ICE STORMS, TREE THROW AND HILLSLOPE SEDIMENT TRANSPORT ON MT. TOBY, WESTERN MASSACHUSETTS

In December 2008 a major ice storm occurred in New England and upstate New York resulting in widespread tree and power line damage. On Mt. Toby, near Sunderland, MA, we document that 694 trees fell over and uprooted within the 108 hectare (1.08 km²) Roaring Brook watershed as a result of this storm. In many cases, clusters of trees appear to have come down side by side or in a cascading fashion. We estimate that ~1300 m³ of root material, unconsolidated sediment, and fractured bedrock was displaced. Conifers (mainly Hemlock trees) typically displaced more sediment than deciduous trees due to their larger root wad diameters and higher root wad diameter-to-thickness ratios. Areas with mature hemlock dominated forest type and steep slopes were more susceptible to uprooting and tree throw, with clusters of tree throw as dense as 125 per hectare. Uprooted trees predominantly fell downhill, indicating that ice storms promote efficient downslope hillslope sediment transport; this contrasts with wind driven tree throw that transports sediment in the dominant wind direction rather than purely downslope. Using published methods of determining rates of tree throw sediment transport, we calculate an ice storm transport rate of 2-5 x 10^-5 m³ m^-1 a^-1 averaged over the entire watershed. This range reflects variation in the recurrence rate for ice storms of this magnitude and the percentage of root wad material that is sediment, rather than degradable root material. Isolated areas with a higher density of tree throw exhibit hillslope sediment transport rates that are much higher, ~4-6 x 10^-4 m³ m^-1 a^-1 averaged over 1 or 2 hectares. Some hillslopes within Roaring Brook exhibited no tree throw and were left unaffected by this ice storm, suggesting that the age and susceptibility of the tree population to tree throw is an important constraint on hillslope sediment transport. Pit and mound microtopography, which mark historic tree throw sites, attests to the continual role of this process on hillslopes throughout the watershed. Regional climate gradients and changing climate, determining the size and recurrence of ice storms, as well as land use history and logging practices, controlling the age and types of forest trees, emerge as important factors in determining future sediment transport by tree throw in the steep forested hillslopes of New England.