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

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. Postdisturbance 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.

Preliminary assessment of effect of disturbance on natural regeneration in gaps of different sizes

“…However, seed germination and seedling establishment were slow due to the improved levels of light conditions in small size gaps. Our finding disagrees with a comment of Tsvetanov et al (2018) that spruce regeneration might preserve their vitality under forest canopy for several years and then grow vigorously when light conditions improve after forest disturbance. Moving on, natural regeneration of larch species was statistically different from spruce in big size gaps across the studied two consecutive growing seasons after gap disturbance respectively.…”

“…So, creating artificial gaps based on stand conditions has emerged as a widespread phenomenon to achieving forest regeneration and succession, including other management objectives (Kollár 2017;Lu et al 2018a). Disturbances that open gaps provide specific microenvironmental conditions in light, soil moisture, and temperature regimes, which often initiate regeneration processes and development (Yamamoto 2000;Tsvetanov et al 2018). Seedling establishment, survival, and growth exclusively depend on light (Kyereh et al 1999;Diaci et al 2008).…”

“…This fine-scale heterogeneity at the forest floor create favourable microsites for seed germination and seedling establishment (Kuuluvainen 1994) and plays an indispensable role in forest dynamics by maintaining species structure and composition following gap disturbance. This serves as a useful predicting tool in assessing the effect of different disturbances intensities in different forest ecosystems Tsvetanov et al 2018). Among lists of gap's physical characteristics; percentage gap area, gap shape, and mode of gapmakers, etc.…”

Recovery and resilience of European temperate forests after large and severe disturbances

“…Although salvage logging is often carried out for economic reasons, it is known to cause direct harm to regeneration and can thus negatively affect forest recovery (Bowd et al, 2021;Royo et al, 2016;Taeroe et al, 2019). Furthermore, the deadwood being removed could otherwise act as a regeneration substrate, especially for spruce, when sufficiently decayed (Bače et al, 2012;Priewasser et al, 2013;Tsvetanov et al, 2018), a microsite that protects seedlings from climate stress (Marangon et al, 2022), or as an obstacle for herbivores (Hagge et al, 2019;Kramer et al, 2014;Marangon et al, 2022). However, a meta-analysis by Leverkus et al (2021) found that post-disturbance salvage logging does not generally hinder forest regeneration, and in some cases, the soil perturbation caused by logging can promote the regeneration of pioneer species by reducing competition with ground vegetation (Ilisson et al, 2007).…”

Young Silver Birch Grows Faster and Allocates Higher Portion of Biomass into Stem Than Norway Spruce, a Case Study from a Post-Disturbance Forest

“…At the same time, the species has long been economically extremely important [7]. Since the regeneration potential of this species is usually high (thanks to seed-producing survivors after disturbance and well-dispersing and germinating seeds), spruce trees often dominate in species composition of young post-disturbance forests [8]. At the same time, in disturbed forest areas, seeds of different species-especially light-demanding, are granted an opportunity to germinate and cover the open soil surface [9].…”