Which results of the standard test for community‐weighted mean approach are too optimistic?

Questions Elevational gradients encompass multiple resource or non‐resource stressors, but how these different gradients drive functional structure is not well understood. Abiotic variables and functional traits were sampled along a 1,400 m tropical elevational gradient to answer the following questions: (a) how do the functional diversity and functional structure of tree communities change with environment; (b) how does the different assembly vary along the elevational gradient? Location Tropical forests on Hainan Island, South China. Methods Along an elevational gradient from 180 m to 1,521 m, sixty 0.05‐ha transects were established. We recorded all woody stems ≥5 cm dbh (diameter at breast height) and measured functional traits and soil features in each transect. The patterns of abiotic variables, functional diversity and community‐weighted mean (CWM) of functional traits were explored along the elevational gradient by using a regression model. The assembly along the elevational gradient was compared with a null model that shuffled species identities. Results The functional richness (FRic), functional dispersion (FDis) and CWMs of specific leaf area, leaf phosphorus and nitrogen content decreased with increasing elevation, as well as with the first axis of the principal components analysis (PCA1) of abiotic variables. The CWMs of leaf dry matter content and maximum height did not show significant change with elevational change, but the CWM of maximum height increased with PCA1. Both FRic and FDis became less divergent and more convergent with increasing elevation and with PCA1, which differed from the predicted outcome under the null model. Conclusions All these observations support that environmental filtering selects species with either acquisitive or conservative strategies at low elevations, but it only selects species with conservative strategies at high elevations. The presence of different stressors, resources (water) and non‐resources (temperature and pH) leads to distinct community assemblages with different functional structures.

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

The effect of environmental filtering on variation in functional diversity along a tropical elevational gradient

Ding

Huang

et al. 2019

“…We also provide synoptic tables containing diagnostic, constant and dominant species, Ellenberg indicator values (EIV; Ellenberg et al, ), and geographical distribution of the clusters as Appendices . Statistical comparison of clusters on the basis of variables dependent on compositional information (e.g., diversity indices, aggregated species attributes) often result in false positive tests due to the so‐called ‘similarity issue’ (Zelený, ; Zelený & Schaffers, ) and other structural biases (Hawkins et al, ) if the same occurrence information had been used for the definition of clusters. To avoid false conclusions we refrain from formal statistical tests in the case of between‐cluster comparisons and present only boxes‐and‐whiskers plots.…”

Section: Methodsmentioning

confidence: 99%

Trait‐based numerical classification of mesic and wet grasslands in Poland

Lengyel

Swacha

Botta‐Dukát

et al. 2020

Questions What vegetation types can be distinguished on the basis of plant functional traits using numerical classification? How do they match syntaxonomical units? Location Poland. Methods Six thousand nine hundred and eighty‐five vegetation plots representing mesic and wet grasslands (Molinio‐Arrhenatheretea, Polygono‐Poetea) were retrieved from the Polish Vegetation Database. Plant functional trait data were assembled from the LEDA and Clo‐Pla databases for most species occurring in the data set. Community‐weighted mean for five traits were calculated for each plot: specific leaf area, canopy height, seed mass, bud bank index and clonality index. Plots were classified using Ward's method and iterative relocation based on silhouette widths. The clusters were interpreted and characterized on the basis of species and trait composition, functional diversity, functional redundancy, Ellenberg indicator values, and geographical distribution. The similarity between the trait‐based classification and the syntaxonomical assignment of plots is evaluated both statistically and by expert knowledge. Results Twelve clusters were distinguished. The classification mirrored the main gradients structuring grasslands in Poland, although, some vegetation types with the strong dominance of functionally unique species appeared more distinct than they are treated in syntaxonomy. Clusters did not differ significantly in functional diversity and redundancy. The differences of clusters in species and trait composition and environmental background are discussed. Conclusion The application of trait data and numerical methods is a promising approach for obtaining vegetation classifications. Such classifications can be in closer relationship with the most important ecosystem processes than floristic classifications because communities comprising different species but similar functional trait distribution are not separated. Trait‐based classifications match phytosociological units to a variable degree. Functional uniqueness and variation of abundance determines how individual species influence the delimitation of vegetation types using our approach.

“…(Ter Braak, Šmilauer, & Dray, ) and implemented in Canoco 5.11. This approach allows testing for links between species composition and species traits, and species composition and the environment (Zelený, ); avoiding the inflated type I error that has been reported for approaches based only on CWMs (Peres‐Neto, Dray, & ter Braak, ). The Canoco projects calculating dc‐CAs are provided as Appendix .…”

Section: Methodsmentioning

confidence: 99%

Salt marsh migration into salinized agricultural fields: A novel assembly of plant communities

Gedan

Fernández‐Pascual

2019

Questions Sea level rise and saltwater intrusion are changing low‐lying coastal landscapes, converting agricultural land and other upland habitats to tidal marsh. Abandoned, saline agricultural fields are affected by a unique combination of environmental filters, those traditionally found in tidal marsh — salinity and flooding — alongside those of cultivated lands — high nutrient availability and a history of disturbance. We asked how species composition and functional trait composition in saline fields compares to those in traditional old fields and natural ecotones, and whether trends in succession can be detected in saline fields during the first years post‐abandonment. Location Chesapeake Bay (Mid‐Atlantic, USA). Methods We surveyed plant communities assembling in saline fields and compared taxonomic and functional trait diversity to those in old‐field and marsh–forest ecotone communities. We also assessed changes in the saline fields after two and three years of abandonment to detect the direction of succession. Results Saline fields occupied an intermediate taxonomic and trait space between old fields and marsh ecotones. From old fields to saline fields to marsh, communities were less weedy, and more wetland, native, and perennial. Specific leaf area decreased across this transition, in concordance with expected changes in response to salinity. Over time, saline fields became less graminoid and less weedy, and more native, wetland, and woody. Conclusions We conclude that marsh migration into abandoned farmland is producing a novel assembly of plant communities. Intermediate functional traits in the saline fields reflect the novel environmental filters imposed by saltwater intrusion and the cultivation legacy. These patterns suggest that abandoned, saline agricultural fields may develop somewhat differently than natural marsh boundaries, with more shrub dominance and greater resilience to Phragmites australis invasion. Importantly, these results suggest that saline fields will provide a facilitating route for marsh migration.