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  • Author or Editor: Roger Kjelgren x
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Infrared sensors were used to quantify canopy temperature and thus detect differences in incipient water stress between a cool-season grass [Kentucky bluegrass (KBG) (Poa pratensis)] and a warm-season grass [buffalograss (BG) (Buchloe dactyloides)]. The infrared sensors, connected to a datalogger, measured average hourly leaf–air temperatures (TL–TA) 1 m above eight replicate plots of Kentucky bluegrass and eight replicate plots of buffalograss. Air temperature and relative humidity from a nearby weather station were used to calculate the average hourly vapor pressure deficit (VPD). In late July, we ceased irrigating and measured TL–TA and soil water content while allowing the turf to dry down for 5 weeks. Soil water content was measured with a neutron probe. Both species exhibited a significant relationship between TL–TA and VPD. As the VPD increased, TL–TA decreased in both species (KBG r 2 = 0.73, BG r 2 = 0.71) on the 2nd day after an irrigation during well-watered conditions. An artifact was created on the first day after an irrigation as a result of excessive surface evaporation. KBG and BG were similar under well-watered conditions. KBG had a higher TL–TA after 4 to 5 days without irrigation. By contrast, BG did not have a higher TL–TA until 25 to 30 days without irrigation. Part of BG's drought avoidance was extraction of soil water down to 0.9 m vs. 0.45 m for KBG.

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Gas exchange and growth of transplanted and nontransplanted, field-grown Norway maple (Acer platanoides L. `Schwedleri') and littleleaf linden (Tilia cordata Mill. `Greenspire') trees were investigated in an arid climate. In the spring of 1995, three trees of each species were moved with a tree spade to a new location within a field nursery and three nontransplanted trees were selected as controls. Predawn leaf water potential, morning-to-evening stomatal conductance and leaf temperature, leaf-to-air vapor pressure difference, midday stomatal conductance and photosynthetic rate, and growth data were collected over a 2-year period. After transplanting, weekly predawn leaf water potential indicated that transplanted trees were under greater water stress than were nontransplanted (control) trees. However, predawn leaf water potential of maple trees recovered to control levels 18 weeks after transplanting, while that of transplanted linden trees remained more negative than that of controls. In 1995, stomatal conductance and photosynthetic rates were lower throughout the day for transplanted trees. In 1996, gas exchange rates of transplanted maple trees recovered to near control levels while rates for transplanted linden trees did not. Sensitivity of stomata to leaf-to-air vapor pressure difference varied with species and with transplant treatment. Each year transplanted trees of both species had less apical growth than did control trees. Although gas exchange and apical growth of transplanted trees was reduced following transplanting, recovery of gas exchange to control rates differed with species.

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We investigated water use and potential drought avoidance of Norway maple (Acer platanoides L.) and green ash (Fraxinus pennsylvanica Marsh) seedlings grown in protective plastic shelters. Gravimetric tree water use and reference evapotranspiration for fescue turf (ETo) were monitored for 1 to 3 days during the growing season. Water use of trees was 8% to 14% of ETo in shelters vs. 29% to 40% for trees not in shelters. Trunk diameter was affected more than whole-tree water relations by lack of irrigation, suggesting that the nonirrigated trees were subjected to only mild water stress. Shelters did not improve drought avoidance, as water potentials were generally more negative and trunk diameter increment was lower for nonirrigated trees in shelters. Maples in shelters were affected more adversely by lack of water than were ash. Higher temperatures in shelters also may have reduced trunk growth. Air temperatures were 13 °C warmer than ambient in nonirrigated shelters, but only 5 °C warmer in irrigated shelters. Tree shelters can reduce transpiration rates by over half, but benefits from reduced water loss may be offset by negative effects of higher air temperatures. Shelters reduced cold hardiness of both species, but maple was affected more than ash.

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Little research has examined water requirements of entire irrigated urban landscapes integrating different types of plants. Three landscape treatments integrating different types of plants—woody, herbaceous perennial, turf—and putative water use classifications—mesic, mixed, xeric—were grown in large drainage lysimeters. Each landscape plot was divided into woody plant, turf, and perennial hydrozones and irrigated for optimum water status over 2 years and water use measured using a water balance approach. For woody plants and herbaceous perennials, canopy cover rather than plant type or water use classification was the key determinant of water use relative to reference evapotranspiration (ETo) under well-watered conditions. For turf, monthly evapotranspiration (ETa) followed a trend linearly related to ETo. Monthly plant factors (Kp) for woody plants, perennials, and turf species under well-watered conditions in this study ranged from 0.3 to 0.9, 0.2 to 0.5, and 0.5 to 1.2, respectively. Adjusted Kp for each hydrozone was calculated based on landscaped area covered by plant types as a percent of total area, and landscape factor (Kl) was calculated based on adjusted Kp for each landscape treatment. Overall, Kl relative to ETo ranged from 0.6 to 0.8 for three water use classifications.

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The response of foliage-air temperature differential (Tl-Ta) to vapor-pressure deficit (VPD) as a means of detecting incipient water stress was investigated in the Illinois planting of the NC-140 Uniform Peach Rootstock Trial. Stomatal conductance, foliage temperature, leaf water potential, air temperature and VPD were followed diurnally on three dates in 1989 for mature `Redhaven' on six different rootstock. On two of three sampling dates where predawn leaf water potential was greater than -0.5 MPa there was no indication of midday stomatal closure and all rootstock exhibited an inverse relationship between T1-Ta and VPD. On the date with the most negative predawn leaf water potential, T1-Ta of two plum rootstock (GF-677 and GF-655-2) was observed to be significantly greater at VPD levels above 2 kPa than the remaining rootstock. All rootstock on this date exhibited greater T1-Ta than at similar VPD levels on the other two dates. These data suggest that transpirational cooling plays a large enough role in foliage temperature regulation of `Redhaven' peach such that incipient water stress and rootstock effects on water relations can be detected through increases in foliage temperature.

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We investigated water use and a water needs index multiplier relative to reference evapotranspiration for a sweetgum cultivar (Liquidambar styraciflua `Moraine') in Logan, Utah, Lubbock, Texas, and Orlando Fla. Three individual trees with ≈80-mm trunk diameter, were potted in to large containers with organic media at each location. Sweetgum water use (Tsw) was measured over the season at each location with load cells and dataloggers, concurrent with measurement of reference evapotranspiration (ETo) from adjacent weather stations. Dawn-to-dusk stomatal conductance (Gs) was measured several times during the season at all locations. Trees were watered daily. At the end of the season, total tree leaf area was collected and used to normalize volumetric water use data to depth units. Tsw was highest in Florida, up to 4 mm/day, as was maximum daily Gs. Tsw only reached 2.5 mm/day in Texas and Utah due in part to stomatal sensitivity to high vapor pressure deficits that moderated transpiration. There was no relationship between Tsw and ETo at ETo levels above 4 mm/day in Texas and Utah, resulting in substantial scatter in the water needs index multiplier relative to ETo that centered on 0.3 in Texas and 0.4 in Utah. Tsw in Florida showed an upper boundary relationship with ETo, under which it varied considerably, resulting in a values relative to ETo centered on 0.6. During a partial dry down in Utah, morning Gs was unaffected while afternoon Gs declined progressively under mild water stress, resulting multiplier values of 0.15-2. The study shows that regional climate affects tree water use independent of effects measured in ETo, increasing the uncertainty of sweetgum water use estimated as a function of ETo.

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We investigated gas-exchange response of norway maple and crabapple to the energy balance of turf, bark-mulch, and asphalt surfaces. In each surface stomatal conductance, leaf temperature (T1), and photosynthesis, were measured during two dawn-to-dusk studies concurrent with soil (To), top surface (Ta), and air temperature (Ta) measurements. Different properties affected the energy balance of each surface. Turf transpiration moderated To and Ts while low thermal conductivity of the mulch resulted in To similar to turf but Ts23C higher. Higher thermal conductivity of the asphalt resulted in higher To but Ts intermediate to mulch and turf surfaces. We did not detect differences in Ta, probably due to close proximity to one another that allowed substantial air mixing between treatments. Higher Ts increased longwave radiation flux that raised midday T1 of trees in the mulch and asphalt 3 to 8C higher than trees in the turf. Differences in T1 between the asphalt and mulch were minimal. Stomatal conductance declined with increasing leaf-to-air vapor pressure gradient in all trees, and was consistently lower for trees in the mulch and asphalt through the day due to larger gradients induced by higher T1. Midday photosynthesis was highest for trees in the turf and lowest for those in the mulch. Foliar interception of higher energy fluxes from mulch and asphalt surfaces apparently limited gas exchange in both species due to over-optimal leaf temperatures as compared to trees in the turf

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Interdisciplinary graduate degrees are becoming increasingly popular, filling both employer needs as well as student goals. The Plants and Soils Department at Utah State University offers an interdisciplinary master of professional studies in horticulture (MPSH) degree program specializing in urban landscape water conservation. The MPSH is a one calendar year degree consisting of a small group cohort with a strong emphasis on communication and policy development geared toward creating and managing water conservation programs. Core to this model is what personality type is drawn to an MPSH degree compared to the traditional, research-based master of science degree. We are comparing the personality types of 16 students in the MPSH to 15 students in, or having completed, the traditional MS degree program by using the Myers-Briggs test (MBTI), Strong Interest Inventory (SII) test, and key informant surveys. Basic MBTI personality categories in extroversion/introversion, sensing/intuition, thinking/feeling and judging/perceiving are being evaluated by comparison in contrast as well as consistency across the two degree types. Key informant surveys quizzed individual preference regarding the two degrees. Preliminary inspection of survey, MBTI and SII results indicate a definite link between type of graduate program and basic personality trait. Students in or having completed the traditional MS degree program that indicated a preference for the MPSH degree shared the same personality types as those in the MSPH program. These preliminary results suggest that an interdisciplinary professional degree in horticulture focused on a particular topic can appeal to horticultural undergraduates that might not otherwise consider a graduate degree.

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Perennial wildflower species are important but not well-understood elements in water-wise landscaping that anchors urban water conservation programs in arid climates. Comparative growth and physiological responses to soil substrate drying of three herbaceous Australian ornamental species from habitats of variable moisture regimes were investigated in the context of isohydric and anisohydric behavior. Clonal Orthosiphon aristatus, Dianella revoluta ‘Breeze’, and Ptilotus nobilis plants were container-grown individually and competitively together in two separate studies. In both studies, plants were water-stressed through cyclical dry downs. We measured stomatal conductance (g S), soil water content, and water potential during each study and osmotic adjustment estimated from pressure-volume data and plant biomass at the end of each study. O. aristatus, a rainforest species, fit a general anisohydric model of high water use and more negative water potential during soil drying until stomatal closure and leaf wilting. D. revolata and P. nobilis, indigenous to Australia's dry interior, fit a general isohydric, drought-tolerant model of stomatal closure from water deficits that moderates leaf water potential but through different mechanisms. P. nobilis and D. revolata moderate water use and maintain acceptable aesthetic performance under water stress, suitable for mixed low-water landscape plantings. O. aristatus would not be suitable for low-water urban landscapes, either isolated or in mixed plantings, because of high soil water depletion and wilting.

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Research was conducted to investigate how energy balance of bark mulch and turf surfaces influence gas exchange and growth of recently transplanted trees. On several occasions over a 3-year period, stomatal conductance and leaf temperature were measured throughout the day on `Emerald Queen' Norway maple (Acer platanoides L.) and `Greenspire' littleleaf linden (Tilia cordata Mill.) trees growing over each surface. Tree water loss was estimated using a general transport flux equation applied to the tree crown apportioned between sunlit and shade layers. Microclimate variables were measured over each surface with a permanent weather station. Tree growth data were collected at the end of each growing season. Soil heat flux data revealed that a greater portion of incoming radiation was prevented from entering the soil below mulch than below turf. Due to this insulating effect, and consequent lack of evaporative cooling, mulch surface temperature was greater, and emitted more longwave radiation, than turf. Leaves over mulch intercepted more longwave radiation, had greater leaf temperature, and greater leaf-to-air vapor pressure difference than leaves over turf. As a result, leaves over mulch had greater stomatal closure than leaves over turf. Estimated tree water loss varied between surface treatments and with climatic conditions. Trees over turf had greater shoot elongation and leaf area than trees over mulch. These data suggest that gas exchange and growth of recently transplanted trees in an arid climate may be reduced if planted over nonvegetative, urban surfaces.

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