When several species of plants are growing together and the yield per hectare is greater than that of a pure stand, the phenomenon is called overyielding. Overyielding may result from more efficient use of resources - land, nutrients, water, and sunlight - because the various species complement each other in the way they use these resources (Trenbath 1986). Some species have roots concentrated near the surface, while others have deep roots. When they grow close to each other, one can take up nutrients and water unavailable to the other. Different species often have different nutrient requirements; one species may need high levels of nitrogen, while another may be a calcium accumulator. Different species have different water requirements. Forests that have a mixture of species with different water requirements may not produce as much biomass as a monoculture of one water-demanding species in a wet year. However, in the long run, the combination of species of low and high demand for water ensures that there will always be at least some growth.

Each species has its own characteristic leaf shapes, colors, and angles. As a result, some species are more efficient in the overstory, while some are more efficient in the understory. Some do better with diffuse light, as occurs on a cloudy day, than in direct sunlight. Each species has a distinct and separate niche. When overyielding is caused by a more efficient use of resources an ecosystem is said to have ecological complementarity. We could also say that overyielding in diverse forests results from a lessening of the intraspecies competition that occurs in a monoculture where adjacent individuals all compete for the same resource at the same time. The idea of overyielding and complementarity was first applied to agricultural systems, where it has been shown that a mixture of crops can have a greater yield than a monoculture (Vandermeer 1990). Because of the difficulty of carrying out a test with replicated plots, it is difficult to confirm the idea that overyielding occurs in natural forests (Ashton 2000). However, Naeem et al. (1994) rigorously tested the theoretical advantage of species mixtures and found that high diversity enhances ecosystem functions such as productivity and resistance to disturbance.

Studies of plantation forests in the USA, Europe, and the tropics have shown that once established, mixtures of compatible tree species have higher yields than single-species plantations (Ashton and Ducey 2000), thus lending credibility to the idea that overyielding and complementarity are important in natural forests. Observations of the characteristics of trees that become established in tree-fall gaps of natural forests also suggest differences in the roles that various species play in natural forest dynamics. Tree-fall gaps occur when mature trees fall over due to death or accident and leave an opening in the canopy that exposes the forest floor to sunlight. If the trunk of the dead tree breaks off but the roots remain in the soil, the seedlings and saplings already present will accelerate their growth due to the new availability of light, nutrients, and soil moisture. These trees have been termed strugglers (Olde-man and van Dijk 1991) because they are able to live for many years struggling to survive under conditions of low light and high competition. When the falling tree is uprooted, and a patch of mineral soil is exposed, the pioneer or "r"-adapted trees that colonize have very different characteristics from those of mature forest species. The pioneer-type species, due to their rapid growth, are better able to take advantage of the opening in the canopy than slower-growing mature forest species. These differences reflect different ways in which species capture light, suggesting that species have evolved separate niches to more fully utilize the available light - in other words, there is complementarity of species.

Diversity in time can be as important as diversity in space, especially when early and late successional species complement each other's roles. Many forest species of mature communities are adapted to the partial shade and humid conditions of the undisturbed forest. When these species are planted in the open sun and bare soil, they often do poorly. Young seedlings and saplings of many rain forest trees do much better when they establish under a nurse species that lessens the severity of the microclimate around the seedlings, par-

Fig. 2.4. Complementarity of structure of species in uneven-aged, mixed-species forest tially suppresses weeds, and provides leaf litter that increases the soil organic matter (Mesquita 1995). Good nurse species often will be early successional species that can establish themselves in harsh microclimate and disturbed soils. Their ecological role complements that of mature forest species.

The mixture of canopy physiognomies illustrated in Fig. 2.4 results from an uneven-aged forest, that is, all the trees did not begin life at the same time. The differences in ages between individuals of different species helps to accentuate the niche differences, because not only are there differences horizontally, there are also differences vertically. In even-aged stands, where trees started growing at the same time, much of the vertical differentiation would be lacking.

Fig. 2.4. Complementarity of structure of species in uneven-aged, mixed-species forest

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