Effects of defoliation on competitive interactions in European white birch

Ecology, Dec, 1997 by Magnus Augner, Juha Tuomi, Matti Rousi

INTRODUCTION

The composition of the neighborhood is important for competitive interactions between plants (Crawley 1986, Grace and Tilman 1990). It can also affect the herbivory load a plant suffers (Tahvanainen and Root 1972, Hay 1986, Hjalten et al. 1993b). That herbivory influences competition between plants has been observed in several instances. A well-described phenomenon, reviewed by Crawley (1983), is an increase in species diversity under moderate grazing. Several studies have shown that herbivory can cause shifts in competitive relations between plants (Cates 1975, Windle and Franz 1979, Berendse 1985, Rai and Tripathi 1985, Fox and Morrow 1992), indicating the potential importance of herbivory for the structure of plant populations and communities.

Both empirical and theoretical studies of plant-herbivore interactions have primarily focused on the effects of herbivory on the attacked plants' own performance. Some plants are extremely tolerant to grazing. It has been shown that grazing-tolerant plants can withstand the effects of moderate (McNaughton 1979, 1983) and even extensive grazing (Paige and Whitham 1987, Paige 1992). In most cases, however, herbivory has deleterious effects on plants (e.g., Belsky 1986, Bergelson and Crawley 1992). Several studies have shown that herbivory has most pronounced negative effects on plant growth when the plants are simultaneously subject also to competition (Bentley and Whittaker 1979, Lee and Bazzaz 1980, Cottam et al. 1986, Maschinski and Whitham 1989, Swank and Oechel 1991, Edwards et al. 1992, Hjalten et al. 1993a, Shabel and Peart 1994).

One way for plants to reduce the potential negative effects of herbivory is to develop mechanical and chemical defenses that decrease either the risk of herbivory or the biomass consumption when being attacked. However, it has been suggested that there can be trade-offs associated with defense characters in that they divert resources that otherwise could be allocated to growth and/or reproduction (Rhoades 1979, Coley et al. 1985, Herms and Mattson 1992), or due to genetic trade-offs between defensive traits and plant growth (Herms and Mattson 1992, Mole 1994).

Herbivory can also affect plant competition if plants benefit from their neighbors being grazed (Bentley and Whittaker 1979, Fowler and Rausher 1985). Such effects may influence competitive relations in plant communities provided that plant types/species respond differently to neighbor defoliation (Augner et al. 1991). Furthermore, they can lead to stable coexistence of defended and undefended plants if selective herbivory on competitively superior, undefended neighbors results in sufficiently large indirect benefits for defended plants (Augner et al. 1991).

We performed a study on two types of European white birch differing in palatability to mammalian herbivores. Our hypotheses were:

1) Both competition and defoliation negatively affect growth.

2) There is a trade-off between defense investment and growth, i.e., palatable plants will grow faster than unpalatable ones when there is neither defoliation, nor competition (for a thorough discussion, see Herms and Mattson [1992]).

3) There is a trade-off between defense investment and competitive ability, so that palatable plants are stronger competitors than unpalatable ones (cf. Rhoades 1979, Coley et al. 1985).

4) Defoliation negatively affects competitive ability. Selective defoliation can thereby alter the competitive relations between plants with different competitive abilities.

5) If unpalatable plants are weaker competitors than palatable plants (hypothesis 3), then selective defoliation of palatable neighboring plants can reduce, or completely negate, the negative competitive effects suffered by unpalatable plants.

METHODS

Plant material

For the experiment we selected two types of European white birch (Betula pendula Roth). These types are results of a breeding program by the Finnish Forest Research Institute aimed at finding fast-growing birch types with high stem quality (for plywood production). Generally, the juvenile growth of the two types has been approximately equal when grown under intratypic competition. However, they differ considerably in susceptibility to damage by small mammals. JR (F1-family) is known to be very susceptible to voles (Rousi et al. 1990) and to hares (Rousi et al. 1993), while 5845 x 5846 (F2-family) has consistently been more resistant (Rousi et al. 1996). For convenience, we call these birch types "palatable" and "unpalatable," respectively.

Resistance of birch seedlings to hares is based on triterpenoids, mainly papyriferic acid. In the late growing season, papyriferic acid is sequestered in resin droplets on the bark of growing shoots. Usually, these droplets dry during the following winter and spring. Plants cannot recirculate papyriferic acid, and, consequently, the resources invested in droplets are lost (Tahvanainen et al. 1991, Taipale et al. 1994). Furthermore, the production of papyriferic acid is costly for plants in terms of the amount of glucose needed to produce a given quantity of papyriferic acid (Bryant et al. 1985). The only known function of papyriferic acid in plants is as a defense against small herbivorous mammals. We analyzed some chemical properties, related to growth and defense, of plants used in the present experiment (M. Augner, J. Tuomi, M. Rousi, and P. Niemel, unpublished manuscript). For this we used six plants of each type from each treatment group. As expected from the susceptibility tests, the unpalatable type had [approximately]1.3 times higher density of resin droplets as compared with the palatable type ([F.sub.1,45] = 24.1, P [less than] 0.001). Consequently, the types differed in defense investment, and, hence, are relevant for tests of hypotheses 1-5.