Ecological Stoichiometry Characteristics Of Two Dominant Shrub Species And Response To Altitude Gradient In The Qilian Mountains

The trade-offs of plant survival strategies are regulated by environment.Altitude,as a key environmental factor,has significant impacts on the plant growth and physiology.By studying the effects of altitude on the ecological stoichiometry of alpine shrub leaves,we can gain a better understanding of the nutrient limitation patterns of alpine ecosystems at a larger spatial scale.The Qilian Mountains,situated in the northwest region of China,serve as a critical water source conservation area.It plays a vital role in regulating regional water cycles and maintaining ecological stability.The distribution of temperature,rainfall and soil nutrients in the region,as well as those of the dominant shrub species,differed significantly along the altitude gradient.These provide an ideal experimental site for studying the effects of altitude on the leaf ecological stoichiometry.In this study,seven alpine shrub communities of low altitude(3100 m,3200 m,3300 m),middle altitude(3400 m)and high altitude(3500 m,3600 m,3700 m)were studied in the Pailugou watershed of Qilian Mountains.The experiment analyzed the distribution patterns of leaf total carbon(C)content,total nitrogen(N)content,total phosphorus(P)content,and stoichiometric ratios of dominant species(Salix gilashanica and Caragana jubata)at various altitudes,as well as their correlations with important value,niche width,soil nutrients,and climatic factors.The results have indicated that:(1)The leaf C content of S.gilashanica increased initially and then decreased with increasing altitude,while the leaf C content of C.jubata continued to increase and was higher than that of S.gilashanica at altitudes above 3400 m.The leaf N content of both species decreased significantly with increasing altitude,and the N content in S.gilashanica leaves were consistently lower than that in C.jubata.Both species showed an upward trend in leaf P content with increasing altitude,but there was no significant difference in leaf P content between the two species at all altitudes.These results indicated that under the same C and N content of the habitat,the C.jubata had a competitive advantage over the S.gilashanica and was able to obtain more C and N nutrients(2)The leaf C:P and C:N ratios of S.gilashanica showed a distribution pattern of increasing first and then decreasing along the altitude gradient,with a peak value335.88 and 20.17 observed at the altitude of 3300 m.In contrast,the C.jubata leaf C:P and C:N ratios showed an overall increasing trend.The N:P of both species exhibited a decreasing trend as altitude increased,indicating a progressive increase in N limitation.Furthermore,the leaf N:P ratio of the S.gilashanica was lower than that of C.jubata at all altitudes,this meant that the level of restriction imposed on S.gilashanica was always stronger than that on C.jubata.(3)The plant height,basal diameter,and crown width of S.gilashanica increased with increasing in altitude,but those of C.jubata decreased.The density,coverage,and biomass of S.gilashanica increased first and then decreased with the increase in altitude,while the density,coverage,and biomass of C.jubata showed an overall increasing trend along the altitude gradient.These findings suggested that C.jubata had a greater ability to adapt to high-altitude environments and to maintain normal growth and development,with higher productivity.The important value and niche width of the two dominant shrub species showed similar trends with their respective changes in abundance,coverage,and biomass along the altitudinal gradient.Moreover,the importance value and niche width of C.jubata were more strongly correlated with leaf C,N,and P content as well as C:N:P stoichiometry than those of S.gilashanica.This implied that even under limited N and P nutrient conditions,C.jubata might possess higher nutrient utilization efficiency.(4)The leaf C and P contents,as well as the C:P ratio of S.gilashanica,showed a significantly positive correlation with corresponding soil indices,while there was no significant correlation among other indicators.There was a highly significant negative correlation between leaf C content,C:N ratio,and C:P ratio of C.jubata and soil organic carbon(SOC)content,SOC:TN and SOC:TP ratios.However,leaf P content and N:P ratio of C.jubata were highly significant positively correlated with soil TP content and TN:TP ratio.These findings suggested that the ecological stoichiometry characteristics of C,N,and P in C.jubata leaves were generally more strongly influenced by soil nutrients than those in S.gilashanica.(5)The mean temperature and total precipitation during the growing season showed a significant positive correlation with the leaf C content,N content,C:P ratio,and N:P ratio of S.gilashanica.In contrast,they exhibited a highly significant negative correlation with leaf P content.For C.jubata,leaf C content,P content,C:N ratio,and C:P ratio displayed an extremely significant negative correlation with temperature and precipitation,whereas leaf N content and N:P ratio exhibited an extremely significant positive correlation with these climatic variables.Generally,the middle-altitude region is more suitable for the growth and development of S.gilashanica.In the harsh environment at higher altitudes,S.gilashanica expends more energy to increase growth rate,maintain its structural advantage,such as plant height and base stem,in order to compete for light resources to the greatest extent possible.Therefore,its distribution is limited along the altitude gradient.However,C.jubata adapts to higher altitudes by utilizing its high assimilation capacity and resource use efficiency,it has a wider distribution and occupies an important ecological position.The results of this study can provide theoretical support for analyzing the nutrient utilization strategies of alpine shrubs under different altitude conditions and their response to climate change.Furthermore,they can play a significant role in enhancing our understanding of the stability of shrub community structure and species adaptation mechanisms,and thus provide scientific evidence for regional ecological environment protection and vegetation restoration and regulation.