Old fronds serve as a vernal carbon source in the wintergreen fern Dryopteris intermedia (Aspleniaceae)

Abstract

Maintaining green leaves beyond the growing season has been hypothesized to benefit plants by supplying either a nutrient or a carbon source. Understanding such ecophysiological aspects of plants will help us to appreciate how a species functions in its environment and predict how it might be affected by future changes in that environment. The wintergreen fern species Dryopteris intermedia does not retranslocate nitrogen and phosphorus from old fronds in spring, but photosynthesis does take place in the old fronds during this season. To determine if carbon fixed in the old fronds is translocated to other parts of the plant, we labeled old fronds with (13)C via photosynthetic uptake and examined old fronds, new fronds, fine roots, and rhizomes for (13)C content 1 day and 1 month after labeling the old fronds. Vernally fixed carbon was translocated to the new fronds but not significantly to the below ground tissues. Old fronds in this species, therefore, serve as a carbon source for vernal growth of new fronds. This is the first study in which a fern was labeled with (13)C to track vernally fixed carbon from old fronds to the rest of the plant in a wintergreen species. Future research should examine the precise timing of this carbon movement and examine other species for a similar or contrasting strategy.

The relative photosynthetic contribution of old and new fronds of the wintergreen fern Dryopteris carthusiana, Ontario, Canada1

“…Findings have not been consistent across published results, with most studies(Van Buskirk and Edwards 1995, Noodén and Wagner 1997, Tessier and Bornn 2007 finding significant contributions to spring carbon gain, and others finding only minor photosynthetic contributions(Tani and Kudo 2005). Still few, if any, studies quantify the overall contribution to total plant-level carbon gain by old versus new fronds.…”

“…Dryopteris carthusiana, as has been described for other members of the genus, maintains intact vascular tissue through the winter under the snowpack (Tessier and Bornn 2007). During the autumn, after leaf fall but before the winter snow pack develops, a short section of the stipe near the ground senesces allowing the frond to sink to the ground (Noodén and Wagner 1997).…”

“…Additionally, a handful of studies have evaluated the contribution of overwintered fern leaves to springtime carbon assimilation (Bauer et al 1991, Van Buskirk and Edwards 1995, Noodén and Wagner 1997, Tani and Kudo 2005, Tessier and Bornn 2007 and resource storage (Minoletti and Boerner 1993, Tani and Kudo 2003, but this latter phenomenon is not the case for all wintergreen ferns (Tessier 2001 Uemura (1994) suggests that in the case of plants which have overwintering leaves with low spring photosynthetic rates, the leaves may primarily serve as storage organs providing resources to accelerate the expansion of new leaves in the spring. Given the similarities in photosynthetic parameters we found for old and new fronds of Dryopteris carthusiana, and the significant contribution to overall carbon gain it does not seem that overwintered fronds should be seen as primarily storage organs, rather in our study they were an important source of vernal carbon gain.…”

“…Specifically we seek to expand on Tessier and Bornn's (2007) finding that old fronds act as a vernal carbon source by testing the hypothesis that vernal carbon gain will be reduced under boreal canopies compared to deciduous canopies due to reduced vernal light availability in the former forest type. Specifically we: 1) quantify the demographic and physiological differences between D. carthusiana found under deciduous vs. boreal forest overstories, 2) quantify the degree to which the wintergreen habit leads to overall growing season net carbon assimilation by following photosynthesis of old and new fronds through the growing season under both boreal and deciduous overstories, and 3) assess the relative difference in D. carthusiana plantlevel carbon gain in a boreal vs. deciduous forest.…”

“…Thus, the old fronds, which are sitting atop the leaf litter following spring snowmelt, are exposed to high spring light levels. Under these high light conditions, the previous summer's leaves, which are able to photosynthesize, provide significant resources to enhance plant growth (Van Buskirk and Edwards 1995) most likely as a spring carbon source for new frond growth (Tessier and Bornn 2007). New fronds begin to unroll slightly more than a week after snow melt and the old fronds begin to senesce during midsummer once the new fronds are fully developed (Mahall and Bormann 1978).…”