Effects of invasive knapweed (Centaurea stoebe subsp. micranthos) on a threatened native thistle (Cirsium pitcheri) vary with environment and life stage

Authors: Rand, Tatyana; LOUDA, SVATA - University Of Nebraska; BRADLEY, KATE - University Of Nebraska; CRIDER, KIM - National Park Service

Submitted to: Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2015
Publication Date: 6/9/2015
Publication URL: http://handle.nal.usda.gov/10113/61785
Citation: Rand, T.A., Louda, S.M., Bradley, K., Crider, K. 2015. Effects of invasive knapweed (Centaurea stoebe subsp. micranthos) on a threatened native thistle (Cirsium pitcheri) vary with environment and life stage. Botany. 93(9):543-558. DOI: 10.1139/cjb-2015-0032.

Interpretive Summary: Although invasive plants can threaten rare native plants, more direct evidence of the strength of their impacts on native plant life stages needed. To determine whether Eurasian spotted knapweed, Centaurea maculosa, affected seedling emergence and establishment, juvenile survival or flowering probability of the rare thistle species, Cirsium pitcheri, we performed two seed addition experiments in two sand dune habitats – lakeshore linear dune, high perched dune – within Sleeping Bear Dunes National Lakeshore, Michigan, USA. Seeds were planted in three plant neighborhoods in each habitat: near C. maculosa, near a native plant, or in the open. Seedling emergence, seedling establishment and early plant survivorship were significantly higher on the perched dune, where surface soils were wetter, than on the lakeshore dune. The influence of plant neighbors varied with habitat and plant life stage. Emergence was reduced by C. maculosa on the lakeshore dune, and on the perched dune, emergence increased near any neighbor. Early plant survival was significantly lower near C. maculosa in both habitats. Therefore, C. pitcheri experienced competition from C. maculosa on the lakeshore dune, and on the perched dune, the interaction shifted from positive (facilitative) to competitive (negative) between successive life stages. The probability of a seedling reaching the adult flowering stage was much lower, and it occurred later, for plants with C. maculosa neighbors, especially on the lakeshore dune. Surface soil moisture was lower on the lakeshore dune, helping explain lower seedling success there. However, plant water status of established C. pitcheri plants, quantified by leaf water content and 13C isotopic analyses, was higher on the lakeshore dune. This paradoxical result suggests that on the lakeshore dune the seedlings that do survive, being closer to lake mean water level, likely have greater access to deeper, subsurface water. Overall, the results demonstrate that C. maculosa has a strong negative effect on survival and flowering of C. pitcheri, but interaction intensity varies with habitat and life stage. Our results suggest that coupling seed addition with watering and invasive plant removal following seedling emergence could increase populations of this rare native plant.

Technical Abstract: Although invasive plants can threaten rare plants, more direct evidence on the type and magnitude of their effects on demographic parameters is needed. To determine whether Eurasian spotted knapweed, Centaurea maculosa, affected seedling emergence and establishment, juvenile survival or flowering probability of the rare Cirsium pitcheri, we performed two seed addition experiments in two dune habitats that varied in underlying abiotic conditions – lakeshore linear dune, high perched dune – within Sleeping Bear Dunes National Lakeshore, Michigan, USA. Seeds were planted in three treatments in each habitat: near C. maculosa, near a native plant, or in the open. Seedling emergence, seedling establishment and early juvenile survivorship were significantly higher on the perched dune, with higher surface soil moisture, than on the lakeshore dune. Treatment effects varied with habitat and plant life stage. Emergence was reduced by C. maculosa on the lakeshore dune, and on the perched dune, emergence increased near to any neighbor. Juvenile survival was significantly lower near C. maculosa in both habitats. Therefore, C. pitcheri experienced competition from C. maculosa on the lakeshore dune, and on the perched dune, the interaction shifted from facilitative to competitive between successive life stages. The probability of a seedling reaching the adult flowering stage was much lower, and it occurred later, for plants with C. maculosa neighbors, especially on the lakeshore dune. Surface soil moisture was lower on the lakeshore dune, helping explain lower seedling success there. However, plant water status of established C. pitcheri plants, quantified by leaf water content and 13C isotopic analyses, was higher on the lakeshore dune. This paradoxical result suggests that on the lakeshore dune the seedlings that do survive, being closer to lake mean water level, likely have greater access to deeper, subsurface water. Overall, the results demonstrate that C. maculosa has a strong negative effect on survival and flowering of C. pitcheri, but interaction intensity varies with habitat and life stage. Our results suggest that coupling seed addition with watering and invasive plant removal following seedling emergence could bolster populations of this rare endemic plant.