Abstract Detail

Ecophysiology

Cole, Catherine [1], Neufeld, Howard [2].

An Unusual Succulent in Southern Appalachian Forests: How Stonecrop (Sedum ternatum) Upsets the Paradigm of Thin Leaves for Understory Shade-Tolerant Species.

Sedum ternatum Michx., also called stonecrop, is a prostrate succulent herb native to Appalachian deciduous forests. It is exceptional in that it thrives in the moist forest understory, yet exhibits a leaf morphology more typical of species in dry, high-light environments, including those in the Sedum genus. This species preferentially grows on shallow soils atop rocks or adjacent to tree trunks. S. ternatum leaves are, paradoxically, much thicker (881 + 51.9 μm) than associated congeneric species: the thickest leaved congener, Erythronium americanum, is 455 + 16.1 μm while the thinnest leaved congener, Laportea canadensis, is 90 + 6.6 μm. The thick S. ternatum leaves at first appear mal-adapted to the low light environment beneath a forest canopy (0.4% full sunlight) and appear to be an exception to the paradigm that shade-tolerant understory species should have broad, thin leaves. However, this species is also evergreen, and leaves are present when the overstory canopy is absent and light levels are high. We hypothesized that 1) the succulent leaf morphology of S. ternatum confers high drought tolerance that permits adaptation to dry, understory microhabitats in which most shade plants are not competitive; and 2) because it is evergreen, S. ternatum achieves most of its carbon gain in early spring before canopy emergence, and in autumn after canopy senescence, functioning as a sun plant when high light reaches the forest floor. With respect to hypothesis 1, plants withstood five weeks of drought in a greenhouse experiment, recovering full gas exchange function and pre-stress water potentials within one or two days upon re-watering. Droughted plants exhibited greater nocturnal acid accumulation than controls, suggesting a form of CAM photosynthesis was engaged by drought. Regarding hypothesis 2, plants had high rates of photosynthesis prior to canopy leaf out and after leaf fall when light levels were highest. Summer rates of gas exchange were minimal and sometimes negative, although plants responded to sunflecks. We suggest S. ternatum avoids competition by using succulent leaves to moderate water stress which allows it to persist in microhabitats with shallow soils prone to frequent drought. As this requires thick leaves that would be mal-adapted to low light environments, it has shifted the majority of its annual carbon gain to spring and fall when light levels are higher. Thus the paradox of a thick-leaves species in the understory is rationalized away because S. ternatum acts as a sun plant during these times.

1 - Appalachian State University, 572 Rivers St., Dept. Of Biology, Boone, NC, 28608, United States
2 - Appalachian State University, Biology, 572 Rivers St., Boone, NC, 28608, United States

Keywords:
Understory
Succulent
S. ternatum
Appalachian.

Presentation Type: Oral Paper
Session: ECOPH2, Ecophysiology II
Location: San Pedro 2/Starr Pass
Date: Tuesday, July 30th, 2019
Time: 2:30 PM
Number: ECOPH2005
Abstract ID:187
Candidate for Awards:None