Abstract
Mountains represent ‘experiments by nature’ that permit testing ecological theory. Using herbarium samples of 92 Rhododendron species collected between 800 and 4500 m a.sl. in the Himalaya-Hengduan Mountains region, SW China, we explored congeneric elevational trends in key plant traits at the among- and at the within-species level. We aimed at identifying the contributing factors to phenotypic trait expression across a climatic gradient, distinguishing effects in response to elevation from those due to phylogenetic relatedness, non-phylogenetic species effect, and within-species trait variability. We found that a substantial fraction of the trait variation (up to 88%) is deeply rooted in phylogeny, with species elevation also revealing a strong phylogenetic signal. While accounting for these phylogenetic influences, we still found consistent associations with elevation of anatomical (increasing leaf thickness and decreasing specific leaf area), morphological (decreasing plant and leaf size, annual length increment), physiological (less negative foliage δ13C signal), and reproductive traits (flower size largely conserved, while leaf size declines, hence relative investment in flowers increases). Our findings support a combined action of phylogenetic inertia and phenotypic adjustments of plants to life conditions at high elevation. Given that our samples come from one genus and a natural climatic gradient (rather than botanical gardens) with temperature the dominant variable, such patterns represent a robust and representative signature of the multiple causes of trait-environment associations in woody species.
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Acknowledgements
We thank the Herbarium of Kunming Institute of Botany, Chinese Academy of Sciences (KUN) for permitting us to sample their vouchers. We also thank Daniel Nelson in Ansgar Kahmen’ s lab in Basel University for stable isotope data.
Funding
YY acknowledges support from the Ten-thousand Talents Program of Yunnan Province (YNWR-QNBJ-2018-318 to YY) and the Yunnan Innovation Team Project (202305AS350004 to YY). JMdV acknowledges support from Swiss National Science Foundation grant (310030_185251 to JMdV).
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CK conceived and designed the work. JML performed data collection. JMdV designed phylogenetic statistical analyses. JML, JMdV, CK and YY jointly performed data analysis and manuscript writing.
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Supplementary file1 Fig.S1 Elevation range of 92 studied Rhododendron species, expressed as the elevation difference between the lowest and highest collections for each species; Fig. S2 Matrix plot of phylogenetic bivariate relations among species means of eight representative trait variables; Fig.S3 a: leaf thickness exhibited significant negative correlation with specific leaf area (SLA); b: leaf thickness exhibited a significant positive correlation with leaf area; c: leaf area exhibited a significant positive correlation with plant height; Fig. S4 Neither length of the one year (a) nor two years (b) old segment of shoot significantly correlated with plant height; Fig. S5 Plant traits exhibited non-significant responses to elevation both among and within species with confidence interval (95% confidence level); Fig. S6 The ratio of shoot length to shoot thickness exhibited significantly negative correlation with elevation; Table S1 Difference of leaf stable C isotope composition (δ13C) of Rhododendron species between shade levels ranked according to the notes on the vouchers on light conditions made by collectors and their interactions with elevation
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Liu, JM., de Vos, J.M., Körner, C. et al. Phylogeny and phenotypic adjustments drive functional traits in Rhododendron across elevations in its diversity hot-spot in W-China. Alp Botany 133, 69–84 (2023). https://doi.org/10.1007/s00035-023-00294-5
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DOI: https://doi.org/10.1007/s00035-023-00294-5