Stand structure and plant diversity characteristics of typical artificial forests after natural recovery in the hilly region of central Hainan

Abstract

Introduction: The global expansion of artificial forests has highlighted the necessity of restoring their ecological service functions and understanding natural succession mechanisms in forest restoration ecology. However, comprehensive analyses of community assembly in tropical artificial forests following long-term natural recovery and their divergence from zonal vegetation remain insufficient.

Methods: In this study, the stand structure and plant diversity were investigated in three typical tropical artificial forests (Acacia mangium, Hevea brasiliensis, and Eucalyptus) after 20 years of natural recovery, alongside 33-year-old natural secondary forests, in the Fengmu Experimental Forest Farm, Hainan Province. The relationships between plant diversity and community structural factors in artificial forests were also examined.

Results: The findings can be summarized as follows. (1) Acacia mangium forests exhibited superior natural regeneration, whereas the naturally regenerated trees in all plantations displayed significantly smaller mean diameter at breast height and height than those in the natural secondary forests. (2) Although the species diversity in certain forest layers of plantations approached that of natural secondary forests, notable differences persisted, and woody plants in plantations lacked the phylogenetic traits observed in natural secondary forests. (3) Redundancy analysis showed that the greater densities and canopy cover of planted trees inhibited arbor layer diversity but promoted phylogenetic dispersion. High tree density facilitated shrub layer establishment, whereas height growth in regenerated trees and shrubs inhibited shrub diversity through resource competition. Additionally, the increased diameter class variation in regenerated trees and taller shrub-herb layers reduced herb layer diversity due to resource limitations.

Discussion: After 20 years of natural recovery, plantations have developed multi-aged, vertically stratified mixed stands. However, growth constraints on woody plants and limited biodiversity recovery persist. Structural optimization is crucial for enhancing niche differentiation and accelerating succession toward climax forest communities.

Keywords: artificial forest transformation; natural succession; plant diversity; stand structure; tropical artificial forest.

Figure 1

Schematic diagram of the study area and sample plot locations. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest.

Figure 2

Number density and average height of arbor layer in different regeneration layers of artificial forest and natural secondary forest. (A) Plant number density; (B) Average plant height. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 3

Number density and average plant height of shrub layers in different forest stands. (A) Plant number density; (B) Average plant height. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 4

Herb layer coverage and average plant height of different forest stands. (A) Coverage; (B) Average plant height. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 5

Plant species diversity indices for different forest stands. (A) Patrick index; (B) Shannon index; (C) Simpson index; (D) Pielou index. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 6

α diversity index of plant phylogeny in different stands. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 7

β diversity index of plant phylogeny in different stands. (A) MPD; (B) MNTD. AM, HB, ER, and SF respectively represent Acacia mangium forest, Hevea brasiliensis forest, Eucalyptus forest, and natural secondary forest. Different lowercase letters indicate significant differences between stands (P < 0.05).

Figure 8

RDA ordination diagram of plant diversity and community structure factors in each layer of the artificial forest. (A) Arbor layer; (B) Shrub layer; (C) Herb layer. R, H, D, E, PD, MPD, and MNTD represent Patrick index, Shannon index, Simpson index, Pielou index, PD, MPD, and MNTD, respectively. DT-d, DT-ch, DT-bh, DT-h, DT-bd, Cd, T-d, T-h, T-bd, S-d, S-h, H-c, H-h, T-DBHSW, T-DBHS, and T-DBHP represent the number density of artificial trees, the average crown width of artificial trees, the average branch height of artificial trees, the height of artificial trees, the average DBH of artificial trees, the canopy coverage of artificial trees, the number density of arbor regeneration layer, the average height of arbor regeneration layer, the average DBH of arbor regeneration layer, the number density of shrub layer, the average height of shrub layer, the coverage of herbs, the average height of herb layer, DBH-Shannon index, DBH-Simpson index, and DBH-Pie index.