Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox - The Ecology of Plants - 2002 / The Ecology of Plants Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox; 2002 / Ch 10

Competition at the Level of Individuals

Size and Density

Plant growth is highly plastic, and the mass, height, number of leaves, and reproductive output of an individual plant can vary over orders of magnitude depending on growth conditions. When plants are grown without close neighbors, they are generally much larger than similar individuals surrounded closely by others, and often have a very different morphology, or form (see Figure 6.6).

In a classic experiment, C. M. Donald (1951) showed that British annual pasture plants sown over a wide range of densities had a remarkably consistent total final dry weight in a given area (Figure 10.1). Whether seeds were planted sparsely or very densely (above a certain minimal density), the total aboveground dry matter at final harvest was constant. The total yield was increased when more resources were supplied, but the same relationship held. While the average plant size at low densities was relatively large, the average plant size became smaller as density increased. In a re-analysis of these data, Tatuo Kira and his colleagues (Kira et al. 1953)

showed that as plant density increased, the mean weight of individual plants decreased in a linear fashion when both were expressed on a log scale (Figure 10.2).

Mean plant size can be a misleading measurement, however. Size relationships among individuals in evenaged, dense single-species plantings have been well studied in greenhouse and in a few field studies. Individual plant sizes are generally extremely uneven in such stands. Typically, a few large individuals dominate the available area, while most individuals remain very small. These highly unequal size distributions are called size hierarchies.

It has been hypothesized that size hierarchies are caused by asymmetric competition for light (Weiner 1990; Schwinning and Fox 1995), in which the largest individuals have disproportionate negative effects on their smaller neighbors. It has been suggested that small initial differences in access to light are responsible for progressively greater inequality in size over time, as long as density remains constant (Figure 10.3). Among a group of seedlings germinating together, a small head start may

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Frequency (%)

Order of emergence (percentile ranking)

Figure 10.4

The effect of relative order of emergence (percentile ranking) on seedling dry weight (mg). Each line shows the relationship (the regression) for one of three different populations of Dactylis glomerata (orchard grass, Poaceae) in a greenhouse experiment. The consequences of emerging sooner or later than one’s neighbors can be enormous: seedlings with the lowest percentile ranking—those that germinated and appeared aboveground first—were more than 1000 times larger than those emerging last. Seedling dry weight (mg) is graphed on a log scale. (After Ross and Harper 1972.)

confer a large advantage. Ross and Harper (1972) showed a strong relationship between the order of emergence and plant size for densely planted Dactylis glomerata (orchard grass, Poaceae; Figure 10.4). In natural populations, this advantage can be offset by variation in hazards (such as frost) and resource availability in early spring.

While size hierarchies have been found in many greenhouse studies and in some tree plantations, much less is known about them in natural populations. Wilson and Gurevitch (1995) examined the spatial relationship of plant sizes in a dense natural stand of Myosotis micrantha (forget-me-not, Boraginaceae), a small winter annual. These plants had extremely unequal sizes (Figure 10.5). The researchers hypothesized that if asymmetric competition was the cause, then large individuals should be surrounded by small, suppressed neighbors.

They found instead that the opposite was true. Large plants had large immediate neighbors, and small plants were associated with small neighbors. Individual plant mass was also highly correlated with the combined mass of neighbors, so that the population formed a mosaic of patches of large plants and patches of small plants. Plants without close neighbors were much larger than plants with neighbors, however, so competition was probably important in affecting plant size. The researchers concluded that asymmetric competition was unlikely to have caused the extreme size hierarchy found in this natural population. Rather, the size hierarchy was probably caused by variation in plant density or patchy resource distribution.

Individual plant mass (g)

Figure 10.5

Distribution (number of plants) of individual plant dry weights (g) in a natural population of the annual Myosotis micrantha (forget-me-not, Boraginaceae), harvested at flowering. There were large numbers of very small plants, with a steep drop in numbers of plants in the larger categories, and very few of the largest individuals. (After Wilson and Gurevitch 1995.)

Mortality, Size, and Density

If seeds of herbaceous plants (usually annuals) are planted in dense monospecific stands and their survival and weights are followed through early life history stages, the seedlings grow until they begin to crowd one another. As crowding becomes severe, some individuals eventually begin to die; usually the smaller and weaker plants succumb most readily. More densely planted stands begin to experience mortality sooner and at smaller individual plant sizes than more sparsely planted stands. Factors such as greater soil fertility or greater initial seed size that result in larger individuals also increase mortality because larger plants cannot maintain as high a density as smaller plants.

This process of density-dependent mortality is known as self-thinning. In a crowded monospecific, even-aged stand of plants, a log-log plot of density against total plant mass will have a negative slope— plant growth can occur only as some individuals die. It is important to realize that the term “self-thinning” does not imply a voluntary, altruistic self-sacrifice of the weaker individuals for the general good (which is certainly not what is happening!). Since the weight of a plant is directly related to its volume (a cubic measure)

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and density is determined by area (a quadratic measure), Yoda and colleagues (1963) proposed what they called the –3/2 thinning law:

w = cN –3/2

where w is the mean dry weight per plant, c is a constant that differs among species, and N is density. On a loglog scale, the relationship between mean dry weight and density is predicted to have a slope of –1.5 (Figure 10.6).

Although the –3/2 thinning law was widely accepted until the 1980s, it has become a matter of considerable debate since then. There is no doubt that mortality occurs in crowded stands; what is in dispute is whether the process is so regular that it can be described by a single numerical relationship for all populations. Studies of single populations have been plagued by serious statistical problems (Weller 1987), as have review papers attempting to evaluate the generality of the relationship (Lonsdale 1990; Weller 1991). On the other hand, Silvertown and Lovett Doust (1993) make a case for the relationship holding well as an upper limit.

Mechanisms of Competition

Plants compete for light, for water and mineral nutrients from the soil, for space to grow and to acquire resources, and for access to mates. Competition for animal vectors

Log density (m–2)

Figure 10.6

Effects of planting at different densities on the mean dry weights of individuals as seedlings age. Each of the green lines represents a different initial planting density. The lowermost point on each line is the initial dry weight at germination (I), and the uppermost point is the final dry weight (F), with weights shown at time intervals t = 1, 2, and 3. (After Kays and Harper 1974.)

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(A)

for pollen and seed dispersal is a special kind of competitive interaction because these mobile biotic resources can respond to plants in ways abiotic resources do not.

Light is in some ways the most peculiar resource for which plants compete (see Chapter 2). In one sense, plants do not really compete for light, because no matter how much light plants take up, an essentially infinite supply is always available directly above their canopies. Yet plant canopies can very effectively mop up photons, reducing light at ground level to less than 1% of the incident sunlight. It can be very dark in many communities, not only for the sapling struggling to grow in a dense forest understory, but also for the seedling emerging in a grassy meadow. One might argue that plants are competing for access to light, or for sunlit space, rather than for light itself. After all, light is available in unlimited supply, but space is not. Perhaps it is most useful to think of plants as competing for light, but in a manner different from that in which they compete for soil resources.

Overgrowing or overtopping neighbors is an obvious means of competing for light. Overtopping is probably a common mechanism leading to successional change from old fields to forests, in which shorter herbaceous plants are replaced by shrubs and eventually by trees (see Chapter 13). Another dramatic example of overtopping is the overgrowth of trees by vines. In the

(B)

Figure 10.7

(A) A strangler fig (Ficus sp., Moraceae), a tropical plant that begins life as a vine, killing the tree that it grows on and subsequently becoming a tree itself. (Photograph courtesy of J.

Thomson.) (B) Celastrus orbiculata (Oriental bittersweet, Celastraceae), a vine native to eastern Asia that is now invasive in the eastern United States, is shown here completely cloaking a tree. (Photograph by J. Gurevitch.)

Tropics, strangler figs (Ficus spp., Moraceae) grow on trees, encircling and eventually killing their hosts (Figure 10.7A). In temperate forests in the eastern United States, trees along forest edges or gaps are sometimes overgrown by either native or invasive exotic vines (Figure 10.7B). It is not always clear whether vines cloak and weaken otherwise healthy trees or overwhelm only trees previously weakened by other factors, such as insects or disease. Once covered by vines, trees become much more likely to be toppled in windstorms and killed. This vulnerability probably results both from the weakening of the root system as a consequence of loss of photosynthate as the vines increasingly shade the leaves and from the tremendous extra weight of the vines.

Most of the competitive interactions experienced by a plant occur on a very local scale. The shade of larger neighbors reduces the plant’s ability to photosynthesize, while the roots of the plants immediately sur-

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interactions between species that might at first appear to be due to competition for resources, but are actually due to a shared predator or herbivore (Holt 1977). Apparent competition might occur if, for example, as the combined density of two plant species increased, they increasingly attracted the attention of herbivores. Thus, both species would suffer from the other’s presence and abundance, but because of herbivory rather than competition.

Apparent competition among plants has been demonstrated only rarely. A recent study showed that a native New Zealand fern, Botrychium australe (adder’s tongue, Ophioglossaceae) was declining in abundance due to apparent competition with an introduced grass, Agrostis capillaris (Sessions and Kelly 2002). The grass may serve as a refuge for an introduced slug. The native fern survives well after fire. However, fire also increases populations of the introduced grass, which leads to an increase in slug density. Increased slug populations following a severe fire led to severe defoliation and mortality of the fern due to the positive effects of the greater grass cover on the slugs.

Populations and Competition

Competitive Hierarchies

Plant ecologists differ strongly on the question of whether consistent competitive hierarchies exist among plant species. Are some species always competitively superior, and is the rank order of subordinates relatively fixed? Some ecologists argue that consistent competitive hierarchies exist. Others argue that fluctuating dominance in competitive networks is the general rule for plant interactions.

What are the implications of these two opposing views? The issue of competitive hierarchies is important because it touches on questions regarding the basic structure of plant communities. If competitive hierarchies were the general rule, ecologists could predict the competitive abilities of plant species based on their traits, such as growth form and size (Herben and Krahulec 1990; Shipley and Keddy 1994). Explanations of species diversity would require an understanding of the factors that prevent competitive exclusion, such as nonequilibrium processes.

In contrast, if communities consisted of competitive networks with frequent reversals of competitive dominance, an understanding of community structure would require studies of niche partitioning and species packing and of how life history or other trade-offs permit stable coexistence. In that case, species diversity in ecological communities would be maintained primarily by competitive interactions, implying that communities are generally deterministic and equilibrial in character.

Connolly (1997) pointed out that most of the data that support the existence of competitive hierarchies are

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based on two-species replacement series experiments (described below), which he argued are biased. Biased or not, extrapolation of the results of these greenhouse experiments to natural plant communities is necessarily limited. We need to have more information, particularly from field experiments, before reaching a conclusion as to the general existence of competitive hierarchies in nature. Unfortunately, while there have been hundreds of studies of plant competition in the field, few have addressed this issue, and thus the data on this question are still too limited to allow any reliable conclusions to be drawn (Goldberg and Barton 1992).

Quantifying Competition

How one measures the intensity of competition will affect the interpretation of the outcome of plant competition studies. Some of the disagreements about the nature of competition among plants can be traced to differences in the way competition has been studied and measured (Grace 1991, 1995). Goldberg and Werner (1983) distinguished between the competitive effect of a plant on its neighbors and the competitive response of a plant to its neighbors. Recognition of these two distinct components of plant competitive ability can help us to understand the ways in which plants interact. The rankings of competitive effect and competitive response among the plants in a community are not necessarily correlated (Figure 10.9; Goldberg 1990).

The interpretation of the results of a competition experiment depends on the units in which the outcome is expressed. In many experiments, the results are calculated on the basis of area (such as biomass per unit area). Alternatively, one can examine the effects of competition per gram of biomass of competitors, or on a perindividual basis.

Not only does each of these measures mean something different, but their usefulness depends on the purpose of the measurement. For example, some models of competition require results that are expressed per individual, while others make the most sense per unit area. Furthermore, while assessing competitive results per individual might be straightforward for many trees or even annuals, it is likely to be impossible (and not even very interesting) for clonal perennials. These issues are clearly a consequence of the great plasticity of plants.

One argument for measuring competition on a pergram basis is that this practice eliminates the effects of size. But is eliminating size always desirable? If two individuals begin at more or less the same size, and one acquires resources more quickly and efficiently than the other and becomes the dominant competitor, is the greater size of that individual a bias that we want to get rid of, or is it the essence of the competitive interaction? Of course, if per-gram effects are calculated at the beginning of such an experiment, the faster-growing

index of performance is the absolute competition index

(ACI), which is simply the difference:

ACI = Pmonoculture – Pmixture

Using these two different indices as measures of the outcome of an experiment may lead to very different conclusions.

Despite its popularity for measuring competition intensity, the RCI is subject to a number of limitations. Ratios expressed on an arithmetic scale have poor statistical properties and are asymmetric (changing the numerator affects the ratio differently than changing the denominator by the same amount). The ACI is also subject to both conceptual and statistical problems. One alternative is to use the log response ratio (LRR):

This index has the advantage of expressing performance relative to potential performance, as RCI does, but has better statistical properties, including symmetry (Hedges et al. 1999). LRR is very similar to the difference in relative growth rates between monoculture and mixture if the initial sizes of the plants are similar and small in comparison with the final sizes.

The debate regarding the superiority of one index of competition over the others rests on the assumption that an index can accurately represent the essential features of a competitive interaction. Any index simplifies reality, however, and thus has certain drawbacks as a tool for understanding competitive interactions (as do indices expressing any kind of relationship, from species diversity to stock market performance). Any attempt to compress complex data into a single index suffers from a loss of information. Other limitations to competitive indices include that they unrealistically assume linear responses to neighbor density; that many different outcomes can result in the same index; and that competition indices tend to be strongly influenced by initial plant size (Grace et al. 1992.)

In making comparisons of competition intensity across environments or species, one alternative is to examine interaction diagrams and test for statistical interactions, using performance directly without converting to indices (Box 10A). Better still would be information over time on the performance of the competitors, following their trajectories to determine the progress of the interaction.

Experimental Methods for Studying Competition

Greenhouse and Garden Experiments

The majority of plant competition experiments have been conducted in greenhouses. Greenhouse experi-