Ectomycorrhizae

FIGURE 5.7 Cross section of an idealized mycorrhizal fungus showing both VA and ecto-mycorrhizal feature. (From Whitford, W. G. and N. Z. ELkins. 1986. Principles and Methods of Reclamation Science. With Case Studies from the Arid Southwest. C. C. Reith and L. D. Potter (eds.). University of New Mexico Press, Albuquerque, NM. With permission.)

FIGURE 5.7 Cross section of an idealized mycorrhizal fungus showing both VA and ecto-mycorrhizal feature. (From Whitford, W. G. and N. Z. ELkins. 1986. Principles and Methods of Reclamation Science. With Case Studies from the Arid Southwest. C. C. Reith and L. D. Potter (eds.). University of New Mexico Press, Albuquerque, NM. With permission.)

seed dispersal. Either the intentional plantings or the natural colonization may dominate the final plant community, but a generalization seems to be emerging that species arriving through natural colonization are more successful when seed sources are near than those intentionally planted by humans. Success of any planting program is determined by natural selection operating on the total biotic input to a site. Thus, the biota self-organizes into a community that exists until conditions change. This process is often called self-design in restoration ecology in recognition of the fact that nature ultimately determines the composition of restored or created communities. Because nature rather than humans selects successful species and because intentional plantings are often expensive, the rationality of planting programs is being examined with greater scrutiny. The question of whether "to plant or not to plant" is being asked (Harmer and Kerr, 1995; Kentula et al., 1992) and self-design is being evaluated as a viable restoration strategy more widely (Middleton, 1999; Whisenant, 1999). William Mitsch is a leader in this effort for wetlands (Metzker and Mitsch, 1997; Mitsch, 1995b, 1998a, 2000; Mitsch and Cronk, 1992; Mitsch and Wilson, 1996; Mitsch et al., 1998) and his long-term, system-wide studies may be the most effective way to determine the optimal planting strategy.

A final consideration concerning biotic inputs is mutualism or symbiotic relationships between organisms. Mutualisms can be critical to the successful establishment of certain species. Animals in particular may play roles in this context when their actions are necessary for plant survival (Handel, 1997; Majer, 1997). Enhancing bird use of a site by providing perches is one example that increases dispersal of certain plant species (McClanahan and Wolfe, 1993; Robinson and Handel, 1993). Perhaps the most important mutualism in regard to restoration is the relationship between certain plants and mycorrhizal fungi. This mutualism occurs in the roots (Figure 5.7), and mycorrhizae literally means "fungus root." There are two economically important types of these fungi: ectotrophic and endotrophic (vesicular-arbus-cular), which differ in their morphology. The fungi acquire all of their carbon for nutrition from the plant and, in return, they aid in nutrient uptake. For both kinds of mycorrhizae, the thallus is located within the cortex of the root, but most of the fungal biomass is in hyphal threads that grow into the surrounding soil. Ectotrophic mycorrhizae directly contribute to the breakdown of soil organic matter, while endotrophic mycorrhizae are especially efficient at nutrient uptake. It is well known that mycorrhizae stimulate host plant growth, and they have even been considered to be keystone species because of this role (Lodge et al., 1996). Their function in restoration ecology is reviewed by Haselwandter (1997), Miller (1987), and Miller and Jastrow (1992). Although strong mutualistic relationships between species such as mycorrhizae are relatively uncommon in nature, E. P. Odum (1969) suggests that they are characteristic of mature ecosystems. Thus, mutualisms should be encouraged in restorations, and their presence is an index of a successful project, according to E. P. Odum's criteria.

Succession as a Tool

Succession is the process through which ecosystems develop over time (see Figure 4.3 in Chapter 4). As such it is one of the fundamental concepts in ecology (Golley, 1977; Mcintosh, 1981). Disturbance is the normal trigger for succession to begin, and different kinds of succession are recognized (primary vs. secondary), depending on the degree to which the ecosystem is set back in the development process. Species abundances change sequentially as succession proceeds because no species is adapted to the full range of environmental conditions that occur at a site from the early pioneer stages through the later, mature stages. Classifications of species strategies in relation to succession have been proposed such as r- vs. K-selection (MacArthur and Wilson, 1967; Pianka, 1970), where the letters refer to coefficients in the logistic population growth equation (see Eq. 3.4). The r-selected and K-selected species form ends of a gradient of adaptation in this theory. The r-selected species have short life-expectancy, large reproductive effort, and low competitive ability, while K-selected species have the opposite: long life-expectancy, small reproductive effort, and high competitive ability. Thus, in relation to the logistic equation, r-selected species emphasize high reproductive rates and are likely to occur in early succession when resources are not limiting. K-selected species emphasize high competitive ability, which is important when resources become limiting, as occurs in later successional stages. Applications of this theory have been criticized, but it is still elegant and useful as a generalization. Grime (1974, 1979) offered a slightly more complicated classification for understanding species strategies: competitive (similar to ^-selected), ruderal (similar to r-selected), and stress-tolerant. His classification is especially significant because of the distinctions drawn between the concepts of stress and disturbance. According to Grime, stress is a forcing function that effects production, while disturbance is a forcing function that effects biomass. Dominance of either stress or disturbance leads to different life history patterns in a predictable fashion. MacMahon (1979) provides a model for different plant life forms in relation to Grime's classification.

A rich variety of life history classifications exists in the literature, sometimes with quite evocative names attached to different strategies: "spenders" vs. "savers" (During et al., 1985), "fugitives" (Hutchinson, 1951; Horn and MacArthur, 1972), "gamblers" vs. "strugglers" (Oldeman and van Dijk, 1991), "bet-hedgers" (Stearns, 1976), and "supertramps" (Diamond, 1974). Van der Valk's (1981) classification is particularly detailed for freshwater wetland plants. Twelve basic life history types are recognized based on three key traits (life span, propagule longevity, and propagule establishment requirements). This classification was developed during long-term studies of succession in prairie wetlands and has been advocated for use as a basis for wetland restoration (Galatowitsch and van der Valk, 1994; van der Valk, 1988, 1998). Whigham (1985) also has successfully applied van der Valk's approach to understanding vegetation in treatment wetlands. Clearly, knowledge of life history patterns can significantly improve restoration plans by aiding in making appropriate choices of species for intentional plantings. Other important references on life history and succession are given by Huston and Smith (1987), Noble and Slatyer (1980), and Whittaker and Goodman (1979).

Succession can be considered both at the population scale, as noted above in terms of species strategies, and also at the ecosystem scale where patterns of change in nutrient cycling and energy flow take place over time. E. P. Odum's (1969) summary is a good introduction to ecological change at both scales.

There is a direct connection between succession and restoration because both concern ecosystem development over time. Some restoration ecologists, especially those who work in terrestrial systems, hold the view that the goal of restoration is to accelerate succession (Bradshaw, 1987) or to otherwise shorten it (MacMahon, 1998). In this sense, succession is used as tool for restoration efforts. Kangas (1983a,b) examined this idea with a simulation model of succession as applied to strip mine reclamation for phosphate mines in central Florida. The model included three stages of succession characteristic of the southeastern U.S. (Figure 5.8) with grass as the pioneer stage, pine trees as the intermediate stage, and hardwoods as the mature or climax stage. Transitions between stages were controlled by shading and the development of a litter layer that regulated seed germination. Figure 5.9 compares the standard run of the model without manipulation to a simulated run with high amounts of seeding and litter addition, as might occur in restoration efforts. In this case, the time to the mature, climax stage of succession was reduced by one half, from 60 years in the standard run to 30 years in the simulation. This type of

Equations for the storages are given below:

Qi =

K2Q1R1 - K3Q1 - K4Q1(+S1 IF R2 < T3 AND Q4 < T1)

(1)

Q 2 =

L2Q2R2 - L3Q2 - L4Q2(+S2 IF Q4 > T2)

(2)

Q 3 =

M2Q3R3 - M3Q3 - M4Q2 + S3

(3)

Q 4 =

K4Q1 + L4Q2 + M4Q3 - N1Q4

(4)

Ri =

J/(1 + K1Q1)

(5)

R2 =

R1A1 + L1Q2)

(6)

R3 =

M1 + M1Q3)

(7)

FIGURE 5.8 Energy circuit model of succession on abandoned phosphate mines in Florida. (Adapted from Kangas, P. 1983b. Analysis of Ecological Systems: State-of-the-Art in Ecological Modelling. W. K. Lauenroth, G. V. Skogerboe, and M. Flug. (eds.). Elsevier, Amsterdam, the Netherlands.)

FIGURE 5.8 Energy circuit model of succession on abandoned phosphate mines in Florida. (Adapted from Kangas, P. 1983b. Analysis of Ecological Systems: State-of-the-Art in Ecological Modelling. W. K. Lauenroth, G. V. Skogerboe, and M. Flug. (eds.). Elsevier, Amsterdam, the Netherlands.)

work suggests that succession can be managed to reduce cost of restoration projects and to increase the ecological value of the resulting systems. The idea of using succession as a tool is to take a systems perspective to restoration. Thus, the goal is "to plant a forest, not trees." In other words, a mature, complex ecosystem is the result of multiple successional stages at a site over time, and it is difficult and costly to skip these stages in restoration. Knowledge of successional history is fundamentally important for understanding and restoring complex ecosystems. Luken (1996) provides a summary of the use of succession as a tool with many examples of strategies related to ecosystem restoration and creation.

While knowledge of succession is clearly useful in restoration ecology, it may have another, more abstract use that is related to engineering. This is the idea of succession as a form of computation and therefore as an abstract tool for problem solving. Several concepts of biology have acted as guides or models for computa-

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