regime in the nursery in 1997 on trunk
diameter growth of Malus 'Sutyzam' in the
landscape following outplanting in 1998.
John E. Lloyd, Daniel A. Herms, and Mary Ann Rose
The objective of this study was to determine if fertilization and irrigation practices in the nursery affect plant performance following outplanting in the landscape. Crabapples (Malus 'Sutyzam') grown in containers under all combinations of low (irrigated at 50% container capacity) and high moisture (irrigated at 25% container capacity) and three fertilizer concentrations (50, 200, and 350 ppm N) in the nursery in 1997 were outplanted in a low-maintenance landscape in 1998. Tree growth in the landscape was highly correlated with nitrogen content of plants when they left the nursery. High fertility regimes in the nursery resulted in faster growth in the landscape, but only for trees exposed to the low-moisture treatment in the nursery (which decreased nitrogen leaching from containers). However, trees receiving the high fertility regime were also less resistant to insects (eastern tent caterpillar, gypsy moth, and whitemarked tussock moth) and less tolerant of drought stress.
Rapid growth of trees in the nursery is necessary to shorten production schedules and maintain profit margins. Furthermore, nutrient loading of plants in the nursery has been proposed as a strategy for increasing growth of transplants in the years following outplanting, the period when nutrient uptake may be limited by root damage and when remobilization of stored nutrients stimulates increased shoot growth (McAlister and Timmer, 1998). However, nurseries are also under increasing pressure to conserve water and limit nutrient runoff. These conflicting management objectives may be resolved through efficient use of water and nutrients if actual plant requirements can be determined. Recent studies have shown that it is possible to decrease fertilizer use in nursery production of containerized plants without sacrificing plant growth (Struve and Rose, 1998; Rose and Wang, in press).
Hamilton et al. (1981) has suggested that conservative use of fertilizer in nursery production may increase establishment and stress tolerance of plants once they reach the landscape. They argue that lower nutrient concentrations increase root growth relative to shoot growth, resulting in increased stress tolerance. Other studies have shown that reduced fertility regimes can increase insect and disease resistance by decreasing the nutritional value of the plants for microbes and insects and by increasing concentrations of plant defense compounds (Herms and Mattson, 1992).
The objective of this study is to determine how fertilization and irrigation practices in the nursery affect performance of trees following outplanting in the landscape. Parameters examined in this study include trunk growth, photosynthesis, stomatal conductance, and insect resistance.
On April 15, 1997, rooted cuttings of Malus 'Sutyzam' (Sugar TymeTM crabapple) were transplanted to 8.6-liter containers and then exposed to all possible combinations of three fertilizer treatments (50, 200, and 350 ppm N) and two moisture levels for the duration of the growing season. The high-moisture-level treatments were irrigated when container moisture was at 25% container capacity; the low-moisture-level treatments were irrigated at 50% container capacity. The experiment was designed as a randomized complete block, with one replicate of each of the six treatment combinations in each of four blocks (Rose, in preparation). The moisture and fertilizer treatments were discontinued in October 1997.
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| Figure 1. Effect of fertilization and irrigation regime in the nursery in 1997 on trunk diameter growth of Malus 'Sutyzam' in the landscape following outplanting in 1998. |
The crabapple trees were moved from Columbus to the Wooster campus of The Ohio State University's Ohio Agricultural Research and Development Center (OARDC) on May 1, 1998, and were transplanted into a turf landscape on May 29, 1998. In order to evaluate the effects of the prior year's nursery treatments, trees were arranged in the same randomized complete block design used in the nursery experiment. A low-maintenance landscape environment was maintained in 1998. Trees were not fertilized and were irrigated only three times -- on the day of transplanting, again one week later, and finally on August 4, during a period of drought.
To determine effects of the prior year's nursery treatments on tree physiology, the authors quantified growth, photosynthesis, and stomatal conductance. Effects on growth were determined by measuring trunk diameter (50 cm from the ground) on June 17 and again on October 23. Photosynthesis (µmol m-2 s-1) and stomatal conductance (cm s-1) were measured using a Licor 6200 portable photosynthesis system on July 30 and 31 during the drought, and again on September 1, two days after significant rainfall.
Three experiments were conducted to determine effects of fertilizer and irrigation on insect resistance. On May 7, prior to outplanting, experiments were initiated with third instar eastern tent caterpillar (Malacosoma americanum) and third instar gypsy moth (Lymantria dispar), both of which are spring-feeding insects. To determine if effects on insect resistance were consistent throughout the field season, a third experiment was initiated on August 25 with first instar whitemarked tussock moth larvae (Orgyia leucostigma), a late-season defoliator. Each of the three experiments was conducted under controlled conditions in the laboratory. Larvae were fed foliage from the experimental trees, and their growth determined by measuring their weight immediately before and after the experiment. Leaves and caterpillars were confined to petri dishes (15 cm in diameter, 2.5 cm high) containing a base of plaster of paris. Water added to the plaster base provided a high-humidity environment, which maintained the turgor of the leaves. Petri dishes were then randomly positioned in a growth chamber maintained at 25°C with an 18:6 day:night photoperiod.
The fertilization regime applied during the previous year in the nursery had a significant effect on tree growth in the landscape. However, the effect of fertilization was dependent on the irrigation regime with which it was combined (Figure 1). When trees were grown under the low-moisture regime in the nursery, growth in the landscape increased at each level of fertilization. In fact, trees that received the high-fertilization rate in combination with the low-moisture treatment in the nursery grew substantially faster in the landscape than trees receiving any other treatment effect.
When trees were exposed to a high level of irrigation in the nursery, the effects of fertilization on subsequent growth in the landscape were not as dramatic. Increasing the rate of N-fertilization in the nursery from 50 to 200 ppm did increase growth following outplanting. However, increasing the rate from 200 to 350 ppm had no additional effect (Figure 1).
The carry-over effect of nursery fertilization on subsequent growth in the landscape is somewhat surprising, since fertilization rate had no effect on tree growth in the nursery, although the irrigation regime had major effects (Rose, in preparation). However, these results can be explained if nitrogen accumulated by the tree in the nursery was stored and then used to support growth the following year. Indeed, growth in the landscape in 1998 was highly correlated with nitrogen concentration of the dormant plant following the 1997 growing season (Figure 2).
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The lack of effect of the high-fertilization rate on tree growth when applied in combination with the high-moisture treatment is consistent with the role of stored nitrogen as an important determinant of plant growth the following year. Dormant trees in the low-moisture treatment had a higher concentration of nitrogen (1.95%) than did trees from the high-irrigation treatment (1.74%), because less nitrogen was leached from containers in the low-moisture treatment (Rose, in preparation).
As plants experience moderate to severe drought stress, photosynthesis becomes limited by closure of stomata (pores in the leaf through which gas and water vapor enter and exit). Stomatal conductance is a measure of the rate at which water vapor moves from the leaf through the stomata to the atmosphere by means of transpiration. As stomata close and transpiration decreases, stomatal conductance declines. Closure of stomata conserves water by decreasing transpiration, but at the same time decreases uptake of CO2 from the atmosphere. Not all plants are affected by drought to the same degree. Drought-tolerant plants maintain higher rates of photosynthesis and stomatal conductance during drought than do plants that are less tolerant of drought stress (Schulze, 1986; Kubiske and Abrams, 1993).
In 1998, photosynthesis and stomatal conductance were measured on July 30 and 31 during drought conditions. There had been no rain the previous week, and over the previous three weeks evaporation exceeded precipitation by 3.88 inches. Photosynthesis and stomatal conductance were also measured on September 1, following the end of the drought (4.36 inches of precipitation fell between August 23 and 29).
The fertilizer regime used in the nursery had clear effects on drought stress tolerance following outplanting. Plants grown under the low-nitrogen treatment (50 ppm) were more tolerant of drought than plants grown under the two higher nitrogen levels. This is indicated by the higher photosynthesis and stomatal conductance rates that low-N plants were able to maintain during drought (Table 1). Trees receiving the two higher fertilization rates had significantly lower rates of photosynthesis and stomatal conductance during the drought period. On September 1, after drought conditions had abated, the photosynthesis and stomatal conductance rates of all trees increased dramatically, and treatment effects disappeared (Table 1).
| Table 1. Effect of Fertilizer Regime Used in the Nursery in 1997 on Net Rate of Photosynthesis (mmol m-2 s-1) and Stomatal Conductance (cm s-1) of Malus 'Sutyzam' in the Landscape Following Outplanting in 1998 (mean ± standard error). Means Within a Column Followed by the Same Letter Are Not Significantly Different. | ||||||
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| July 30 | July 31 | September 1 | ||||
| Nitrogen Fertilization Rate (ppm) | Photosynthesis Rate | Stomatal Conductance | Photosynthesis Rate | Stomatal Conductance | Photosynthesis Rate | Stomatal Conductance |
| 50 | 4.3 ± 0.5 a | 0.05 ± 0.01 a | 4.3 ± 0.5 a | 0.06 ± 0.01 a | 10.5 ± 0.6 a | 0.32 ± 0.02 a |
| 200 | 3.4 ± 0.5 ab | 0.04 ± 0.01 ab | 2.5 ± 0.5 b | 0.03 ± 0.01 b | 12.3 ± 0.6 a | 0.32 ± 0.02 a |
| 350 | 2.4 ± 0.5 b | 0.03 ± 0.01 b | 2.1 ± 0.6 b | 0.03 ± 0.01 b | 11.6 ± 0.6 a | 0.26 ± 0.02 b |
There are a number of reasons why high rates of N-fertilization may decrease drought stress tolerance. Increased nitrogen availability generally stimulates shoot growth to a greater degree than root growth, thus decreasing the root:shoot ratio of the tree (Linder and Rook, 1984). In this way, fertilization can simultaneously increase tree water demands while decreasing the tree's ability to acquire water during drought. Trees receiving the low-fertilizer treatment in this experiment had a higher root:shoot ratio when they left the nursery (Rose, in preparation), which could have been responsible for their increased tolerance of drought stress.
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| Figure 3. Effect of fertilization regime in the nursery in 1997 on growth of eastern tent caterpillar, gypsy moth, and whitemarked tussock moth larvae feeding on Malus 'Sutyzam' in 1998. |
High rates of fertilizer may also decrease drought stress through effects on the chemical composition of leaves. A well-documented effect of high fertilization rates is to decrease foliar concentration of tannins and other secondary metabolites that provide trees with stress tolerance and insect resistance (Herms and Mattson, 1992). Tannins impregnate the outer wall of epidermal cells, making them more impervious to water, and thus contribute to water conservation under stress (Bussotti et al., 1998). Analysis of foliar tannin concentrations of these trees is under way.
Numerous studies provide strong evidence that fertilization almost always decreases resistance of trees to defoliating insects. This is because fertilization generally increases the nutritional value of the plant and decreases concentrations of the trees' defensive chemicals (Herms and Mattson, 1997). The results of this study are consistent with this pattern. As the fertilizer rate used in the nursery increased, so did the insect growth rate (Figure 3). This was true for both early-season (gypsy moth and eastern tent caterpillar) and late-season (whitemarked tussock moth) species. Only eastern tent caterpillar was affected by the irrigation regime used in the nursery. Larvae grew faster on plants receiving the high-moisture treatment.
The fertilization regime used in the nursery during 1997 had major effects on the growth, stress tolerance, and insect resistance of crabapple in a low-maintenance landscape in 1998, even though it had little effect on tree growth in the nursery. Conversely, the irrigation regime used in the nursery had little effect on trees following outplanting, although it had major effects on tree growth in the nursery.
The growth rate of trees in the landscape was highly correlated with their nitrogen concentration when they left the nursery. The higher the nitrogen content of the plant, the faster it grew following outplanting. However, increased fertilizer rates in the nursery also decreased the drought stress tolerance and insect resistance of trees once they were in the landscape, possibly because they had lower root:shoot ratios and decreased concentrations of naturally occurring defensive chemicals. Trees receiving the high-fertilizer rate in the nursery had lower rates of photosynthesis during drought, although there were no differences when soil moisture was favorable. Growth rates of eastern tent caterpillar, gypsy moth, and whitemarked tussock moth all increased as the rate of fertilizer increased.
Nutrient-loading in the nursery has been proposed as a strategy for increasing growth and hastening establishment of trees following transplanting (McAlister and Timmer, 1998) and thus could represent a value-added component of nursery production. This is especially true in situations where fertilization is undesirable, such as in forest regeneration where fertilization favors competing vegetation. However, this study suggests that nutrient-loading may be most beneficial when trees are growing under favorable conditions and may be detrimental under stressful conditions. With the exception of a moderate drought from mid-July to early August, growing conditions for trees were good in Wooster in 1998, and there was little insect pressure in the experimental plots. The decreased stress tolerance and insect resistance of trees heavily fertilized in the nursery may counteract positive effects of nutrient-loading during years of more severe drought and insect outbreaks sometimes experienced in low-maintenance landscapes.
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