SILICA BODIES IN CONIFER FOLIAGE: RESULTS OF A COMMON-GARDEN EXPERIMENT AND IMPLICATIONS FOR THE HISTORY OF THE TERRESTRIAL SILICA CYCLE

What does the terrestrial silica cycle look like before the evolution of silica-accumulating angiosperms, such as grasses? Conifers are the dominant plants of the extant boreal forests, of forests worldwide during Mesozoic Era, and within upland environments in the Late Paleozoic Era. Before the evolution and radiation of angiosperms, conifers were the dominant trees. However, conifers today accumulate small but variable amounts of silica in their tissues, and lack a dedicated molecular mechanisms for accumulation of silicic acid from groundwater. Therefore, the magnitude of the plant contribution to the terrestrial silica cycle in a conifer-dominated world remains unknown.

To examine silica biomineralization among extant conifers, we used a modified dry-ashing protocol to measure the quantity of silica bodies in conifer foliage from plants grown in the Haverford College Arboretum. We sampled more than 23 genera of conifers grown in a common environment and used a local bamboo stand as an experimental control.

Results show low concentrations of silica bodies, or a complete absence, in most conifer foliage, supporting molecular evidence for passive, rather than active, Si-accumulation in the conifer lineage. It is likely that the terrestrial plant silica pool was low in the upland environments of the Late Paleozoic and continued to be low in the absence of ferns, lycophytes, and horsetails until the radiation of grasses and other Si-accumulating angiosperms beginning in the Cretaceous Period and continuing in the Cenozoic Era.