Germination response indicates the ability of a species to overcome conditions that potentially limit early establishment. Congeners might be expected to have similar germination responses due to similar evolutionary history. However, when congeners vary by native or introduced origin, artificial selection and other factors associated with species introductions may lead to measurable differences in germination. We assessed germination rates and timing of introduced, invasive Lonicera maackii and Rosa multiflora; introduced, naturalized L. xylosteum; and native L. canadensis under controlled temperature and photoperiod. We hypothesized that introduced shrub species would have higher germination rates than native species because pre- and post-introduction selection would favor individuals with higher germination.
In a growth chamber experiment, one hundred seeds per species were exposed to average temperatures and photoperiods typical of a field site in NE Ohio where a concurrent field study was taking place. Temperatures and photoperiods simulated winter (Nov. – Jan.) or spring (Mar. – May), and were adjusted every 4 weeks. Germination was assessed weekly for 14 weeks, and survival curves were generated using a Kaplan-Meier estimator, where “survival” indicates the probability of not germinating. Log-rank tests of homogeneity were used to assess similarity among the survival curves.
Results/Conclusions
Germination varied among species under the simulated spring treatment, but no seeds of any species germinated under the simulated winter treatment. Although average germination was highest for L. canadensis (>95%), germination began >= one week after introduced L. xylosteum (66%) and => two weeks after invasive L. maackii (up to 72%). Average germination was <10% in R. multiflora, and germination began 0-3 weeks after all other study species. Two-way comparisons showed no differences in survival curves between the two batches of L. canadensis seeds (p = 0.128), while differences between L. canadensis and L. xylosteum varied by seed batch (p = 0.66 and p < 0.01). The survival curve of L. maackii seeds with the highest total germination differed significantly from both batches of L. canadensis seeds (p < 0.001) and from L. xylosteum (p < 0.001).
Invasive L. maackii shows an extended phenology relative to native species; our results indicate that this pattern also extends to germination relative to a native congener. Although germination varied widely by seed batch in L. maackii, and was low in R. multiflora, high propagule production, and space preemption attributed to early germination could overcome low germination rates in these invasives.