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Effects of Herbivores on the Competition of C3 and C4 Graminoids.
1. A study was conducted to determine the effects of herbivores on the competition of C3 and C4 graminoids at the Central Plains Experimental Range (the CPER) in northern Colorado, USA., and several places in southwestern Japan: Nara Park, Miyajima Island, Mt. Azuma, the Oita Prefecture Animal Husbandry Range (the OPAHR) on Mt. Kuju, Mt.Aso, and Toi-Misaki Point. Herbivore effects were mainly derived from cattle at the CPER, Mt. Azuma, the OPAHR on Mt. Kuju and Mt. Aso, from horses at Toi Misaki Point, and from half-tamed deer (Cervus nippon) at Nara Park and Miyajima Island.
2. The main objective of the present study was to demonstrate to what extent herbivores correlate with two physiological function systems in C3 and C4 graminoids. The hypothesis investigated was: the abundance of C3 and C4 grammoids is dependent on direct herbivore influence, and also on indirect influences brought about by a process of micro-environmental modification caused by herbivores, such as that in water resources and irradiance, which are physiologically advantageous or disadvantageous to C3 and C4 plants.
3. The importance values of C3 and C4 graminoids were expressed as summed dominace ratio (SDR), relative importance value (SDR'), cumulative value for C3 and C4 graminoids (C3- and C4-SDR'), the number of C3 and C4 graminoid species, and index value (C3- or C4-SDR' multiplied by the number of C3 or C4 graminoids species respectively).
4. To examine the indirect influence (in the relationship among herbivore - abiotic factors - vegetational aspects), the soil properties: bulk density, maximum water holding capacity (MWHC), moisture percentage (MP), water content ratio (WCR), soil organic matter (carbon and nitrogen content) and soil hardness were measured. Sampling for vegetation and soils was done along an ecological cline, from the upland to the lowland of the hillslope where those factors listed above exhibited sequential alternations. This procedure was repeated in differently grazed pastures (heavy, moderate, light and non-grazing) and different exclosure systems (inside and outside the fence).
5. The results were:
(1) As a function of grazing intensity, using discrete nominal variables (from heavy to non-grazing), the importance of C4 graminoids markedly increased and that of C3 grammoids generally declined.
(2) Studies in the exclosure regime excluding the intensive deer grazing in Nara Park also showed similar trends: tolerance of C4 graminoid and vulnerable traits of C3 grammoidsi The extent of grazing effects depends on the exclosure condition - open or closed - with or without canopies. However, the results on Miyajima Island were not distinct in terms of responses of C3 and C4 graminoids to the grazing.
(3) C3 and C4 importance values responded to the ecological gradient in progressive or retrogressive succession, which were expressed as a replacement of a japonica and a Miscanthus sinensis type community, and were attributed to the different grazing intensities. This also suggests that the abrupt increase in light intensity and temperature caused by the removal of the canopy (Miscanthus sinensis)influenced the competitive relationship between C3 and C4 graminoids.
(4) There was a close relationship between the abundance of the C3, C4 graminoids and topographical features of the hillslope (the upland, the mid-slope and the lowland), which regulate the spatial use by herbivores, indicating that the distributional difference of C3 and C4 graminoids along the hillslope is largely due to the discriminative herbivore use of the pasture.
(5) Simple regression techniques were used to define the relationships between soil properties and C3, C4 graminoid distribution. The importance of C3 and C4 graminoids showed sxgnificant correlations mostly with soil properties, especially water-related factors and bulk density. This suggests that the physiological functions of those two types of graminoids responded to these soil factors.
(6) Herbivore activity, such as grazing, trampling and resting on the ground, altered the soil condition. Differences in soil bulk density, hardness and space structure were distinct among the differently grazed areas.
6. The evidences listed above support the hypothesis that herbivores influence C3 and C4 graminoid competition: directly with grazing which stimulates tolerant and intolerant characteristics of C3 and C4 graminoids, and indirectly with modified soil condition and light intensity, which fit or do not fit the physiological traits of C3 and C4 species.
7. The fact that these trends occurred consistently over all the study areas regardless of major climate differences and herbivore type suggests that the herbivore is a primary regulator in the competition between the C3 and C4 grammoids. However, if the grazing pressure upon the vegetation is extremely high as seen in herbaceous communities On Miyajima Island, the relations of herbivores and C3 and C4 graminoid distribution are indistinctive.
the Journal of Science of the Hiroshima University. Series B. Division 2 (Botany), Vol. 20, 1985
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