How forest gaps shaped plant diversity along an elevational gradient in Wolong National Nature Reserve?

摘要

Understanding the underlying ecological processes that control plant diversity within (α-diversity) and among (β-diversity) forest gaps is important for managing natural forest ecosystems,and it is also a prerequisite for identifying the formation and maintenance mechanisms of forest plant communities.In this study,we focused on the interrelationships among habitat type (gapon-gap plots),gap size,elevation and environmental factors,and we explored their effects on plant diversity (α-diversity and β-diversity).To do this,a total of 21 non-gap (i.e.,closed canopy) plots (100 m2) and 63 gap plots,including 21 with large gaps (200-410 m2),21 with medium gaps (100-200 m2) and 21 with small gaps (38.5-100 m2),were selected along an elevational gradient in a subalpine coniferous forest of southwestern China.Using structural equation models (SEMs),we analyzed how forest gaps affected plant diversity (α-diversity and β-diversity) along an elevational gradient.The results showed that (1) as elevation increased,unimodal patterns of α-diversity were found in different-sized gaps,and β-diversity showed a consistent sinusoidal function pattern in different-sized gaps.The gap size was positively related to α-diversity,but this effect disappeared above 3500 masl.Moreover,the patterns of α-diversity and β-diversity in non-gap plots were irregular along the elevational gradient.(2) SEMs demonstrated that many environmental factors,such as the annual mean air temperature (AMAT),ultraviolet-A radiation (365 nm,UV-A365),ultraviolet-B1 radiation (297 nm,UV-B297),moss thickness (MT),soil carbonitrogen ratio (C/N ratio),NH4-N and NO3-N,were significantly affected by elevation,which then affected α-diversity and B-diversity.The photosynthetic photon flux density (PPFD),UV-A365 and UV-B297 were significantly higher in plots with forest gaps than in the non-gap plots.Moreover,the PPFD and UV-A365 were positively and directly affected by gap size.Surprisingly,except for the NH4-N and the C/N ratios,the below-ground environmental factors showed little or no relationships with forest gaps.All of these effects contributed to plant diversity.Overall,the above-ground environmental factors were more sensitive to gap-forming disturbances than the below-ground environmental factors,which affected α-diversity and β-diversity.The predicted pathway in the SEMs of the elevational effects on α-diversity and β-diversity was relatively complicated compared with the effects of forest gaps.These results can provide valuable insights into the underlying mechanisms driving the diversity-habitat relationship in the subalpine coniferous forests of southwestern China.