Study On Biomass And Nutrient In Pinus Massoniana-Hardwood Mixed Forests

Pinus massoniana is one of the major fast-growing timber tree species in south of China. However, long-term development of Pinus massoniana monoculture plantations has led to soil degradation, productivity decline, and outbreak of pests, e.g. Dendrolimus punctatus, which threatened the sustainable forest management. Mixed-species stands are generally believed to be superior to monocultures in improving the stability, stress resistance, and complexity of the ecosystems, thus the interest in the ecology and management of mixed-species stands has increased in recent decades. A large number of studies have been focused on Masson's pine forest management, but very few concerned with multiple mixed stand patterns on one site. Since the middle of 1980's, Xinkou Experimental Forest Farm of Fujian Agriculture and Forest University has started to plant hardwood species, including Michelia macclurei, Castanopsis fissa, Castanopsis sclerophylla, Castanopsis kawakmii, Cyclobalanopsis myrsinaefalia, and Castanopsis lamontii, under the canopy of Pinus massoniana plantations. After sixteen years of development, six types of uneven-aged mixed stands were formed. Biomass, primary productivity, and mineral nutrient status in the six mixed stands and the pure Masson's pine plantation were analyzed to study the response of the ecosytems to the interplanting 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 227.811 t·hm^-2,272.813 t·hm^-2,230.536 t·hm^-2,235.996 t·hm^-2,262.494 t·hm^-2,and 210.124 t·hm^-2, respectively, but only 213.072 t·hm^-2 in the pure Pinus massoniana stand. As to the vertical stratification of biomass, the arbor layer had the highest yield, undergrowth came second, and forest floor the least. The percent allocation accounted for 62.03%⁷1.32% of the total to stems, 16.33%— 18.61% to roots, 8.99% 16.94% to branches, and 1.074%⁴.32% to foliar in all the seven stands. Undergrowths flourished under the pure Pinus massoniana stand with a total biomass of 3.678 t·hm^-2, the highest among the seven stands, but producing the least forest floor standing crops compared to the mixed stands. The litter production at L layer occupied 63.2% of the forest floor accumulation in the mixed plantations, but the L-layer percentage was 86.3% in the pure stand.Spatial structure of biomass in the pure and mixed stands was different. The living and dead branches started to appear at the height of 10 meters in the mixed stands, but 13—14 meters in the pure forest. In the mixed stands, the root biomass of hardwoods were mostlydistributed in the soil depth of 0-40 cm, but roots of Pinus massoniana penetrated below the 60cm of soils. This kind of structure formed under the mixed stands was benefit for water and nutrient absorption, because organic matter and moisture are relatively richer in the surface soil.The net primary productivity (NPP) in the mixed forests was between 10604.62 and 15248.12 kg-hm'V1, and 7338.48 kg-hm"V in the pure forest. Difference of NPP existed between hardwood species and Masson's pine in the mixed forest. The total NPP of Pinus massoniana was greater than that of hardwoods in the mixed forests except Pinus massoniana - Castanopsis fissa communities. The foliar NPP of hardwoods was higher than that of Pinus massoniana. Though the coarse root biomass was 76 174 times greater than fine roots, NPP of coarse root was only 1.4².5 times more than fine roots . NPP of the roots was the highest in the pure forest, coming next with branches, leaves, and barks. However, foliar and bark NPP was higher than the roots.The concentrations of five macroelements in the arbor trees of the mixed stands decreased in the order: Ca>N>K>P>Mg, but the sequence in the pure stand varied somewhat (Ca>N>K>Mg>P). Nutrient contents of stem in the Pinus massoniana ranged from 0.030% to 0.900 %, which was less than that of branches, leaves and roots. The concentrations of N% K and Ca in the roots decreased with the size. In the mixed forests, phosphorus contents in the roots of Pinus massoniana decreased in the order: fine roots>coarse roots>middle-sized roots and Mg contents increased with the root size. The nutrient contents in the hardwoods differed from that in the Pinus massoniana. Nitrogen and Magnesium contents in the coarse and middle roots of Pinus massoniana was greater than that of hardwoods, but the trend was opposite in the fine roots.Nutrient contents of undergrowths in the seven communities were not identical. Potassium and calcium contents in the aboveground were greater than that of belowground in the mixed forests, but exhibiting the opposite trends in the pure stand. Significant differences existed in nutrient contents of forest floor between the forest types and between the organs of the same tree, but showing the similar trends for the element concentrations: Ca>N>Mg or K>P.The total nutrient accumulation in the ecosystems varied from 176.29 t -hm"2to 194.96 t -hm'2 for the seven forest types. The largest nutrient pool was found to reside in the soils, accounting for about 98.0% (97.7%⁹8.7%) of the total, followed by the arbor layer, forest floor and undergrowth. The accumulation of five elements in the soil layer decreased in the...