Succession in Forest Ecology

Disturbance in the forest environment is a common event, whether caused by slash and burn agriculture, tree fall, logging, storm damage or flooding. Deforestation, often cited as an event with permanent causes, is often reversed after abandonment with the forest once again reclaiming its former space, whilst another area is cleared for exploitation. In short, the forest as an entity is adaptive, thus enabling recovery after disturbance. How this process occurs is called succession, a theory which aims to provide an almost universal understanding of the ecology of an ecosystem within which organisms are naturally selected to fulfil a role. The role that organisms fulfil, or the strategy they have adopted are said to exist along a continuum from ‘r to k’, a theory first elucidated by MacArthur and Wilson (1967). The ‘r to k continuum’ evolved from the simple idea that natural selection favours a different suite of adaptations for when resources are abundant than when resources are in short supply (Pianka 1970). The ‘r - k’ Continuum The simplest way of understanding the ‘r’ to ‘k’ continuum is to consider each letter as one end of a polar dichotomy, with ‘r’ representing a fluctuating environment that is highly dynamic such as early successional forest made up of pioneer species, and ‘k’ representing a steady state such as late primary forest with mostly climax species. Thus species that are adapted for ‘r’ conditions must have specific attributes that enable maximum exploitation of abundant resources, and species adapted for ‘k’ conditions having specific attributes that enable maximum competitive ability in a resource restricted environment. In forest tree species typical ‘r’ characteristics are: 1) Frequent, Early and Rapid Reproduction: This maximises a species availability as a coloniser in the case of infrequent disturbance. 2) Abundant Small Light Seeds, often with Wings (samaras): Facilitates widespread (wind) dispersal and distribution enabling colonization of large areas. High mortality rates are compensated by the high abundance of seeds. 3) Rapid Germination: Ensures successful establishment and counters low seed survival rates due to lack of endosperm. 4) High Light Demander: Typically requires close to, or full, light conditions. Morphologically (structurally) adapted with large sun leaves at the top of the plant to avail of open conditions. Normally contains no lower branches or shade leaves. 5) Monocultural, Even Aged and Rapid Growth: Plants grow uniformly and fast, blanketing an area. Growth is apical (straight upwards). The canopy is highly regular. This combined with no lower branches typically suggests no sub-canopy and thus high light conditions on the forest floor. A robust understorey of shrubs and ‘k’ successional species dominates the forest floor. 6) Short Life Span: Species live for only a short length of time as energy is invested in reproduction and rapid growth, as opposed to structural integrity and competitive traits. Cecropia sp. is by far the best example of an ‘r’ type species in Amazonian forests, however other examples include, Pourama sp., and Ochroma sp. Travelling along the Tambopata, one can see the successional forests of Cecropia developing in the abandoned fields of farms that intersperse the river front primary forest. This becomes even more apparent as one heads to the Tambopata Research Center. Upriver island forests, subject to frequent flooding display that typical monocultural successional forest, also blanketed by Cecropia sp., amongst other species, as the successional pathway is constantly interrupted by the changing river. In forest tree species typical ‘k’ characteristics are: 1) Infrequent and Slow Reproduction: Reproduces more during optimal conditions. 2) Limited Number of Large Seeds: Attracts mammalian and in some cases avian dispersers. High endosperm content provides a good food source for mammals ensuring the consumption of the seeds, or provides resources for long seed life. Therefore seed mortality is typically low. 3) Slow Germination: Awaits optimal climatic conditions such as relative humidity or water levels before germinating. 4) Shade Tolerant: Adapted to growing slowly in small gaps or even under closed canopy in low light conditions whilst waiting for a gap to appear. In such conditions it grows lots of branches with shade leaves to maximise light utilisation. 5) Irregular and Multi-storeyed Canopy: Climactic conditions with lots of ‘k’ species usually ensure lots of shade. Open understorey is typical often with very low shrubs present. 6) Long Life Span: Competitive traits are maximized by reducing the energy cost in reproduction. Plants are tall and structurally solid with adaptations for the long-term survival in their specific environment. Dipteryx sp. is a good example of a climax species that has typical ‘k’ characteristics. It is a very strong structural tree that is known in English as the ‘ironwood’. Large buttress roots ensure stability, so that it rarely falls outside of old age. It grows slowly, has large seeds and casts a strong shade. It is distributed by bats and buried by agoutis. The ‘r – k’ continuum in general ecology The concept of ‘r’ and ‘k’ is applicable and used widely in general ecology[i]. Consider the contrast between insects or small rodents such as rats and higher mammals such as humans. The former has a boom bust cycle so when resources are high there is rapid and frequent reproduction producing numerous offspring, each of which quickly mature without parental supervision, and quickly begin reproducing themselves. As the available resources decline there is a large reduction in the population until resources bottom out and begin increasing. They are typically small in stature and have a short life span. Humans in contrast have few young, spend a long time parenting, reach maturity quite late, and have a long lifespan[ii]. References MacArthur, R. H. & Wilson, E. O. (1967). The theory of island biogeography. Princeton University Press, Princeton, N.J. 203 p. Pianka E R. (1970). On r and K selection. Amer. Naturalist 104: 592-597. http://uts.cc.utexas.edu/~varanus/rKcit.html [i] Many ecologists have long since ceased using this continuum for they believe it is focused too much on reproduction and not enough on behavioural adaptations throughout periods of growth. They believe that ‘r-k’ is too rigid and does not include the psychological adaptations of higher mammal species for example. See Michael N. Bruton's (ed.) 1989 book “Alternative Life-History Styles of Animals” Kluwer, Dordrecht. Similarly in forest ecology the ‘struggler v gambler’ dichotomy has been cited as an alternative that might incorporate strategies that don’t sit readily in the ‘r-k’ continuum. [ii] Some might argue with a relative degree of success that for humans it really depends on where on the planet a human lives that determines where on the r-k continuum one exists.