Abstract
In the process of subtropical forest succession, it has long been recognized that population decline of Masson pines in coniferous-broadleaf mixed forest is caused by shading from broadleaf trees. However, little is known about the mechanism underlying the interaction between them. Here, we first chose two sets of Masson pine plots approximately aged 60 years in subtropical mountainous areas in eastern China (i.e., pure coniferous forest vs. coniferous-broadleaf mixed forest). Then, we measured and compared tree height, diameter at breast height, first branch height (FBH), live crown ratio (LCR) of Masson pines between the two sets of plots, and also determined the difference in growth performance of Masson pines relative to their neighboring broadleaf trees in the mixed forest stand. Compared with plots in pine forests, Masson pines in mixed plots had lower tree height and crown breadth, higher FBH, lower LCR, and leaf area. Furthermore, the difference of mean FBH between reference trees (Masson pines) and their neighboring trees (i.e., broadleaf trees) in mixed forest plots was greater than that in pine forest plots, and the ratio of LCR between Masson pines and their neighbors (0.46) in mixed forest was significantly smaller than in pine forest (1.05), indicating that those broadleaf trees around Masson pines probably affected their growth. The mean distance between Masson pines and neighboring trees (1.59 m) in mixed forest plots was significantly shorter than in pine forest plots (2.77 m) (p < 0.01), suggesting that strong competition may occur between reference trees and their neighbors. There was a significant difference in the ratio of crown volume between reference tree Masson pine and its neighboring trees in mixed forests (p < 0.01), indicating that the ratio of biomass synthesis to consumption of pines was much lower than their nearby broadleaf trees in mixed forest. Our results have demonstrated for the first time that Masson pines’ population decline is affected by shade-tolerant broadleaf late-successional species, which can be primarily attributed to the distinctive light transmittance of dominant species nearby (pure pine vs. mixed forest). This study provides a new perspective for future studies on the mechanism of forest succession.
Highlights
The secondary succession of subtropical forests in eastern China can be divided into six major phases: naked lands, grasses, shrublands, coniferous forests, coniferousbroadleaf mixed forests, and deciduous broadleaf forests [1]
Our results have demonstrated that the shading effect of adjacent broadleaf trees on the lower branches of pioneer conifers plays an important role in mixed forest, resulting in population decline of conifers during early succession
Our study shows a considerable difference in growth performance between Masson pines (Pinus massoniana) and neighboring trees in coniferous-broadleaf mixed forests compared with coniferous forests
Summary
The secondary succession of subtropical forests in eastern China can be divided into six major phases: naked lands, grasses, shrublands, coniferous forests, coniferousbroadleaf mixed forests, and deciduous (or evergreen) broadleaf forests [1]. There are sometimes a few full-grown Masson pines in the near climax community during the late forest succession of subtropical eastern China [6] These studies indicate that soil condition is not the basic reason for the decline of Masson pine population. Based on allelopathy studies from laboratory, Chen et al [7] posit adverse effects of pioneer species on later successive species in forest succession, which is similar to the results of a recently performed pot experiment [8]. Both investigations lack the support of field experiments. This appears to contradict another popular hypothesis, i.e., the “Relay Floristics Hypothesis,” that species in the early stage of succession may provide favorable conditions for those in the late stage [9,10]
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