PARENT SESSION
Organized Oral Session 51: Predictability of vegetation dynamics: From quadrats to landscapes, from years to decades
Organizer(s): MR Fulton and PA Harcombe
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 511 C, Level 5, Palais des congrès de Montréal

Old-growth forests are rarely equilibrial, but they are sometimes predictable.

Woods, Kerry¹, ¹ Bennington College, Bennington, VT, USA

ABSTRACT- Is predictability related to successional status in forests? In early-mid succession, competitive relationships may drive predictable compositional changes at decadal scales. In late succession, less pronounced competitive hierarchies may make dynamics less predictable. If old-growth stands approach compositional equilibrium (a 'climax-like' state), change may consist primarily of fluctuations around that equilibrium and be generally unpredictable. I use multiple measurements of over 250 long-term (40-70 yr) permanent plots in old-growth forests in northern MI to assess dynamics and predictability. Sites vary in soil chemistry and range from dry-mesic to permanently saturated peaty soils. Canopy composition ranges from hemlock-northern hardwood to swamp forests dominated by Fraxinus nigra, Thuja occidentalis. A locally severe wind-disturbance in 2002 allows comparison of predictability under small-gap disturbance regime with disturbance with larger patch size. In upland stands, dynamics under small-gap disturbance regime are generally predictable although only locally convergent in composition space. This appears to be due either to convergence towards dominance by shade-tolerant Acer saccharum, Fagus grandifolia or Tsuga canadensis, or to consistent decline in less-tolerant Betula alleghaniensis. However, stands of high equitability and stands already strongly dominated by Acer saccharum follow less predictable trajectories. Rare, intermediate wind-disturbance may prevent compositional convergence, but disturbance patterns are also relatively predictable. Swamp forests show more heterogeneous dynamics and lower predictability; plots of similar initial composition can have divergent and varying trajectories. However, declines in Picea mariana and Larix laricina generate predictable compositional change where these species were originally important. Swamp forests have more open canopy and higher mortality than upland forests, and are more diverse. These differences may introduce more dependence on historically contingent factors, and reduce predictability. Predictability in these stands may be highest when initial composition includes significant representation of less shade-tolerant species.

Key words: northern hardwood, succession, forest dynamics, old-growth