Persistent Suppression in Dwarf Birch after Release from Heavy Summer Browsing by Caribou

Abstract

The Rivière George Caribou Herd (RGCH), in northern Québec, erupted during a few decades to culminate at approximately 800,000 animals by 1990. The RGCH appeared regulated by competition for summer forage. Repeated browsing and trampling reduced summer range productivity to 50% compared to ungrazed areas, lichens and dwarf birch having been mostly responsible for this reduction. A reconnaissance flight in 1991 and intensive field work the following two summers suggested that caribou use of dwarf birch leaves was light following the period of peak numbers. We hypothesized that birch stands would show signs of rapid and vigorous recovery in 1993-1994. We surveyed five heavily browsed birch stands, and two inaccessible, lightly browsed ones. Although utilization of birch leaves by caribou ranged between 0 and 6% during the 1994 growing season, heavily browsed stands exhibited no signs of rapid and strong recovery. Young plants did not abound in heavily browsed stands compared to lightly browsed ones, neither was wood accumulation faster in the former than in the latter. Stem density and height were similar in heavily browsed and lightly browsed stands. However, the leaf/wood ratio and the percent dry weight of leaves and wood were higher in lightly browsed than in heavily browsed stands, which resulted in approximately twice as high leaf biomass in lightly browsed than in heavily browsed stands. We hypothesized that previous intense caribou browsing caused a persistent suppression of dwarf birches, and that stems could not accumulate sufficient reserves during a growing season to deploy more leaves in the next spring in order to return the leaf/wood ratio to a normal value. We discuss the consequences of such a reaction of dwarf birch to herbivory on the demography of the RGCH.