Biomass and nutrient allocation in Douglas-fir and ...

Tree Physiology 19, 59--63 © 1999 Heron Publishing----Victoria, Canada

Biomass and nutrient allocation in Douglas-fir and amabilis fir seedlings: influence of growth rate and temperature B. J. HAWKINS,1 S. B. R. KIISKILA2 and G. HENRY1 1

Centre for Forest Biology, University of Victoria, P.O. Box 3020, Victoria, B.C. V8W 3N5, Canada

2

Pacific Regeneration Technologies, Red Rock Nursery, R.R. 7, RMD 6, Prince George, B.C. V2N 2J5, Canada

Summary Allocation of biomass and nutrients to currentyear and one-year-old shoots and roots of two-year-old conifer seedlings with differing rates of growth was studied. Differences in growth rate were achieved by selecting fast- and slow-growing populations of the relatively fast- and slowgrowing conifer species, Pseudotsuga menziesii (Mirb.) Franco and Abies amabilis Dougl. ex Loud, respectively. Environmentally controlled differences in growth rate were induced by placing half of the seedlings in a 10 °C growth chamber and half in a 20 °C growth chamber in their second growing season. Seedling samples were harvested in May, before the temperature treatment, and in July and November of the second growing season, and biomass and nutrient concentrations of current-year and one-year-old shoots and roots were determined. Seedling biomass and nutrient allocation differed among the high-growth treatments. Seedlings exhibiting high growth in response to the 20 °C treatment and faster growing populations within species both showed increased allocation to new shoots, whereas seedlings of the fast-growing species showed greater allocation to old shoots than to new shoots. Increased growth increased nutrient uptake, but nutrient concentration decreased with growth rate as a result of dilution, so that faster-growing seedlings had greater nutrient-use efficiency than slowergrowing seedlings. Retranslocation of P and K was seen in the second year only in slow-growing populations at 10 °C. Nutrient concentrations of one-year-old plant parts decreased in the second year, indicating new growth was a stronger sink for nutrients than second-year growth. Keywords: Abies amabilis, nutrient uptake, nutrition, Pseudotsuga menziesii, retranslocation.

also be retranslocated from the bark and wood (Nelson et al. 1970) and from roots under conditions of drought stress (Ferrier and Alexander 1991). The mechanism controlling nutrient retranslocation has not been elucidated. Some studies indicate a positive correlation between nutrient pool and retranslocation (Chapin and Kedrowski 1983, Nambiar and Fife 1987), whereas other studies show a negative correlation (Miller et al. 1976). Retranslocation rates have also correlated with shoot growth rate (Nambiar and Fife 1991). Recent work indicates that the ecological characteristics of a species control retranslocation (Munson et al. 1995, Hawkins et al. 1998), in conjunction with soil nutrient availability and sink strength. Growth rates of plants are determined by many environmental and genetic factors. Temperature is an important environmental determinant of growth rate because most plant processes are temperature dependent (Kramer and Kozlowski 1979). Growth rates also differ among species, but physiological causes for genetic differences in growth are often difficult to determine. The objective of this study was to compare the allocation of biomass and nutrients to one-year-old and current-year shoots and roots of seedlings of two conifer species with different rates of growth caused by both environmental and genetic factors. Genetic differences in growth rates were achieved by selecting fast- and slow-growing populations of fast- and slow-growing conifer species. Environmentally controlled differences in growth rate were produced by growing seedlings of the four populations at two temperatures. We tested the hypothesis that the effects of growth rate on nutrient uptake and allocation differ depending on whether growth rate is controlled by environmental or genetic factors.

Materials and methods Introduction Movement of nutrients from older to newer plant tissues is important in plant nutrition. In closed-canopy conifer stands, up to 66% of the nutrients required for growth can be obtained through retranslocation from older foliage (Miller 1995), and in young conifers, 32 to 40% of N used for leaf growth is derived from remobilization (Millard 1996). Nutrients may

Seeds from a fast- and a slow-growing population of both amabilis fir (Abies amabilis Dougl. ex Loud.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) were obtained from the B.C. Ministry of Forests, Victoria, Canada (Hawkins et al. 1998). Seeds were sown in peat in Styroblock containers (60 × 34 × 15 cm deep, with 45 × 336 ml cavities) in late June 1993 and grown in an unheated greenhouse (Hawkins et al. 1998).

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Received September 23, 1997

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Results and discussion Seedling biomass allocation differed among the various highgrowth treatments, and nutrient allocation generally followed the same pattern. The slow-growing species, amabilis fir, had a greater proportion of biomass in new shoots than the fastgrowing species, Douglas-fir (50 ± 12% versus 43 ± 7%; mean ± SD), and Douglas-fir allocated proportionately more biomass to old shoots than amabilis fir (24 ± 2% versus 17 ± 2%) (Figure 1). There was no significant difference between species in the proportion of biomass allocated to roots. In contrast, fast-growing populations of amabilis fir allocated significantly more dry weight to new shoots than slow-growing populations (54 ± 11% versus 45 ± 13%, respectively; P < 0.0001), whereas slow-growing populations allocated more biomass to old shoots and new roots than fast-growing populations (Figure 1). Douglas-fir populations did not differ significantly in biomass allocation. Increased growth in response to high temperature resulted in a significantly (P < 0.05)

Yijkl = µ + Ti + C(T )j(i) + Pk + TPik + C(T )Pj(i)k + e(ijk)l ,

where µ is the overall mean; Ti is the effect of the ith temperature (i = 1,2); C(T)j(i) is the effect of the jth controlled environment chamber nested within the ith temperature (j = 1,2); Pk is the effect of the kth population (k = 1,2); TPik is the interaction between the ith temperature and the kth population; C(T)Pj(i)k is the interaction of the jth controlled environment chamber nested within the ith temperature and the kth population; and e is the error associated with measurements of pairs of seedlings. To determine significance levels, Ti was tested against C(T)j(i) and TPik was tested against C(T)Pj(i)k.

Figure 1. Mean dry weights of new and old shoots and roots of fast(F) and slow-growing (S) populations of (a) Douglas-fir and (b) amabilis fir seedlings. May values indicate starting condition before bud flush. July and November values are for seedlings grown at 20 and 10 °C. Note that the scale differs between species. Root weights are positive below the ‘zero’ line. Error bars represent the standard deviation in total dry weight.

TREE PHYSIOLOGY VOLUME 19, 1999


Three weeks after germination, 1 liter of nutrient solution (with nitrogen (N) included as NH4NO3 at 100 mg l −1, phosphorus (P) as NaH2PO4⋅H2O at 20 mg l −1, and potassium (K) as K2SO4 at 100 mg l −1) was applied to each Styroblock (45 seedlings). The seedlings were fertilized weekly until the end of September by which time all had set bud. Seedlings remained in the greenhouse throughout the winter and were irrigated as required. In May 1994, 12 seedlings from each population were harvested and measured for height and root collar diameter. The seedlings were divided into root and shoot portions that were randomly bulked into two groups of six per population, dried at 78 °C for 48 h and weighed. These bulked portions were then ground to pass a 1-mm sieve and analyzed for total N, P, K and micronutrients (Hawkins et al. 1998). Of the remaining seedlings, 32 from each population were transplanted to 1.1-liter polyethylene bags (Menne Nursery Corp., Amherst, NY) containing sand ammended with dolomite lime and micronutrients (Hawkins et al. 1998). Eight seedlings per population were randomly placed in each of four controlled environment chambers. Two chambers were set at 20 °C and two at 10 °C. All chambers provided a 16-h photoperiod, and nutrient solution was applied weekly with additional irrigation as needed. In mid-July, after shoot extension was complete, and in mid-November, when seedlings were dormant, four randomly selected seedlings per population were harvested from each controlled environment chamber. The sampled plants were measured for height and root collar diameter, and then bulked in pairs for measurements of dry weight and nutrient concentrations of current-year (new) and one-year-old (old) shoots and roots. Current-year roots were defined as those that had extended into the sand, and old roots were defined as those inside the peat plug, comprising both live and dead material. Because of a significant species × temperature interaction, growth and nutrient data were analyzed by month and species with a nested analysis of variance using the PROC ANOVA procedure of the SAS statistical software package (SAS Institute Inc., Cary, NC). Temperature and population were considered to be fixed factors, whereas the effect of the controlled environment chamber was considered random. The model was as follows:

BIOMASS AND NUTRIENT ALLOCATION IN CONIFER SEEDLINGS

In general, fast-growing species and populations and seedlings grown in the 20 °C treatment had lower nutrient concentrations for a given content than slow-growing species and populations and seedlings grown in the 10 °C treatment, indicating greater nutrient-use efficiency in faster growing plants than in slower growing plants (Figure 3). Although plants exposed to the 10 °C treatment had a higher root:shoot ratio than plants exposed to the 20 °C treatment (0.63 ± 0.04 at 10 °C versus 0.37 ± 0.03 at 20 °C)----which could account for their higher nutrient concentrations----slow-growing species and populations did not differ significantly in root:shoot ratio from their fast-growing counterparts. In these seedlings, genetic limitations on growth resulted in reduced nutrient-use efficiency. In all treatments, a rapid decrease in nutrient concentrations was observed in old roots and shoots at some time during the growing season (Figure 4). Nutrient content and concentration increased steadily in new shoots and roots of all seedlings

Figure 2. Mean increase in N (a and d), P (b and e) and K (c and f) content from May to July and July to November in new (N) and old (O) shoots and roots of fast- (F) and slow-growing (S) populations of Douglas-fir (a, b, c) and amabilis fir (d, e, f) seedlings grown at 20 and 10 °C. Note that the scale differs between species and element. Error bars represent the standard deviation in element content in November.

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increased proportion of biomass allocated to new and old shoots and a decreased allocation to roots (Figure 1). High temperature increased biomass allocation to new shoots by 13% in Douglas-fir and 20% in amabilis fir in November. High growth rates due to species, population or temperature resulted in increased amounts of nutrients being taken up by seedlings, and increased allocation of nutrients to new shoots (Figure 2). In Pinus sylvestris L., increased root temperature also increased N allocation to current-year shoots at the expense of the older shoots, but allocation to roots was unaffected by root temperature (Vapaavuori et al. 1992). Low rates of uptake of P and K in slow-growing seedlings at low temperature resulted in more P and K being retranslocated from old roots and shoots than was supplied to these organs between May and July (Figures 2b, c, e and f). In most seedlings, however, nutrient content of old shoots and roots increased between May and July, masking any retranslocation that may have taken place.

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throughout the second growing season, especially in the 20 °C treatment (data not shown). These data indicate that new shoots and roots are stronger nutrient sinks than old shoots and roots (cf. Fife and Nambiar 1984). In Douglas-fir at high temperature, N concentration in old shoots declined between May and July, whereas at low temperature, allocation to new growth occurred primarily between July and November (Figure 4a). In contrast, amabilis fir showed no allocation of N to old shoots between May and July, and N only accumulated after July (Figure 4c). Hawkins et al. (1998) observed greater recovery of N concentrations in old shoots of amabilis fir compared to Douglas-fir, and speculated that this could be an adaptation maintaining photosynthetic function in longerlived amabilis fir needles. Our study supports this suggestion because some recovery of N concentration was seen in all old amabilis fir shoots, whereas N concentration declined substantially in old Douglas-fir shoots at low temperature. Hawkins et al. (1998) showed that export of nutrients from Douglas-fir and amabilis fir shoots did not occur until the third growing season. In the present study of these species in their second growing season, a comparison of the effects of temperature and growth rate on nutrient retranslocation could not

be made because retranslocation was only observed in slowgrowing seedlings at low temperature. Our results support the findings of Munson et al. (1995) and Hawkins et al. (1998) indicating that a combination of species’ characteristics and sink strengths, rather than growth rate alone, controls biomass and nutrient allocation. The observed nutrient depletion in slow-growing seedlings at low temperature has practical implications for nursery managers. Nutrient loading (Ingestad and Lund 1986) of slow-growing species in the nursery would provide seedlings with a greater pool of nutrients to draw from after planting and improve establishment on cold sites.

Acknowledgments The authors thank Dr. Jack Woods, Rod Meredith, Scott Dunn and J. Ingram of the B.C. Ministry of Forests for supplying Douglas-fir and amabilis fir seed; and Dr. R. van den Driessche for arranging nutrient analyses. Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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Figure 3. Change in N (a and c) and P (b and d) concentration and content from May to July to November in fast- (F) and slow-growing (S) populations of Douglas-fir and amabilis fir grown at 20 and 10 °C. Note that the scale differs between species and element. Error bars represent standard deviation of element content.

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Figure 4. Change in N concentration and content from May to July to November in old shoots and roots of fast(F) and slow-growing (S) populations of Douglas-fir and amabilis fir grown at 20 and 10 °C. Note that the scale differs between species and element. Error bars represent standard deviation of element content.
