Allocation pattern of biomass and productivity in the mixed uneven-aged stands of Masson's pine and hardwood species

One-year old seedlings of six hardwood species, Michelia macclurei, Castanopsis fissa, Castanopsis sclerophylla, Castanopsis kawakamii, Cyclobalanopsis myrsinaefolia and Castanopsis lamontii, were separately planted under the overstory of 25-year-old Masson's pine plantation, and developed into closed mixed stands after 16 years of management. By harvesting and measuring the average sample trees collected from the plots, a group of allometric growth equations were established to estimate the allocation patterns of biomass and net primary productivity in the above six mixed stands and also the pure Masson's pine plantation. The whole-tree biomasses were 216.41 t/hm~2, 260.06 t/hm~2, 221.92 t/hm~2, 221.65 t/hm~2, 246.13 t/hm~2 and 201.04 t/hm~2, respectively, for the mixtures, and 201.04 t/hm~2 for the pure pine stand. The percent allocation to aboveground biomass was in the range of 81.4% to 83.7%, with little difference among the stand types. In the mixtures, the pine trees in the upper canopy accounted for 72.5%85.4% of the total biomass, the lower hardwoods for 27.5%14.6%. For all the stand types, allocation to stems was the greatest, making up 56.4%64.8% of the sum, followed by roots (16.3%18.6%), branches (9.0%16.9%), barks (4.9%7.3%), with foliage the least (1.1%4.3%). For pine trees in the mixtures, average allocation percentages to stems, barks, branches, foliage, and roots were 63.9%, 6.5%, 10.8%, 1.0% and 17.9%, respectively, but for the hardwoods 42.0%, 4.6%, 27.6%, 12.2% and 13.6%, respectively. In the mixtures, the average dead-to-living branch ratio was 0.47, quite higher than the pure stand, suggesting the upward growth by hardwoods in the mixtures accelerated the natural pruning of the forests. The spatial pattern of the biomasses varied greatly between pure and mixed stands, with 67.1% of the total biomass being distributed in the height of 09 meters for the mixtures, and 53.7% for the pure. In addition, the first whorl of living branches and foliage were present at the height of 23 meters above the ground in the mixtures, but 1314 meters in the pure pine stand. In the mixtures, roots of the hardwoods were mostly distributed at the soil depth of 040 cm, occupying 74%99% of the total belowground biomass, but very sparse under 60 cm; pine roots were mainly concentrated at the depth of 020 cm and under 60 cm, making up 26% and 49% of the biomass, respectively. In the mixtures, more fine roots were distributed in the soils under 40 cm, but in the pure stand, more fine roots found in the surface soil (040 cm). The net primary productivity (NPP) was estimated to be between 10.60 and 15.25 t/(hm~2·a) for the mixtures, and 7.34 t/(hm~2·a) for the pure. The NPP for each component of the pine trees varied with the stand types, but decreased in the order: stemfoliagebranchbarkroot. For the hardwoods except Castanopsis fissa, foliage exhibited the greatest NPP, followed by stems, branches, and roots, barks the least. Significant nonlinear relations were found between NPP and two biomass proportion parameters: the photosynthetic organs/aboveground ratio (X_1) and fine roots/belowground ratio (X_2), developed into a predicatable formula: NPP=5.5745+1.1985 X_1+2.6479 X~2_2. In all the studied stands, fine roots (d2 mm) averagely held about 0.2% of the total biomass, but 2.9% of the NPP.