| The canopy of the urban forest community is the most closely interactive interface between forest and the external environment.Forest canopy changes the understory light regimes by intercepting the solar radiation,which in turn affects the species composition and distribution of the understory vegetation.As the important components of urban forest communities,canopy structure and understory vegetation play an irreplaceable role in maintaining ecosystem diversity and ecological function stability.Testing the effects of canopy structure and understory light conditions on understory vegetation of urban forest communities can provide fundamental data for the sustainable management of urban forests.The understory vegetation of Huanghuagang Park in central area,Maofengshan Forest Park in the suburb and Shimen National Forest Park in the exurb were investigated by using classical community survey methods and hemispherical imaging techniques.The effects of forest canopy structure on understory vegetation in different regions of Guangzhou were quantitatively detected using correlation analysis,non-parametric tests(Kruskal-Wallis test),Canonical Correspondence Analysis(CCA),Indicator Species Analysis(ISA)and other methods.The main findings are as follows:(1)The forest canopy structural attributes differed among various urban forest communities.The canopy openness and understory light conditions showed the consistent variation trends among various urban forest communities,highest in exburb forest followed by suburb forest and lowest in urban forest,while the leaf area index showed the opposite variation trend.The canopy structure and understory light conditions in the urban area significantly differed from those in the suburbs and the suburbs(P < 0.001).The canopy structural attributes were significantly correlated with understory light regimes in the three regions(P < 0.05).(2)Understory plants were richest in the exburban forest communities in Guangzhou.The number of species and diversity indices of understory plants of 3 various urban forest communities were highest in the exburban,followed by the suburban,and lowest in central area.The species diversity of shrubs was greater than those of herbs in the suburb and exburb areas,and lower than that of herbs in the urban areas.Indicators of understory vegetation of urban forest communities in urban areas were largley herbs and shrubs,while that in suburb and exburb were largely arbor seedlings.(3)The effects of forest canopy structure on understory plants differed among various forest communities.The correlation between canopy structure and understory plants was the most significant in the exburb forest(P < 0.05);particularly,the canopy structure exerted strongest effect on species richness of the shrub layer.There was little difference in the forest canopy structure within the same region.Thus,the understory plants showed consistent responses to different classes of canopy openness.The understory plants of urban forests communities in different regions responded differently to the understory light conditions,and the indicator species were also different.Mussaenda pubescens and Rubus reflexus indicated strongly with understory light conditions in the suburbs.Canonical Correspondence Analysis(CCA)of canopy structures and understory vegetation in different layers of different regions revealed that canopy structure and understory light conditions all affected the species distribution of understory plants,among which the leaf area index,transmitted direct light and canopy open strongly affected the species distribution,especially the arbor seedling distribution.In general,forest canopy structure and understory vegetation attributes significantly differed among urban forest communities in different regions of Guangzhou.The forest canopy mainly affected the spatial distribution of understory light conditions and species diversity of understory vegetation through the synthetic effects of solar radiation.In addition,due to the differences in habitats and environmental requirements between shrub layer and herb layer,the responses shrubs and herbs to canopy structure and understory light conditions were also different. |
