| Before aging,plants transfer available nutrients to living tissues for direct use in plant growth,known as Nutrient resorption（NuR）.This strategy can reduce the dependence of plants on external nutrients to adapt to the poor nutrient environment.Yantai coastal shelter forest provincial nature reserve is a coastal sandy soil with poor soil nutrients.Therefore,taking Pinus thunbergii,the constructive species in Muping section of nature reserve,as the main research object,through the determination of leaf functional traits（chemical functional traits,structural functional traits）and understory main soil nutrients and stoichiometric characteristics,the basic growth indexes（height and DBH）of Pinus thunbergii were measured,and the plots were divided into different stand densities(high density:1150 plants·hm-2,medium density:535 plants·hm-2,low density:285 plants·hm-2)and different coastal distances（200m,400m,600m,800m,1000m,1200m,1400m）were classified to explore the changes of leaf nutrient resorption rate under different stand densities and different coastal distances.The relationship between leaf functional traits,soil nutrient characteristics and individual growth indexes and leaf nutrient resorption rate was analyzed.The changes of leaf functional traits,soil nutrient characteristics and individual growth indexes with the distance between stand density and coast were also analyzed.In order to provide basis for protection and management of Yantai coastal shelter forest.Through the research,it is found that:（1）The average values of NRE（nitrogen resorption efficiency）,PRE（phosphorus resorption efficiency）,KRE（potassium resorption efficiency）and CaRE（calcium resorption efficiency）were 52.44%,48.53%,64.22%and 13.50%,respectively.Na and Mg were enriched in the senescing leaves,and the mean value of NaRE and MgRE were-39.26%and-49.81%respectively.The nutrient resorption rates of all elements were the highest in the middle stand density(535 plants·hm-2),indicating that the nutrient uptake and utilization efficiency of Pinus thunbergii was the highest in the middle stand density in the study area.Except for NRE,leaf nutrient resorption rates of other elements had significant changes with the increase of coastal distance（P<0.05）.（2）There were some differences in leaf functional traits among different stand densities and coastal distances.The contents of N and Ca in mature leaves and Na,Ca and Mg in aged leaves showed significant differences among different stand densities（P<0.05）.Except for the contents of C in mature leaves and Na in senescent leaves,other nutrient indexes had significant changes with the increase of coastal distance（P<0.05）.The changes of mature leaves and senescent leaves with the density of stand and the distance of coast are basically the same,and there is synergy.According to the threshold of leaf stoichiometric ratio,the growth and development of Pinus thunbergii in the study area may be limited by N.In the middle stand density,the SLA of the leaves was relatively low and the LDMC was relatively high,while the high density was the opposite.The resource utilization strategies were different among different stand densities.LA,LL and SLA decreased significantly with the increase of coastal distance（P<0.05）,indicating the adjustment and adaptation of Pinus thunbergii to external environmental stress.Leaf NRE and MgRE were significantly correlated with the elemental nutrient contents of mature and senile leaves,CaRE was significantly correlated with the elemental nutrient contents of mature leaves,and PRE,KRE and NaRE were significantly correlated with the elemental nutrient contents of senile leaves.In contrast,nutrient characteristics of senescent leaves had stronger correlation with nutrient resorption efficiency.N:K and NRE,C:Mg and MgRE in mature and senescent leaves;K:P and PRE,C:K,N:K,K:P and KRE,C:Na and NaRE;The correlation between C:Ca and CaRE in mature leaves reached a significant level,which could indicate the nutrient resorption capacity of Pinus thunbergii leaves.At the same time,LL（leaf length）,LA（leaf area）,SLA and SLW could reflect the nutrient resorption of leaves except K in the study area to a certain extent.（3）Soil AP,AK,TN,TP,C:N,C:P,N:P showed significant changes under different stand densities（P<0.05）.Except for soil pH of 20-50 cm,other indexes showed significant changes with the increase of coastal distance（P<0.05）.There was a significant negative correlation between NRE and PRE and soil available N and P contents（P<0.05）,which was consistent with the"relative resorption hypothesis",while there was a significant positive correlation between KRE and soil AK（P<0.05）.Soil C:N,C:P,N:P had a strong correlation with NRE and CaRE,and had a better indication of nutrient resorption efficiency.（4）There was a certain correlation between the height and DBH of Pinus thunbergii and leaf functional traits,understory soil nutrients and stoichiometric characteristics.The density of Pinus thunbergii height in middle stand was significantly higher than that of other stands（P<0.05）.With the increase of coastal distance,the height and DBH basically increased gradually.PRE was significantly correlated with DBH,while KRE,CaRE and MgRE were significantly correlated with high DBH.In conclusion,the nutrient resorption of leaves of Pinus thunbergii in the study area was higher in the middle stand density,but the variation trend of nutrient resorption of leaves of different elements to the coastal distance did not show a uniform law.At the same time,the influencing factors of leaf nutrient resorption rate,such as leaf functional traits,growth index（height and DBH）and understory soil nutrient characteristics,also responded to the changes of stand density and coastal distance to a certain extent.The study provides a certain basis for the construction and management of Yantai coastal shelter forest nature reserve with Pinus thunbergii as the main afforestation species and relatively poor soil nutrient. |
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