Habitat construction by terrestrial plants

It is well known that plants are highly plastic developmentally, adjusting the number, form, and structure of their organs in response to environmental conditions. The converse is equally true: although their habitat-constructing activities may be less conspicuous than those of beavers and elephants, plants substantially shape their external environments both above and below the ground, beyond the simple effects of resource uptake (and in addition to photosynthetic oxygen production; see Section 5.1.1). Through the processes detailed below, individual plants modify the soil surface microclimate, shape the physical, chemical, and biotic properties of the soil, and create spatial patterns of water and chemical deposition. Recently, evolutionary ecologists have begun to focus not only on these external changes but on how these changes in turn affect plants and cohabiting species (e.g., Ridenour and Callaway 2001; Stinchcombe and Schmitt 2006; Wolkovich 2010; see also the discussion of plant flammability and fire regimes in Chapter 2, Section 2.3.3). In view of the diverse abiotic and biotic effects noted below, such studies (under both field and controlled conditions) could provide a wealth of new insights into ecological and evolutionary dynamics in terrestrial systems.

These external impacts scale according to plant size and longevity: individual trees typically alter their surroundings across a 5-15 m spatial radius and at a temporal scale from decades to centuries or, in some cases, millennia (Binkley and Giardina 1998). At the level of plant communities, different types of vegetation create characteristic environmental impacts. For instance, the canopy formed collectively by forest trees (i) intercepts solar energy and radiant heat to maintain cooler daytime and warmer nighttime soil surface temperatures, (ii) leads to more intensive water cycling due to heavy evapotranspiration, and (iii) provides high inputs of organic matter to the soil and its microbial communities, thus supplying a sustained nutrient supply (Binkley and Giardina 1998).

Such collective effects create a generally positive feedback loop through which plants promote conditions favorable to their own growth. However, certain plant taxa or communities can create conditions that instead promote their replacement by other types of plants. A familiar example is the deep shade cast by pine trees of the north temperate region. This environment provides too little light for the growth of pine seedlings but just the right conditions for the shade-tolerant seedlings of several broad-leaf deciduous trees. It is this type of self-suppressing habitat modification that provides the motive force for ecological succession (Bazzaz 1979).

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