Cambial activity and phloem-xylogenesis of three plant species in an elevation gradient in the Sierra Nevada, México

Background: Secondary xylem and secondary phloem are vascular cambium derivatives and their dynamics (phloem-xylogenesis) has been studied in conifers and dicotyledons that grow in the same place. Studies that evaluate the phloem-xylogenesis in species of wide elevation gradients and with different life forms are scarce. Question: Are there differences in cambial activity and phloem-xylogenesis between species with different life forms and differential distribution in an altitudinal gradient? Studied species: Alchemilla procumbens, Acaena elongata and Ribes ciliatum. Study site and years of study: Cerro Tláloc, Sierra Nevada, Mexico State, Mexico, from 2015 to 2017. Methods: Branches were collected for 24 months per species per site and their tissues were processed through the methyl acrylate-embedding technique. Results: The three species can either form phloem before xylem, form both tissues simultaneously (as in Alchemilla) or the phloem become non-conductive before xylogenesis ends (in Acaena). The periods of cambial activity and of phloem-xylogenesis were longer (17 weeks) in Acaena (hemicriptophyte) and Ribes (phanerophyta); while in Alchemilla (cryptophyte) they were shorter but intermittent over the year and both were further reduced as altitude increases. Conclusions: The cambial activity and xylogenesis among individuals of different altitudes suggest that both are plastic and respond to environmental factors at each site. In the three species, phloem-genesis is less variable than xylogenesis as it has been recorded in other species. The rapid response of vascular cambium in Alchemilla suggests that it takes advantage of the pulses of environmental factors beneficial to the phloem-xylogenesis.