Arbuscular mycorrhizae (AM) occur in most terrestrial ecosystems and are
crucial to understanding community structure and function. However, their
role in primary succession is poorly understood. Common pioneers are AM
when in densely vegetated areas and sometimes in lupine patches, but not at
other microsites. On the Pumice Plains of Mount St. Helens AM are the
result of invasion by vascular plants, rather than the reverse. The
vegetation of the Pumice Plain is composed primarily of facultatively
mycotrophic species which are currently nonmycorrhizal. AM inocula were
added to a series of microsites, without significant growth or survival
effects on pioneer species - there was a trend for negative impacts on
plant biomass. Mycorrhizae may play a pivotal role in later primary
succession, but so far they do not. This may be due to limited nutrient
availability and the facultatively mycotrophic nature of the colonizing
plant species. Microsites, especially dead lupines, continue to be
important to plant olonization.
Two greenhouse studies examined the role of AM in Mount St. Helens pioneer species under three nutrient regimes and four competitive scenarios. Nutrient levels were complete, complete without phosphorus (-P), and tap water. In tap water a negative effect from AM colonization was observed perhaps due to parasitic action of the AM fungi. A weak but apparent benefit from AM occurred in a without phosphorus treatment. AM assisted a facultatively mycotrophic species in competition with a nonmycotrophic species and a lack of AM improved the competitive ability of the nonmycotrophic species. The next step is to broaden the investigation to other interactions and trophic levels.
Lupinus lepidus was initially expected to dramatically alter community development on Mount St. Helens. However, the expected impact has been tempered by its slow spread in recent years. Specialist herbivores severely limit lupine spread by feeding only in edge patches. The number of predators and parasites is much higher in core areas than edge areas. What will happen to the now-stalled lupine reinvasion as parasitoids or predators possibly spread outward from their colonization foci in the core region? And how much does L. lepidus' direct and indirect facilitative effects extend beyond ruderal invaders to later successional species? At this time, in lupine patches it may be possible to study the shift to a successional phase where biotic interactions become more important. We are doing this by focusing on three questions.
1. Do resource or consumer gradients associated with lupine patches cause spatially structured herbivore dynamics during primary succession?
An experimental manipulation of herbivore resource quality and consumers will be used. Food quality will be manipulated by transplanting core plants to edge areas and vice versa. If plant quality is a factor, we predict that core plants transplanted to edge areas, where herbivores are abundant, will have lower herbivore loads. Predators and parasitoids will be removed in some areas and supplemented in others and their effect on herbivore populations censused.
2. How do lupines and lupine herbivory affect plant community dynamics?
Because live lupines competitively suppress other ruderal species, whereas dead lupines facilitate these same species herbivore-induced lupine mortality may accelerate succession at the local scale, while decreased rates of lupine spread may decelerate succession at the landscape scale. In this experiment we seek to directly quantify the effects of lupine herbivory on local successional rates and trajectories by examining the interaction of lupine facilitative effects, herbivory, and mycorrhizal innoculum on plant community development, including the development of a vertical vegetation component.
3. How is the tritrophic reinvasion progressing and impacting primary succession?
The sequential spatial spread of a tritrophic invasion will be pursued by the use of transect sampling to quantify the spatial spread of lupines, herbivores and parasitoids over time.