, but they have received relatively little detailed study compared to catastrophic disturbances and small gap dynamics. In this study, we examined the immediate impact of moderate-intensity wind storms on stand structure, opening sizes, and light regimes in three hemlock-hardwood forests of northeastern Wisconsin. These were compared to three stands managed by single-tree and group selection, the predominant forest management system for northern hardwoods in the region. Wind storms removed an average of 41% of the stand basal area, compared to 27% removed by uneven-aged harvests, but both disturbances removed trees from a wide range of size classes. The removal of nearly half of the large trees by wind in two old-growth stands caused partial retrogression to mature forest structure, which has been hypothesized to be a major disturbance pathway in the region. Wind storms resulted in residual stand conditions that were much more heterogeneous than in managed stands. Gap sizes ranged from less than 10 m2 up to 5000 m2 in wind-disturbed stands, whereas the largest opening observed in managed stands was only 200 m2. Wind-disturbed stands had, on average, double the available solar radiation at the forest floor compared to managed stands. Solar radiation levels were also more heterogeneous in wind-disturbed stands, with six times more variability at small scales (0.1225 ha) and 15 times more variability at the whole-stand level. Modification of uneven-aged management regimes to include occasional harvests of variable intensity and spatial pattern may help avoid the decline in species diversity that tends to occur after many decades of conventional uneven-aged management. At the same time, a multi-cohort system with these properties would retain a high degree of average crown cover, promote structural heterogeneity typical of old-growth forests, and maintain dominance by late-successional species.

Ecological importance of intermediate windstorms rivals large, infrequent disturbances in the northern Great Lakes

“…In forest ecosystems, LIDs typically damage trees across several tens to hundreds of thousands of hectares or more, inflicting widespread catastrophic damage approaching or exceeding 70% canopy loss (Canham and Loucks 1984, Foster and Boose 1992, Hanson and Lorimer 2007. However, LIDs may also damage a significantly smaller area of trees depending on the location and characteristics of specific disturbances, necessitating explicit descriptions of disturbance size, intensity, and recurrence interval applicable for disparate regions and disturbance types (Foster et al 1998).…”

“…Hence, one might surmise that more frequently occurring intermediate disturbances also play a pivotal role in shaping complex mosaics of trees on forested landscapes (Woods 2004, Busby et al 2009). In fact, intermediate disturbances may damage extensive areas, foster the development of landscape heterogeneity, and exert a strong influence on species composition, structure, demography, and ecosystem processes (Frelich and Lorimer 1991a, Dyer and Baird 1997, Boose et al 2001, White and Jentsch 2001, Woods 2004, Hanson and Lorimer 2007. Compared to LIDs, damage from intermediate disturbances tends to be patchier, less catastrophic, and usually occurs over tens of thousands of hectares or less.…”

“…Compared to LIDs, damage from intermediate disturbances tends to be patchier, less catastrophic, and usually occurs over tens of thousands of hectares or less. Damage typically removes 30-60% of the canopy, but patchy areas of catastrophic damage (.70% canopy loss) are common (Dyer and Baird 1997, Hanson and Lorimer 2007, Fraver et al 2009). Criteria for identifying intermediate disturbances are inevitably subjective, but the critical elements appear to be an event markedly less severe (in terms of storm intensity and area impacted) and more frequent than rare benchmark LIDs, yet strong enough to inflict patchy catastrophic damage and not considered a gap disturbance.…”

Emulating Natural Disturbance Regimes: an Emerging Approach for Sustainable Forest Management

“…The diagonal lines between the rare, large-scale and more frequent, small-scale ovals are a reference for the bounds (longest return interval and smallest scale) of each forest type's natural disturbance regime. The Northeastern hardwood diagram modifies one in Seymour et al (2002), adding a hypothesized intermediate disturbance regime suggested by recent research (Millward and Kraft 2004;Woods 2004;Hanson and Lorimer 2007) some researchers (Hunter 1999;Lindenmayer and Franklin 2002) have suggested using to evaluate management activities. In addition to Seymour et al's (2002) choice of scale and frequency, we suggest a third evaluation criterion, the level of biological legacies left by historic disturbances.…”

“…>10 ha). However, recent studies suggest that intermediate intensity disturbances, such as ice storms and microburst wind events, may be more prevalent than previously recognized (Ziegler 2002;Millward and Kraft 2004;Woods 2004;Hanson and Lorimer 2007). These events tend to produce partial to high canopy mortality across a moderate to large sized area, but they can leave abundant residual live and dead or damaged trees (Keeton unpublished data).…”