Species replacement probabilities during successions

forest succession can be represented as a tree-by-tree replacement model .

A model of succession developed by Horn (1981) sheds some light on the successional process. Horn recognized that in a hypothetical forest community it would be possible to predict changes in tree species composition given two things. First, one would need to know for each tree species the probability that, within a particular time interval, an individual would be replaced by another of the same species or of a different species. Second, an initial species composition would have to be assumed.

Horn considered that the proportional representation of various species of saplings established beneath an adult tree reflected the probability of an individual tree's replacement by each of those species. Using this information, he estimated the probability, after 50 years, that a site now occupied by a given species will be taken over by another species or will still be occupied by

Table 16.1 A 50-year tree-by-tree transition matrix from Horn (1981). The table shows the probability of replacement of one individual by another of the same or different species 50 years hence.

Occupant 50 years hence

Table 16.1 A 50-year tree-by-tree transition matrix from Horn (1981). The table shows the probability of replacement of one individual by another of the same or different species 50 years hence.

Occupant 50 years hence

Present occupant

Grey birch

Blackgum

Red maple

Beech

Grey birch

0.05

0.36

0.50

0.09

Blackgum

0.01

0.57

0.25

0.17

Red maple

0.0

0.14

0.55

0.31

Beech

0.0

0.01

0.03

0.96

the same species (Table 16.1). Thus, for example, there is a 5% chance that a location now occupied by grey birch will still support grey birch in 50 years' time, whereas there is a 36% chance that blackgum will take over, a 50% chance for red maple and 9% for beech.

Beginning with an observed distribution of the canopy species in a stand in New Jersey in the USA known to be 25 years old, Horn modeled the changes in species composition over several centuries. The process is illustrated in simplified form in Table 16.2 (which deals with only four species out of those present). The progress of this hypothetical succession allows several predictions to be made. Red maple should dominate quickly, whilst grey birch disappears. Beech should slowly increase to predominate later, with blackgum and red maple persisting at low abundance. All these predictions are borne out by what happens in the real succession (final column).

The most interesting feature of Horn's so-called Markov chain model is that, given enough time, it converges on a stationary, stable composition that is independent of the initial composition of the forest. The outcome is inevitable (it depends only on the matrix of replacement probabilities) and will be achieved whether the starting point is 100% grey birch or 100% beech, 50% blackgum and 50% red maple, or any other combination (as long as adjacent areas provide a source of seeds of species not initially present). Korotkov et al. (2001) have used a similar Markov modeling approach to predict the time it should take to reach the climax state from any other stage in old-field successions culminating in mixed conifer-broadleaf forest in central Russia. From field abandonment to climax is predicted to take 480-540 years, whereas a mid-successional stage of birch forest with spruce undergrowth should take 320-370 years to reach the climax.

Since Markov models seem to be capable of generating quite accurate predictions, they may prove to be a useful tool in formulating plans for forest management. However, the models

Table 16.2 The predicted percentage composition of a forest consisting initially of 100% grey birch. (After Horn, 1981.)

Age of forest (years)

Table 16.2 The predicted percentage composition of a forest consisting initially of 100% grey birch. (After Horn, 1981.)

Age of forest (years)

Species

D

SD

1DD

150

200

oo

Data from old forest

Grey birch

100

5

1

0

0

0

0

Blackgum

0

36

29

23

18

5

3

Red maple

0

50

39

30

24

9

4

Beech

0

9

31

47

58

86

93

are simplistic and the assumption that transition probabilities remain constant in space and over time and are not affected by historic factors, such as initial biotic conditions and the order of arrival of species, are likely to be wrong in many cases (Facelli & Pickett, 1990). Hill et al. (2002) addressed the question of spatiotemporal variation in species replacement probabilities in a subtidal community succession including sponges, sea anenomes, polychaetes and encrusting algae. In this case, the predicted successions and endpoints were similar whether replacement probabilities were averaged or were subject to realistic spatial or temporal variation. And the outcomes of all three models were very similar to the observed community structure (Figure 16.12).

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