Gap Size in Hyrcanian Forest Affects the Lignin and N Concentrations of the Oriental Beech (Fagus orientalis Lipsky) Fine Roots but Does Not Change Their Morphological Traits in the Medium Term

Research Highlights: Fine roots play an important role in plant growth as well as in carbon
(C) and nutrient cycling in terrestrial ecosystems. Gaining a wider knowledge of their dynamics
under forest gap opening would improve our understanding of soil carbon input and belowground
carbon stock accumulation. Single-tree selection is increasingly recognized as an alternative
regime of selection cutting sustaining biodiversity and carbon stock, along with timber production,
among ecosystem functions. However, the fine root response in terms of morphological and chemical
composition to the resulting harvest-created gaps remains unclear. Background and Objectives:
This paper investigates the effect in the medium term (i.e., 6 years after logging) of differently sized
harvest-created gaps on fine root dynamics and chemical composition. Materials and Methods: A total
of 15 differently sized gaps (86.05–350.7 m2) and the adjacent 20 m distant closed canopies (control)
were selected in a temperate Fagus orientalis forest (Hyrcanian region, Iran). Eight soil cores were
collected at the cardinal points of the gap edge, including four facing the gap area—the same at
the adjacent intact forest. Results: For the selected edge trees, the different size of gaps, the core
position, and the tree orientation did not affect the investigated morphological traits, except for the
slightly higher specific root length (SRL) for the larger fine root fraction (1–2 mm) in the side facing
the gap area. Differently, the investigated chemical traits such as N concentration and cellulose:lignin
ratio significantly increased with increasing gap size, the opposite for C:N ratio and lignin. Moreover,
N concentration and C:N significantly decreased and increased with the fine root diameter,
respectively. Conclusions: This work highlighted that, in the medium term and within the adopted
size range, artificial gap opening derived from single-tree selection practice affected the chemistry
rather than the biomass and morphology of gap-facing fine roots of edge trees. The medium term of
six years after gap creation might have been long enough for the recovery of the fine root standing
biomass to the pre-harvest condition, particularly near the stem of edge trees. A clear size threshold
did not come out; nevertheless, 300 m2 may be considered a possible cut-off determining a marked
change in the responses of fine roots.