Biogeog, Johnson 2008

What is a chronosequence?

Using space-for-time substitution to infer succession.

What key assumption do chronosequences make?

Sites differ only in age and share the same history.

What is the paper's main conclusion?

Chronosequences should not be relied on uncritically.

Why do chronosequences often fail?

Sites differ in disturbance history, soils, propagules, and environment.

What did Cowles' dune succession propose?

A predictable sequence from annuals → grasses → trees → climax forest.

Why was Cowles' dune model challenged?

Disturbance, not age, explained species distributions.

What controlled Cakile and Ammophila distributions?

Seasonal wave disturbance.

Why don't cottonwoods replace Ammophila on dunes?

They require moist germination sites, not dune facilitation.

What did Olson (1958) find on Lake Michigan dunes?

Same-aged dunes had very different vegetation.

What was more important than dune age?

Topography and disturbance history.

What factor most strongly shapes coastal dune vegetation?

Sand burial and erosion.

What is hydrarch succession?

The succession from open water to forest in ponds and bogs.

Why was the classic hydrosere model challenged?

Local disturbance and water-level changes altered pathways.

What did Glacier Bay studies show about succession?

Colonization order mainly reflected dispersal ability.

Did alder facilitate spruce establishment at Glacier Bay?

No.

What determines colonization in old-field succession?

Propagule availability and species growth rates.

Were late-successional species absent from young fields?

No, many were present very early.

Why do abandoned fields follow different pathways?

Different land-use histories, crops, and surrounding vegetation.

What methods are preferred over chronosequences?

Long-term studies, stratigraphy, palynology, and stand reconstruction.

What do these methods show?

Real successional pathways are often idiosyncratic.

Exam summary?

Succession is driven by disturbance, dispersal, and site history, not a single predictable sequence.