H. W. Hunt

Simulation model for the effects of climate change on temperate grassland ecosystems.

Abstract We studied the responses of temperate grasslands to climate change using a grassland ecosystem model which simulates seasonal dynamics of shoots, roots, soil water, mycorrhizal fungi, saprophytic microbes, soil fauna, inorganic nitrogen, plant residues and soil organic matter. Forty-year simulations were made for several climate change scenarios. The model was driven with observed weather and with combinations of elevated atmospheric CO 2 , elevated temperature, and either increased or decreased precipitation. Precipitation and CO 2 level accounted for most of the variation among climate change treatments in the responses of soil, plants, animals and microbes. Elevated temperature extended the growing season but depressed photosynthesis in the summer, with little net effect on annual primary production. Doubling CO 2 (1) caused persistent increases in primary production, in spite of greater nitrogen limitation, and (2) led to greater storage of carbon in plant residues and soil organic matter. The increased carbon storage was not great enough to keep pace with the present rate of increase in atmospheric CO 2 .

Bias and Random Errors in Estimators of Net Root Production: A Simulation Approach

Simulation of net primary production in a grassland and subsequent calculations of belowground net production revealed substantial sources of error that may be common to all estimates of belowground net production from field data. Inherent bias in the most frequently used estimators and a large counterintuitive effect of sample variability require that techniques be carefully considered before conclusions are drawn based upon estimates of belowground net production.

The response of mycorrhizal colonization to elevated CO2 and climate change in Pascopyrum smithii and Bouteloua gracilis

Large intact soil cores of nearly pure stands of Pascopyrum smithii (western wheatgrass, C3) and Bouteloua gracilis (blue grama, C4) were extracted from the Central Plains Experimental Range in northeastern Colorado, USA and transferred to controlled environment chambers. Cores were exposed to a variety of water, temperature and CO2 regimes for a total of four annual growth cycles. Root subsamples were harvested after the completion of the second and fourth growth cycles at a time corresponding to late winter, and were examined microscopically for the presence of mycorrhizae. After two growth cycles in the growth chambers, 54% of the root length was colonized in P. smithii, compared to 35% in blue grama. Field control plants had significantly lower colonization. Elevation of CO2 increased mycorrhizal colonization in B. gracilis by 46% but had no effect in P. smithii. Temperatures 4° C higher than normal decreased colonization in P. smithii by 15%. Increased annual precipitation decreased colonization in both species. Simulated climate change conditions of elevated CO2, elevated temperature and lowered precipitation decreased colonization in P. smithii but had less effect on B. gracilis. After four growth cycles in P. smithii, trends of treatments remained similar, but overall colonization rate decreased.

Root turnover and production by14C dilution: implications of carbon partitioning in plants

SummaryEstimates of belowground net primary production (BNP) obtained by using traditional soil core harvest data are subject to a variety of potentially serious errors. In a controlled growth chamber experiment, we examined the aboveground-belowground, labile to structural tissue, and plant to soil dynamics of carbon to formulate a14C dilution technique for potential successful application in the field and to quantify sources of error in production estimates.Despite the fact that the majority of net14C movement between above- and belowground plant parts occurred between the initial labeling and day 5, significant quantities of14C were incorporated into cell-wall tissue throughout the growing period. The rate of this increase at late sampling dates was greater for roots than for shoots. Total loss of assimilated14C was 47% in wheat and 28% in blue grama. Exudation and sloughing in wheat and blue grama, respectively, was 15 and 6% of total uptake and 22 and 8% of total plant production.When root production estimates by14C dilution were corrected for the quantities of labile14C incorporated into structural carbon between two sampling dates, good agreement with actual production was found. The error associated with these estimates was ±2% compared with a range of −119 to −57% for the uncorrected estimates. Our results suggest that this technique has potential field application if sampling is performed the year after labelling.Sources of errors in harvest versus14C dilution estimates of BNP are discussed.

Estimating aboveground net primary production in grasslands: A simulation approach

Abstract A simulation model which reflected the seasonal dynamics of biomass in a North America mixed prairie was employed to evaluate methods of calculating aboveground net primary production from harvest data. The methods of calculating aboveground net primary production were evaluated under conditions with and without variability in the data. A multivariate normal random number generator was utilized in the variance case. In the no variance case, methods which utilized only live biomass underestimated aboveground net primary production by approximately 42%. Methods which utilized live and recent dead (senesced) biomass produced estimates very close to the model value. Variance in the data resulted in overestimation of aboveground net primary production with the degree of overestimation related to the sampling frequency. Use of a simple simulation model which simultaneously makes use of all the data to arrive at an estimate of aboveground net primary production may prove to be superior to the statistical estimators.

Responses of a C3 and C4 perennial grass to CO2 enrichment and climate change: Comparison between model predictions and experimental data

Abstract Ecological responses to CO 2 enrichment and climate change are expressed at several interacting levels: photosynthesis and stomatal movement at the leaf level, energy and gas exchanges at the canopy level, photosynthate allocation and plant growth at the plant level, and water budget and nitrogen cycling at the ecosystem level. Predictions of these ecosystem responses require coupling of ecophysiological and ecosystem processes. Version GEM2 of the grassland ecosystem model linked biochemical, ecophysiological and ecosystem processes in a hierarchical approach. The model included biochemical level mechanisms of C 3 and C 4 photosynthetic pathways to represent direct effects of CO 2 on plant growth, mechanistically simulated biophysical processes which control interactions between the ecosystem and the atmosphere, and linked with detailed biogeochemical process submodels. The model was tested using two-year full factorial (CO 2 , temperature and precipitation) growth chamber data for the grasses Pascopyrum smithii (C 3 ) and Bouteloua gracilis (C 4 ). The C 3 C 4 photosynthesis submodels fitted the measured photosynthesis data from both the C 3 and the C 4 species subjected to different CO 2 , temperature and precipitation conditions. The whole GEM2 model accurately fitted plant biomass dynamics and plant N content data over a wide range of temperature, precipitation and atmospheric CO 2 concentration. Both data and simulation results showed that elevated CO 2 enhanced plant biomass production in both P. smithii (C 3 ) and B. gracilis (C 4 ). The enhancement of shoot production by elevated CO 2 varied with temperature and precipitation. Doubling CO 2 increased modeled annual net primary production (NPP) of P. smithii by 36% and 43% under normal and elevated temperature regimes, respectively, and increased NPP of B. gracilis by 29% and 24%. Doubling CO 2 decreased modeled net N mineralization rate (N_min) of soil associated with P. smithii by 3% and 2% at normal and high temperatures, respectively. N_min of B. gracilis soil decreased with doubled CO 2 by 5% and 6% at normal and high temperatures. NPP increased with precipitation. The average NPP and N_min of P. smithii across the treatments was greater than that of B. gracilis . In the C 3 species the response of NPP to increased temperatures was negative under dry conditions with ambient CO 2 , but was positive under wet conditions or doubled CO 2 . The responses of NPP to elevated CO 2 in the C 4 species were positive under all temperature and precipitation treatments. N_min increased with precipitation in both the C 3 and C 4 species. Elevated CO 2 decreased N_min in the C 4 system. The effects of elevated CO 2 on N_min in the C 3 system varied with precipitation and temperature. Elevated temperature decreased N_min under dry conditions, but increased it under wet conditions. Thus, there are strong interactions among the effects of CO 2 enrichment, precipitation, temperature and species on NPP and N_min. Interactions between ecophysiological processes and ecosystem processes were strong. GEM2 coupled these processes, and was able to represent the interactions and feedbacks that mediate ecological responses to CO 2 enrichment and climate change. More information about the feedbacks between water and N cycling is required to further validate the model. More experimental and modeling efforts are needed to address the possible effects of CO 2 enrichment and climate change on the competitive balance between different species in a plant community and the feedbacks to ecosystem function.

Inferring trophic transfers from pulse-dynamics in detrital food webs

In semiarid ecosystems, decomposers are active during numerous short periods following rainfall events, and most inactive in the intervening dry periods. Many studies concern season-long dynamics of decomposer populations, but less is known of the short-term dynamics during wet periods. These short-term dynamics may provide the key to understanding interactions between microbes and fauna.The dynamics of populations in the detrital food web were followed after wetting large intact soil cores that had been removed from native shortgrass steppe, winter wheat, and fallow plots. The cores were sampled over a ten day period for bacteria, fungi, protozoa, and various functional groups of microarthropods and nematodes. The native sod had appreciably greater biomass of fungi, nematodes and microarthropods than did the cultivated plots, but there was no difference in bacteria or protozoans. The observed dynamics after wetting were different in two experiments which differed in temperature, soil water level, and the initial sizes of the populations. These results were interpreted in relation to a model of the structure of the detrital food web, and estimates were made of the rates of trophic transfers in the web. Consumption by protozoa was great enough for them to account for bacterial turnover, but consumption by fungivorous nematodes and microarthropods appeared to be too small to account for fungal turnover.Progress in understanding the dynamics of detrital food webs requires a better definition of the functional groups of soil organisms, their resources, predators and population parameters, and the effects of soil structure and water content on trophic relationships.

A simulation model of intraseasonal carbon and nitrogen dynamics of blue grama swards as influenced by above- and belowground grazing

Abstract This study examines the effects of various kinds and levels of herbivory on plant processes and productivity. A model was developed to simulate growth of blue grama under field conditions. Two submodels represent uptake of carbon (C) and nitrogen (N) and their allocation within the plant. Shoots and roots were divided into structural material, characterized by high C : N ratio, and metabolic or storage material characterized by lower C : N ratio. We assumed that root and shoot growth rates were governed by C : N ratio of metabolic components of plant tissue. The model was run for three consecutive years using climatic data as driving variables. We calibrated the model with field data for 1971 and validated it against field data for 1972 and 1973. Model validation exhibits a relatively good correlation with Clarks (1977) data. Simulation of belowground grazing show that nematodes could have the biggest effect on net primary production among all grazers, but do not affect shoot production to root production ratio. Nitrogen cycling in the plant-soil system is affected much more by belowground grazing than by aboveground grazing. Sensitivity analysis was performed to determine the effects of above- and belowground grazing on net primary production and nitrogen allocation. Grasshopper grazing increases net primary production by stimulating aboveground production and increases N availability in the soil through the recycling process.

Simulated carbon and nitrogen dynamics in blue grama swards subject to above- and belowground grazing, irrigation and fertilization part II. The grazing optimization notion

Abstract The study was designed to explore responses to grazing of primary-production processes, such as patterns of C (carbon) and N (nitrogen) allocation, to better understand the mechanisms by which herbivores affect ecosystem function. A model was created to simulate growth of blue grama in the field. Shoots and roots were divided into structural material (characterized by high C:N ratio) and metabolic/storage material (characterized by lower C:N ratio) to facilitate representation of the quality of the grass subjected to grazing. This plant model was linked to a model of C and N dynamics in grassland soils, and run in conjunction with an abiotic model. The simulation was run for three consecutive years and tested by sensitivity analysis. This paper concentrates on the effects of above- and belowground grazing. Production was maximized at moderate aboveground grazing intensities. However, the difference between maximal primary production and that predicted in the absence of grazing was too small (less than 5%) to be detected in the field.