A further example of a successional niche being responsible for species replacement is worth highlighting. Trembling aspen (Populus tremuloides) is a tree that appears earlier in successions in North America than northern red oak (Quercus rubra) or sugar maple (Acer saccharum). Kaelke et al. (2001) compared the growth of seedlings of all three species when planted along a gradient of light availability ranging from forest understory (2.6% of full light) to small clearings (69% of full light). The aspen outgrew the others when relative light availability exceeded 5%. However, there was a rank reversal in relative growth rate in deep shade; here the oak and maple, typical of later stages of succession, grew more strongly and survived better than aspen (Figure 16.13). In his resource-ratio hypothesis of succession, Tilman (1988) places strong emphasis on the role of changing relative competitive abilities of plant species as conditions slowly change with time. He hypothesized that species dominance at any point in a terrestrial succession is strongly influenced by the relative availability of two resources: not just by light (as demonstrated by Kaelke et al., 2001) but also by a limiting soil nutrient (often nitrogen). Early in succession, the habitat experienced by seedlings has low nutrient but high light availability. As a result of litter input and the activities of decomposer organisms, nutrient availability increases with time - this can be expected to be particularly marked in primary successions that begin with a very poor soil (or no soil at all). But total plant biomass also increases with time and, in consequence, light penetration to the soil surface decreases. Tilman's ideas are illustrated in Figure 16.14 for five hypothetical species. Species A has the lowest requirement for the nutrient and the highest requirement for light at the soil surface. It has a short, prostrate growth form. Species E, which is the superior competitor in high-nutrient, low-light habitats, has the lowest requirement for light and the highest for the nutrient. It is a tall, erect species. Species B, C and D are intermediate in their requirements and each reaches its peak abundance at a different point along the soil nutrient-light gradient. There is scope for further experimental testing of Tilman's hypothesis.
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