PARENT SESSION
Poster Session 2: Forest Ecology
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Tip-up mounds in large gaps play an important role in old-growth forest dynamics.

Brese, Daniel^*,1, Woods, Kerry¹, ¹ Bennington College, Bennington, VT, USA

ABSTRACT- Following a major wind-disturbance (ca. 25% canopy loss, estimated return time 100-300 yr) in 2002 in an old-growth hemlock-hardwood forest in northern Michigan, we assessed understory response to soil disturbance (tip-up mounds) and variation in light conditions. Two years after the disturbance, we compared survival of pre-existing seedlings, establishment of new seedlings, and herbaceous understory composition among several microsite-types: mineral soil of mound-tops; tipped-up forest floor of mound side; pit; pit perimeter; and adjacent, intact forest floor. We measured soil moisture and temperature, canopy openness, and light regime for the same microsites. Intact forest floor supported diverse herbaceous communities with many pteridophytes; there was no compositional response to variation in light levels. Intact forest floor had high seedling cover dominated by pre-existing, shade-tolerant Acer saccharum and Fagus grandifolia; there were very few new germinants. Virtually no plants had established in pits. Tipped-up forest floor typically remained structurally intact, but experienced very high mortality of both tree seedlings and herbaceous species (especially pteridophytes); persistent species were predominantly tuberous/rhizomatous ephemerals and opportunists (e.g., Erythronium, Maianthemum). Both mineral-soil mound ridges and the tipped-up forest floor were drier and hotter than intact forest floor or mound periphery. Nonetheless, mound ridges, particularly those with higher light, had high densities of new seedlings of shade-intolerant woody species (particularly Betula alleghaniensis). Canopy openness and soil moisture on mounds were positively correlated; thus, Betula seedlings were also most abundant in microsites with lowest soil-moisture. This result supports the notion that rare, significant, patchy disturbances are important in shaping canopy structure and maintaining diversity in late-successional forests, and suggests that requirements for Betula establishment are not simply related to moisture availability. Implications for understory communities in general are less clear, but new mounds do not appear to be congenial microsites for establishment or survival of herbaceous species.

Key words: Betula alleghaniensis, patch dynamics, mound-pit, disturbance