Hydraulic adjustments are of limited use in bracken fern

Invasive plant species are successful in part due to their ability to adapt to different environmental conditions. The bracken fern Pteridium aquilinum is a highly successful weedy species found throughout the temperate and subtropical regions of the world. It is persistent, toxic to humans and livestock, and can even pollute water sources. Unlike most other ferns, which are relegated to shady understorey habitats by their intolerance for high evapo-transpiration, P. aquilinum tolerates full sun. Also unlike most other ferns, which use only tracheids for water transport, the species has evolved true xylem vessels, considered to be a major advantage in angiosperms.

In a recent study published by Annals of Botany, Alex Baer and colleagues set out to test whether the success of P. aquilinum can be attributed to its xylem vessels and related hydraulic characteristics by comparing sun and shade populations of the fern. The researchers measured a number of hydraulic traits such as hydraulic conductivity and embolism resistance, as well as structural traits such as xylem conduit diameter and length, stomatal density, and vein density.

The authors predicted that hydraulic features related to xylem would play a major role in the ability of P. aquilinum to colonize such a wide range of habitats. Surprisingly, while these traits did contribute, they did so much less than expected. Neither the xylem conductivity nor the resistance of petioles to drought-induced embolism varied significantly between the sun and shade plants. Gas exchange, however, was nearly twice as high in sun plants, with proportional increases in water-use efficiency and stomatal and vein density. These increased densities were largely the result of a more compact leaf in sun plants.

“Taken together, vascular acclimation of P. aquilinum to light is accomplished primarily by reductions in leaf area and a concurrent increase in xylem content,” the authors write. The presence of xylem vessels, contrary to expectations, gave no particular advantage to sun plants. What’s more, the ferns’ adjustments to a high light environment were based more in adding additional tissues rather than “fine-tuning” what was already present. “This may be part of a broader suite of limitations among ferns that may be related in part to their ancestral physiology and the absence of a vascular cambium.” In other words, the presence of xylem may not confer full advantage to these ferns on account of other evolutionary limitations in their physiology.