Tools To Study Succession

Retrospective approaches

Retrospective studies of succession include historical accounts, repeat photography, dendrochronology, and analysis of organic carbon or

Figure 4.3 Dense, "over-stocked" coniferous forest in the Blue Mountains of eastern Oregon. A tradition of fire suppression often leads to densely packed stands with high fuel loads, which are susceptible to catastrophic wildfires or outbreaks of insects. Photo by Stephen DeStefano.

biogenic opal in soil. These approaches cannot be used to test hypotheses about ecosystem dynamics, although they may be useful for describing changes that have occurred on specific sites and for generating hypotheses. Retrospective techniques can typically be used to assess species-level changes in plant distribution only with dominant woody plants. A few studies of vegetation change are, however, exceptional in their fine taxo-nomic resolution and spatial scale (e.g., Neilson and Wullstein 1983; Neilson 1986; Wondzell and Ludwig 1995). Nonetheless, these efforts are similar to other retrospective studies in that they are correlative and therefore cannot be used to distinguish between the many confounding factors associated with succession. Additional limitations of specific types of retrospective studies are described below.

Historical accounts of ecosystem change (e.g., land survey records, early maps, and notes of early travelers, surveyors, and military scouts) are usually anecdotal and imprecise, and thus do not allow the accurate determination of historical vegetation physiognomy or species composition. In addition, historical accounts are often contradictory and colored by fallacies (Bahre 1991).

Repeat ground photography has a limited and oblique field of view, and historical photographs usually portray anthropogenic manipulation of landscapes. These characteristics seriously limit the usefulness of repeat photography for determining changes in the distribution of species (Bahre 1991). Repeat aerial photography is also constrained by the date of the earliest photographs. In addition, extensive coverage of aerial photographs was not available until after broad-scale ecosystem changes had already occurred.

Dendrochronology is limited to woody plants, usually trees, and is based on correlations between tree age and cross-sectional ring number. Dendrochronological assessments are used to describe the dates of establishment, defoliation, or stem injury of individual woody plants. These assessments are then extrapolated to stands of trees in an attempt to describe periods of recruitment, mortality, rapid growth, or disturbance (Fritts 1976; Johnson and Gutsell 1994). However, if trees were once present but are currently absent, then reconstructions of stand age structure cannot be used to elucidate this important change. Perhaps more importantly, the characteristics of dominant woody plants in many ecosystems are poorly suited for dendrochronological assessment because: (1) current dendrochronological techniques are usually unsuitable for the determination of the stem age of several species, and (2) stem age does not necessarily represent individual plant age, since many species resprout after top removal.

Analyses of stable carbon isotopes have been used to assess vegetation change in grasslands and savannas. Stable isotope analysis relies on differential fractionation of carbon isotopes during photosynthesis. Nearly all woody plants possess the C3 pathway of photosynthesis, whereas the dominant grasses in subtropical and tropical ecosystems have the C4 metabolic pathway. These two metabolic pathways ultimately affect the stable carbon isotope ratio (13C/12C) of living plant tissue, which is retained and incorporated into soil organic material after plant mortality and decomposition. Therefore, the stable carbon isotope ratio in the soil can be used as an indicator of previous vegetation on a site (Figure 4.4).

The isotopic composition of soil organic carbon does not accurately reflect the past dynamics of C3 and C4 vegetation if: (1) the isotopic composition of the surface soil differs from that of the overlying vegetation; (2) soil depth is not an appropriate surrogate for time (if relatively new carbon is transported beneath older soil carbon via soil mixing -e.g., soils may be mixed by burrowing animals, freezing and thawing cycles, or alluvial processes); (3) deep-rooted C3 plants (e.g., shrubs, trees) deposit soil carbon beneath C4 plants; or (4) current or former dominant grasses possess the C3 photosynthetic pathway. Because these conditions often occur within some ecosystems, stable isotope analysis is not appropriate for studying succession within these systems (Dzurec et al.

Figure 4.4 Stable carbon isotopic composition and 14C dates for soil organic matter at different soil depths along a transect through a Quercus savanna into a C4 semi-desert grassland in southern Arizona, United States. Each line represents a single soil core. Soil cores A-D are 500 m above the savanna-grassland ecotone, cores E-H are 150 m above the ecotone, I-L are at the savanna-grassland ecotone, and cores M and N are 150 m below the ecotone in the grassland. Solid lines are cores collected beneath trees and dashed lines are cores collected beneath grasses >5 m from a tree canopy. Values adjacent to lines are 14C dates (in years) at different depths; all depths without dates had postmodern signatures. Reproduced with permission from McClaran and McPherson (1995).

-20 -17 813C Value

Figure 4.4 Stable carbon isotopic composition and 14C dates for soil organic matter at different soil depths along a transect through a Quercus savanna into a C4 semi-desert grassland in southern Arizona, United States. Each line represents a single soil core. Soil cores A-D are 500 m above the savanna-grassland ecotone, cores E-H are 150 m above the ecotone, I-L are at the savanna-grassland ecotone, and cores M and N are 150 m below the ecotone in the grassland. Solid lines are cores collected beneath trees and dashed lines are cores collected beneath grasses >5 m from a tree canopy. Values adjacent to lines are 14C dates (in years) at different depths; all depths without dates had postmodern signatures. Reproduced with permission from McClaran and McPherson (1995).

1985; McClaran and McPherson 1995). However, analysis of stable carbon isotopes is useful for identifying past shifts in boundaries between systems dominated by plants with different photosynthetic pathways (e.g., subtropical forest/grassland boundaries).

Vegetation changes may be inferred by assessing biogenic opal (i.e., plant microfossils or opal "phytoliths") in soils (e.g., Kalisz and Stone 1984), and the technique is conceptually similar to stable isotope analysis. Grasses produce more biogenic opal than woody plants, and the opal from grasses is morphologically distinct from the opal of woody plants (Witty and Knox 1964; Kalisz and Stone 1984). Biogenic opal is comprised of silica dioxide, which is very resistant to decomposition. Thus, the abundance and type of opal in the soil can be used to indicate previous vegetation on a site. Biogenic opal can be used to distinguish between some members of the grass family, and has, therefore, been used to study conversions from perennial to annual grasslands (e.g., Bartolome et al. 1986). The limitations of biogenic opal analysis are similar to those of stable isotope analysis: both techniques rely on chemical or morphological differences between plant taxa (especially grasses and woody plants) and make similar assumptions about deposition in the soil.

The concurrent use of several different retrospective techniques may facilitate the appropriate interpretation of past changes in ecosystems. However, different retrospective techniques may generate conflicting interpretations of the same phenomena, as illustrated by the following example (McPherson and Weltzin 2000):

Reports of past changes in the oak savanna/semidesert grassland boundary are varied. Paleoecological data suggest that oak savannas have shifted up-slope in concert with warmer and drier conditions since the Pleistocene. This interpretation is consistent with upslope movement of most woody species in the last 40,000 years, as determined by paleoecological research (Betancourt et al. 1990). In contrast, research based on stable carbon isotope technology and radiocarbon dating indicated that oaks at the savanna/grassland boundary had encroached into former grasslands within the last 1,500 years, which implied that oak savannas had shifted downslope into semidesert grasslands (McPherson et al. 1993; McClaran and McPherson 1995). The latter finding matches Leopold's (1924) interpretation of downslope movement of oaks, based on observations of progressively smaller trees from the savanna into the grassland. On a more contemporary temporal scale, use of repeat ground photography led Hastings and Turner (1965) to conclude that the oak savanna/semidesert grassland boundary moved upslope during the last century. Finally, Bahre (1991) examined surveyor's records, repeat ground photography, and repeat aerial photography, and concluded that the distribution of oak savannas had been stable since the 1870s. Thus, boundaries between oak savannas and adjacent semidesert grasslands have been variously reported as shifting upslope, remaining static, or shifting downslope. Although these differences may be attributable in part to variation in temporal and spatial scales, they are largely the result of different interpretations.

This example indicates that the disadvantages associated with retrospective techniques cannot be overcome simply with the use of multiple methods.

There are two primary, overarching limitations to using retrospective approaches to study succession. First, it is virtually impossible to reconstruct accurately the events and conditions that contributed to past changes, even at well-studied localities. Second, even if this were possible, conditions responsible for historical or prehistoric species distributions are unlikely to be repeated in the future. Earth is entering an unprecedented era in terms of atmospheric gas concentrations, climatic conditions, land use, and land cover; thus, even a complete understanding of past climates and assemblages of organisms will not allow the confident prediction of future changes. This situation is exacerbated by species-specific response patterns that are often not linear or predictable, even within life forms (Tilman and Wedin 1991; Archer 1993). Finally, results of retrospective investigations do not elucidate mechanisms of ecosystem changes (sensu Simberloff 1983; Campbell et al. 1991) because confounding between various factors precludes identification of mechanisms and introduces the potential for spurious correlations.

Consider the relatively recent large-scale changes in vegetation physiognomy that have occurred in former grasslands and savannas throughout the world. Dramatic transitions from grasslands and savannas to closed-canopy woodlands have captivated the scientific community, but the mechanisms underlying the changes remain unknown after more than three decades of detailed investigation (Archer 1989). For example, increased woody plant abundance in most grasslands and savannas has been attributed to changes in atmospheric or climatic conditions, reduced fire frequency, increased livestock grazing, or combinations of these factors (as reviewed by Archer 1994). Differing opinions about the causes of vegetation change have contributed to acrimonious debate. For example, Bahre (1991:105), in a critique of work conducted by Hastings and Turner (1965), concluded that "probably more time has been spent on massaging the climatic change hypothesis than on any other factor of vegetation change, and yet it remains the least convincing." Such debate hardly seems beneficial for scientific advancement or appropriate management, yet it is a natural product of retrospective approaches.

Regardless of scientific progress toward consensus on the mechanisms of past ecosystem change, elucidation of these mechanisms would provide little or no predictive power to current and future management of natural ecosystems. Events that may have contributed to past changes in ecosystem structure (e.g., cattle grazing, decreased fire frequency, specific timing of precipitation) may fail to produce similar responses today because of other, more profound changes in the physical and biological environments over the last century. For example, ecosystems now experience increased atmospheric concentrations of greenhouse gases (e.g., carbon dioxide, methane, nitrous oxides), increased abundance of woody perennial plants and introduced plants, and decreased abundance of some plant and animal species. Finally, there are no historical analogs for the conditions which are now widespread. For example, a return to the fire regimes which characterized prehistoric ecosystems is unlikely to occur without major cultural inputs to extant ecosystems. Even if we could reproduce prehistoric fire regimes, the decision to do so should be based on clearly defined, site-specific goals and objectives, rather than on the misguided hope that restoration of disturbance regimes will necessarily restore prehistoric ecosystems. Our knowledge of the past should guide contemporary management, not constrain it (Figure 4.5).

The descriptive nature of retrospective approaches, coupled with the complex site-specific interactions underlying ecosystem change, render these approaches unsuitable for the determination of the mechanisms of ecosystem change. Retrospective approaches are constrained by fundamental conceptual and philosophical limitations, and they are hampered by various technical weaknesses. Several of the technical obstacles associated with specific retrospective techniques have been removed by significant technological advances, and we expect continued progress in this area of research; however, the more important conceptual and philosophical limitations can be overcome only by traveling back in time. Thus, although it is widely acknowledged that understanding mechanisms of ecosystem change is central to the interpretation and prediction of species and ecosystem responses to disturbance or climate change, there is no evidence to suggest that such mechanistic understanding can be achieved with retrospective approaches.

Monitoring

Monitoring involves direct observations of changes in ecosystem structure over time. As such, managers usually monitor in an attempt to discover and document changes in ecosystem structure as they occur. This information serves as a primary basis for evaluating and possibly altering management strategies. Thus, monitoring is an important component of adaptive management.

Many attributes can be monitored, including soil, life forms, and species. However, monitoring programs usually focus on species composition. This approach is similar in most respects to the retrospective

Figure 4.5 Second-growth forest of Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) following timber harvest in the coastal mountain range of western Oregon. Species composition at the time of disturbance, land-use patterns, and the nature of perturbations over time contribute to postdisturbance vegetation patterns. Photo by Stephen DeStefano.

Figure 4.5 Second-growth forest of Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) following timber harvest in the coastal mountain range of western Oregon. Species composition at the time of disturbance, land-use patterns, and the nature of perturbations over time contribute to postdisturbance vegetation patterns. Photo by Stephen DeStefano.

approaches discussed above, with two notable exceptions: (1) spatial and temporal scales generally are finer and (2) taxonomic resolution usually is higher. These two attributes engender confidence that changes in ecosystem structure result from events that precede the changes. For example, a reduction in livestock density that precedes a shift in species composition may be interpreted as causal. The confidence associated with this conclusion typically increases with decreased time between sampling intervals, increased site specificity, and increased taxonomic resolution. However, this interpretation must be tempered with the knowledge that this approach is correlative and phenomenological, even with infinitely short sampling intervals, complete knowledge of changes on a particular site, and perfect taxonomic resolution. Considerable caution is warranted before changes in management are invoked as causal. Shifts in species composition or other measures of ecosystem structure that are attributed to management may have resulted from subtle and unperceived environmental events or other confounding factors. Management is not an experiment, regardless of the degree to which the management is "adaptive:" treatments are not assigned at random to experimental units, experimental units are rarely homogeneous or replicated, and managers frequently change many factors simultaneously. These characteristics preclude confident determination of causality.

Although changes in species composition may not be confidently attributed to management, observing these changes is an important component of effective management. Several metrics can be monitored, as described in Chapter 3. Ordination can then be used to describe changes in species composition over time (sensu Austin 1977). The direction and distance that quadrats "move" in ordination space over time may reflect successional patterns (Figure 4.6).

Comparative studies

Succession has frequently been inferred from studies that compare sites with different elapsed times after disturbance (i.e., chronosequences, or "space-for-time" substitutions). The taxonomic resolution of these comparative studies is usually superior to the resolution associated with retrospective approaches, and is similar to that obtained with monitoring. Comparative studies are similar to retrospective approaches in several respects; in particular, they are useful for generating hypotheses and for describing changes that have occurred on specific sites, and they cannot be used to test hypotheses about succession. This approach assumes that all sites under study have identical histories of disturbance, biotic influence, and environmental conditions (Luken 1990); this assumption is rarely valid, which makes this approach susceptible to serious criticism (e.g., Miles 1979). Nonetheless, chronosequences may be the best approach for describing successional sequences that predate Anglo settlement, and they have been widely used to suggest patterns of vegetation change in terrestrial ecosystems with long-lived organisms (Burrows 1990).

Comparative studies in the mesic central United States have suggested that old-field succession can be described on the basis of changes in models of community organization. Specifically, early-successional plant communities are characterized by a geometric model; as succession

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