Primary Succession on Mount St. Helens: Rates, Determinism, and Alternative States

Abstract

We explored vegetation recovery on primary surfaces after the 1980 eruption including whether different communities might exist under similar conditions. Data were from repeat sampling of permanent plots, grids and depressions that formed during the eruption (potholes). We assessed succession rates using turnover, community similarity and trajectory complexity. We estimated turnover from community type change, while information theory described trajectory complexity. Similarity between repeat samples assessed succession rate. Succession vectors defined by detrended correspondence analysis (DCA) of repeatedly measured plots assessed floristic change. We determined the degree to which habitat variables predicted species composition using redundancy analysis. We tested the hypothesis that only one CT could occupy a particular habitat type (HT) by comparing species composition within CTs and HTs.

Successional rates declined with increasing elevation, proximity to colonists accelerated succession and succession may be arrested when strong dominants establish quickly. Biotic effects from Lupinus strongly alter successional rates by changing the rules for subsequent invasion. Environmental stress affects seedling establishment (drought) and biomass accumulation (infertility).

We explored the effects of determinant factors. The correspondence between vegetation and predictive factors was initially weak, but sometimes increased, limited by stochastic, transient and contingent effects. Contingent events also limited floristic convergence. Dispersal limitations and priority effects may have limited potential convergence. Unexplained variance in studies of plant to environment relations is largely due to contingent events, unmeasured variables and suboptimal statistical models.

We conducted a search for alternative states. While multiple communities may exist in some habitats, these may converge. Where indications that alternative states exist, more detailed habitat analysis may reveal differences that support alternative communities. Our data suggests that often there is a reasonable correlation between a community and a habitat, thus supporting the single state hypothesis. This long term study on Mount St. Helens provided significant insights into ecosystem recovery processes and has improved restoration methods.

Key terms in bold italic face are defined in the glossary.

Glossary

Alternative states

Two or more community types found in the same habitat.

Alternative stable states

Two or more community types seeming to persist in the same habitat.

CCA

Canonical correspondence analysis, constrained ordination based on DCA.

Colonization

The process of arrival and establishment, both are selective of species.

Competition

Negative impacts of one species on another due to use of limited resources.

Constrained ordination

Method of describing species patterns in terms of environmental factors.

Convergence

Occurs when two communities become increasingly similar as they mature.

DCA

Detrended correspondence analysis, a method to summarize vegetation change and forming trajectories based on nonlinear assumptions.

Dispersal

The process by which an organism or its reproductive units are transferred between habitats.

Divergent communities

Communities that become increasingly distinct as they mature.

Facilitation

Effects that promote establishment and growth of one species by another.

Euclidean distance, d

A measure of relationship between two samples based on DCA scores: dij = square root [Σ(xik – xjk)2], i and j are values in two samples, and k = number of species over which the comparison is made.

Habitat complexity

A measure of the number of CTs and their residence time calculated by the Shannon information statistic H′ = –Σ pi log pi; pi is the proportion of time occupied by each CT.

Habitat type

Environmental unit based on available habitat values.

Lahar

A slurry of mud and debris, here resulting from melting ice during an eruption.

Microsite

Small-scale habitat (see safe-sites).

Mosaic

A patchwork of vegetation on the landscape.

Mycorrhizae

Fungi that form mutualistic interactions with roots.

PCA

Principal components analysis, a linear method to assess matrix variation.

Percent similarity (PS)

A measure of relationship between two samples based on species cover: PS_ij = 200 Σ_min(x_ik, x_jk)/Σ(x_ik+x_jk), where min = minimum of two values xik, xjk, remaining terms as in Euclidean distance, d.

Permanent plots

Marked sites that are repeatedly sampled.

Primary succession

Ecosystem development on barren surfaces initially lacking in soil or biota.

Priority effects

The consequences of establishment sequence that condition later compositional changes.

Pumice

A silica-rich volcanic rock usually ejected during explosive eruptions.

Pyroclastic flow

Rapidly descending current of superheated gas and tephra hugging the ground; deposits are often of very fine texture and easily eroded.

RDA

Redundancy analysis, a constrained ordination based on PCA.

Relict site (vegetation)

Sites protected by topography and snow that supported surviving vegetation.

Safe-site

Any location that provides suitable conditions for establishment.

Stochastic

Referring to a random, chance-driven process; opposed to deterministic.

Stresses

Any factors that limit production.

Successional rate

The change in species composition through time.

Tephrafall

A rain of volcanic particles to the ground following ejection into the atmosphere by an explosive eruption. Tephra is a collective term for particles of any size, shape, or composition ejected in an explosive eruption.

Trajectory

The temporal path traveled by vegetation communities, often determined by DCA.

Trajectory complexity

A measure of community turnover measured by Hˈ. See habitat complexity.