Problems with Succession as a Model of Vegetation Change

GEOG*3110 Biotic and Natural Resources

Tommy Thompson

Succession as a model of vegetation change has numerous problems beyond the fact that

Clements (1916) underappreciated the spatial range of disturbance. Many variables occur in

nature that influence the successional pathways towards the final climax species.

Chronosequencing assumes that old communities developed under the same conditions that

occur during growth of young communities. As a result of these observations, the way

succession and the succession model is viewed has changed.

The succession model of vegetation change does not take into consideration the

proximity of propagule sources to recently disturbed sites. According to Fastie (1995) and his

research from Glacier Bay, Alaska, distance of seed sources at young sites can result in the delay

or exclusion of seral communities, changing the number of successional pathways. This is a

different idea from what Clements (1916) proposed in that succession is sequential and

unidirectional, and ends at a stable climax community. This model more closely follows

Gleason’s (1917) individual continuum hypothesis, arguing that the availability of nearby species

determines successional pathways.

Environmental variability is another factor causing problems for the succession model of

vegetation change. Fastie (1995) believes that a species' life history plays a factor in successional

pathways. Not all species are capable of germinating and establishing populations in particular

environments, and therefore, post disturbed sites may have a different climax species than nearby

undisturbed sites. This is consistent with Olson (1958) who observed that the climax species is a

function of initial environmental conditions.

Environmental variability was a factor used by Walker (1973) to reject the hydrarch

succession hypothesis in Chesapeake Bay. According to Clements' (1916) water tables lower

overtime, and as land dries, dryland communities replace wetland communities. Walker (1973)

used paleoecological records to determine that water levels fluctuated in Chesapeake Bay.

Walker (1973) also observed that seral communities did occur during lower water levels, but as

water levels increased, peat bog drowned out tree communities. Based on this, it was concluded

that the peat bog was climax species. This is inconsistent with Clements (1916) successional

model of vegetation change.

Another variable that affects successional pathways is landscape position of young sites.

Cowles (1899) believed that the Lake Michigan sand dunes followed the xerarch succession

hypothesis towards the climax species of Beech-Maple (Fragus grandifolia & Acer saccharum)

forests. Colinvaux (1993) was able strip down Cowles' observations arguing that they were

based solely on landscape positioning. The growth and persistence of Black Oak (Quercus

velutina) at higher elevations, within what should be a Beech-Maple climax, was due to the fact

that rain water reduced soil alkalinity by carrying carbonates down slope. Fastie (1995) had

similar observations, believing that landscape position can exclude the arrival of certain species.

This is also inconsistent with Clements' (1916) successional model of vegetation change.

Interspecific interactions, according to Fastie (1995), is another factor causing problems

in the succession model of vegetation change. The traditional model of succession by Clements

(1916) is stepwise; each seral community modifying the environment for the next species until a

climax is achieved. Sitka Alder (Alnus sinuata) are believed to be a seral community before the

climax community of Sitka Spruce (Picea sitchensis). Fastie’s (1995) research shows the

opposite of what would be predicted according to Clements in that the Sitka Alder actually

inhibit and reduce the growth of Sitka Spruce as the species interact and compete for light.

Succession as a model for vegetation change as proposed by Clements in 1916 has many

problems. Fastie (1995) was able to argue against the model using spatial variables including

propagule proximity to recently disturbed sites. Additionally, arguing that environmental

variability and species' life history also alter successional pathways towards climax species

(Fastie, 1995). Walker (1973) was able to reject the hyrarch succession hypothesis using

paleoecologial records. As well, Colinvaux (1993) was able to argue against xerarch succession

solely based on topography. Lastly, Fastie also argued that interspecific interaction between

successional species was not considered by Clements. Based on these observations, Clements

model of succession for vegetation change is invalid.

Works Cited:

Clements, Frederic Edward. Plant succession: an analysis of the development of vegetation. No.

242. Carnegie Institution of Washington, 1916.

Colinvaux, P. A. "Pleistocene biogeography and diversity in tropical forests of South America."

Biological Relationships between Africa and South America (1993): 473-499.

Cowles, Henry Chandler. The ecological relations of the vegetation on the sand dunes of Lake

Michigan. The University of Chicago Press, 1899.

Fastie, Christopher L. "Causes and ecosystem consequences of multiple pathways of primary

succession at Glacier Bay, Alaska." Ecology 76.6 (1995): 1899-1916.

Gleason, Henry Allan. "Some applications of the quadrat method." Bulletin of the Torrey

Botanical Club 47.1 (1920): 21-33.

Olson, Jerry S. "Rates of succession and soil changes on southern Lake Michigan sand dunes."

Botanical Gazette (1958): 125-170.

Walker, Richard A. "Wetlands preservation and management on Chesapeake Bay: The role of

science in natural resource policy." Coastal Management 1.1 (1973): 75-101.