Responses Of Dalbergia Odorifera Seedling To Neighbors With Different Niches Under Conditions Of Drought And Nitrogen Application

Mixed stands can be more productive if growth facilitation via niche segregation occurs.This process is species-specific and depends on the resource availability.Leguminous species could acquire nitrogen via both absorption from the soil and biological nitrogen fixation and also play a key role for measuring the impacts of external conditions on the dynamics of forestry systems,especially in the context of climate change.However,information regarding whether the woody Leguminous Family Dalbergia odorifera benefit from neighbor with niche differentiation and this interactive effect is modulated by water level and N application were obscured.In addition,woody Leguminous Family D.odorifera,precious redwood and endemic to Hainan Island,are greatly limited by abiotic stress(such as drought and nutrient deficiency)and biological stress(intraspecific competition in pure stands).Thus,it is of great ecological and economic value to study the response dynamics of woody legume in mixed forest under the background of global climate change,and to seek for the best planting models of D.odorifera.D.odorifera seedlings as leguminous woody target species were grown with D.odorifera of same N-fixing niche,Delonix regia that belongs to a heterogenous but nonleguminous family,and with a species from a different family Swietenia mahagoni(different N-fixing niche).All seedlings were treated with the root interaction and isolated planting models under two watering regimes(well-watered and drought condition)and two nitrogen applications(application and non-application),respectively.Comprehensive evaluation on the competitive response of D.odorifera via subordinate function analysis and principal component analysis on the basis of the performances of growth and development,phenotype of root system,eco-physiology and competitive indexes were performed to evaluate the responses of D.odorifera under different interactive models.In addition,the correlation between competitive advantage of roots and the competitiveness of the whole seedling was evaluated via correlation analysis.The responsive mechanism of woody Leguminous Family D.odorifera to different niche neighbors and the responsive dynamics after drought and nitrogen application were studied.Meanwhile,the best mixed models to improve the growth and tolerance of D.odorifera under specific conditions was explored.The major findings of this study are as following:(1)D.odorifera under the root system contact models would perform different responses to different niche neighbors in growth and development(dynamic changes of plant height(H),basal diameter(D),branch number and leaf number),root morphological characteristics(root length,root surface area,root diameter,root volume,number of root tips and root branch,specific root length,specific density of root branch and specific root tip number,root system class and nodule),and physiology(biomass accumulation and distribution,photosynthetic parameters,water use,ecological stoichiometry,non-structural carbohydrates,antioxidant enzymes and osmotic regulation).D.odorifera was benefited from different niche neighbors(D.regia and S.mahagoni)but inhibited from conspecifics in drought,N deficiency or the combined stresses under root system contact.In addition,D.odorifera was benefited from S.mahagoni but inhibited from D.regia in combined well-watered and N application condition.(2)Compared with the root system contact models under the 100% FC and 30%FC conditions,D.odorifera competing with D.regia and S.mahagoni under the root system isolated models showed markedly poor performances(lower levels in H,D,number of branch and leaves,stem dry matter(SDM),LDM,root dry matter(RDM)and total dry matter(TDM),RCI,Pn,leaf water content,stem water content,root water content,water content of whole seedling,root length and surface area of total root,coarse root and fine root,root diameter,root volume and root branching number)but optimal performances(higher level in H,D,SDM,LDM,RDM and TDM,Pn,leaf water content,stem water content,root water content,water content of whole seedling,root length and surface area of total root,coarse root and fine root,root diameter,root volume and root branching number)for D.odorifera competing with conspecifics.After N application under the two water regime conditions,D.odorifera planting with S.mahagoni under the root system interact than root system isolation had poorer performances(lower levels in H,D,number of branch and leaves,LA,LDM,SDM,TDM,RCI,Pn,PNUE,PPUE,chlorophyll content,leaf water content,stem water content,root water content and water content of whole seedling).However,few or insignificant differences were observed in the growth and development,root morphological characteristics,physiology and relative competitive indexes of D.odorifera among the three competition models under root system isolation in all treatments.The competitive response of D.odorifera to different niche neighbors was dominated by belowground interaction and belowground interaction for resource may be the only way to affect the differential response of D.odorifera due to the negligible aboveground interaction.(3)Drought stress strengthened the competitive intensity and aggravated the inhibitory effects on D.odorifera from D.odorifera.However,N application alleviated the inhibitory effects of drought stress on D.odorifera from D.odorifera and S.mahagoni.In addition,N application stimulated the promoted effects on D.odorifera from S.mahagoni but enhanced competition intensity of D.odorifera from D.regia under the 100% FC condition,and converted the facilitative to inhibitory effects.Thus,water level and N application might alter responses of D.odorifera to different niche neighbors under the root system interaction.Appropriate N application maybe alleviate the inhibitory effect of drought stress on D.odorifera in its mixed forests.Collectively,D.odorifera under the root system interaction models would perform different responses to different niche neighbors.D.odorifera was benefited from S.mahagoni but inhibited from conspecifics in all treatments under root system contact.Compared with root system isolation,root system interaction would have more positive effects on D.odorifera.Furthermore,the competitive response of D.odorifera to different niche neighbors was dominated by belowground interaction rather than the negligible aboveground one,so interaction for aboveground resource was the only way to affect the differential response of D.odorifera.Under the condition of root system interaction,water and nitrogen application may change the response of D.odorifera to different niche neighbors.In addition,mixture with S.mahagoni under appropriate N application could be the favorable planting model for improving the growth and tolerance in D.odorifera planation management.