Forest canopy reduction stimulates xylem production and lowers carbon concentration in fine roots of European beech

Forest canopy reduction is well known to affect the coarse and fine root biomass production. Seasonality, to the same extent, is acknowledged to vary the non-structural carbon content. However, the fine root response in terms of carbon concentration to both canopy cover and seasonal change remains unclear. To this aim, morphological and anatomical traits like xylem percentage, carbon concentration and starch content were measured in mid-summer and early-fall on fine roots of three beech stands differing in canopy cover and basal area. The results highlighted a significant effect of canopy cover on the xylem area percentage throughout the <2 mm diameter fine root population, as the lower the canopy cover is, the larger the xylem area is. Moreover, an inverse relationship occurred between carbon concentration and xylem area percentage, highlighting the key role played by this anatomical trait. In order of magnitude, the significant carbon concentration decrease observed 5 years after felling was 15 kg ha−1 for a mean fine root biomass of 200 g m−2. For a given xylem percentage, starch concentration seasonal change partially explained the carbon concentration decrease with the incipient dormancy. Root tissue density significantly decreased with soil depth in mature and 15-yr-old conversion stands, whereas it did not in recently cut (5-yr-old) conversion stand. Outcomes are that canopy closure in the mature stand, which increases the fine root standing crop, produced a higher total carbon input into the soil. Moreover, fifteen years since felling appeared sufficient for detecting a clear trend in the recovery of fine root biomass to pre-thinning levels.