Photosynthetic and growth responses of Pinus koraiensis seedlings to canopy openness: Implications for the restoration of mixed-broadleaved Korean pine forests

Abstract

The mixed-broadleaved Korean pine (Pinus koraiensis Sieb. et Zucc.) forest (MBKPF) is a zonal climax ranging from Northeast China to the Far East region of Russia. However, since the 1950s, most of the MBKPFs in Northeast China have become the mixed broadleaved secondary forests due to the heavy forest exploitation by timber harvesting. The natural regeneration of Korean pine is generally poor because of the light limitation in Northeast China. This study was conducted to understand how Korean pine seedlings adapt to different canopy openness (CO) and what would be the minimum CO for its establishment and growth in the natural forest stands. In order to test the hypothesis that Korean pine seedlings have acclimated to low light regimes, the responses of photosynthetic, morphological and growth parameters to light environments were examined in Korean pine seedlings that were field-grown in secondary forests. The results can provide the references for the restoration of MBKPFs in Northeast China. Three-year-old Korean pine seedlings were under-planted in secondary forest stands with three levels of CO at 30%, 15% and 7%, which were created by modifying the branches of the secondary forest stands. After one year of acclimation under the secondary forest stands, the variations in the physiological and morphological characteristics of the needles as well as the growth parameters were measured. The maximum net photosynthesis (Amax), dark respiration (Rd), needle chlorophyll (Chl) contents and total needle nitrogen (N) contents all responded positively to the increased CO. The total non-structural carbohydrate (TNC), Amax, Rd, and N contents in the 7% stand were significantly lower than those in the 15% and 30% stands (p<0.05). The capture efficiency of excitation energy by open Photosystem II (PSII) reaction centres (Fv′/Fm′) in the 7% stand was the lowest and was significantly different from those in the 15% and 30% stands (p<0.05), although no significant differences were found between 15% and 30% CO (p>0.05). These results indicated that 7% CO seedlings were experiencing some degree of low light stress. The leaf mass area (LMA) and height growth decreased as the CO decreased from 30% to 15% to 7%, while the lateral shoot growth increased. Therefore, needle morphology, tree height and crown growth of Korean pine seedlings displayed a large plasticity in response to variations in light intensities. These findings suggest that Korean pine seedlings can strongly acclimate in the forest canopy by adjusting physiological traits that are associated with light energy distribution in the PSII and by adjusting the morphological features of their needles. These results suggest that the minimum canopy openness required for the survival of P. koraiensis seedlings (4 years old) grown under a canopy should be more than 7%.