Latitudinal Variation In Nitrogen And Phosphorus Resorption In Leaves And Fine Roots Of Pinus Koraiensis And Fraxinus Mandshurica

Nutrient resorption is an important nutrient conservation strategy during plant growth,especially nitrogen（N）and phosphorus（P）elements play an important role in the nutrient cycling process of ecosystems.The study of N,P resorption in leaves and fine roots not only improves the understanding of the resource acquisition and utilization strategies of above-and below-ground parts of plants,but also reflects the adaptation of plants to environmental diversity and is an important basis for studying the response of forest ecosystems to global changes.Therefore,in this study,Pinus koraiensis and Fraxinus mandshurica,which are valuable timber species in Northeast China,were sampled at five sites with latitudinal differences in their natural distribution areas for leaf,root and soil sampling.The middle-aged（>40 years）P.koraiensis plantations and the middle-aged（40-60 years）F.mandshurica stands in adjacent natural secondary forests were selected as sampling wood at each study site.We measured N,P in mature and senescent leaves,live roots and recently dead roots to compare the nutrient resorption characteristics and influencing factors of leaves and fine roots at different latitudes,and to analyze the relationship between nutrient resorption strategies of leaves and fine roots.The main objectives of this study were to（1）clarify the trends of N,P resorption in leaves and fine roots of P.koraiensis and F.mandshurica with latitude,（2）reveal the effects of traits and environmental factors on nutrient resorption in leaves and fine roots of P.koraiensis and F.mandshurica,and（3）understand the linkages between above-ground and below-ground N and P resorption strategies of P.koraiensis and F.mandshurica.The main findings were as follows:（1）There was a latitudinal variation pattern in nutrient resorption in leaves,related to tree species type and element type.The NRE and PRE of P.koraiensis were higher than those of F.mandshurica,67.25%and 54.02%,respectively,while the NRP and PRP of F.mandshurica were higher than those of P.koraiensis,16.50 mg g^-1 and 1.30 mg g^-1,respectively.The NRP of F.mandshurica leaves showed a trend of decreasing and then increasing with increasing latitude.The PRE of P.koraiensis leaves showed a significant positive correlation with latitude（P<0.05）,while the PRP and NRE of leaves of the two species did not differ significantly from each other at each location,so there was no significant trend of change.The relative resorption of P.koraiensis leaves was significantly negatively correlated with latitude（P<0.05）,and the relative resorption of F.mandshurica leaves first decreased and then increased with latitude.The N,P resorption characteristics of P.koraiensis and F.mandshurica are influenced by the leaf nutrient content and ecological stoichiometry characteristics,and the correlations are both similar in regions and interspecific differences.In addition,unlike F.mandshurica,the relative resorption of P.koraiensis leaves is also influenced by leaf morphological traits including leaf tissue density and specific leaf area.In particular,the PRE and relative resorption of P.koraiensis leaves were more significantly correlated with climate and soil factors,and the NRP was affected by climate factors.（2）The resorption of N and P existed in the absorptive fine roots of P.koraiensis and F.mandshurica,and there was a trend of latitudinal variation,but there were interspecific differences in the variation trend.In general,the NRE and PRE of the absorptive fine roots of F.mandshurica were higher than those of P.koraiensis（14.77%and 37.85%,respectively）,and the NRP and PRP of F.mandshurica were higher than those of P.koraiensis(21.43 mg g^-1 and0.75 mg g^-1,respectively).The NRP of absorptive fine roots of P.koraiensis increased and then decreased with latitude,while that of first order rootof F.mandshurica decreased and then increased with latitude.The PRP of absorptive fine roots of P.koraiensis decreased with increasing latitude,and the PRP of absorptive fine roots of F.mandshurica significantly and positively correlated with latitude（P<0.05）.Relative resorption of the absorptive fine roots of F.mandshurica tends to increase and then decrease with latitude.There was a correlation between the traits and ecological stoichiometry of absorptive fine roots and nutrient resorption,and the NRP and PRP were more easily affected by the nutrient content and ecological stoichiometry of fine roots.The NRP and PRP of absorptive fine roots of P.koraiensis were both significantly positively correlated（P<0.05）with the N,P contents of absorptive fine roots and negatively correlated（P<0.05）with C:N and C:P.The Nu RE of absorptive fine roots of P.koraiensis was only influenced by the effective soil nutrients,while the Nu RP of first order root of F.mandshurica was related to climatic factors,reflecting the different adaptation strategies of the absorptive fine roots of the two tree species in response to environmental changes.（3）The N,P resorption characteristics of leaves and fine roots were relatively independent of each other,and only a few indicators were correlated with each other.The NRE of P.koraiensis leaves was negatively correlated（P<0.05）with the NRP of fine roots,while the rest of the indicators were not significantly correlated.Ecological stoichiometry of leaves and fine roots of P.koraiensis and F.mandshurica was somewhat related,but differed between species.The NRE and PRE of leaves of the two tree species were significantly and negatively correlated with the corresponding NRP and PRP,respectively,reflecting the negative correlation between nutrient resorption rates and nutrient concentrations of senescing leaves.In addition,the relative resorption of both leaves and fine roots were significantly negatively correlated with the PRE of the corresponding organs（P<0.05）.In conclusion,this study confirmed that there was a latitudinal variation pattern in the nutrient resorption characteristics of both P.koraiensis and F.mandshurica leaves and fine roots,but there were differences in the trends of different elements.Ecological stoichiometry characteristics and climatic and soil factors had different degrees of influence on nutrient resorption in both leaves and fine roots.Nutrient resorption characteristics of leaves and fine roots of the two tree species were relatively independent from each other.Our understanding of the nutrient use strategies of tree species can provide a scientific basis for the construction of a nutrient cycling model of forest ecosystems at the regional scale.