Canopy gaps characteristics of pure and mixed stands in the Hyrcanian forests of north Iran.

Canopy gaps play an important role in forest ecology helping to preserve biodiversity, influence nutrient cycles, maintain the complex stand structure, and also they substantially contribute to regeneration of forests. To survey the characteristics of canopy gaps in the Hyrcanian forests of Iran, three main old-growth and intact stands were studied. For each canopy gap, two parameters were measured: the length (L) as the longest distance within the gap and the width (W) as the largest distance perpendicular to the length. The gap maker (DBH≥20 cm) was considered to be a tree from the upper tree layer whose death caused an opening in the canopy. A total of 89 canopy gaps were studied in three old-growth stands (48 gaps in hornbeam-ironwood, 21 in hornbeam, and 19 in beech-hornbeam stands). The given canopy gaps cover 5.4% of the land area with an average of eight gaps per hectare. The mean size of the canopy gaps were 291, 353 and 565m 2 for hornbeam-ironwood, hornbeam, and beech-hornbeam stands, respectively. Frequency distribution of the gap sizes the mentioned three forest stands showed a lognormal distribution. The number of gap makers ranged from one to eight with a median of 4.5. Amongst the studied 261 gap makers in all canopy gaps, 61.2% belonged to hornbeam-ironwood stand. Also, there were canopy gaps formed by one to four gap makers within the three forest types, while gaps formed by five, six and eight gap makers were only observed within hornbeam stand. The results showed that the characteristics of the canopy gaps were different in terms of the composition, complexly of structure, environmental characteristics, size, number and species of gap makers in different forests. As a whole, hornbeam-ironwood stands present in lower altitudes, are more susceptible to wind-throw and create smaller gaps but more gap makers.