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Bert Cregg*

Lilac (Syringa vulgaris L) seedlings are commonly grown in many seedling nurseries in Michigan. Typically seedlings are lifted in the fall and stored prior to shipment or stored by the customer. A major problem in field production of lilacs is that seedlings often retain their leaves late in the fall. If the leaves are not removed prior to storage or shipment, the seedlings will mold and deteriorate. Therefore, growers must spend additional labor to remove the leaves, often by hand. The goal of this research was to evaluate chemical alternatives to defoliate lilac seedlings in field nurseries. Two on-farm research trials were conducted in 2001 and 2003 in cooperation with a seedling grower in Saugatuck, MI. In Experiment 1, Florel (1/2 and ¼ dilution) and chelated copper (0.5% and 1% solution) were sprayed by and onto lilac in the seedling bed. Florel and chelated copper effectively reduced leaf area of lilac seedlings. Less than 20% of the initial leaf area remained on the 1% copper and ½ Florel-treated seedlings. The ½ Florel and 1% chelated copper completely defoliated 67% and 40% of the seedlings, respectively, whereas only 17% on the control seedlings lost all their leaves prior to lifting. Both levels of Florel and the 1% copper treatment reduced growth of seedlings after planting. In experiment 2, we applied chelated copper treatments at varying rates (0.25% and 0.5%) and times (1 application and 2 applications) using the cooperators' spray equipment. Repeated applications of chelated copper were more effective in reducing seedling leaf area than a single application at both concentrations tested.

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Bert M. Cregg and Robert Schutzki

Landscape mulches are widely promoted to improve soil moisture retention, suppress weeds, and improve the growth of landscape plants. The objective of this project was to determine the effect of common landscape mulches (pine bark, hardwood fines, cypress mulch, color-enhanced ground pallets) on soil moisture, soil pH, weed control, and physiology and growth of landscape shrubs. Two additional treatments were not mulched: no mulch + no weed control and no mulch + weed control. Growth was measured on eight taxa (Euonymus alatus ‘Compactus’, Spiraea ×bumalda ‘Goldflame’, Weigela florida ‘Java red’, Taxus ×media ‘Runyan’, Thuja occidentalis ‘Golden Globe’, Hydrangea paniculata ‘Tardiva’, Viburnum dentatum ‘Synnestvedt’, Viburnum trilobum ‘Compactum’). Leaf gas exchange [net photosynthesis and stomatal conductance (g s)] were measured on Hydrangea paniculata, V. dentatum, and V. trilobum only. All mulches increased soil moisture compared with no mulch + weed control. There was no difference in soil pH or foliar nitrogen among treatments. All mulches, except cypress mulch, increased plant growth of most shrub taxa compared with no mulch without weed control. Mulches increased g S relative to no mulch without weed control. Photosynthetic rates of plants mulched with cypress mulch were less than the other mulches and not different from no mulch. Overall, the result suggests that, except for cypress mulch, the organic mulches tested are equally effective in improving growth of landscape plants. Reduced photosynthetic efficiency and growth of shrubs with cypress mulch suggest potential allelopathic effects.

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Pascal Nzokou and Bert M. Cregg

Growth and nutrient uptake of containerized fraser fir (Abies fraseri) seedlings in response to irrigation and fertilization was investigated for 2 years in a greenhouse experiment. Height and stem diameter growth increased 12% to 35% and 4% to 32%, respectively, with increased irrigation. There was an inverse relationship between irrigation and foliar nitrogen content and no irrigation effect on foliar phosphorus, potassium, magnesium, and manganese. Irrigation increased foliar calcium. Approximately 2.0% to 4.5% of applied nitrogen was lost through leaching. Increases in total biomass in high irrigation treatments were caused by higher root and stem biomass. Higher irrigation treatments increased nitrogen use efficiency (NUE) and assimilatory nitrogen use efficiency probably as a result of increase in carbon assimilation efficiency leading to increase in net primary productivity. There was no clear effect on the root weight ratio, but the index nitrogen availability per unit of foliage indicated a higher availability in plants receiving the lowest irrigation. This suggests that under water stress, the decrease in assimilation and NUE may be buffered by an increase in the plant's ability to provide nitrogen and other nutrients to various organs.

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Soo-Hyung Kim and Bert Cregg

Innovative approaches are required for improving crop productivity and quality to meet the increasing demand for providing food, energy, and other services for growing populations in a changing climate. The colloquium sponsored by the Environmental Stress Physiology (STRS) Working Group at the 2011 American Society for Horticultural Science (ASHS) annual conference served as a forum to bring together several of the emerging methods for diagnosing, monitoring, and mitigating crop environmental stress with an emphasis on horticultural, physiological, and ecological approaches. These methods are likely to be readily applicable for many research areas in specialty crops in the context of climate change. The colloquium articles in this volume provide a foundation and context to lead dialogues and initiate research themes for developing adaptive strategies to minimize climate impacts on horticultural crop production in a changing climate.

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Grant Jones*, Bert Cregg, and Katrina Schneller

The genus Abies is a member of the family Pinaceae and consists of approximately 55 species found mainly in upper latitudes or higher elevations in the northern hemisphere. Firs generally prefer cool, well-drained sites with acidic soil. Fir trees are occasionally used as landscape trees but are more commonly grown as Christmas trees. Recently, the Michigan State Univ. Forestry Dept. and the Michigan Christmas tree industry initiated a field test of exotic firs (Abies sp.) for potential use as Christmas trees. In the present study we expanded the evaluation of these exotic fir species to include their suitability as landscape trees and to characterize their tolerance to various environmental stresses. We compared foliar nutrition, gas exchange, chlorophyll fluorescence, and shoot morphology of 23 Abies species and hybrids planted at three horticultural research stations in Michigan. Foliar nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), boron (B), zinc (Zn), and copper (Cu) were correlated (P < 0.05) with soil pH. Species and hybrids varied (P <0.05) in maximum photosynthetic rate, water use efficiency, chlorophyll fluorescence, and the ratio of total needle area to projected needle area. We will discuss the significance of these traits in relation to selecting trees adapted for use in the Midwestern landscape.

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Katrina L. Schneller*, Bert Cregg, and Grant Jones

Thirteen species of shade trees including four oak and three maple species were planted from bare root whips in Spring 2001 at the Michigan State Univ. horticulture station near Benton Harbor, Mich. Forty trees of each species were planted and placed under a randomized complete block design consisting of four fertigation treatments in a 2 × 2 factorial (with and without irrigation; with and without nitrogen fertilization). Fertilized trees received 168 kg per hectare of ammonium nitrate annually via the drip system. We measured tree height, stem diameter, and stem volume index (D2 H) at the end of each growing season. We also measured photosynthetic gas exchange with a portable photosynthesis system. After three growing seasons irrigation increased stem volume and height growth. Fertilization has not increased stem volume and height growth, yet there is an interaction between irrigation and fertilization meaning that among irrigated trees fertilizer produced significantly more growth. Species effect on growth was significant (P < 0.001); honey locust trees have grown the most with an average height increase of 1.28 meters in 2003. `Crimson King' Norway maple and goldenchain trees grew the least averaging less than.305 meters of height growth per year. There is no interaction between species and fertigation responses meaning that all species had similar responses to the treatments. From light response curves obtained with a portable photosynthesis system Amax (maximum rate of CO2 assimilation) varied significantly by species and treatment, but showed no interaction between species and treatment effects. Implications for the nursery industry along with further photosynthetic responses will be discussed.

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Bert M. Cregg, Pascal Nzokou, and Ron Goldy

We evaluated height growth, diameter growth, and survival of newly planted fraser fir and colorado blue spruce Christmas trees in southwest Michigan in response to mulch, weed control, and irrigation. Mulches included black polyethylene, white polyethylene, VisPore mulch mats, and wood chips. Seedlings were also established with or without raised beds and with or without complete weed control. Weed control (mulches or a combination of chemical weed control and hand weeding) improved survival and growth of both species after 2 years. Growth was similar for trees in irrigated plots or with wood chip mulch without irrigation. Polyethylene mulch increased growth compared with similar production systems with raised beds and bare ground. Among production systems, variation in growth and survival reflected patterns of predawn water potential and midday shoot gas exchange, suggesting that differences were largely related to plant moisture stress. White mulch improved growth relative to similar production systems with black mulch and wood chip mulch improved growth compared with similar production systems without irrigation. Overall, the ranking of magnitude of growth response effects were weed control > irrigation > mulch. These results underscore the importance of weed control for establishment and maintenance of high-quality Christmas tree plantations.

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Pascal Nzokou, Nicholas J. Gooch, and Bert M. Cregg

Irrigation of fraser fir (Abies fraseri) in Christmas tree production is gaining importance in the upper midwestern United States because of the intensive planting of this species out of its natural range. However, current scheduling practices rely on empirical observations with no monitoring of soil moisture and no use of automated irrigation system. The goal of this project was to design, construct, and implement a tensiometer-based automated irrigation system for fraser fir Christmas tree plantations that would (1) use existing technologies, (2) apply water based on changes in soil moisture content, (3) provide operational flexibility, and (4) interface with a computer for system changes, data collection, and system modifications. Soil tensiometers equipped with 4- to 20-milliampere transducers were installed at two drip-irrigated tree farms. Water on demand was controlled by soil moisture tension levels that triggered the stimulation of a relay wired to solenoids delivering irrigation water to the various treatments. The system functioned according to the design as expected. However, several issues associated with the need for regular maintenance of tensiometers, computer programming, and system wiring created some challenges regarding the reliability and transferability of similar system to commercial facilities.

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N.J. Gooch, Pascal Nzokou, and Bert M. Cregg

Containerized conifers are increasingly marketed and used as live Christmas trees worldwide. However, prolonged exposure to indoor conditions may reduce cold hardiness. We examined physiological and morphological changes of three species black hills spruce (Picea glauca), balsam fir (Abies balsamea), and douglas fir (Pseudotsuga menziesii var. glauca) subjected to in-home conditions for 10 and 20 days. Shoot cuttings were subjected to artificial freeze testing (AFT) and the physiological and morphological changes were evaluated by chlorophyll fluorescence, bud mortality, and needle damage. After 7 days indoors, bud temperature at 50% lethality (LT50) was −24.5 °C for douglas fir, −23.5 °C for black hills spruce, and −22.5 °C for balsam fir. After 20 days indoors, bud LT50 increased to −18 °C for black hills spruce and balsam fir, and −21 °C for douglas fir. The effect of the indoors exposure on needle damage was very limited for black hills spruce and balsam fir; however, severe needle damage was apparent on douglas fir even at just 3 days of indoor exposure (LT50 = −21 °C). This negative impact worsened with indoor exposure time with LT50 for after 20 days of indoor exposure at −7 °C. Chlorophyll fluorescence values followed a similar trend with needle damage with black hills spruce and balsam fir showing no difference, while douglas fir values were significantly affected. These results confirm the hypothesis that live trees kept indoors for extended periods progressively deharden and become very sensitive to cold damage when moved outdoors following the indoor exposure. However, whole plant survival after transplantation in the field did not corroborate results obtained from the AFT. Further studies are needed to investigate the potential causes of the high transplantation mortality following the display treatments.

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Amanda J. Taylor, R. Thomas Fernandez, Pascal Nzokou, and Bert Cregg

The objective of this study was to quantify the effects of cyclic irrigation on growth and physiology of container-grown conifer species in pot-in-pot (PIP) production in the upper Midwest. Trees of four conifer species (Picea glauca var. densata, Picea pungens, Abies fraseri, and Pinus strobus) were grown in 25-L containers and assigned to one of four combinations of irrigation rate (low or high) and daily irrigation cycle frequency (one or four). Irrigation rates were based on common nursery practice in the region (2 cm rainfall equivalent/day) and one-half the standard rate (1 cm rainfall equivalent/day). Cyclic irrigation increased relative height growth and relative caliper growth of Pinus strobus by over 80% and 35%, respectively, compared with once-daily irrigation. The high-rate irrigation increased relative caliper growth of Picea pungens by 40% compared with the low rate. The effects of irrigation regime on needle- or shoot-level gas exchange varied by species and date of measurement. Carbon isotope discrimination (Δ13C) of needle and wood tissue was positively correlated (r ≥ 0.64, P < 0.001) with needle conductance to water vapor (gwv) and negatively correlated (r ≤ −0.60, P< 0.001) with intrinsic water use efficiency (WUEi). Carbon isotope discrimination of wood and needle tissue decreased with the low irrigation rate, indicating increased WUEi associated with reduced gwv. Cyclic irrigation had relatively little effect on Δ13C except for Pinus strobus. Our findings suggest that carbon isotope composition of wood and needle tissue provides a sensitive and accurate representation of plant response to varying moisture availability. From a water management perspective, identifying optimal irrigation rates appears to be more important than number of daily cycles for these crops grown in the midwestern United States.