Amount Of Aboveground Biomass Research Articles

Corn stover harvest strategy effects on grain yield and soil quality indicators

The development of technologies to use cellulosic biomass as a feedstock for biofuel production was recognized as an important research focus because cellulose is a more widely-available feedstock than corn starch. Our objective was to compare various corn (Zea mays L.) stover harvest strategies to determine which would be most sustainable. A complete block design with 2 ha plots, each replicated three times, was imposed on a 50 ha (125 acre) Clarion-Nicollet-Webster soil Association site near Emmetsburg, Iowa, U.S.A. before harvesting the 2008 corn crop. Hand samples were collected from a 1.5 m2 area in each plot to establish the potential amount of above-ground biomass that could be potentially harvested using one of seven stover harvest strategies. Surface soil samples (0 to 15 cm) were analyzed following each harvest to monitor fertility changes and to make subsequent fertilizer recommendations. Grain yields averaged 11.4, 10.1, 9.7, and 9.5 Mg ha-1 in 2008, 2009, 2010, and 2011, respectively, but were not signifi cantly affected by stover harvest treatments. Relative yield for the various treatments ranged from 97 to 107% of the conventional treatment for which none of the residue was harvested. Four-year average stover yields ranged 1.0 to 5.2 Mg ha-1 which was 12 to 60% of the above-ground biomass. Three years of plant tissue data at anthesis indicated N management needed to be improved as the values were below the critical concentration of 27 g kg-1. Sulfur concentrations were just barely above the critical value of 15 g kg-1. Soil test analyses showed substantial fi eld variability but no signifi cant stover harvest treatment effects. There was a slight decrease in soil organic carbon, unrelated to the stover harvest treatments, that is attributed to the intensity of tillage and crop yields that were lower than expected. Overall, this study is consistent with other studies in the U.S. Corn/Soybean Belt that indicate to sustain soil resources within this region, corn stover should generally not be harvested if average grain yields are less than 11 Mg ha-1 (175 bu ac-1). Strategies for achieving those levels include implementing more rigorous soil-testing and plant analysis programs, installing tile drainage where needed, improving overall nutrient management programs, reducing tillage intensity, incorporating cover crops and rotating corn with other crop such as soybean [Glycine max (L.) Merr.], small grain, or forage.

How does the precipitation gradient interact with common disturbances to structure prairies?

How plant communities are structured, and the relative roles of gradients and disturbances in that structuring, has long been of interest. Here I use plots in replicate tallgrass, mixedgrass, and shortgrass prairies across Northern Oklahoma to address this issue by sampling plant percent cover three years after applying treatments realizing common prairie disturbances of burning, grazing, and soil turnover. I found (1) shortgrass plots had the least amount of aboveground biomass (AGB), with burning and soil turnover plots also having low AGB in the other two prairies, (2) tallgrass plots had the most total cover, with soil turnover plots having the least in the other two prairies, (3) tallgrass plots had the most species, with soil turnover plots having the least in the other two prairies, (4) control plots in mixedgrass had the smallest evenness and plots in shortgrass had the highest, and (5) a high degree of functional similarity in all three prairies. In addition to controlling these different aspects of population and community structure in prairies, results also show that the most severe disturbances can lead to a prairie plant composition and structure more similar to that found in the drier, most western prairie areas.

Festuca campestris alters root morphology and growth in response to simulated grazing and nitrogen form

Summary 1. Large herbivores are known to spatially concentrate and alter the form of nitrogen (N) in grassland systems, which can modify aboveground plant chemical composition and productivity, and result in shifts in grazing pressure in these areas. Root responses to grazing under high N inputs may facilitate these changes, but their responses are less well understood. 2. We examined biomass, root morphology and demography, and nitrogen pool responses of Festuca campestris Rydb. seedlings grown in pots in an environmental chamber and subject to heavy fertilization with isotopically‐labelled N in a NO3‐N or NH4‐N fertilizer treatment, and to no clipping or clipping to 4 cm twice during the experiment. 3. Festuca campestris exhibited re‐growth patterns after defoliation that reflected preferential allocation to aboveground shoots, with percent recovery of shoot biomass twice that of roots. Clipping did not increase the risk of root mortality. Instead, reduced root biomass in clipped plants resulted from slowed elongation and decreased production of new roots. Although roots with smaller diameters and at lower density had a greater risk of root mortality, clipping did not alter the mean root diameter or level of root branching. 4. Under high N fertilization, NH4‐N increased the amount of aboveground biomass, ameliorated the effects of clipping on root production and further reduced the risk of root mortality in clipped plants. N‐form had no effect on root morphology. 5. While total plant N uptake and the percent utilization of fertilizer were lower in clipped plants, total plant nitrogen accumulated per root biomass (g g−1) at harvest was higher in clipped plants. 6. Our findings indicate that where defoliation is coupled with high nitrogen return, Festuca campestris recuperates by increasing nitrogen uptake, shifting production to favour recovery of aboveground biomass, and mitigating carbon losses by decreasing investment in new, actively growing roots. Because application of a more mobile form of nitrogen (NO3‐N) did not alter root morphology, root physiological responses may be most important for re‐growth when nitrogen is not limiting. Root reductions associated with grazing may have a greater impact in locations where grazer‐mediated nitrogen return is spatially decoupled from defoliation.

Predicting the Above-ground Biomass of Scots Pine (Pinus sylvestris L.) Stands in Turkey

In this study, biomass equations are presented for scots pine stands within the Erzurum Regional Forestry Management Area. Thirty-three sample plots, each of 0.04 hectares were chosen in order to define the biomass equations of black pine, one of the common needle-leaved species in Turkey. A tree that is the most similar to mean tree according to basal area was cut in each sample area as a sample tree. Various models were tested, utilizing the diameter (d) and the height (h) as independent variables, and the most suitable models were determined. Using these models, above-ground biomass amounts can easily be acquired for single trees and stands.

Are ‘ancient wheat species’ more adapted to hostile environments than modern bread wheat?

This preliminary study investigated whether three ‘ancient wheat species’, spelt (Triticum spelta L.), emmer (Triticum dicoccum) and einkorn (Triticum monococcum L.) are more adapted to hostile environments than modern bread wheat (Triticum aestivum L.) based on their physiological characteristics and comparative water use efficiencies. Spelt was found to have higher radiation use efficiency (RUE) than the other species and produced a similar amount of above ground biomass. However, bread wheat produced the highest yield, mainly due to a superior harvest index. Emmer had the lowest carbon isotope discrimination (Δ13C) which is expected to translate to the highest water use efficiency (WUE).

The impact of co‐occurring tree and grassland species on carbon sequestration and potential biofuel production

AbstractWe evaluated how three co‐occurring tree and four grassland species influence potentially harvestable biofuel stocks and above‐ and belowground carbon pools. After 5 years, the tree Pinus strobus had 6.5 times the amount of aboveground harvestable biomass as another tree Quercus ellipsoidalis and 10 times that of the grassland species. P. strobus accrued the largest total plant carbon pool (1375 g C m−2 or 394 g C m−2 yr), while Schizachyrium scoparium accrued the largest total plant carbon pool among the grassland species (421 g C m−2 or 137 g C m−2 yr). Quercus ellipsoidalis accrued 850 g C m−2, Q. macrocarpa 370 g C m−2, Poa pratensis 390 g C m−2, Solidago canadensis 132 g C m−2, and Lespedeza capitata 283 g C m−2. Only P. strobus and Q. ellipsoidalis significantly sequestered carbon during the experiment. Species differed in total ecosystem carbon accumulation from −21.3 to +169.8 g C m−2 yr compared with the original soil carbon pool. Plant carbon gains with P. strobus were paralleled by a decrease of 16% in soil carbon and a nonsignificant decline of 9% for Q. ellipsoidalis. However, carbon allocation differed among species, with P. strobus allocating most aboveground in a disturbance prone aboveground pool, whereas Q. ellipsoidalis, allocated most carbon in less disturbance sensitive belowground biomass. These differences have strong implications for terrestrial carbon sequestration and potential biofuel production. For P. strobus, aboveground plant carbon harvest for biofuel would result in no net carbon sequestration as declines in soil carbon offset plant carbon gains. Conversely the harvest of Q. ellipsoidalis aboveground biomass would result in net sequestration of carbon belowground due to its high allocation belowground, but would yield lower amounts of aboveground biomass. Our results demonstrate that plant species can differentially impact ecosystem carbon pools and the distribution of carbon above and belowground.

Predicting the above-ground biomass of calabrian pine (Pinus brutia Ten.) stands in Turkey

Biomass equations are presented for calabrian pine stands within the Adana-Karaisalý Regional Forestry Management Area. Thirty three sample plots, each of 0.04 ha, were chosen in order to define the biomass equations of calabrian pine, the most common needle leave species in Turkey. A tree which is the most similar to mean tree according to basal area was cut in each sample area as a sample tree. Various models were tested, utilizing the diameter (d) and the height (h) as independent variables and the most suitable models were determined. Using these models, above-ground biomass amounts can easily be acquired for single trees and stands.

Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.

Environmental controls on photosynthetic production and ecosystem respiration in semi-arid grasslands of Mongolia

The Mongolian steppe zone comprises a major part of East Asian grasslands. The objective of this study was to separately evaluate the quantitative dependencies of gross primary production (GPP) and ecosystem respiration ( R eco) on the environmental variables of temperature, moisture, radiation, and plant biomass in a semi-arid grassland ecosystem. We determined GPP and R eco using transparent and opaque closed chambers in a grassland dominated by Poaceae species in central Mongolia during five periods: July 2004, May 2005, July 2005, September 2005, and June 2006. Values of GPP were linearly related to live aboveground biomass (AGB) enclosed by the chamber. The amount of GPP per unit ground area differed among the study periods, whereas GPP normalized by the amount of AGB did not differ significantly among the periods, suggesting that plant production per unit green biomass did not depend on the phenological stage. GPP/AGB fit well a rectangular hyperbolic light–response curve for all the study periods. When the air and soil were dry, considerable reduction in GPP was observed. The GPP/AGB ratio was also expressed as individual functions of air temperature, vapor pressure deficit, and volumetric soil water content. R eco was exponentially related to the soil temperature and the relationship was modified by soil moisture. The amount of R eco and its temperature sensitivity ( Q 10) declined with decreasing soil moisture. Sharp increases of R eco after rainfall events were observed. The values of R eco, even including the rain-induced pulses, were expressed well as a bivariate function of soil temperature and soil moisture near the soil surface.

Manipulating nutrient and water availability in a maritime pine plantation: effects on growth, production, and biomass allocation at canopy closure

• We present here the results of a water and nutrient manipulation experiment in a five-year-old plantation of maritime pine in south-western France. • Water and nutrient levels were manipulated in a factorial design with two levels of irrigation (control receiving only rainfall (C) and irrigated (I)) and three levels of fertilisation (control with no added nutrients (C), P-only (P) and annual addition of a complete nutrient mix (F)) in order to quantify growth limitations of plantation forest in this particular area. • The treatments applied during five years increased aboveground biomass annual increment by 4% (I) to 58% (IF) with respect to the control (C). The fertilised plots had a slightly non-significant lower root-to-shoot ratio. The effect of irrigation was maximal in 2002, resulting in 6%, 7% and 12% higher growth rate on the F, C and P plots, respectively. A windstorm disturbed the experiment in 1999 and has affected preferentially the fertilised plots, with IF plots displaying 60% damage. • The higher growth rate of fertilised and irrigated plots was attributed to both an increase (estimated at 5 to 15%) in the amount of light absorbed by the canopy, and an increase (estimated at 26% for IF plots) in the amount of above-ground biomass produced annually per unit leaf area.

Local and regional processes in low‐productive mountain plant communities: the roles of seed and microsite limitation in relation to grazing

It is becoming widely accepted that plant community structure is determined not only by local scale factors, but that regional factors may play considerable role. The research studying the associated processes in different environments with different species assemblages is still limited. We conducted a two‐year seed sowing experiment to test whether a plant community in a low‐productive mountain snowbed is limited by seed or microsite availability and how these variables depend on natural grazing. In a factorial design, half of the plots received a mixture of seeds of fourteen species naturally occurring at the study site and above ground biomass was removed from half of the plots. These treatments were applied to plots with long term grazer exclosures and to plots accessible to grazers. Both sowing and biomass removal increased the number of seedlings, the species richness of seedlings and total species richness. The number of seedlings was higher in open plots than in exclosures in the second year. Both seedling richness and total species richness were higher in open plots. Seedling recruitment was negatively related to the amount of above ground biomass and positively to the initial species richness. These results suggest that even fairly low‐productive environments can be both seed and microsite limited and that these depend on grazing pressure. Natural grazing by mammal herbivores (e.g. lemmings and reindeer) favours species colonization and seedling emergence. Low‐productive mountain snowbeds are prone to colonization from the local species pool and even high species richness may not constrain ingression of new species.

Modeling soil quality thresholds to ecosystem recovery at Fort Benning, GA, USA

The objective of this research was to use a simple model of soil carbon (C) and nitrogen (N) dynamics to predict nutrient thresholds to ecosystem recovery on degraded soils at Fort Benning, Georgia, in the southeastern USA. Artillery, wheeled, and tracked vehicle training at military installations can produce soil disturbance and potentially create barren, degraded soils. Ecosystem reclamation is an important component of natural resource management at military installations. Four factors were important to the development of thresholds to recovery of aboveground biomass on degraded soils: (1) initial amounts of aboveground biomass, (2) initial soil C stocks (i.e., soil quality), (3) relative recovery rates of biomass, and (4) soil sand content. Forests and old fields on soils with varying sand content had different predicted thresholds for ecosystem recovery. Soil C stocks at barren sites on Fort Benning were generally below predicted thresholds to 100% recovery of desired future ecosystem conditions defined on the basis of aboveground biomass. Predicted thresholds to ecosystem recovery were less on soils with more than 70% sand content. The lower thresholds for old field and forest recovery on more sandy soils were apparently due to higher relative rates of net soil N mineralization. Calculations with the model indicated that a combination of desired future conditions, initial levels of soil quality (defined by soil C stocks), and the rate of biomass accumulation determine the predicted success of ecosystem recovery on disturbed soils.

THE EFFECT OF PHOSPHATE APPLICATION TO COVER CROPS ON THE YIELDS OF SUCCEEDING MAIZE (<i>Zea mays</i> L.) ON A KANHAPLIC HAPLUSTALF IN NORTHERN GUINEA SAVANNA OF NIGERIA

Phosphorus deficiency is a major constraint that limits food production in Nigeria, especially under acidic soil conditions. Presently, successful production of high nutrient requiring plants such as maize is only possible with the use of inorganic phosphate fertilizers that are very costly and often unavailable to the farmer. In an attempt to address this problem, field trials were conducted on a Kanhaplic Haplustalf at Samaru (11 o 11'N, 7o 38'E) to evaluate the response of ten (10) leguminous cover crops and maize to native and applied phosphate, and to evaluate the effects of these treatments on the performance and P nutrition of succeeding maize in a rotation. Among the cover crops evaluated, Crotalaria ochroleuca was the most P efficient in terms of total amount of above ground biomass per unit P taken up, while Phaseolus vulgaris was the least efficient. The yield of maize preceded by Lablab purpureus, Chamaecrista rotundifolia, and Mucuna pruriens was increased by over 50% in the first year after cover cropping. The positive effects of these cover crops on yield of succeeding maize did not however manifest in the second year. Effect of Sokoto Phosphate Rock (SPR) on maize yields and available soil P started manifesting only in the third year of cropping when the soil P was sufficiently depleted to force the dissolution of phosphate rock. Nigerian Journal of Soil Science Vol.4 2003: 25-34

Tephrosia species and provenances for improved fallows in southern Africa

Seeking an alternative to Sesbania spp. tree fallows, a Tephrosia species and provenance trial was conducted at Msekera Research Station, Chipata (Zambia) to evaluate eleven Tephrosia vogelii and three Tephrosia candida provenances. They were tested for biomass production, quality of biomass, resistance to root-knot nematodes, nitrogen release, and for their effects on soil nitrogen dynamics. At the end of 1.5 years, the T. candida provenances 02970, 02971 and 02972 from Madagascar produced two times greater amount of aboveground biomass than the T. vogelii provenances. There was little variability among the T. vogelii provenances in terms of litter and biomass production. Weed growth was significantly greater under T. vogelii than T. candida provenances. While Tephrosia vogelii provenance 98/02 from Zambia and T. candida 02972 were highly tolerant to the Meloidogyne incognita nematodes, T. vogelii provenances 02977, 98/03, 02973 from Kenya, Zambia and Malawi, respectively, were highly susceptible to the nematodes. The Tephrosia species and provenances showed a wide variability in terms of N, lignin and polyphenol concentration in their foliage. Mineralization of N in the foliage of T. candida provenances 02970 and 02971 and T. vogelii provenances 98/04 and 02974 from Malawi occurred rapidly within 14 weeks of incubation. At the end of the 2-year growth period, there was significantly greater total inorganic N under T. candida provenance 02972 (12.5 mg kg−1) than T. vogelii (5 mg kg−1) provenance Mungwi 98/02. Maize (Zea mays L.) yields after T. candida provenances were greater than those after T. vogelii provenances. Further testing of the most promising provenances is needed for their effects on subsequent maize yields under a range of farm conditions.

Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2

The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, δ18O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C3/C4 grassland to doubled atmospheric [CO2] using Open-Top Chambers (OTC). Elevated atmospheric [CO2] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the δ18O of soil CO2 as a proxy for soil water, after establishing a strong relationship between δ18O of soil CO2 from non-chambered control (NC) plots and δ18O of soil–water from an adjacent area of native grassland. Soil–CO2 δ18O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.

Yield and Growth Response of Cuphea to Sowing Date

ABSTRACTSelect germplasm of Cuphea, developed from an interspecific hybridization of C. viscosissima Jacq. and C. lanceolata f. Silenoides W.T. Aiton, shows good potential for commercial production in short‐season temperate climates. Cuphea seed oil could serve as a domestic source of medium‐chain fatty acids, which are in high demand by the chemical manufacturing industry. However, little is known about proper management practices for Cuphea agronomic production. A field study was conducted in west central Minnesota to determine optimum time of sowing in the northern Corn Belt region and describe influences of sowing date on growth and seed yield components of a semidomesticated germplasm line (PSR23). Seed was sown on 15 April, 1 May, 15 May, 1 June, and 15 June 1999 and 2000. Sowing in May resulted in greatest seed and seed‐oil yields, which were as high as 1.09 and 0.29 Mg ha−1, respectively. Seed yield declined as much as 31% when sown 15 April and 65% when sowing was delayed until 15 June. Plants developed from seed sown before June tended to form more branches and accumulated a greater amount of aboveground biomass. Typically, there was a distinct decrease in plant biomass accumulation and most seed yield components when sowing date was delayed until June. Cuphea PSR23 can be successfully grown in the northern Corn Belt, with early to mid May being the best time for sowing.

Estimates of above-ground biomass and nutrient accumulation in Mimosa scabrella fallows in southern Brazil

Naturally regenerated stands of bracatinga ( Mimosa scabrellaBentham) are harvested for firewood after six to eight years of unregulated growth, debris burnt and the area planted to one cycle of intercropped maize (Zea mays L.) and beans (Phaseoulus vulgaris L.). Burning breaks dormancy of bracatinga seed (> 80% germination) marking the onset of a new fallow-crop cycle. This production system has been practiced for nearly 100 years in Southern Brazil, covering some 60,000 ha in 3,000 small farms. An estimation of above-ground biomass and nutrient accumulation was made using literature data on stand age, population numbers, tree sizes, tree biomass partitioning and concentration of major nutrients in tree tissues. A simple simulation model, used to quantify above-ground nutrient pathways and their temporal dynamics, confirmed that six to eight years is the optimal rotation length. Biomass and nutrients deposited onto the soil, peak at stand age six years, which may result in significant soil fertility improvement prior to crop planting. At year six, estimated total above-ground biomass amounts to 83 Mg ha ‐1 ; 44 Mg ha ‐1 available as firewood and 39 Mg ha ‐1 to be returned to the soil. Roughly half the amount of nutrients fixed in the above-ground bracatinga biomass would be exported in firewood and subsequent grain crops.

Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2

SummaryAlthough legumes showed a clearly superior yield response to elevated atmospheric pCO2 compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N2 fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO2 response. Under elevated atmospheric pCO2, effectively nodulating lucerne, thus capable of symbiotically fixing N2, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO2 when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO2, although not as strongly as effectively nodulating plants. The CO2‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N2 fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO2. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N2 fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO2. This supports the assumption that symbiotic N2 fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO2‐rich world.

Regional assessment of boreal forest productivity using an ecological process model and remote sensing parameter maps

An ecological process model (BIOME-BGC) was used to assess boreal forest regional net primary production (NPP) and response to short-term, year-to-year weather fluctuations based on spatially explicit, land cover and biomass maps derived by radar remote sensing, as well as soil, terrain and daily weather information. Simulations were conducted at a 30-m spatial resolution, over a 1205 km(2) portion of the BOREAS Southern Study Area of central Saskatchewan, Canada, over a 3-year period (1994-1996). Simulations of NPP for the study region were spatially and temporally complex, averaging 2.2 (+/- 0.6), 1.8 (+/- 0.5) and 1.7 (+/- 0.5) Mg C ha(-1) year(-1) for 1994, 1995 and 1996, respectively. Spatial variability of NPP was strongly controlled by the amount of aboveground biomass, particularly photosynthetic leaf area, whereas biophysical differences between broadleaf deciduous and evergreen coniferous vegetation were of secondary importance. Simulations of NPP were strongly sensitive to year-to-year variations in seasonal weather patterns, which influenced the timing of spring thaw and deciduous bud-burst. Reductions in annual NPP of approximately 17 and 22% for 1995 and 1996, respectively, were attributed to 3- and 5-week delays in spring thaw relative to 1994. Boreal forest stands with greater proportions of deciduous vegetation were more sensitive to the timing of spring thaw than evergreen coniferous stands. Similar relationships were found by comparing simulated snow depth records with 10-year records of aboveground NPP measurements obtained from biomass harvest plots within the BOREAS region. These results highlight the importance of sub-grid scale land cover complexity in controlling boreal forest regional productivity, the dynamic response of the biome to short-term interannual climate variations, and the potential implications of climate change and other large-scale disturbances.

Effects of wheat leaf rust on interactions between wheat and wild oats planted at various densities and proportions

The importance of competition as a major influence on the composition and structure of plant communities has recently been questioned, because other types of interactions can cause significant compositional changes. The goal of this research was to broaden our understanding of disease as a process structuring plant communities under a variety of competitive scenarios. Two cultivars of spring wheat (Triticum aestivum L., cv. Twin and cv. Penawawa) and wild oats (Avena fatua L.) were planted at three densities and at five proportions. One-half of the experimental material was inoculated with uredospores of Puccinia recondita. Increasing the proportion of wheat or oats in mixtures led to significant increases in the amount of aboveground biomass and total seed weight for that species. The seed weight and aboveground biomass per culm or per planted seed decreased for wheat and wild oats as the proportion of wild oats increased in mixtures, indicating a competitive advantage for wild oats when grown with wheat. Wild oats generally did not respond significantly to the effects of leaf rust on wheat, while wheat performance declined. Lowered wheat performance in inoculated stands was the main reason for lower relative biomass ratios of wheat to wild oats. Puccinia recondita infections occurred late in the life cycle of wheat, thereby decreasing the potential impact on wild oats' adults through competitive interactions.