CAUSES OF DECREASES IN PRODUCTIVITY IN UNDISTURBED TALL GRASS PRAIRIE

There is abundant evidence that protection of the tall grass prairie from grazing, mowing, or burning for even a few years results in accumulation of much litter and reduced productivity. This reduced productivity has been attributed primarily to either the insulating effect of the litter, a lowered rate of mineral recycling, or a presence of toxins in the litter. Considerable evidence exists in the literature concerning the first suggestion, but no studies have been reported concerning the last two in grasslands. A comprehensive project was conducted during the growing season of 1977 in a tall grass prairie in Oklahoma to investigate the three suggestions. It was clear from our data that increased productivity as a result of mowing and burning was not due to the elimination of toxins nor to an increased rate of turnover of organic matter. Soil moisture stress was greater in the mowed and burned plots than in the controls. Thus, the single factor examined which was consistently different, and likely to explain the increased productivity, was the higher soil temperature in the burned and mowed plots early in the growing season. The four dominant grasses in the study plots are non-Festucoid species which are known to have high optimum temperatures for growth. SEVERAL investigators have reported that protection of the tall grass prairie from grazing, mowing, or burning for even a few years results in an accumulation of considerable litter and reduced productivity (Weaver and Fitzpatrick, 1934; Weaver and Rowland, 1952; Kelting, 1954; Kucera and Koelling, 1964). Mechanical removal of prairie litter by clipping or mowing usually results in increased productivity and increased numbers of flowering culms (Curtis and Partch, 1950; Penfound, 1964b; Hulbert, 1969; Richards, 1969). Many investigators in many parts of the tall grass prairie also have reported increases in dry matter production as the result of burning (Curtis and Partch, 1948; Kelting, 1957; Kucera and Ehrenreich, 1962; Ehrenreich and Aikman, 1963; Hadley and Kieckhefer, 1963; Kucera and Koelling, 1964; Kucera and Dahlman, 1968; Hulbert, 1969; Richards, 1969). Many of these reported marked increases in flowering are also due to burning. There have been three basic reasons suggested for the decrease in productivity of an undisturbed prairie, and a fourth implied. Several investigators have suggested that the standing plant remains and litter serve as an insulation which keeps the ground cooler until later in the spring, thus shortening the growing season (Penfound and Kelting, 1950; Weaver and Rowland, 1952; Ehrenreich and Aikman, 1963; Hulbert, 1969; Peet, Anderson, and Adams, 1975). Others 1 Received for publication 24 April 1978; revision accepted 21 July 1978. have suggested that the accumulation of litter may result in a decreased rate of mineral cycling (Daubenmire, 1968). It has been reported that changes in numbers of soil microorganisms occur after burning of forests and other woody areas, but there are no such reports for the tall grass prairie (Ahlgren, 1974). Vogl (1974) and Peet et al. (1975) suggested that the litter may contain toxins which leach out and inhibit growth of the prairie plants. Some investigators have implied that the primary effect of the heavy litter layer is that it serves as a mechanical barrier to seedlings and culms of the grasses and decreases light energy which reaches them (Weaver and Fitzpatrick, 1934; Penfound, 1964a). There is considerable evidence in the literature to support the first suggestion concerning the insulating effect of the litter, but virtually no research has been done in grasslands to evaluate the other suggestions (Daubenmire, 1968; Ahlgren, 1974; Vogl, 1974). We decided, therefore, to investigate the three basic suggestions-insulating effect, lowering of rate of mineral recycling, production of toxins-during the same growing season in a tall grass prairie in Oklahoma. MATERIALS AND METHODS-Nine 3 x 3-m plots with 1-m borders were measured and staked in October 1976 in the protected tall grass prairie section of the University of Oklahoma Grasslands Investigations Plots. These plots are located approximately 12 km southwest of Norman along State Highway 9 in McClain