Carbon stocks of three secondary coniferous forests along an altitudinal gradient on Loess Plateau in inland China

Abstract

Natural forests in inland China are generally distributed in montane area and secondary due to a semi-arid climate and past anthropogenic disturbances. However, quantification of carbon (C) stock in these forests and the role of altitude in determining C storage and its partition among ecosystem components are unclear. We sampled 54 stands of three secondary coniferous forests (Larix principis-rupprechtii (LP) forest, Picea meyerii (PM) forest and Pinus tabulaeformis (PT) forest) on Loess Plateau in an altitudinal range of 1200-2700m a.s.l. C stocks of tree layer, shrub layer, herb layer, coarse wood debris, forest floor and soil were estimated. We found these forests had relatively high total C stocks. Driven by both higher vegetation and soil C stocks, total C stocks of LP and PM forests in the high altitudinal range were 375.0 and 368.4 t C ha-1 respectively, significantly higher than that of PT forest in the low altitudinal range (230.2 t C ha-1). In addition, understory shrubs accounted for about 20% of total biomass in PT forest. The proportions of vegetation to total C stock were similar among in the three forests (below 45%), so were the proportions of soil C stock (over 54%). Necromass C stocks were also similar among these forests, but their proportions to total C stock were significantly lower in LP and PM forests (1.4% and 1.6%) than in PT forest (3.0%). Across forest types, vegetation biomass and soil C stock simultaneously increased with increasing altitude, causing fairly unchanged C partitioning among ecosystem components along the altitudinal gradient. Soil C stock also increased with altitude in LP and PT forests. Forest floor necromass decreased with increasing altitude across the three forests. Our results suggest the important role of the altitudinal gradient in C sequestration and floor necromass of these three forests in terms of alleviated water conditions and in soil C storage of LP and PM forests in terms of temperature change.

Fig 1. Schematic maps showing the location of the study area on the Loess Plateau, China and the mean annual temperature and precipitation changes along the altitudinal gradient on Guandi mountain range.

MAT, mean annual temperature; MAP, mean annual precipitation. MATs and MAPs along the altitudinal gradient are presented using the estimated data given by Xiao et al.[23].

Fig 2

Vegetation (a), tree layer (b), shrub layer (c) and herb layer biomass (d) and MAI (e) along the altitudinal gradient in three coniferous forests on the Loess Plateau, China. MAI, mean annual increment; LP, Larix principis-rupprechtii forest; PM, Picea meyerii forest; PT, Pinus tabulaeformis forest. Shrub and herb layer biomass were presented after square-root transformation. Solid lines represent significant linear relationships between altitude and vegetation biomass (r² = 0.19, p<0.001), tree layer biomass (r² = 0.27, p<0.001), shrub layer biomass (r² = 0.62, p<0.001) and herb layer biomass (r² = 0.22, p<0.001) and MAI (r² = 0.29, p<0.001) respectively across the three forests. Dotted lines represent a significant linear relationship between altitude and shrub layer biomass (r² = 0.62, p<0.001) and a significant quadric relationship between altitude and MAI (r² = 0.32, p = 0.057) respectively in LP forest. Long dash line represents a significant linear relationship between altitude and shrub layer biomass (r² = 0.47, p = 0.002) in PM forest. Dot-dash line represents the significant linear relationship between altitude and herb layer biomass (r² = 0.43, p = 0.003) in PT forest.

Fig 3

CWD and forest floor necromass (a) and forest floor necromass (after natural-log transformation) along the altitudinal gradient (b) in three coniferous forests on the Loess Plateau, China. LP, Larix principis-rupprechtii forest; PM, Picea meyerii forest; PT, Pinus tabulaeformis forest. Different letters in (a) indicate significant differences of the forest floor necromass (p<0.05) among three forest types. Solid line in (b) represents a significant linear relationship (r² = 0.16, p = 0.003) between altitude and forest floor necromass across the three forests.

Fig 4

Percentage of C stock in different soil horizons (a) and total soil C stock along the altitudinal gradient (b) in three coniferous forests on the Loess Plateau, China. LP, Larix principis-rupprechtii forest; PM, Picea meyerii forest; PT, Pinus tabulaeformis forest. Solid, long dash and dotted lines represent significant relationships between soil C stock and altitude across the three forests (r² = 0.61, p<0.001), within LP forest (r² = 0.49, p = 0.001) and within PM forest (r² = 0.29, p = 0.021) respectively in (b).

Fig 5

Total C stock (a) and proportions of vegetation (b), necromass (c) and soil C stocks (d) to total C stock along the altitudinal gradient in three coniferous forests on the Loess Plateau, China. LP, Larix principis-rupprechtii forest; PM, Picea meyerii forest; PT, Pinus tabulaeformis forest. The proportion of necromass to total C stock was log-transformed. Solid lines represent significant linear relationships between altitude and total C stock (r² = 0.56, p<0.001) and the proportion of necromass to total C stock (r² = 0.22, p<0.001) respectively across the three forests. Dotted and long dash lines represent significant linear relationships between altitude and total C stock within LP forest (r² = 0.39, p = 0.006) and PM forest (r² = 0.25, p = 0.033) respectively.