Overstory and understory leaf area index as indicators of forest response to ice storm damage

Leaf area index (LAI) was measured with the tracing radiation and architecture of canopies (TRAC) optical instrument in three consecutive summers from 1999 to 2001 in sugar maple forests across eastern Ontario to monitor recovery from ice storm damage suffered in January 1998. The study sites were experimental blocks of the Ontario Ministry of Natural Resources (OMNR) designed for measurement and monitoring of the effects of fertilizer and lime treatments on maple recovery. Understory vegetation survey data, collected in 1999 and again in 2001, were converted to understory gap fraction, and processed in the same manner as the TRAC data to obtain understory LAI. Subtraction of understory LAI from total LAI measured with the TRAC allowed monitoring of productive, overstory trees tapped for syrup production. Annual LAI measurements and LAI change were evaluated in relation to percent crown loss estimates and plot treatments. Understory, overstory and total LAI increased from year-to-year in most plots. Of the single year measurements, total LAI measured in 1999 was significantly related to damage, while 2000 and 2001 LAI accounted for progressively less of the variation in damage, indicating a change in canopy condition between seasons. Understory LAI increased dramatically in response to overstory crown loss, while overstory increased more in absolute terms, but less in relative terms. Between-season LAI change was more significantly related to damage than any of the annual LAI measurements to which it was compared, indicating that LAI response is a better indicator of damage than single year LAI. LAI change was not, however, significantly related to plot treatment.