LB 22-235 - Cautious coexistence: quantifying species interactions and performance effects of two associated forest invaders on a co-occuring native ephemeral

Background/Question/Methods:

Co-occurrence of multiple invasive species is increasingly common, and the effects of potential interactions among them are unclear. Many Eastern North American floodplain forests are invaded by the shrub Amur honeysuckle (Lonicera maackii) and, more recently, the spring ephemeral lesser celandine (Ficaria verna). Both species initiate leaf-out before co-occurring natives and form dense canopies that reduce light availability throughout the spring growing season. White trout lily (Erythronium albidum) is a native ephemeral that commonly co-occurs with both invaders. Erythronium albidum reproduces almost exclusively vegetatively, with the shoot system consisting of a single stem and leaf. Despite this simple architecture, we observed substantial variation among Erythronium in apparent allocation to stem versus leaf growth.



We hypothesized that resource availability near celandine and/or honeysuckle may influence resource allocation in Erythronium, with potential costs via reduced photosynthetic capacity. Assuming stems are not photosynthetic, resources invested in stem growth could result in fewer resources returned to roots for future growth and fecundity.


We sampled Erythronium along transects containing uninvaded areas and those containing honeysuckle, celandine and both species. We measured light levels, soil temperatures, and celandine/honeysuckle abundance to assess resource availability. Erythronium leaf area, stem height, and total height were measured to quantify performance.

Results/Conclusions:

Celandine reduced light at the soil surface by 98% (p< .0001, r2 =. 68). Prior to its canopy leaf-out, honeysuckle reduced plant-available light by 43% across the entire sampling area (p< .0001).



Stem height of Erythronium growing amidst celandine was 294% greater than stems of individuals growing in leaf litter alone and 243% greater than stems of individuals growing in litter-free areas (p< .0001, r2= .48). Across the entire dataset, Erythronium leaf area was negatively correlated with stem height (p< .05); however, within celandine stands they were positively correlated (p< .0001), suggesting that stem versus leaf growth may not represent a resource allocation tradeoff in this environment. Nevertheless, leaf area was lowest within celandine stands (p< .05), suggesting that resource competition may negatively affect its long-term performance. Honeysuckle did not affect any Erythronium measures (p >.1), suggesting that honeysuckle invasions may only negligibly impact this species.



Erythronium appears to etiolate in response to low light levels produced by celandine canopies, but etiolation did not come at an apparent cost of leaf area reduction. These findings raise questions about the physiological response of Erythronium to celandine invasions and have implications for the trajectory of native ephemeral community composition in invaded forests.