Biomass allocation of Vincetoxicum rossicum and V. nigrum in contrasting competitive environments.

PREMISE: Understanding how drought and biomass allocation patterns influence competitive ability can help identify traits related to invasiveness and guide management. Vincetoxicum nigrum and V. rossicum are increasingly problematic herbaceous perennial vines in the northeastern United States and southeastern Canada. METHODS: Using a greenhouse experiment, we investigated how biomass allocation and competition intensity of Vincetoxicum spp. responded to four competitive regimes at two levels of soil water availability in the presence of conspecific or congeneric neighbors. RESULTS: Soil moisture was the most important influence on growth and biomass allocation. Vincetoxicum nigrum had a greater capacity for growth and reproduction than V. rossicum, especially under drought. Drought reduced the probability of reproduction for V. rossicum. Vincetoxicum rossicum had a higher root-to-shoot ratio than V. nigrum under adequate soil moisture. This difference more than doubled under drought. Under interspecific competition, V. nigrum maximized its biomass, while V. rossicum limited aboveground growth and reproduction. Root-only competition increased shoot and root biomass relative to shoot-only competition. The effects of root and shoot competition were additive under interspecific competition, but interacted under intraspecific competition (negative interaction under drought and positive interaction under sufficient soil moisture). CONCLUSIONS: Management strategies targeting mixed populations of V. rossicum and V. nigrum are most important under ample water availability. Under drought conditions, strategies focused on V. nigrum should effectively limit Vincetoxicum growth and seed reproduction. Phenotypic plasticity and the positive competition intensity associated with drought in monocultures may contribute to drought resistance in these invasive species.

Does polyembryony confer a competitive advantage to the invasive perennial vine Vincetoxicum rossicum (Apocynaceae)?

Determining which traits may allow some introduced plant species to become invasive in their new environment continues to be a key question in invasion biology. Vincetoxicum rossicum is an invasive, perennial vine colonizing natural and seminatural habitats primarily in the northeastern United States and southeastern Canada. More than half its seeds exhibit polyembryony, a relatively uncommon condition in which a single seed produces multiple seedlings. For evaluating the potential consequences of polyembryony on invasiveness, V. rossicum plants derived from seeds of three embryonic classes-singlets, doublets, and triplets (one, two, and three seedlings per seed, respectively)-were paired in all combinations intraspecifically and with the co-occurring native herbs Solidago canadensis and Asclepias syriaca in a greenhouse study. Vincetoxicum rossicum biomass was 25-55% greater and follicle production 55-100% greater under intraspecific competition compared with interspecific competition. However, within a competitive environment, follicle production varied little. Regardless of competitive environment, V. rossicum originating from seeds with a greater number of embryos typically performed no better than plants arising from seed with fewer embryos (singlets = doublets = triplets)-except intraspecifically where doublets outperformed singlets, and with S. canadensis where triplets outperformed singlets. Our findings suggest that overall performance and fitness of V. rossicum is higher in monocultures than in mixed stands and that its ability to invade new habitats may not be attributable to the production of polyembryonic seeds.

Survival and performance of the invasive vine Vincetoxicum rossicum (Apocynaceae) from seeds of different embryo number under two light environments.

The nonnative vine Vincetoxicum rossicum threatens several ecosystems in the Lower Great Lakes Basin of North America. One feature that may contribute to its invasiveness is the production of some seeds with multiple embryos (polyembryony), which may be beneficial as a bet-hedging strategy in variable environments. However, lower seed reserves per embryo in polyembryonic seeds may entail costs in low-light environments. The effect of seed from three embryonic classes (1, 2, or 3 embryos/seed) on V. rossicum survival and growth was studied under two forest understory light environments: full canopy (shade) or canopy gaps (light) in New York state. Two seedling cohorts were planted, in May 2004 and in May 2005. The survival and growth of seedlings was monitored biweekly for two (2005 cohort) or three (2004 cohort) seasons. For both cohorts, plants grown in canopy shade had reduced survival and growth compared with those grown in gaps. Contrary to expectations, seed embryo number had no effect on the final height, survival, or dry mass of plants in either habitat. Our results suggest that any fitness advantage provided by polyembryony may be habitat (light) dependent and not a general trait that affords V. rossicum a benefit in all habitats colonized.

A Model of the Effects of Tillage on Emergence of Weed Seedlings.

A simple model is developed in which the density of weed seedlings emerging in a field is related to (1) the ability of seedlings to emerge from various depths in the soil, (2) the survival of seeds at different depths, and (3) the depth of seed burial in no tillage, rotary tillage, and plow tillage. Other tillage regimes are considered by analogy. Literature is reviewed to determine biologically reasonable functions describing seedling emergence, seed survival, and distribution of seeds with depth, and parameters of these equations are estimated from data in the literature. Problems related to the mathematical description of these phenomena are discussed, and it is noted that some commonly held beliefs regarding survival of seeds in the soil are mutually incompatible. Although many studies have investigated the persistence of seeds as a function of depth in the soil, few have distinguished death from the production of seedlings. The model indicates that in the first year following input of seeds to the soil, no tillage will have more seedlings than tillage, but in later years no tillage will likely have fewer seedlings unless innate or induced dormancy is high or seed survival near the soil surface is unusually good. If seed return is allowed, no tillage or minimum tillage will have more seedlings perennially. Recovery of good weed control following a year with substantial seed input may be easiest if the soil is plowed deeply to bury the seeds, and then shallow or no tillage is used in subsequent years to avoid returning seeds to the surface. Much of the literature on the effects of tillage on weed density is difficult to interpret because little indication is given of the vertical distribution of seeds in the soil at the beginning of the experiment.