Windthrow of Forest Trees and Its Effect on Tree Regeneration and Diversity of Understory in the Hyrcanian Temperate Upland Forests of Iran

Windthrow is a major natural disturbance shaping canopy dynamics in temperate forests, yet high-elevation (upland) Hyrcanian forests remain poorly studied, particularly with respect to regeneration responses and topographic controls. This study examined windfall effects in Dashte Daman forests, northern Iran (1027–1629 m elevation). First, 90 systematically distributed 50 × 50 m sample plots were established across a 300 × 300 m grid on a stratified map including affected and unaffected areas. For each fallen tree (broken or uprooted), diameter at breast height, trunk height, and crown length were measured. Understory vegetation composition, including herbaceous cover and tree regeneration, was also recorded. The influence of topographic variables (slope, elevation, aspect) and forest type on windfall abundance was analyzed, alongside correlations with slope, elevation, distance to nearest road, stand density, and canopy cover. Comparisons with undisturbed control areas were made to assess regeneration patterns. Results indicated that oriental beech (Fagus orientalis) was the most windfall species, followed by common hornbeam (Carpinus betulus), whereas chestnut-leaved oak (Quercus castaneifolia) and common ash (Fraxinus excelsior) were rare. Windfall frequency averaged eight fallen and 4.7 uprooted (a total of 12.7) trees per hectare, with corresponding volumes of 31.6 m3/ha and 18.56 m3/ha, respectively. Considering the abundance of wind-fallen trees, windfall frequency was highest on south-, southwest-, and west-aspects, at slopes of 20%–40%, and elevations of 1000–1200 m. Valleys exhibited greater tree breakage, whereas uprooting dominated hillsides. Pure oriental beech stands had significantly higher windfall rates than mixed beech-hornbeam or oak-hornbeam forests. Negative correlations were observed between windfall frequency and distance to road (r = −.34) and elevation (r = −.53), underscoring the role of roads in facilitating wind penetration into the forest, while also demonstrating that higher elevations experience reduced windthrow, likely due to the more wind-firm structure of trees, characterized by lower height and a higher stability coefficient. Regeneration of beech was present in all sample plots in the windthrow area, whereas hornbeam regeneration occurred in 27.4% of plots. In the control area, regeneration rates were 94% for beech and 23% for hornbeam. These results indicate that windthrow did not inhibit the regeneration of these climax species. However, the presence of herbaceous plants, which may interfere with regeneration in 65% of plots, underscores the need for future monitoring to ensure successful seedling establishment in disturbed areas. Given the susceptibility of pure oriental beech stands to windfall, promoting mixed-species composition through targeted management is recommended.