Energy and Carbon Balances in Cheatgrass: An Essay in Autecology

Biotic and abiotic pathways of incident energy are rarely considered together in comparable detail, so little is known about whether energy or carbon relations remain constant in contrasting energy environments. An experiment to determine the fates of energy and carbon in cheatgrass (Bromus tectorum L.) was carried out on steep (40°) north— and south—facing slopes on a small earth mound, using many small lysimeters to emulate swards of cheatgrass. Meteorological conditions and energy fluxes that were measured included air and soil temperatures, relative humidity, wind speed, incoming and reflected shortwave radiation, net all—wave radiation, heat flux to the soil, and evaporation and transpiration separately. The fate of photosynthetically fixed carbon during spring growth (31 March to 31 May) was determined by separation of the plant tissues into mineral nutrients, ash, crude protein, crude fat, crude fiber, and nitrogen—free extract (NFE) (the proximate analysis scheme routinely used for feed analysis) for roots, shoots, and seeds separately. Temperatures and humidities were not notably different between slopes. However, all terms in the radiation balances were significantly larger on the south exposure, and the sensible heat flux to the atmosphere on the south exposure was much greater than on the north. The energy—saturated south exposure was subjected to water stresses much earlier than the north exposure. Total transpired water, the fraction of energy used to transpire water, and soil water content simultaneously showed an abrupt decrease in slope at about 10% soil water content (—14 bars average water potential) in the 60—cm lysimeter soil profile, indicating a rather sudden decrease in water availability to the plants. This change occurred near day 30 on the south exposure, and near day 45 on the north exposure, implying that water was readily available to the north exposure for about 2 wk longer than on the south exposure. Root and seed production were both about 30% greater on the north exposure than on the south, but shoot production was not different. Heats of combustion (caloric content) differed between roots, shoots, and seeds, but not between exposures. Total production and total energy fixation were not statistically different on the two exposures, because the greater biomass and variability of the shoots overwhelmed the statistically significant differences in roots and seeds. Incident shortwave radiation was very much greater on the south exposure than on the north, so the south exposure was much less efficient in converting solar energy into chemical energy stored in plant tissues. The clearest difference in carbon pathways occurred for crude protein in shoot tissues at the end of the growing season (23 g m— ² on the north compared to 30 g m— ² on the south). Since both exposures began the growing season with 33 g m— ², shoot protein was translocated to other tissues during the season, to a greater extent on the north exposure than on the south. The crude fiber and NFE (g m— ²) in seed tissues were both greater on the north exposure, reflecting only a greater total seed biomass; but for roots and shoots there were no appreciable differences between exposures other than crude protein. The average individual seed weight was only about 10% heavier on the north exposure, although the north produced more than a third more total seed biomass. The composition of an average seed indicated that the difference in seed weight was caused primarily by about 8% more NFE in north exposure seeds, probably starches in the endosperm. By contrast, the crude protein in an average seed was almost identical between slopes–possibly a very important compensation for seedling size and vigor. Homeostasis (self—regulation) was evident as a gradation decreasing in the order seed viability (very strongly homeostatic) > seed composition (strong) > shoot and root composition (moderately strong) > average seed weight = shoot production (moderate) > seed number = mineral content (weak) > total root and seed production (no detectable homeostasis).