Effect of canopy openness and canopy type on the natural regeneration of Abies numidica in a mixed forest of the Babor Massif, Algeria

Natural regeneration is a key process for the persistence and long-term conservation of the endangered Numidian fir (Abies numidica de Lannoy), a relict endemic conifer restricted to the Babor-Tababort massif in northern Algeria. Light availability is a fundamental factor influencing juvenile establishment and survival across forest stand types, while canopy structure strongly regulates the amount and spatial distribution of light reaching the understory. Yet, their influence on the regeneration of A. numidica remains poorly documented. This study aimed to evaluate the effects of canopy characteristics and light availability on the natural regeneration of A. numidica using hemispherical photography acquired with a smartphone-based imaging approach. The study was conducted in the mixed Numidian fir forests of Babor-Tababort National Park, Algeria. Hemispherical photographs were collected at 30 sampling sites distributed within the mixed forest according to the dominant canopy type (A. numidica, C. atlantica, and Q. canariensis-P. tremula mixed canopy). At each site, one hemispherical photograph was taken at the sampling point, and the density of A. numidica juveniles was recorded within a 100 m2 sampling area around the corresponding point. The 30 sites were selected to represent the range of canopy conditions encountered in the study area, then we processed in R using the hemispheR package to derive canopy structural attributes, including Gap Fraction and Diffuse Non-Interceptance (DIFN). The density of A. numidica juveniles was recorded around each sampling point. Gaussian regression models were then applied to investigate the relationship between regeneration density, light availability, and canopy type. The results revealed a non-linear response of regeneration to light availability. Juvenile density was highest under intermediate canopy openings, indicating the existence of optimal light conditions for establishment and early growth. Very dense canopies limited regeneration by reducing light penetration, whereas large canopy openings were associated with lower juvenile abundance. Regeneration also differed among canopy types, being generally most abundant under mixed (Quercus canariensis and Populus tremula) canopies, intermediate beneath A. numidica canopies, and lowest under Cedrus atlantica canopies. These findings demonstrate that moderate canopy openings and heterogeneous stand conditions promote the regeneration of A. numidica. They also highlight the usefulness of smartphone-based hemispherical photography as a cost-effective tool for assessing canopy structure in montane forests. The study provides valuable insights for the conservation and sustainable management of this endangered endemic conifer in the Babor massif.