Stoichiometric Characteristics And Influencing Factors Of Assimilative Branches Of Four Shrubs In The Junggar Desert

Assimilative branches are special organs of xerophytes that replace degraded leaves for photosynthesis.Shrubs with assimilative branches（ABs）are unique,drought-resistant,and constructive species in temperate deserts in Central Asia.Understanding the nutrient traits and driving factors of ABs is crucial to understand the life history strategies of desert shrubs and predict their responses and adaptations to environmental changes.Four dominant shrubs with ABs（i.e.,Ephedra przewalskii,Calligonum mongolicum,Haloxylon ammodendron,and Haloxylon persicum）widely distributed in the Junggar Desert in northwestern China were selected as the objects,the intra-and interspecific differences in the stoichiometric traits（also known as nutrient traits）,allometric relationships and homeostatic indexes of four shrubs with ABs at regional and local scales and their variability under different environmental gradients（i.e.,latitude and longitude,precipitation,soil texture types）were systematically and comparatively explored.The results provided a scientific basis for understanding the convergence or divergence of desert plants.The results showed that:（1）At the regional scale,the nutrient traits of ABs were different.E.przewalskii had the highest carbon（C）concentration(395.09±1.69 mg g^-1),H.ammodendron had the lowest C concentration(295.36±7.71 mg g^-1)while the nitrogen（N）concentration was the highest(10.77±0.70 mg g^-1).Compared with the E.przewalskii and C.mongolicum,H.ammodendron and H.persicum had lower phosphorus（P）concentrations(0.68±0.04 and0.56±0.03 mg g^-1,respectively)and higher N:P（15.59±0.55 and 15.54±0.29,respectively）;According to the Relative Growth Rate Hypothesis,the results confirmed the slower growth rate characteristics of H.ammodendron and H.persicum.Compared with plants in other regions,the C,N,P concentrations and N:P of four shrubs of ABs were lower and C:N、C:P were higher in the Junggar desert.According to the theory of stoichiometric ratio,E.przewalskii and C.mongolicum were most likely limited by N,while H.ammodendron and H.persicum were co-limited by N and P.A common N–P scaling exponent（1.171）among the four shrubs was determined,revealing higher N allocation rate than P.Except that the C:P of H.persicum was small homeostasis,the other nutrient traits of ABs of the four shrubs exhibited strong or absolute homeostasis.（2）The C concentrations of ABs of the four shrubs hardly changed with latitude and longitude,precipitation,and soil type under different environmental gradients at regional scales,and the interspecific variability also did not change with varying precipitation and soil texture,while other nutrient traits showed differential trends under different precipitation gradients and soil texture types.The N-P scaling exponent of E.przewalskii was the smallest（0.873）at high precipitation gradients,and the other three shrubs were the smallest at moderate precipitation gradients.As precipitation increased,the homeostasis of N and P of E.przewalskii decreased,and N of H.ammodendron and P of H.persicum showed lower homeostasis under moderate precipitation gradients.The N-P scaling exponents of E.przewalskii and C.mongolicum were the smallest under sand dust soil（1.036 and 1.080,respectively）,and the exponent of H.persicum under sand soil（1.075）were significantly smaller than the other three shrubs.C:N and C:P of E.przewalskii showed lower homeostasis under sand dust soil,N,C:N and N:P of H.persicum showed lower homeostasis under coarse sand soil,and the fit of homeostatic index to ABs nutrient traits of other shrubs was not significant.（3）At the local scale（southeastern of the study area）where the precipitation was relatively high,as soil particle size decreased and soil nutrients increased,the homeostasis of the ABs of small shrubs（E.przewalskii and C.mongolicum）decreased,and their stoichiometric traits mostly varied with changes in soil properties,whereas large shrub（H.persicum）still maintained its own strong homeostasis.Interspecific differences in the nutrient traits of ABs of shrubs did not change with changes in soil properties.Similar to the regional-scale results,the N-P scaling exponents of ABs of the three shrubs were also significantly higher than the global average.（4）At the regional scale,the intra-and interspecific stoichiometric traits exhibited different longitudinal and latitudinal patterns,and their relationships with climatic and soil factors also differed.Soil properties presented the greatest influence for E.przewalskii,while C.mongolicum were mainly affected by the interaction of soil properties and climate.The influencing factors on different stoichiometric traits of two saxaul species were diverse and inconsistent,and climatic,soil,and geographic factors played different roles;however,climate showed greater impacts on the N and P contents than soil.At the local scale,the stoichiometric traits of small shrubs（E.przewalskii and C.mongolicum）were strongly correlated with soil properties,whereas the correlations between the large shrubs（H.persicum）and soil properties were weak.Consequently,during the long-term coping with high temperature,drought and soil nutrient deprivation conditions in the Junggar desert,four shrubs formed obvious adaptive differentiation in nutrient traits of ABs.The findings provide a certain scientific basis to further enhance the understanding of ecological adaptation and survival strategy of desert shrubs in arid desert environments.