Gap Characteristics and Disturbance Regime in an Intact Hyrcanian Oriental Beech Forest, Iran

Disturbance regimes and gap characteristics of the forest stands are the conceptual foundations for the core of silviculture. The gap characteristics and disturbance regime were investigated in an intact old-growth oriental beech stand in a forest reserve of Hyrcanian region, north of Iran. Seventy six canopy gaps were sampled at 5 transects 625 to 2000 m. The gap area was estimated using two field methods of measurement: elliptical (ELLIP) and non-convex polygonal (NOPO). The mean canopy gap fraction was 8.4% and 10.8% based on NOPO and ELLIP, respectively. The mean expanded gap fraction calculated by ELLIP (22.0%) was greater than that by NOPO (18.1%). Mean canopy gap size was 159 m(2) and 208.7 m(2) for NOPO and ELLIP methods, respectively. Mean expanded gap size was 515.7 m(2) and 626 m(2) for NOPO and ELLIP methods, respectively. Most of canopy gaps (68.4%) were smaller than 200 m(2), whereas only 1.4% was bigger than 500 m(2). Nearly 90% of number and area of whole expanded gaps belonged to classes of 200-1000 m(2). As canopy gap size increased the gap area-perimeter relationship was obviously differed from circular shape, indicating that gap geometry became more complex with gap size. The expanded gaps also followed this pattern, but in expanded gaps (< 450 m(2)) complexity was less than that in canopy gaps. The maximum of gap-makers was 6 trees per gap but 53% of gaps were formed by the death of two trees or more. Near 43% of gaps were expanded after initial gap formation, due to recurrence of disturbance events. The species composition of gap-makers was similar to those of gap border and forest canopy. We could not identify the mode of mortality of 12.3% gap-makers; however, 60.4% and 19.3% of them were snapped-alive and uprooted, respectively. Exogenous agents such as chronic winds could have a prominent role in initial formation and expansion of gaps.