The utility of Trillium and Maianthemum as phyto-indicators of deer impact in northwestern Pennsylvania, USA

Abstract

Trillium spp. and Maianthemum canadense Desf. are preferred deer forage throughout their range and have potential as indicators of deer impact throughout the eastern United States. This paper evaluates the use of these plants as indicators at a landscape level within the Kinzua Quality Deer Cooperative (KQDC, ∼30,000 ha) in McKean County, Pennsylvania where a high level of deer impact has been sustained over the last 60 years. We hypothesized that deer impacts to Maianthemum and Trillium spp. populations across the KQDC landscape would resemble those reported at much smaller scales in previous studies and would indicate spatial variation in deer impact within the KQDC area. We found that most of the Trillium and Maianthemum characteristics measured in the KQDC were below the “healthy” standards established in the literature and comparable to characteristics associated with high deer impacts: the percentage of Trillium flowering in the KQDC was 9.1%, compared to the standard of 21–34%; the average Trillium heights in 27% of the KQDC plots were below the 12–14 cm recommended for healthy Trillium populations; Maianthemum mean leaf length and percent flowering were 39 mm and 0.3%, compared to standards of 42 mm and 0.5% for plants in browse accessible areas and 55 mm and 20% flowering for plants in refugia. Despite these high levels of impact, no significant correlations were found between the indicator variables and a direct measure of deer impact, a browse index based on browsing measurements of preferred woody plant species. Based on these results we conclude that these two species do not track spatial variation in deer impact across the KQDC. Relationships between deer impact and indicator characteristics could be clouded by factors such as environmental variables not measured in our study and legacy effects of high deer abundance. Both species will bear further investigation in a long-term monitoring project within the KQDC and could serve as a measurement of recovery in areas that have experienced high deer impact when deer populations are lowered and monitoring sites are analyzed through time.