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.

Combining leaf gas-exchange and stable carbon isotopes to assess mycoheterotrophy in three species of Pyroleae

Determining the extent of mycoheterotrophy (MH) in plants, primarily through the use of stable isotope methods, has gained considerable attention in the last decade. The aim of this study was to characterize the rates of photosynthesis (PS) and several gas-exchange parameters, as well as stable carbon isotope composition (δ13C) of partially mycoheterotrophic (PMH) Pyroleae compared with autotrophic reference species of Ericaceae. An end-member mixing model was applied to δ13C, deriving estimates of % C gained via fungi (CDF). The δ13C was significantly enriched for Orthilia secunda and Pyrola chlorantha (relative to autotrophs) resulting in estimates of CDF ranging from 13.8% to 20.8%. Despite significantly lower PS rates for O. secunda and P. chlorantha, as well as lower conductance and transpiration, there were no significant differences in the Ci:Ca ratios across all of the species, suggesting that the C isotope inferences for these two species were reflective of fungal C gains. By contrast, results for all of the variables indicated primarily autotrophic C nutrition for Chimaphila umbellata. Further studies, such as isotope labelling experiments or assessments of biochemical constraints to autotrophy, may resolve the uncertainties in these species, allowing more accurate understanding of the complex nutritional mode of these plants.