Elevational trends of biodiversity and plant traits do not converge—a test in the Helan Range, NW China

When elevational gradients include combinations of different climatic gradients, such as a decline in temperature combined with an increase in moisture, vegetation and plant trait responses are difficult to explain. Here, we used plant species richness and morphometric traits data across steep elevational gradients in the Helan Range of Northwestern China in an attempt to separate general trends (temperature related) from regional peculiarity (moisture related). Based on the floristic data of the Helan Range as well as plot-based data, both drought (at low elevation) and low temperature (high elevation) are associated with low species richness presumably also explaining the peak in diversity at mid-elevation, where climatic conditions are moderate. However, this mid-elevation peak in diversity is not mirrored in trends of plant traits such as leaf size and inflorescence size, which show either unidirectional trends or no change with elevation (with impacts of drought and low temperature perhaps gradually replacing each other). Our analysis illustrated the taxonomic and plant functional type (PFT)-related biases in functional trait studies and showed that consistent patterns only emerge after careful data stratification, with taxonomy (family level) holding more promises than PFTs. Inflorescence size increased with elevation in major insect-pollinated families, a trend not seen in wind-pollinated (graminoid) families. However, the reproductive effort expressed as inflorescence/leaf length ratio increases with elevation in the majority of plant families, irrespective of their pollination system. The fact that these biometric responses to elevation do not correlate with responses in species richness (which peaks at mid-elevation) may reflect contrasting drivers of trait selection and biodiversity. Based on our plot-based data, this analysis also confirmed the usefulness of floristic archive data for testing ecological theory related to elevational gradients.