Pioneer trees play a key role in the recovery of a reclaimed ecosystem that is nutrient-deficient in the initial stage of reclamation. Clarifying the growth of pioneer trees and their response to intra- or interspecific competition is of great importance to help stewards manage reclaimed forest. Nevertheless, such documents are lacking. We utilized forestry inventory data from a R. pseudoacacia and P. tabuliformis mixed forest plot with an area of 8000 m2 to find out the effect of trees interaction on their growth (indicated by the amount of biomass carbon per stem) in a nutrient-lacking reclaimed ecosystem 17 years after reclamation in Pingshuo opencast coal mine, Shanxi Province, China. In total, 2133 tree individuals were measured and tagged; the kriging interpolation method was applied to map spatial variation of forest-biomass carbon (C). Univariate and bivariate mark correlation functions were employed to examine the effect of intra- and interspecific interactions on tree’s biomass C accumulation. The results showed that tree biomass C was 27.84 Mg ha−1 in the forest. C class structure of planted R. pseudoacacia followed inversed J-shaped distribution and seeded-in R. pseudoacacia and Ulmus pumila (a spontaneous species) had L-shaped distribution. P. tabuliformis, however, followed a nearly normal distribution. Patchy distribution was observed with regard to C spatial arrangement of all tree species. In terms of the intraspecific correlation of biomass C, no significant competitive or facilitative interactions (GoF p ≥ 0.05) was observed among conspecific trees of seeded-in R. pseudoacacia and P. tabuliformis. In contrast, significant negative interactions (GoF p < 0.05) or repulsion correlations between biomass C of planted R. pseudoacacia individuals and U. pumila individuals were found at the scale of 1–2 and 0–8.5 m, respectively. In term of the interspecfic spatial correlation of biomass C, a significant positive interaction between the heterospecific individuals of planted R. pseudoacacia and seeded-in R. pseudoacacia was observed at the scale of 2.5–4 and 12.5–15 m. Similarly, seeded-in R. pseudoacacia and U. pumila were found to be attracted by P. tabuliformis at 7–9 and 0–2 m, respectively. Conversely, significant departure effect was observed at the scale of 1–3 m between the biomass C of U. pumila and planted R. pseudoacacia and 4.5–5.5 m between the biomass C of U. pumila and seeded-in R. pseudoacacia. Consequently, R. pseudoacacia and P. tabuliformis could coexist for a long term and P. tabuliformis was a species that facilitated the accumulation of C of other tree species. Overall, complex intra- and interspecific interactions in nutrient-limit reclamation ecosystem affected biomass C accumulation. R. pseudoacacia–P. tabuliformis mixed forest could be an efficient reclamation pattern to restore biomass C in the Loess Plateau area.

Vegetation and soil surveys were made within three mixed forests and four monoculture forests on opencast coal mine spoils in the Loess Plateau (China) to reveal the tree species effect on reclaimed mine soil (RMS) and carbon (C) sequestration.Results showed that tree biomass, annual litter biomass, and reserved litter biomass varied among forests, with Robinia pseudoacacia L.-Pinus tabuliformis Carr. recording the highest tree biomass, leaf C, and leaf nitrogen (N) values. The greatest C and N contents in RMS were recorded in the R. pseudoacacia -P. tabuliformis forest in both small macroaggregate (250-2000 μm) and large macroaggregate (>2000 μm) fractions. However, no significant differences among forests were observed in microaggregate (53-250 μm) and silt+clay (<53 μm) associate C and N. Significant positive correlations were observed among soil C and vegetation biomass and vegetation biomass associated C. It was in line with the C isotope analysis, which indicated that C flowed in the order: leaf!litter/root!soil macroaggregate!soil microaggregate/ silt+clay. Moreover, negative correlations between soil C and soil δ 15 N in large and small macroaggregate fraction were recorded, indicating C sequestration in macroaggregate was potentially associated with soil N cycling. Overall, Tree species effect on soil C sequestration was derived from the amount and quality of the vegetation biomass and it was mediated by soil N cycling to some extent. R. pseudoacacia -and P. tabuliformis mixed forest could be proposed as a favourable reclamation choice, due to these species prominent C sequestration ability.