The influence of ectomyorrhizae on drought tolerance characteristics of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings

Douglas-fir seedlings were inoculated with different species of ectomycorrhizae-forming fungi in order to test the concept that ectomycorrhizae enhance the drought tolerance of seedlings and to investigate the mechanisms responsible for this effect. Seedlings were transplanted at age 6 to 8 weeks into pots containing pasteurized loam soil and inoculated with either Rhizopogon vinicolor (Rv), Laccaria laccata (L1), or Hebeloma crustuliniforme (Hc), or left uninoculated. Rv and He colonization produced abundant hyphal growth, while Ll produced much less hyphae. After 4 months under well-watered greenhouse conditions, neither Rv- or L1- colonized seedlings had significantly different dry mass and leaf N, P, K, and Ca concentrations compared to nonmycorrhizal controls. Higher nutrient concentrations of Hc-colonized seedlings resulted from suppressed growth, since total amounts of these nutrients were equal to or less than for nonmycorrhizal controls. Seedlings were transferred to a growth room where photosynthesis, stomatal conductance, and plant water potential components were measured under well-watered and soil water-limiting conditions. Drought tolerance, as evaluated by net photosynthesis rate over the soil water potential range of -0.05 to -0.6 MPa, was clearly enhanced by Rv, somewhat enhanced by Hc, and decreased by Ll compared to nonmycorrhizal controls. Stomatal conductances closely followed net photosynthesis rates. Compared to control seedlings, leaf water potentials of mycorrhizal seedlings were lower (Rv by 0.2 to 0.3 MPa) or similar (L1 and Hc) over the entire range of soil water potential. Significantly reduced root lengths (Rv 65% of control; Hc 70% of control; Ll 90% of control) may have counteracted a mycorrhizal benefit of efficient water absorption. It is hypothesized that higher net photosynthesis rate and stomatal conductance despite lower leaf water potential, as observed for Rv-colonized seedlings, can arise from an ectomycorrhizae-altered carbon economy of the plants. According to this hypothesis, net photosynthesis rate and stomatal conductance are correlated with photosynthate sink demand, which here would be increased by export to the mycorrhizal fungus. Strong mycorrhizal demand, which occurs at some cost to plant growth, stimulates photosynthesis, to which the stomata respond by opening in spite of water stress. The degree to which this effect was observed in this study correlated with the visual abundance of hyphal growth which each fungal species developed

A Design Aid for Determining Width of Filter Strips

Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based Vegetative Filter Strip Model to evaluate the relationship between filter strip width and trap¬ping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models

A Design Aid for Determining Width of Filter Strips

Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based Vegetative Filter Strip Model to evaluate the relationship between filter strip width and trap¬ping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models

MODELING SEDIMENT TRAPPING IN A VEGETATIVE FILTER ACCOUNTING FOR CONVERGING OVERLAND FLOW

Vegetative filters (VF) are used to remove sediment and other pollutants from overland flow. When modeling the hydrology of VF, it is often assumed that overland flow is planar, but our research indicates that it can be two-dimensional with converging and diverging pathways. Our hypothesis is that flow convergence will negatively influence the sediment trapping capability of VF. The objectives were to develop a two-dimensional modeling approach for estimating sediment trapping in VF and to investigate the impact of converging overland flow on sediment trapping by VF. In this study, the performance of a VF that has field-scale flow path lengths with uncontrolled flow direction was quantified using field experiments and hydrologic modeling. Simulations of water flow processes were performed using the physically based, distributed model MIKE SHE. A modeling approach that predicts sediment trapping and accounts for converging and diverging flow was developed based on the University of Kentucky sediment filtration model. The results revealed that as flow convergence increases, filter performance decreases, and the impacts are greater at higher flow rates and shorter filter lengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact than convergence that occurred in the filter (in-filter). An area-based convergence ratio was defined that relates the actual flow area in a VF to the theoretical flow area without flow convergence. When the convergence ratio was 0.70, in-filter convergence caused the sediment trapping efficiency to be reduced from 80% for the planar flow condition to 64% for the converging flow condition. When an equivalent convergence occurred in-field, the sediment trapping efficiency was reduced to 57%. Thus, not only is convergence important but the location where convergence occurs can also be important

AgBufferBuilder: A Filter Strip Design Tool for GIS

AgBufferBuilder is a GIS-based computer program for designing vegetative filter strips around agricultural fields that utilizes terrain analysis to account for spatially non-uniform runoff (Figure 1). The core model is derived from the process-based Vegetative Filter Strip Modeling System (VFSMOD-W). A detailed description of the core model and its development is provided in Dosskey et al. (2011). The GIS program runs with ArcGIS (ESRI, Redlands, CA)

TESTING A SIMPLE FIELD METHOD FOR ASSESSING NITRATE REMOVAL IN RIPARIAN ZONES

Being able to identify riparian sites that function better for nitrate removal from groundwater is critical to using efficiently the riparian zones for water quality management. For this purpose, managers need a method that is quick, inexpensive, and accurate enough to enable effective management decisions. This study assesses the precision and accuracy of a simple method using three ground water wells and one measurement date for determining nitrate removal characteristics of riparian buffer zones. The method is a scaled-down version of a complex field research method that consists of a large network of wells and piezometers monitored monthly for over two years. Results using the simplified method were compared to those from the reference research method on a date-by-date basis on eight sites covering a wide range of hydrogeomorphic settings. The accuracy of the three-well, 1 day measurement method was relatively good for assessing nitrate concentration depletion across riparian zones, but poor for assessing the distance necessary to achieve a 90% nitrate removal and for estimating water and nitrate fluxes compared to the reference method. The simplified three-well method provides relatively better estimates of water and nitrate fluxes on sites where ground-water flow is parallel to the water table through homogeneous aquifer material, but such conditions may not be geographically widespread. Despite limited overall accuracy, some parameters that are estimated using the simplified method may be useful to water resource managers. Nitrate depletion information may be used to assess the adequacy of existing buffers to achieve nitrate concentration goals for runoff. Estimates of field nitrate runoff and buffer removal fluxes may be adequate for prioritizing management toward sites where riparian buffers are likely to have greater impact on stream water quality