A. de Moraes

Grassland ecophysiology and grazing ecology.

Environmental constraints and plant responses to defoliation morphogensis of pasture species and adaptation to defoliation animal interactions sustainable grazing management of natural pastures.

Campos in southern Brazil.

The Brazilian subtropical region is located between the extreme southern border ofthe country (approximately 33°S) and the Tropic of Capricorn. This chapter dis-cusses the main grazing ecosystems found in this region (states of Rio Grande doSul, Santa Catarina and Parana), based on a tradition of beef cattle livestock pro-duction, which started at the beginning of Brazilian colonization at Rio Grande doSul and, little by little, spread north to the grasslands of Santa Catarina and Parana.Few places in the world present such diversity in native forage species, withalmost 800 grasses and 200 legumes.

Effects of grazing on the roots and rhizosphere of grasses.

Fertilizer inputs are currently being reduced in many areas (Commission, European Communities, 1992) and the resultant drop in pasture fertility will reduce the stock carrying capacity. As a consequence, individual plants will be defoliated less frequently (Curll and Wilkins, 1982) and less nitrogen will be deposited as urine (Thomas et al., 1988), thus influencing plant competition and composition. Research to date has concentrated on the effects of grazing on above-ground aspects, and it has recently been stated that ‘the effects of herbivory on the timing, mass and quality of below-ground inputs remains one of the greatest unresolved issues of the dynamics of nutrient cycling’ (Ruess and Seagle, 1994). The soil microbiota in grasslands consists of populations of microorganisms, including bacteria, fungi, protozoa, nematodes, and microand macroarthropod groups (Ingham and Detling, 1986). These all rely for their growth, at least in part, on carbon or nitrogen substrates via litter, root production, sloughage and exudation. Figure 4.1 illustrates the main links in the detrital trophic food web and shows the primary role of plant roots, the connectivity and some of the many trophic interactions. Although nearly all soil organisms belong to the detrital food web, significant numbers of root herbivores exist in grassland soil (Curry, 1994), also relying on plant roots for their survival. Any alteration in plant-derived carbon, such as through defoliation, will have consequences at many levels in the food web (Fig. 4.1). Since microbial activity, supported in part by root-derived carbon, drives soil nutrient cycling, the production and use of carbon from root systems is also a key issue in the functioning of soil ecosystems (van Veen et al., 1989). 4

Reserve formation and recycling of carbon and nitrogen during regrowth of defoliated plants.

Most herbaceous plants store carbon and nitrogen. There are several possible ecological advantages that the ability to store resources confers upon a species, which can be particularly important when considering plant competition and vegetation dynamics. These advantages include: (i) allowing growth to occur when either the availability of the external nitrogen is low (e.g. in the spring for growth of Molinia caerulea (Thornton and Millard, 1993)) or a short growing season means that uptake of soil nitrogen alone is not sufficient for growth (Jaeger and Monson, 1992); (ii) supporting reproduction by recycling resources from vegetative to reproductive growth – for example, in species exhibiting monocarpic senescence (Millard, 1988) and particularly in biennials (Heilmeier et al., 1986); and (iii) enabling more rapid recovery from catastrophic events, such as defoliation (e.g. Thornton et al., 1993a). It is this last role for storage that is considered in this review. Before we consider the quantitative significance of storage of both carbon and nitrogen in grasses in relation to defoliation, we shall first examine briefly the physiological strategies for storage used by herbaceous species. Experiments to quantify the storage and remobilization of both carbon and nitrogen will then be discussed and their limitations in determining the ecological significance of these processes in relation to vegetation dynamics highlighted. In order to understand competitive interactions between defoliated plants there is a need for field experiments that quantify storage and remobilization. Recent research is discussed which has developed a range of techniques that might allow such field experimentation in the future. 5

Defoliation Patterns and Herbage Intake on Pastures

This chapter is concerned with the herbage intake of grazing ruminants and thedefoliation patterns that result from the interaction between the sward and theanimal’s mouth. In the early history of grassland research, many assumptions weremade about defoliation patterns and their role in determining animal productiv-ity. It was assumed for instance that, under continuous stocking, defoliation wasso frequent as to reduce herbage production and therefore animal production.Work during the 1950s indicated that this was mostly not the case and that ani-mal production levels were similar in rotational and continuous stocking(McMeekan and Walshe, 1963). Subsequently, when more detailed studies werecarried out, it was found that intervals between defoliations under continuousstocking were not so different from those under rotational stocking (Hodgson,1966; Hodgson and Ollerenshaw, 1969) and probably not sufficient to have alarge effect upon herbage production. Meanwhile, following the paper of Mott(1960), emphasis was being placed on stocking rate, rather than grazing method,as being the principal determinant of animal productivity.The factors affecting herbage production and animal production under humidtemperate conditions continued to be studied under cutting and under rotationalgrazing up until the beginning of the 1980s. Since that time, there would appear tohave been a predominance of grazing studies carried out under continuous stockingor under controlled conditions similar to continuous stocking conditions. This hasbeen associated, on the one hand, with considerable advances in the technology ofgrazing management, particularly the use of sward height to control both swardgrowth and herbage intake (Hodgson, 1990). On the other hand, headway has also

Plant-animal interactions in complex plant communities: from mechanism to modelling.

It is widely acknowledged that ruminant herbivores have a major impact on the floristic composition and stability of grassland and shrub vegetation (McNaughton, 1979; Miles, 1981; Crawley, 1983; Hodgson and Illius, 1996). If overexploitation by grazing animals or undesired vegetation changes are to be avoided, protocols need to be developed for vegetation management through the manipulation of the herbivore population. To do this, it is first necessary to identify and quantify the processes by which vegetation dynamics are modified by the activities of herbivores in time and space (e.g. Fig. 10.1). The centre of Fig. 10.1 represents the hierarchical levels within vegetation – the individual plant, the population of individuals, mixed species associations (i.e. the plant community) and mixed community associations (Senft et al., 1987; Birske, 1989). An understanding of events at the level of the individual plant and the plant populations is required for a full appreciation of the pattern and process at the vegetation community level (Bullock, 1996). This diagram demonstrates the need to understand the responses of plants to the intensity and frequency of herbivore impact in relation to environmental conditions and the animal factors, which affect not only the intensity and frequency of impact but also the distribution of that impact. While this review is primarily concerned with the activities of herbivores (i.e. defoliation, excretion and treading) as they influence vegetation, it is necessary to be aware of the importance of the abiotic factors and the possibility that interactions might occur such that individual plant and plant community responses to herbivores may be modified by variation in these factors. 10