Belowground morphology and population dynamics of two forest understory herbs of contrasting growth form

Forest understory herbs exhibit a large range of variation in morphology and life history. Here we expand the reported range of variation by describing the belowground structures of two very different species, Xerophyllum tenax and Chimaphila umbellata. We excavated individuals in forests of the Cascade Mountains, Pacific Northwest, USA. Xerophyllum tenax has short rhizomes, but an extensive root system that is exceptionally large among forest understory species. The roots reach 4 m in length and may occupy an area 50 times that of the aboveground canopy. In contrast, Chimaphila umbellata has very small roots, but an extensive rhizome system. The largest plant we excavated had 57 m of connected rhizomes and still had a seedling source. Both species have long-lived individuals but differ in response to disturbance. Based on monitoring of 151 permanent 1 m2 plots in an old-growth forest, X. tenax increased only minimally in density over 40 years following tephra deposition from the 1980 eruption of Mount St. Helens, whereas density of C. umbellata increased substantially. The very different morphology of these two species highlights the large range of variation found among forest herbs, which needs to be considered when examining the forest understory.

Relationship Between Xerophyllum tenax and Canopy Density in the Southern Cascades of Washington

Large-scale commercial harvest of beargrass (Xerophyllum tenax) has been taking place in the Cascades of Washington and Oregon for the past 15 to 20 years. The long, slender leaves are either used fresh or dried and dyed for use in the floral industries in the United States and Europe. Our objectives were to develop a better understanding of beargrass production under different tree canopy (overstory) densities in the Pacific silver fir/big huckleberry/beargrass and the mountain hemlock/big huckleberry/beargrass plant associations in and around the Cispus Adaptive Management Area. We examined differences in beargrass production for different overstory canopy conditions on 10 sites in each association. Results indicated that beargrass quality is not of commercial grade under open canopies (<60% overstory density). For medium and high densities, the interaction between plant association and overstory density was significant for all response variables except harvestable dry mass. Harvestable dry mass of beargrass did not differ between the two associations, but was greater under medium- compared with high-density conditions. For the Pacific silver fir association, the high-overstory-density class had greater basal area of beargrass per site, and plants were larger with longer leaves compared to medium-canopy-density sites. We did not find this relationship for the mountain hemlock association, except for the longest leaf variable. It is unclear why basal area and size of beargrass were more closely related to overstory conditions for the Pacific silver fir association. Evaluation of the sustainability of beargrass as a nontimber forest product will require long-term study of the relationships among environmental variables, beargrass productivity, and beargrass population dynamics. West. J. Appl. For. 19(2):82–87.