Characteristics Of Biomass And Nutrient Storage In Stands Of Pinus Massoniana Mixed With Broad-leaved Species

Pinus massoniana is one of the major fast-growing timber tree species in southern China. However, long-term management of monoculture plantations was found to cause soil degradation and productivity decline. In the meanwhile, large area of single species stands created an environment easily being infected by Dendrolimus punctatus and attacked by wild fire, which threatened the sustainable forest management. Mixed-species stands are commonly believed to be superior to monocultures in improving the stability, stress resistance, and complexity of the ecosystems, thus the great interest has been caused in the ecology and management of mixed-species stands in recent decades. Biomass, primary productivity, and mineral nutrient status in the four mixed stands and a pure Masson pine plantations were investigated to study the ecosystem responses to the mixed planting practice.The results showed that the total biomass, including arbor layer,undergrowth and forest floor standing crops in the mixed stands were estimated to be 176.52 t/hm~2 for P. massoniana—Liriodendron chinense mixture, 160.77 t/hm~2 for P. massoniana—Dalbergia hupeana mixture, 153.56 t/hm~2 for P. massoniana—Quercus acutissima mixture, 115.31 t/hm~2 for P. massoniana—Liquidambar formosana mixture, but only 110.53 t/hm~2 for the pure P. massoniana stand. As to the vertical stratification of biomass, the order was as arbor layer > forest floor > undergrowth. The biomass allocation for the arbor layer indicated that the stems contributed 62.0%-67.8% of the total biomass; branches, 14.6%-19.1%; roots, 11.6%-19.1%; and foliar, 4.5%-6.0% in both mixed and pure stands. Undergrowths flourished under the pure Pinus massoniana stand with a total biomass of 2.58 t/hm~2, which was the highest among the five sampling stands. However, the amount of forest floor standing crops was lowest in the pure pine stand. For the forest floor material, L layer contributed 55.8% of the forest floor accumulation in the mixed stands, but was 87.2% in the pure stand.Distribution of root biomass of P. massoniana in the pure and mixed stands was rather different. The root biomass of P. massoniana in pure stand was increased with the soil depth. The root biomass of P. massoniana were distributed mostly in the depth of 0-20 cm and below 60 cm in the mixed stands. This kind of structure under the mixed stands was benefit for water and nutrient absorption. The fine root biomass were decreased with the soil depth in the mixed stands. The fine root biomass was distributed almost in the depth of 0-40 cm in the pure stand.The net primary productivity (NPP) in the mixed forests was between 8532.87 and 13654.35 kg/hm~2/a, and 8192.13 kg/hm~2/a in the pure forest. Difference of NPP existed between broadleaved species and masson pine in the mixed forest. The total NPP of P. massoniana was greater than that of broadleaved species in the mixed forests except P. massoniana - Liriodendron chinense and P. massoniana - Liquidambab formosanaand stands. Though the coarse root biomass was 29.1-37.9 times greater than fine roots, the mean NPP of coarse root was only 1.2 times more than fine roots. NPP of the stems was the highest and barks the least in the five sampling stands.Nutrient contents of undergrowth in the five sampling stands were rather different. P, K, Ca contents in the aboveground were greater than those of belowground in the mixed forests, but exhibiting the opposite trends in the pure stand. Significant differences existed in nutrient contents of forest floor among the forest types and the different organs of the same tree. However, a similar trend appeared in element concentrations which ranked as Ca > N > Mg > K > P.The total nutrient accumulation in the ecosystems varied from 76.35 to 128.14 t/hm~2 for the five sampling stands. The largest nutrient pool was in the soils, accounting for about 97% of the total pool. The next was the arbor layer, and followed by forest floor and undergrowth. The accumulation of five nutrient elements in the soil decreased in the order by K > Mg > Ca > N > P. Nutrient accumulations in the arbor was the highest, undergrowth came second, and forest floor the least. For all nutrient elements, its accumulations in the forest floor showed the same sequence: the pure < the mixture, but N and Mg storages in the undergrowth was higher in the pure stand than in the mixed ones.