Facilitation

Cases of competition-colonization trade- . .

the importance of offs and/or successional niche relations , ... .

facilitation - but are prominent in virtually every succes-

not always sion that has been described, including all those in the previous section. In addition, we have seen cases where early species may change the abiotic environment in ways (e.g. increased soil nitrogen) that make it easier for later species to establish and thrive. Thus, facilitation has to be added to the list of phenomena underlying some successions. We cannot say how common this state of affairs is. However, the converse is by no means uncommon; thus, many plant species alter the environment in a way that makes it more, rather than less, suitable for themselves (Wilson & Agnew, 1992). Thus, for example, woody vegetation can trap water from fog or ameliorate frosts, improving the conditions for growth of the species concerned, whilst grassy swards can intercept surface flowing water and grow better in the moister soil that is created.

Figure 16.12 (left) Simulated recovery dynamics (Markov chain models) of three of the species that make up a subtidal community starting from 100% bare rock for spatially varying, time varying or homogeneous replacement probabilities: (a) the bryozoan Crisia eburnea, (b) the sea anenome Metridium senile and (c) encrusting coralline algae. The points at the end of each plot (±95% confidence intervals) are the observed abundances at a site in the Gulf of Maine, USA. (After Hill et al., 2002.)

Integrated PPFD (mol m-2 day-1)

Figure 16.13 Relative growth rate (during the July-August 1994 growing season) of trembling aspen (+), northern red oak (•) and sugar maple (□) in relation to photosynthetic photon flux density (PPFD). (After Kaelke et al., 2001.)

Integrated PPFD (mol m-2 day-1)

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