How Significant are Conifers for the Silicon Biogeochemical Cycle?
The conifers were generally shown by Hodson et al. (2005) to be low silicon accumulators. The apparent reason is that conifers (and many gymnosperms) largely lack NIPIIs, aquaporins that are involved in silicon transport in plants (Trembath-Reichert et al. 2015). This led Trembath-Reichert et al. to propose that the terrestrial silicon biogeochemical cycle would be decreased in the Mesozoic Era because of the dominance of conifers.
However, scrutiny of the supplementary data from Hodson et al. suggests that there are some conifer species (e.g. Abies alba, Larix decidua, Picea abies) where silica is over 2% dry weight in the needles, well within the range for the silicon-accumulating Poales (which include the grasses). Moreover, in species that retain their needles longer than one year, silicon accumulates steadily over several seasons (Hodson and Sangster, 1998; Wyttenbach et al. 1991).
Gerard et al. (2008) modeled the silicon biogeochemical cycle in a Douglas fir (Pseudotsuga menziesii) forest. They found that the needles contained up to 2.7% silica, that 60% of the biogeochemical cycle was controlled by biological processes, and it was calculated that 37% of silicon was taken up actively by the trees. The available data suggest that some important conifer species, including fir, larch, spruce and Douglas fir, can accumulate significant amounts of silica in their needles, and that silicon cycling in conifer-dominated systems will not necessarily be lower.