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Pascal Nzokou and Paligwende Nikiema

The effects of three plant growth regulators (PGRs) (prohexadione–calcium, paclobutrazol, and flurprimidol) on the resistance of Fraser fir (Abies fraseri) and Colorado blue spruce (Picea pungens) to cold injuries were investigated. Treated and untreated seedlings were first allowed to harden before exposure to warm temperatures in a greenhouse. The seedlings were then subjected to an artificial freezing test to simulate the return to normal winter conditions. Chlorophyll fluorescence, relative electrolyte leakage, bud survival, and posttreatment plant survival were recorded to evaluate the effectiveness of the treatments. Results showed that treatments with paclobutrazol and flurprimidol maintained the photosynthetic ability of the plants and reduced the extent of needle cold injuries. There was no effect on bud and plant survival, possibly as a result of the timing of the PGR application. Further studies with adjustment of the timing and rates of PGR treatments are needed to validate these results.

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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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Yingqian Lin, Alexa R. Wilson, and Pascal Nzokou

High rates of inorganic fertilizers are used in conventional intensive production systems such as Abies fraseri (fraser fir) cropping systems for Christmas trees. Groundcovers can be used as green mulches, help reduce the use of farm chemicals, and provide several environmental benefits. We investigated the performance of a low-input cropping system by combining two legume cover crops [Dutch white clover (Trifolium repens) and alfalfa (Medicago sativa)] in combination with low rates of inorganic fertilizers as a step toward a more sustainable production system. The randomized block design comprised one cover crop and one of three applications of reduced rate inorganic fertilizer (75%, 50%, and 25% of the recommended rate). A conventional system using herbicides for weed control and the 100% rate of inorganic fertilizer was used as a control. Parameters measured included tree morphology, foliar nitrogen concentration, soil mineral nitrogen, and nitrate-N leaching below the root zone. A significant positive growth response (height and diameter) was obtained in all alfalfa-based cropping systems. This was accompanied by foliar nutrient concentrations similar to conventional plots and a reduction in nitrate-N leaching. However, in white clover-based cropping systems, the growth response was reduced (both height and diameter), suggesting competition for soil resources. In addition, the total nitrate-N leaching was higher in this system, suggesting an imbalance between mineral nitrogen availability and use in white clover-based cropping systems. We conclude that if the potential competition between cover crops and trees can be properly managed, legume cover crops can be effectively used to make intensive production tree-based systems more sustainable. Further studies related to mineralization and macronutrient flows are needed before any definite recommendation can be made about the use of these systems in large-scale production systems.

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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.

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Wendy S. Klooster, Bert M. Cregg, R. Thomas Fernandez, and Pascal Nzokou

Container production of landscape conifers, including pot-in-pot (PIP) production, is increasing relative to field production in the northern United States. Because much of the research on PIP has been performed in the southern United States, this study focused on characterizing the growth and physiological response of PIP-grown conifers to fertilizer and substrate to improve production for growers in northern climates. In May 2006, we potted 90 seedlings each of Abies fraseri, Picea glauca var. densata, P. pungens glauca, and Pinus strobus into 11.2-L containers. Substrate consisted of pine bark (B) and peatmoss (PM) in ratios of 90:10, 80:20 or 70:30 (vB:vPM). Trees were top-dressed with controlled-release fertilizer (15N–4P–10K) at rates of 0.25, 0.5, and 1.0 g of nitrogen per liter of container (g·L−1). After 2 years, growth response to substrate varied by species; however, all species grew as well or better in the 80:20 mix than in the other mixes. In response to fertilizer addition, adding 0.5 or 1 g N/L increased height growth compared with 0.25 g. Increasing the fertilizer rate from 0.5 g N/L to 1 g did not increase height growth. Foliar nitrogen increased with each fertilizer addition although height growth did not increase beyond 0.5 g·L−1, indicating possible luxury consumption. Furthermore, net photosynthesis rates of spruce trees declined with fertilization in the second year of the study, possibly as a result of increased water stress due to greater total leaf area per tree. Chlorophyll fluorescence was not consistently correlated with foliar nutrition. From a practical standpoint, results of the study indicate that 0.5 g N/L will provide adequate nutrition for these crops. A substrate mix of 80% bark:20% peatmoss produced maximal or near-maximal growth for all four species tested.

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Şemsettin Kulaç, Pascal Nzokou, Deniz Guney, Bert Michael Cregg, and Ibrahim Turna

Four-year-old seedlings of Abies fraseri [(Pursh) Poir] (fraser fir) were grown in semicontrolled conditions in hoop houses with five watering regimes (0.00, 0.62, 1.25, 2.50, and 3.70 cm/week) with the goal of determining the seasonal variation in the physiological response to drought stress. Drought stress was monitored by measuring predawn (Ψpd) and midday (Ψmd) potentials in a subset of plants from each treatment. Physiological variables monitored were chlorophyll fluorescence (F v/F m), Chl a, Chl b, total carotenes, and total carbohydrate concentrations. Morphological characteristics including height growth, root collar diameter, and terminal shoot growth were also measured. Predawn stem water potential values were generally higher (–0.8 to –1.9 Mpa) than midday values (–1.3 to –2.9 Mpa). Irrigation consistently increased Ψpd and Ψmd compared with non-irrigated treatments. Photosynthetic pigments (Chl a, Chl b, and carotenes) decreased midseason (14 July) and increased toward the end of the season (25 Aug.) in predawn and midday measurements. There was a significant effect (P < 0.05) of drought stress on photosynthetic pigment concentrations in predawn and midday samples in the late-season measurements (25 Aug.). These results were accompanied with a similar significant difference in F v/F m between non-irrigated and irrigated trees. We concluded that significant effects observed on photosynthetic pigment concentrations in some of the treatments did not affect carbohydrate concentrations. Exposure of A. fraseri to water stress did not cause a reduction in supply of metabolic carbohydrates; consequently, the decline and mortality in water-stressed plants can only be the result of hydraulic failure caused by xylem cavitation leading to cessation of water flow in tissues, desiccation, and cellular death. Further studies are needed to confirm these preliminary conclusions.