Short- and long-term natural regeneration after windthrow disturbances in Norway spruce forests in Bulgaria

Abstract

Abstract: Norway spruce forests are among the forests most affected by natural disturbances in Europe. One of the key aspects is the regeneration of the disturbed areas, which is decisive for later forest development. We studied the natural regeneration after two windthrows that occurred 30 (1983) and 50 years ago (1962) in an old-growth forest over 150-year-old in the Parangalitsa Reserve and a recent windthrow (2001) in a 130-year-old single cohort forest in the Bistrishko branishte Reserve in Bulgaria. We set up study plots along transects, counted regeneration and substrates, and analyzed age using tree rings. Post-disturbance regeneration made up 62-81% of all recorded trees and was more important than advance regeneration, but it strongly differed among the windthrows. Our data indicated two discrete peaks of post-disturbance regeneration. The first peak started immediately after the windthrows and was dominated by Norway spruce and rowan, while the second one started about 30 years later and was dominated by spruce. Pioneers such as Populus tremula, Salix caprea and Pinus sylvestris were less prominent than expected, contributing up to 21% of the total regeneration. Despite the fact that the highest density of initial regeneration was found on mounds from uprooted trees, the largest total number in the three studied areas was on intact forest floor, which hosted between 69 and 80% of all regeneration. The importance of coarse woody debris rose significantly two to three decades after the disturbances and was particularly important for the secondary regeneration, which consisted of Picea abies and Abies alba.