BELL, TIMOTHY J.1,2* and MARLIN BOWLES2. 1Chicago State University, Department of Biological Sciences, 9501 South King Drive, Chicago, IL, 60628; 2The Morton Arboretum, Lisle, IL, 60532. - Elasticity analysis of Pitcher’s thistle (Cirsium pitcheri) and Mead’s milkweed (Asclepias meadii) restorations.
We evaluate elasticity analysis of different life stage contributions
to population growth (l) as a method to
guide management application for restorations of two federal
threatened plant species with contrasting life history strategies.
Pitcher’s thistle (Cirsium pitcheri) is a short-lived
monocarpic herb endemic to western Great Lakes sand dunes where it
colonizes successional habitats and requires frequent cohort
replacement to maintain populations. Mead’s milkweed (Asclepias
meadii) is a long-lived iteroparous herb of late-successional
midwestern tallgrass prairie and glades, with infrequent natural
reproduction. Long-term demographic monitoring data were used to
develop matrix models for a natural population and a restoration of
Pitcher’s thistle and for two restorations of Mead’s milkweed. We
grouped elasticities into those representing combined effects of
changes to growth (G), stasis and retrogression (L) and fecundity (F)
on l. Although most C. pitcheri
matrices had G/L/F elasticity ratios expected for semelparous plants,
several were more similar to those expected for iteroparous herbs,
whereas ratios for A. meadii were similar to those expected for
late-successional plants. For C. pitcheri l tends to increase with increasing F in both the
natural and restored populations, suggesting that we should be able to
increase l by increasing recruitment. For
A. meadii L and G had much stronger effects on l than F, suggesting that restoration management
should focus on growth and stasis rather than increasing reproduction,
which seems applicable because of the time required for flowering,
expected longevity, and apparently low fecundity of this species.
Because of its short life span and rapid cohort development, C.
pitcheri provided useful transitions for each stage, and
demographic data from a natural population strengthened interpretation
of elasticities. Interpretation of A. meadii elasticities
supported a logical management need to increase growth despite
limitations due to slow growth, low fecundity and lack of data from
natural populations.
Key words: Asclepias meadii, Cirsium pitcheri, elasticity analysis, restoration