A forest gap is not forever: Towards an objective standard to determine when a gap is considered closed in temperate forests

• Published in: Agricultural and forest meteorology Vol. 340; p. 109598
• Main Authors: Lu, Deliang, Zhu, Jiaojun, Zhang, Guangqi, Fang, Shuai, Sun, Yirong, Zhu, Chunyu, Zhang, Jinxin, Wang, G.Geoff
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2023
• Subjects: Crown base height; Gap filling; Gap-closure standard; Natural regeneration; Tree diversity; Gap filling; Gap-closure standard; Crown base height; Natural regeneration; Tree diversity
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2023.109598

•We propose a gap closure standard based on height and stability of regeneration.•(1) Dominant gap regeneration layer reached canopy base height.•(2) The regeneration layer maintained stably in density, richness, and evenness.•Repeated gap disturbance was needed for successful gap filling in small gaps.•Light-demanding species filled large gaps but were negligible in small gaps. Forest gaps create environmental heterogeneity and drive forest dynamics through gap-phase regeneration. However, it is unclear when a forest gap can be considered closed, limiting a global synthesis of forest succession that integrates gap patterns and dynamics. We created twelve forest gaps (six large gaps, RD/H [ratio of gap diameter to gap border tree height] = 1.3; six small gaps, RD/H = 0.6) and monitored tree regeneration for seventeen years to establish an objective gap-closure standard for regeneration filling in a temperate secondary forest. Our results indicated that a forest gap could be considered closed or entering a gap-closure phase when the dominant gap regeneration layer (i.e., the top 10% gap-fillers) (1) reached the crown base height (8.9 m on average) of canopy trees and (2) entered a period of stability in density, richness, and evenness. Linear mixed-effects models found that these two conditions met each other in the same period (14.5 years) in the large gaps, with a mean density, richness, and evenness of 0.1 (stem m−2), 4.9 (species gap−1), and 0.9, respectively. However, if no repeated gap disturbance occurred, small gaps were closed by lateral extension rather than by height growth. Light-demanding species (e.g., Juglans mandshurica) mainly contributed to the closure of large gaps and played negligible roles in the small gaps, but intermediate or shade-tolerant species (e.g., Acer mono) in the small gaps had potential for successful gap filling following repeated gap disturbance. Our gap-closure standard links gap structure to species composition and provides a reference for large-scale research on gap patterns and dynamics, which helps to understand the ecological process during forest succession. The standard also provides a relatively clear boundary between forest gaps and closed forest stands, which is a premise to develop appropriate forest management and shorten the restoration period of degraded secondary forests.