Plant Leaf And Fine-root Functional Traits And Diversity In Typical Communities Of The Mu Us Desert

During the first two decades of the 21^stcentury,China has made remarkable progress in desertification control.The area of desertified and degraded grasslands has been decreasing while vegetation cover has been increasing.Desertification has been reversed dramatically.However,our knowledge remains limited on plant leaf and root functional traits and diversity in typical communities over the course of vegetation restoration.This knowledge gap hinders a full assessment of the effectiveness of desertification control efforts.This study investigated plant leaf and root functional traits of typical communities in different successional stages（i.e.,semi-fixed dunes,fixed dunes,fixed dunes covered with biological soil crusts,fixed dunes with abundant herbaceous plants）in the Mu Us Desert of northern China.The objectives of this study were（1）to examine the interspecific variations in leaf and root functional traits and their correlations,（2）to compare traits and trait correlations among functional groups and successional stages,（3）to quantify species diversity,functional diversity,and phylogenetic diversity in typical communities at different successional stages and to further reveal the mechanisms underlying communities assembly for different successional stages.The key findings are as follows.（1）Leaf nitrogen and phosphorus contents（LN,LP）of the 48 species investigated were higher than the average values reported in global synthesis studies.The average leaf nitrogen-to-phosphorus ratio（LN:LP）in our study was 11.66,indicating that plant growth was mainly limited by nitrogen.Our results on leaf trait correlations partially supported the leaf economics spectrum（LES）theory.Specific leaf area（SLA）was negatively correlated with leaf thickness（LT）,leaf dry matter content（LDMC）,leaf tissue density（LTD）,carbon-to-nitrogen ratio（LC:LN）,and positively correlated with LP,but was not significantly correlated with LN.LP and LN were also largely decoupled.The relationships between paired leaf traits were inconsistent among functional groups（PFGs）.In addition,the overall pattern of leaf trait correlations suggested differentiation among PFGs.SLA and LN were lower while LC:LN was higher in semi-fixed dunes than in the other three stages.Although leaf-trait syndrome did not differ significantly among successional stages,the relationships between paired leaf traits were inconsistent,suggesting that plants are capable of adjusting leaf trait relationships to maximize survival and growth.（2）Fine-root nitrogen content（RN）of species in study region was close to the average value at the global scale,while the phosphorus content（RP）was higher than the global average.Fine-root nitrogen-to-phosphorus ratio（RN:RP）was lower than the global average,indicating that root growth in the study area,like leaf growth,was mainly N-limited.Specific root length（SRL）was not correlated with other measured fine-root traits.Fine-root tissue density（RTD）was positively correlated with RN and RN:RP,but negatively correlated with RP.Therefore,fine-root trait correlations did not mirror the leaf-trait syndrome,and the variations in fine-root traits did not support the root economic spectrum（RES）.The relationships between paired fine-root traits were inconsistent among PFGs,and fine-root trait syndrome differentiated significantly among PFGs.The relationships between paired fine-root traits was inconsistent among successional stages.In addition,RTD,RP and fine-root trait syndrome varied significantly among successional stages.（3）Fine-root tissue density,nitrogen and phosphorus contents,nitrogento-phosphorous ratio,and carbon-to-nitrogen ratio all increased with analogous leaf traits,whereas SRL was not correlated with SLA.After phylogenetic effects were considered,nutrient contents and their ratios still displayed stronger coordination between leaves and fine roots than morphological traits.The N and P contents and the N:P ratio were higher in leaves（more active organ）.The plasticity of nutrient contents was higher in fine root than in leaves.The relationships between analogous leaf and fine-root traits were inconsistent among PFGs and successional stages,and the whole-plant trait syndrome varied significantly among PFGs and successional stages.This suggests that the effects of functional groups and habitat conditions on trait association and tradeoff should be considered when predicting root traits from leaf traits.（4）Forα-diversity,Shannon-Wiener diversity,species richness,simpson index,functional richness,functional dispersion,Rao Q quadratic entropy and phylogenetic diversity were lowest in plant communities at the phase of fixed dunes covered with biological soil crusts.All of theseα-diversity parameters were not significantly different among plant communities during the other three stages.Furthermore,these biodiversity indices were positively correlated with each other,suggesting coordinated changes in species,functional,and phylogenetic diversity during community succession.The pairwise differences in species composition,functional traits,and phylogeny between community succession were mainly attributable to species turnover.In addition,allβ-diversity indices increased with the number of transitions between stages,indicating continued changes in community species composition during community succession.Species composition,functional traits,and phylogeny all changed dramatically during the transition from semi-fixed to fixed dunes,resulting in a large dissimilarity between communities at the two stages.The phylogenetic structure of plant communities tended to diverge at the three later stages of community succession,indicating that competitive exclusion was the key factor driving community organization.However,the phylogenetic structure of plant communities on semi-fixed dunes did not exhibit any consistent patterns,implying that community assembly was affected by the combined effects of habitat filtering and competitive exclusion.In summary,our results suggest that fine-root functions in the studied desert communities were probably coordinated with leaf functions with respect to nutrient allocation and use,although fine-root traits did not mirror leaf-trait syndrome during community succession.In addition,both functional groups and habitat conditions affected trait correlations and tradeoffs.Species diversity,functional diversity,and phylogengtic diversity were positively correlated with each other.In addition,habitat filtering,competitive exclusion all played an important role in shaping plant communities during community succession.Our findings contribute to an improved understanding of plant functional traits,resource utilization strategies,and biodiversity in desert ecosystems,aid in the prediction of aboveground and underground ecological processes,and also provide important implications for conservation practices in deserts.