Prediction of Cheatgrass Field Germination Potential Using Wet Thermal Accumulation

Invasion and dominance of weedy species is facilitated or constrained by environmental and ecological factors that affect resource availability during critical life stages. We compared the relative effects of season, annual weather, site, and disturbance on potential cheatgrass (Bromus tectorum L.) germination in big sagebrush (Artemisia tridentata Nutt.) communities. Soil water status and temperature in the seedbed were measured continuously for 4 years on 9 big sagebrush sites in Nevada and Utah. Field plots at lower-, middle-, and upper-elevation sites were either undisturbed, or were burned, sprayed with herbicide, or both sprayed and burned. Spraying removed perennial herbaceous vegetation, whereas burning removed sagebrush. We used thermal-germination data from laboratory incubation studies of 18 cheatgrass seedlots and field soil moisture and temperature measurements to model and predict potential germination in the field plots for periods when seedbeds were continuously wet (above -0.5, -1, or -1.5 MPa) and across intermittent wet and dry periods. Season had the greatest effect on potential cheatgrass germination, followed by annual weather, and site variables (elevation and location); the effects of disturbance were minimal. Potential germination was predicted for most sites and years in spring, a majority of sites and years in fall, and few sites or years in winter. Even though disturbance has limited effects on potential germination, it can increase cheatgrass invasion and dominance by reducing perennial herbaceous species resource use and allowing increased cheatgrass growth and reproduction.     

Evaluation of soil erosion risk and identification of soil cover and management factor (C) for RUSLE in European vineyards with different soil management

Vineyards show some of the largest erosion rates reported in agricultural areas in Europe. Reported rates vary considerably under the same land use, since erosion processes are highly affected by climate, soil, topography and by the adopted soil management practices. Literature also shows differences in the effect of same conservation practices on reducing soil erosion from conventional, bare soil based, management. The Revised Universal Soil Loss Equation (RUSLE) is commonly adopted to estimate rates of water erosion on cropland under different forms of land use and management, but it requires proper value of soil cover and management (C) factors in order to obtain a reliable evaluation of local soil erosion rates. In this study the ORUSCAL (Orchard RUSle CALibration) is used to identify the best calibration strategy against long-term experimental data. Afterwards, ORUSCAL is used in order to apply the RUSLE technology from farm based information across different European wine-growing regions. The results suggest that the best strategy for calibration should incorporate the soil moisture sub-factor (S_m) to provide better soil loss predictions. The C factor, whose average values ranged from 0.012 to 0.597, presented a large spatial variability due to coupling with local climate and specific local management. The comparison across the five wine-growing regions indicates that for the soil protection management, permanent cover crop is the best measure for accomplishing sustainable erosion rates across the studied areas. Alternate and temporary cover crops, that are used in areas of limited water resources to prevent competition with vines, failed to achieve sustainable erosion rates, that still need to be addressed. This raises the need for a careful use of C values developed under different environmental conditions.     

Process-based modelling of the severity and impact of foliar pest attack on eucalypt plantation productivity under current and future climates

We examined the impacts of a defoliating pest, Mycosphaerella leaf disease (MLD), on rotation-length Eucalyptus globulus plantation productivity under current and future climates by using the ecoclimatic species niche model CLIMEX to generate severity, frequency and seasonality scenarios for MLD for specific E. globulus sites. These scenarios were used as inputs to the process-based forest productivity model CABALA. Climate projections from two global climate models were used to drive CABALA with either no or full acclimation of photosynthesis to elevated atmospheric CO₂ assumed. In addition we varied water and nitrogen availability to examine the impacts of different severities of MLD on plantation productivity across environmental gradients. We predicted that, under current climatic conditions, rotation-length reductions in V associated with MLD damage would be no greater than 12%, with an across-site average of 6%. There was considerable between-site variation in predictions that reflected variation in site productivity. Under future climates, we predicted that MLD may reduce rotation length V by as much as 42%, although the reduction averaged across all sites was 11%. The predicted impact of MLD on V was greatest at lower productivity sites. The importance of N and water availability in recovery following MLD attack was highlighted. Uncertainty in model predictions revolved around the climate models used and assumptions of degree of photosynthetic acclimation to elevated CO₂. Large differences in predicted impact of MLD were associated with this uncertainty. Our results suggest that the effects of defoliation due to pests on plantation productivity should not be ignored when considering future management of forest plantations. The approach developed here provides managers with a tool to appraise risk and examine possible impacts of management interventions designed to reduce or manage risk.     

Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations

More accurate projections of future carbon dioxide concentrations in the atmosphere and associated climate change depend on improved scientific understanding of the terrestrial carbon cycle. Despite the consensus that U.S. terrestrial ecosystems provide a carbon sink, the size, distribution, and interannual variability of this sink remain uncertain. Here we report a terrestrial carbon sink in the conterminous U.S. at 0.63 pg C yr⁻¹ with the majority of the sink in regions dominated by evergreen and deciduous forests and savannas. This estimate is based on our continuous estimates of net ecosystem carbon exchange (NEE) with high spatial (1 km) and temporal (8-day) resolutions derived from NEE measurements from eddy covariance flux towers and wall-to-wall satellite observations from Moderate Resolution Imaging Spectroradiometer (MODIS). We find that the U.S. terrestrial ecosystems could offset a maximum of 40% of the fossil-fuel carbon emissions. Our results show that the U.S. terrestrial carbon sink varied between 0.51 and 0.70  pg C yr⁻¹ over the period 2001-2006. The dominant sources of interannual variation of the carbon sink included extreme climate events and disturbances. Droughts in 2002 and 2006 reduced the U.S. carbon sink by ∼20% relative to a normal year. Disturbances including wildfires and hurricanes reduced carbon uptake or resulted in carbon release at regional scales. Our results provide an alternative, independent, and novel constraint to the U.S. terrestrial carbon sink.     

Induced N-limitation of bacterial growth in soil: Effect of carbon loading and N status in soil

Application of C-rich plant residues can change the soil system from C-limitation for microbial growth to limitation by other nutrients. However, the initial nutrient status of the soil may interact with the added amount of residues in determining limitation. We studied this interactive effect in soils from the Harvard Forest LTER, where annual addition of N since 1988 has resulted in soils with different N-status: No N (Unfertilized), 50 (Low N) and 150 (High N) kg N ha⁻¹. We hypothesized that adding C-rich substrate would change the soil from being C- to being N-limited for bacterial growth and that the extent of N-limitation would be higher with increasing substrate additions, while becoming less evident in soil with increasing N-status. We compared the effect of adding two C-rich substrates, starch (0, 10, 20, 40 mg g⁻¹ soil) and straw (0, 20, 40, 80 mg g⁻¹), incubating the soils for up to 3 and 4 weeks for starch and straw, respectively. Nutrient limitations were studied by measuring bacterial growth 3 days after adding C as glucose and N as NH₄NO₃ in a full factorial design. Initially bacterial growth in all soils was C-limited. As hypothesized, adding C-rich substrates removed the C-limitation, with lower amounts of starch and straw needed in the unfertilized and Low N soils than in the High N soil. Combinations of different N-status of the soil and amendment levels of starch and straw could be found, where bacterial growth appeared close to co-limited both by available C and N. However, at even higher amendment levels, presumable resulting in N-limitation, bacterial growth still responded less by adding N then C-limited soils by adding C. Thus, in a C-limited soil there appeared to be N available immediate for growth, while in an N-limited soil, easily available C was not immediately available.     

Influence of Experience on Browsing Sagebrush by Cattle and Its Impacts on Plant Community Structure

Mechanical and chemical methods used historically to rejuvenate sagebrush-steppe landscapes are cost prohibitive. A low-cost alternative is to fashion systems of management in which locally adapted animals use sagebrush as fall and winter forage to reduce feeding costs and to enhance the growth of grasses and forbs during spring and summer. We evaluated the practicality of fall browsing of sagebrush (Artemisia tridentata ssp. tridentata, ssp. wyomingensis) by cattle. To do so, we assessed 1) the foraging behavior and body weights of cattle with varying levels of experience browsing sagebrush, and 2) the ensuing responses of sagebrush, grasses, and forbs to cattle grazing. In spatially and temporally replicated trials from 2007 to 2009, cattle were challenged to eat sagebrush. Pregnant cows with calves (2007 and 2008), bred yearling heifers (2008), and first-calf heifer/calf pairs (2009), supplemented with protein and energy, learned to eat sagebrush as a significant portion of their diet (up to 63% of scans recorded during grazing). Experienced animals consistently ate more sagebrush and lost less weight, or gained more weight, than naive animals in 2008 and 2009 (P < 0.05). Cover, production, and percent composition of grasses and forbs maintained or dropped slightly from 2007 to 2008 but then rebounded sharply in 2009 to much greater levels than in 2007 or 2008 (P < 0.05). A corresponding reduction in shrub cover, production, and percent composition accompanied the increase in forbs and grasses (P < 0.05). Our research suggests grazing by cattle during fall and winter can be effective, biologically and economically, and can lead to habitat renovation and resilience by creating locally adapted systems of management in ways that landscape manipulations with chemical and mechanical treatments or prescribed fire cannot.     

Optimising crop production and nitrate leaching in China: Measured and simulated effects of straw incorporation and nitrogen fertilisation

The sustainability of growing a maize—winter wheat double crop rotation in the North China Plain (NCP) has been questioned due to its high nitrogen (N) fertiliser use and low N use efficiency. This paper presents field data and evaluation and application of the soil–vegetation–atmosphere transfer model Daisy for estimating crop production and nitrate leaching from silty loam fields in the NCP. The main objectives were to: i) calibrate and validate Daisy for the NCP pedo-climate and field management conditions, and ii) use the calibrated model and the field data in a multi-response analyses to optimise the N fertiliser rate for maize and winter wheat under different field managements including straw incorporation.The model sensitivity analysis indicated that a few measurable crop parameters impact the simulated yield, while most of the studied topsoil parameters affect the simulated nitrate leaching. The model evaluation was overall satisfactory, with root mean squared residuals (RMSR) for simulated aboveground biomass and nitrogen content at harvest, monthly evapotranspiration, annual drainage and nitrate leaching out of the root zone of, respectively, 0.9 Mg ha⁻¹, 20 kg N ha⁻¹, 30 mm, 10 mm and 10 kg N ha⁻¹ for the calibration, and 1.2 Mg ha⁻¹, 26 kg N ha⁻¹, 38 mm, 14 mm and 17 kg N ha⁻¹ for the validation. The values of mean absolute deviation, model efficiency and determination coefficient were also overall satisfactory, except for soil water dynamics, where the model was often found erratic. Re-validation run showed that the calibrated Daisy model was able to simulate long-term dynamics of crop grain yield and topsoil carbon content in a silty loam field in the NCP well, with respective RMSR of 1.7 and 1.6 Mg ha⁻¹. The analyses of the model and the field results showed that quadratic, Mitscherlich and linear-plateau statistical models may estimate different economic optimal N rates, underlining the importance of model choice for response analyses to avoid excess use of N fertiliser. The analyses further showed that an annual fertiliser rate of about 300 kg N ha⁻¹ (100 for maize and 200 for wheat) for the double crop rotation with straw incorporation is the most optimal in balancing crop production and nitrate leaching under the studied conditions, given the soil replenishment with N from straw mineralisation, atmospheric deposition and residual fertiliser.This work provides a sound reference for determining N fertiliser rates that are agro-environmentally optimal for similar and other cropping systems and regions in China and extends the application of the Daisy model to the analyses of complex agro-ecosystems and management practices under semi-arid climate.     

Forest property rights in the frame of public policies and societal change

Property rights over natural resources became a distinct area of inquiry in environmental economics and policy in the last decades, but their role has not yet been investigated thoroughly. Transition countries represent an excellent material of analysis of various policies and institutional developments concerning the regime of use and management of natural resources. The processes of societal transformation had deep impacts on the forestry sector, entailing land reforms and subsequent changes to its institutional and organisational framework. This paper presents an analysis of the reciprocal relationship between the evolving forest property rights and the conduct of policy and economic actors, in connection with their outcomes. The theoretical framework consists of institutional economics, whose core concept is that patterns of interaction between institutions and actors produce physical outcomes, assessable by criteria such as equity and efficiency. The study concentrated on three distinct periods and the applicable property regimes. Within this framework, the impacts of the characteristics of property regimes and the general framework of socio-economic conditions on the exercise of property rights were analysed. The analysis of the conduct of forest owners in relation to the institutional design was completed by the analysis of the other actors influential for land reforms. Romanian forestry sector, in evolution from the period shortly before World War II to present, represents the case of the study. Research data were collected by interviewing/questioning, participant observation and literature review. They were analysed through an integrated method of content analysis and a matrix analysis. An important conclusion is that not only the regime of forest property is determinant for the outcomes of resource use and management, but also the general settings in which they are embedded. Another essential point is that land reforms driven by either efficiency or equity rationales are not mere diversions of the benefit stream, but complex processes with serious implications for the status of the resource at stake.     

Land reclamation using reservoir sediments in Tigray,northern Ethiopia

Accelerated soil erosion leads to sedimentation in reservoirs and a decline in their life span. As many reservoirs in northern Ethiopia are dry at the end of the dry season, we were able to evaluate the potential of using reservoir sediments for land reclamation. Stripped land from which construction material for the reservoirs had been excavated was covered with 0, 15 and 30 cm of sediment and planted with a local garlic cultivar (Allium sativum). The applied reservoir sediments had low to medium organic C and total N contents and were high in available P and exchangeable cations. The yield of garlic increased with additional available water and the application of sediments. The results show that total biomass and bulb yield were three times higher on the reclaimed plots than on the control ones (11.7 t/ha vs. 3.6 t/ha for the biomass; 7.7 t/ha vs. 2.0 t/ha for the yield). When sediment transport and labour costs were taken into account, plots with 15 cm of sediments had in the first cropping season a cost‐benefit ratio of 3, whilst those with 30 cm had a cost‐benefit ratio of 0.9. This study demonstrates that the use of relatively small quantities of reservoir sediments is an economically viable strategy for land reclamation. The result can be improvement in income for resource‐poor farmers by as much as 76%, and the life expectancy of the reservoirs is also increased.