The Changes Of Plant Functional Traits Of Lower Subtropical Evergreen Broad-leaved Forests Along The Urban-rural Gradient

Plant functional traits can affect the"morphological-physiological-phenological"characteristics of species fitness by influencing the"growth-reproduction-survival"strategies.They also reflect the responses of plants to environmental factors and impact on ecosystem.With the rapid development of urbanization,the vegetation of evergreen broad-leaved forests has been largely destroyed and thus has a series of environmental issues.At present,there are many studies on plant functional traits,but the changes in mechanisms and how traits respond to urbanization along the urban-rural gradient is relatively few.However,such the studies on plant functional traits under urbanization provide a scientific basis for species protection and ecological restoration.This study aims to investigate the secondary lower subtropical evergreen broad-leaved forests under the urban-rural gradient?urban sites,Baiyun Mountain and Pugang,a suburban site,Luogang,a rural site,Daling Mountain?.We analyze the plant functional traits and functional diversity,and explore the change rule of plant functional traits under the urban-rural gradient as well as the changes in environmental variables during urbanization.The main results are as follows:1.The functional traits of the dominant species changed significantly along the urban-rural gradient.Plants in urban area were higher and had larger specific leaf area?SLA?and leaf nutrient content?LN,LP?,but lower leaf dry matter content?LDMC?and maximum photosynthetic rate(A_max).No significant difference was observed in the carbon content?LC?and water use efficiency?WUE?of leaves.SLA and nutrient content were key traits that affected the distribution of plants and reflected the species difference along the urban-rural gradient.2.There were changes in the trade-off between functional traits in the secondary lower subtropical evergreen broad-leaved forests under urbanization partly because of the changes in environmental variables.The LDMC in urban area was positively correlated with tree height,but the correlation became weaker,and the negative correlation between A_max and WUE became positive along the gradient.However,due to internal dynamic balance,relations between certain traits remained unchanged.For example,the LN and LP of the leaves were positively related to each other and both were negatively related to C:N and C:P.3.The functional richness?FRic?and functional dispersion varitation?FDiv?increased significantly along the gradient,but the functional evenness?FEve?did not show clear pattern.Functional diversity index generally showed an increasing trend.The correlation between functional diversity indices was weak,only the correlation between FRic and FEve was positive.4.The environmental factors of the forest communities were significantly different under the urban-rural gradient.The temperature decreased and the amount of the litter increased with the gradient.Urbanization led to soil acidification and loss of nutrients?TN?SOC?K?Ca?Mg?.The content of heavy metals?Cu?Pb?Ni?Cr?in the urban and suburban areas in forest soils was significantly higher than that in the far suburban Daling Mountain and exceeded the background value of the surface soil in Guangdong province.According to the differences of urban soil factors,the gradient could be divided into the urban areas of Baiyun Mountain,Pugang,Luogang,and the suburban Daling Mountain.5.The environmental factors of the forest communities had significant effects on plant functional traits along the urban-rural gradient.Environmental factors mainly affected the specific leaf area?SLA?and nutrient content of the leaves.Plants with higher SLA and nutrient content?LN?LP?usually grew in high temperature and soils with lower nutrient,while the plants growing in soils with rich nutrient had high LDMC,C:N and C:P.Therefore,urbanization favoured the growth of"fast growing"species.6.Soil nutrient elements?TN,TP,K,Mg?and heavy metal elements?Cu,Pb and Ni?of the forest communities had remarkable influence on functional diversity along the urban-rural gradient.The soil nutrient content?19.31%?was higher than the content of heavy metals?8.60%?,and the former mainly explained the functional diversity.The functional diversity tended to be higher in areas with high nutrient content.