Root biomass and productivity in a grazing ecosystem: the Serengeti

Ecology, March, 1998 by S.J. McNaughton, F.F. Banyikwa, M.M. McNaughton

INTRODUCTION

A definitive, though now dated, review of the effects of defoliation on individual plants, largely pot studies, provided overwhelming evidence that clipping consistently, though not universally, reduces short-term root growth (Jameson 1963). This view has become ingrained in the thinking of some ecologists (Belsky 1986, Painter and Belsky 1990), although field studies indicate that grazing can have no effect (Cargill and Jeffries 1984a, b, Milchunas and Lauenroth 1989) or even increase belowground biomass or productivity (Sims and Singh 1978a, b, van der Maarel and Titlyanova 1989). Similarly, more recent pot experiments have indicated that the effect of defoliation on grass root growth is complex: sometimes inhibitory, sometimes nil, and sometimes stimulatory (McNaughton and Chapin 1985, McNaughton 1986, Oesterheld and McNaughton 1988, 1991, Georgiadis et al. 1989, Schmid et al. 1990). Allometric constraints (Coughenour et al. 1984, Coleman et al. 1993, 1994, Coleman and McConnaughay 1996) incorporated into simulation models (Coughenour et al. 1985) indicate that it is impossible, over any extended time period (e.g., a full growing season), for aboveground production to remain constant or increase in response to herbivory while belowground growth is inhibited. Inhibition of root growth, if it occurs as a consequence of grazing, cannot be translated into an increase or maintenance of aboveground growth (Oesterheld and McNaughton 1988, 1991); hormonal mechanisms and mineral nutrient shortages preclude that result (Aiken and Smucker 1996). If root growth is inhibited by defoliation, so also shoot growth will be inhibited.

Experiments in the Serengeti ecosystem indicate that grazing increases aboveground net productivity over both brief periods (McNaughton 1979a, b) and full growing seasons (McNaughton 1985), and a variety of mechanisms have been identified that can contribute to such compensatory growth (McNaughton 1983a, b). Some of those mechanisms are intrinsic, including increased photosynthetic rate and brief reallocation of resources (Detling et al. 1979, Oesterheld and McNaughton 1988, 1991), and others are extrinsic, including nutrient recycling (Floate 1981, Holland et al. 1992) and modified radiant-energy profiles (McNaughton 1983a). Nevertheless, the question remains, does intense grazing, with over 90% of aboveground production consumed on an annual basis at some locations in the Serengeti ecosystem (McNaughton 1985), translate into (a) an inhibition of belowground productivity and, therefore, (b) a long-term decline in root biomass under grazing? This paper presents experiments answering both interrelated questions in the negative.

MATERIALS AND METHODS

Eleven study sites were situated along the rainfall - fertility - grazing gradient (McNaughton 1985, 1988, 1990) of Tanzania's Serengeti National Park from the extreme northwest, near the Kenya border, to the extreme southeast, near the Ngorongoro Conservation Area. Briefly characterized, the northwest has mean annual rainfall of 100-120 cm, poor soils, and intermediate grazing; the central region has mean annual rainfall of 70-90 cm with moderate-fertility soils and moderate grazing; the southeast has mean annual rainfall of 35-50 cm, fertile soils, and heavy grazing (McNaughton 1983c, 1985, 1990). The nominal rainy season is November to May, although it is shorter in low-rainfall areas and longer in higher rainfall areas (McNaughton 1985); April is the wettest month and June the driest, on average.

Each site had two or three chain-link fences erected for the study year and two or three permanently marked open plots (see McNaughton [1985] for details). All sampling was random within those plots. Soil cores 20 cm deep by 5 cm in diameter were separated into upper and lower samples at the 10-cm depth. All wet-season sampling was at monthly intervals, peak dry-season sampling was every two months. At the beginning of the dry season, the period of peak root biomass, soil cores 50 cm deep and separated at 10-cm intervals also were collected at short, mid-height, and tall grass sites in the southeast, middle, and northwest of the ecosystem, respectively. Similarly, 30-cm samples were collected inside and outside fences at two short grass and two tall grass sites where fences (replicate profiles: three replicates per treatment per site) were erected 2225 yr prior to these studies. Plant tissues were separated into live roots by bright color and soft texture, with other organic debris in the profile categorized as belowground litter. Dried samples were returned to Syracuse University, washed free of soil through 0.5-mm mesh sieves, redried, and weighed with ash (8 hr at 500 [degrees] C) weight subsequently subtracted out. Data were transformed as necessary to meet the distribution requirements of parametric statistical tests; most means reported are geometric means.

RESULTS

There was a distinct ([F.sub.9, 923] = 29.9, P [less than] 0.000001) ecosystem-wide pattern of seasonal variation in root standing crop [ILLUSTRATION FOR FIGURE 1 OMITTED]. Minimum root biomass, averaged across all sites, was reached at the end of February, the beginning of the heaviest part of the rainy season, and maximum root biomass was reached in June, the first month of the dry season. The rapid transition from May to June indicates a burst of belowground translocation and root growth by the grasses immediately prior to dormancy, just about doubling belowground standing crop. Over most of the year, root biomass to a 20 cm depth was between 150 and 250 g/[m.sup.2]. The low confidence limits for each month indicate a substantial degree of synchrony among widespread locations. Confidence limits were particularly low during the peak of the rainy season from February through May and were broader at other times because of spatially isolated and temporally variable showers in higher rainfall regions of the Serengeti. Average biomass was depth-related ([F.sub.1, 923] = 63.4, P [less than] 0.000001), averaging over the year 129 g/[m.sup.2] at 0-10 cm and 78 g/[m.sup.2] at 10-20 cm.