Seedling herbivory by slugs in a willow hybrid system: developmental changes in damage, chemical defense, and plant performance

We evaluated feeding preference and damage by the slug, Arion subfuscus, on seedlings of two willow species, Salix sericea and S. eriocephala, and their F₁ interspecific hybrids. Trays of seedlings were placed in the field and excised leaves were presented to slugs in choice tests. Slugs preferred feeding on and caused the most damage to S. eriocephala seedlings. S. sericea seedlings were least preferred and least damaged. F₁ hybrid seedlings were intermediate in preference and damage. Slug preference of and damage to these seedlings decreased over time, suggesting developmental changes in resistance. Seedlings were sampled for phenolic glycoside and tannin chemistry weekly to coincide with the field and laboratory experiments. Concentrations of phenolic glycosides and tannins increased linearly with seedling age, coincident with changes in slug preference and damage, indicating a developmental change in defense. Slug deterrence was not detected at low concentrations of salicortin when painted on leaves or discs, but both salicortin and condensed tannins deterred slug feeding at concentrations between 50 and 100 mg/g, levels found in adult willows. Seedling performance was related to damage inflicted by slugs. Due to lower levels of damage when exposed to slugs in the field, S. sericea plants had significantly greater biomass than S. eriocephala plants. Biomass of F₁ hybrids was equal to S. sericea when damaged. However, undamaged S. eriocephala and F₁ hybrid plants had the greatest biomass. Because F₁ hybrid seedlings performed as well as the most fit parent in all cases, slugs could be an important selective factor favoring introgression of defensive traits between these willow species.     

Regeneration of Salix arbuscula and Salix lapponum within a Large Mammal Exclosure: The Impacts of Microsite and Herbivory

Over‐grazing or browsing by large herbivores may result in the loss of individual plant species or entire plant communities. Restoration schemes often involve exclusion of large mammals, but the resulting changes in vegetation may alter other important ecological processes such as regeneration, via changes in microsite availability for seed germination or increases in populations of seedling predators. Working within a large fenced area from which large mammals were excluded, we experimentally tested the effects of microsite, small herbivores, and their interactions on post‐dispersal seed and early seedling mortality of one nationally scarce (Salix arbuscula) and one nationally rare (S. lapponum) species of montane willow. Seeds were sown in three different microsites: natural vegetation, mown vegetation (mimicking grazed sward), and bare ground. Small exclosures and slug pellets were used to examine the effects of small mammal and slug predation, respectively. Survival of seedlings was monitored during the summer following planting. The presence of bare ground, rather than the absence of herbivores, was of over‐riding importance for early seedling survival and establishment. Protecting seedlings from small mammals made no difference to the levels of survival; however, protecting seedlings from slugs (Arion spp.) resulted in approximately 45% of seedlings surviving until the end of the summer compared to only 30% when seedlings were available to slugs. Although excluding large herbivores may increase seed production of existing individuals, the impacts of changes to plant communities on processes such as regeneration need to be considered if restoration projects are to be fully successful.     

Interactive effects of substrate,hydroperiod, and nutrients on seedling growth of Salix nigra and Taxodium distichum

The large river swamps of Louisiana have complex topography and hydrology, characterized by black willow (Salix nigra) dominance on accreting alluvial sediments and vast areas of baldcypress (Taxodium distichum) deepwater swamps with highly organic substrates. Seedling survival of these two wetland tree species is influenced by their growth rate in relation to the height and duration of annual flooding in riverine environments. This study examines the interactive effects of substrate, hydroperiod, and nutrients on growth rates of black willow and baldcypress seedlings. In a greenhouse experiment with a split-split-plot design, 1-year seedlings of black willow and baldcypress were subjected to two nutrient treatments (unfertilized versus fertilized), two hydroperiods (continuously flooded versus twice daily flooding/draining), and two substrates (sand versus commercial peat mix). Response variables included height, diameter, lateral branch count, biomass, and root:stem ratio. Black willow growth in height and diameter, as well as all biomass components, were significantly greater in peat substrate than in sand. Black willow showed a significant hydroperiod–nutrient interaction wherein fertilizer increased stem and root biomass under drained conditions, but flooded plants did not respond to fertilization. Baldcypress diameter and root biomass were higher in peat than in sand, and the same two variables increased with fertilization in flooded as well as drained treatments. These results can be used in Louisiana wetland forest models as inputs of seedling growth and survival, regeneration potential, and biomass accumulation rates of black willow and baldcypress.     

Responses of Jeffrey Pine on a Surface Mine Site to Fertilizer and Lime

Controlled‐release fertilization with two formulations, Forestcote 22‐4‐6 + Minors and Gromax 21‐6‐2 + Minors, and dolomitic lime were evaluated for their capacity to facilitate establishment and enhance nutrition of bareroot Jeffrey pine (Pinus jeffreyi Grev. & Balf.) on an acidic Sierra Nevada surface mine. All amendments were applied at outplanting to the backfill of augered planting holes, with a low rate of 8 g and a high rate of 16 g per seedling for the two fertilizer formulations, whereas a single rate of 30 g was used for lime. Liming induced excessive seedling mortality throughout the study, whereas both fertilizer formulations also reduced survival but only as an interactive effect with the lime amendment. Growth stimulation by the fertilizer applications was readily apparent after three growing seasons, with the response to the high rate surpassing that to the low rate and the response to the Forestcote formulation exceeding that to Gromax. Conversely, the lime amendment depressed seedling growth, and it is likely that a propensity to exacerbate the stress imposed by dry growing season soils through interference with water uptake accounts for the overall poor performance of the seedlings that received this amendment. Improved N and P nutrition, with the former likely the most critical, was largely responsible for the growth stimulation resulting from fertilization. However, this treatment also suppressed the absorption of certain micronutrients as well as that of Al, and the amelioration of potential phytotoxicities may have also contributed to the favorable performance of fertilized seedlings. Further evidence for this conclusion was supplied by base cation/metallic element molar ratios involving Al and Mn, which were frequently increased by fertilization. In contrast, the influence of the lime amendment on seedling nutrition was marginal, with positive responses too infrequent to improve seedling performance.     

Effects of Nitrogen Exponential Fertilization on Growth and Nutrient Concentration of <i>Hydrangea macrophylla</i> Seedlings

Slow growth rate restricts the development and growth of seedlings due to nutrients deficiency or nutrient imbalance. Exponential fertilization can enhance the internal nutrient reserves in seedlings at the nursery-stage and strengthen their resistance to adverse conditions. In this study, nitrogen requirements for producing Hydrangea macrophylla ‘Hanatemari’ that robust seedlings, nutrient dynamics, biomass and growth, was examined utilizing exponential fertilization. The potted seedlings were fertilized with urea under exponential regime at rates of 0.5, 1.5 and 2.0 g nitrogen/plant (EF1, EF2, and EF3), respectively. In addition, an unfertilized group treated with equal volume of deionized water was used as control. The results showed that seedlings under 1.5 g N/plant (EF2) had the highest plant growth index and total biomass. The nutrient concentrations of different organs varied in different fertilization treatments. Based on the results of current study, it is concluded that 1.5 g N/plant (EF2) is suitable exponential fertilization treatment for the culture of hydrangea seedlings. Our treatments results showed that 2.0 g N/plant is not suitable for seedling culturing, because of serious nutrient toxicity. These findings will help to improve seedling quality and strengthen the production of H. macrophylla for plantation.     

A method to study slug predation on seedlings in the field

Dandelion is a common Asteraceae species that populates disturbed sites and gaps within swards where it becomes an important competitor of grasses. The natural control against dandelion includes seedling predation, with slugs, particularly Arion lusitanicus, being the most important in the Czech Republic. However, the study of slug seedling consumption is difficult because naturally established seedlings are not always available. Therefore, we developed a method of exposing laboratory‐grown seedlings as bait for slug predation. Dandelion seeds were sown in plastic cups containing a bottom layer of moist substrate. The emerged seedlings were thinned to 20 and displayed in an open area. During 2008–2010, the seedling baits were placed at 15 sites at 1 month intervals throughout the dandelion vegetative season, in parallel with plasticine baits that monitored slug feeding activity. For each 1 month interval, seedling survival was observed for a period of 8 days, and the estimated time to death was calculated; the percentage of surviving seedlings was then recorded. This method of seedling presentation demonstrated that local and temporal variation in seedling survival is correlated with slug feeding activity. The advantage of this technique is that the seedlings in baits may be presented at any place and time, as required by the experimental design; however, we found that the estimated time to seedling death was shorter for the exposed baits than for the naturally established seedlings. The method is suitable for the study of plant species other than dandelion, and also for aims other than the study of spatiotemporal trends in seedling consumption by slugs.     

Colonization by nitrogen-fixing <Emphasis Type="Italic">Frankia</Emphasis> bacteria causes short-term increases in herbivore susceptibility in red alder (<Emphasis Type="Italic">Alnus rubra</Emphasis>) seedlings

Carbon allocation demands from root-nodulating nitrogen-fixing bacteria (NFB) can modulate the host plant’s chemical phenotype, with strong bottom–up effects on herbivores. In contrast to well-studied rhizobia, the effects of other important NFB on plant chemistry and herbivory are much less understood. Here, combining field surveys in the Oregon Coast Range, USA with laboratory experiments, we analyzed how N₂-fixing Frankia bacteria influenced plant growth, chemistry, and herbivory on Alnus rubra (red alder) seedlings. In the field, we quantified Frankia nodulation, herbivore damage, and plant size. In the laboratory, we grew seedlings with Frankia (F+), Frankia-free but nitrogen-fertilized (N+), or both uncolonized and unfertilized (F?N?) and assessed growth and leaf chemistry. We further conducted choice trials with black slugs, Arion rufus, a natural red alder herbivore. In the field, Frankia nodulation was significantly positively correlated with herbivory and negatively with seedling height. In contrast, in the lab, F+ as well as N+ seedlings were significantly taller than the F?N? controls. Seedlings from both treatments also had significantly increased leaf protein concentration compared to controls, whereas carbon-based nutritive compounds (carbohydrates) as well as leaf palatability-decreasing condensed tannins, lignin, and fiber were decreased in F+ but not in N+ treatments. In the choice assays, slugs preferred leaf material from F+ seedlings, but the effects were only significant in young leaves. Our study indicates that colonization by Frankia causes short-term ecological costs in terms of susceptibility to herbivory. However, the ubiquity of this symbiosis in natural settings suggests that these costs are outweighed by benefits beyond the seedling stage.     

Effect of nitrogen fertilization upon the secondary chemistry and nutritional value of quaking aspen (Populus tremuloides Michx.) leaves for the large aspen tortrix (Choristoneura conflictana (Walker))

Summary We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.     

施肥对短枝木麻黄幼苗总酚和可溶性缩合单宁含量的影响

研究了施用氮肥和磷肥对短枝木麻黄幼苗总酚(total phenolics,TP)和可溶性缩合单宁(extractable condensed tannin,ECT)含量的影响,探讨短枝木麻黄单宁形成的养分效应.结果表明:施加氮肥使短枝木麻黄幼苗小枝的TP和ECT含量显著降低,支持碳氮平衡假说和生长分化平衡假说,但对氮含量没有显著影响,从而导致TP/N和ECT/N降低;施加磷肥对TP和ECT含量没有显著影响;随着处理时间的延长,短枝木麻黄幼苗小枝TP含量升高了9.91%～14.32%,而ECT含量降低了14.32%～298.88%;TP或ECT与有机物质含量的关系则相反,表明不同类型单宁的合成途径不同,但由于TP和ECT均与氮含量无显著相关性,故不支持蛋白质竞争模型;在贫瘠土壤条件下,TP/N和ECT/N的水平较高,有利于提高短枝木麻黄的防御水平,降低凋落物的分解率,减少养分损失,从而保持较高的生产力.     

Palatability of silver birch seedlings to root voles Microtus oeconomus

We used laboratory feeding trials to test the palatability of silver birch Betula pendula seedlings to root voles Microius oeconomus Seedlings of two B pendula families (A and B) were grown in growth chambers on three soil types and under two light intensities. Seedlings from family A grew taller and had more resin droplets on their bark than seedlings from family B The more light or nutrients the seedlings received the taller they grew and the more resm droplets they had on their bark We offered 10 root voles both basal and top 5–10 cm segments of winter-dormant seedlings one treatment at a time (no-choice arrangement) in a random order The seedlings grown on low-fertility soil were eaten more than those grown on non-fertilized or fertilized peat The top segments of seedlings grown in low light were slightly more eaten than those grown in high light Birch family did not have any effect on the consumption Consumption correlated negatively with the amount of resin droplets on the bark when the seedling material was compared over different treatments These results indicate that root voles would prefer young birch seedlings that have low amount of resin droplets on the bark and that grow in poor soils or under low light conditions     

Does the differential seedling mortality caused by slugs alter the foliar traits and subsequent susceptibility of hybrid willows to a generalist herbivore?

Abstract 1. Many Salicaceae species naturally form hybrid swarms with parental and hybrid taxa that differ in secondary chemical profile and in resistance to herbivores. Theoretically, the differential mortality in the seedling stage can lead to changes in trait expression and alter subsequent interactions between plants and herbivores. This study examines whether herbivory by the generalist slug Arion subfuscus, which causes extensive mortality in young willow seedlings, causes shifts in (a) the foliar chemistry of F2 willow hybrids (Salix sericea and Salix eriocephala), and (b) the subsequent susceptibility to Japanese Beetles, Popillia japonica. 2. In 2001, two populations of F2 seedlings were generated: those that survived slug herbivory (80–90% of seedlings placed in the field were killed by the slugs) were designated as S-plants, whereas C-plants (controls) experienced no mortality. 3. Common garden experiments with cuttings from these populations, in 2001 and 2002, revealed extensive variation in the phenolic chemistry of F2 hybrids, but revealed no significant difference between S- and C-plants, although the levels of foliar nutrients, proteins and nitrogen tended to be higher in S-plants. 4. Concentrations of salicortin and 2′-cinnamoylsalicortin explained 55 and 38% of the the variation in leaf damage caused by Japanese beetles, and secondary chemistry was highly correlated within replicate clones (salicortin R²= 0.85, 2-cinnamoylsalicortin R²= 0.77, condensed tannins R²= 0.68). 5. Interestingly, Japanese beetle damage and condensed tannins were positively correlated within the S-plants, but not in the C-plants, suggesting that slugs had selected for plants with a positive relationship between tannins and P. japonica damage. This is unlikely to be a consequence of a preference for tannins, but is suggested to be related to the elevated nutrient levels in the S-plants, perhaps in combination with the complex-binding properties of tannins. 6. The damage was highly correlated within replicate clones and a model choice analysis suggested that Japanese beetle damage may be explained by four factors: concentrations of salicortin, condensed tannins, and nitrogen, as well as the specific leaf area (thick leaves were damaged less).     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

Root growth characteristics, biomass and nutrient dynamics of seedlings of two larch species raised under different fertilization regimes

The effects of different fertilization regimes on root growth characteristics, nutrient uptake and biomass production of Japanese larch (Larix kampferi Sarg.) and its hybrid larch (L. gmelinii × L. kampferi) seedlings were examined for one growing season. Seedlings were raised in the greenhouse under three fertilizer levels (10, 20, 40 mg N seedling^–1 season^–1) and two delivery schedules, conventional (C) and exponential (E) for 12 weeks. Root growth, biomass allocation and nutrient loading capacity of seedlings were measured for a 3-week interval. By the end of growing season, seedlings fertilized with low dose conventionally (10C) and exponentially (10E) developed relatively longer root and larger root surface areas than those fertilized at high dose exponential loading (40E). At final harvest, the 40E treated Japanese larch had 134% and 155% more shoot mass as compared with those raised under 20E and 10E treated seedlings, respectively. The seedlings fertilized under 10C and 10E showed a high root mass ratio, while 40E treated seedlings showed a low root mass ratio. These data indicated that different nutrient levels (10 mg, 20 mg and 40 mg) strongly affected root growth characteristics. The same seasonal dose (10 mg) applied exponentially (10E) accumulated more N in seedlings compared to the 10C treatment. Exponential fertilization enhanced an increase in N concentration of the whole plant suggesting exponential delivery schedule is an efficient fertilization technique for greater nutrient uptake of plants. In contrast, N concentration of whole plant was declined for seedlings treated with conventional fertilization due to growth dilution. Late in the growing season, seedlings raised under 40E did not significantly improved dry mass production of root, but nutrient accumulation increased without a concomitant increased in root dry mass production. The result suggests that seedlings fertilized exponentially at medium and high dose rates (20E and 40E) induced luxury nutrient consumption within the plant.     

Plant responses to fertilization and exclusion of grazers on an arctic tundra heath

This study investigated the impacts of fertilization and grazing by Norwegian lemmings (Lemmus lemmus), grey‐sided voles (Clethrionomys rufocanus), and reindeer (Rangifer tarandus) on a diverse tundra plant community dominated by deciduous shrubs. Four out of eight study areas, having a size of 2500 m² each, were fertilized with a N‐P‐K fertilizer and four areas served as unfertilized controls. Two types of exclosures were used within each study area, one to exclude solely reindeer, and one to exclude both rodents and reindeer. Open, grazed plots served as controls. During 5 years following the fertilization event the changes in vegetation inside and outside the exclosures were monitored using a point frequency method. The densities of rodents on the fertilized and unfertilized areas were investigated by live trapping and by counting nests of overwintering individuals. Reindeer do not graze on the study area during the growing season but migrate through this area in autumn and spring. Fertilization increased the abundance of vascular plants while grazing by reindeer and rodents decreased the abundance of vascular plants significantly on both fertilized and unfertilized areas. Rodents preferred clearly the fertilized areas during winter, decreasing the abundance of Vaccinium myrtillus and Vaccinium vitis‐idaea, while very little grazing occurred during summer. Graminoids showed the strongest positive response to fertilization and dominated the plant community on ungrazed plots, while winter grazing by both reindeer and rodents significantly decreased the abundance of graminoids. Deciduous shrubs (Betula nana, Vaccinium myrtillus) increased slightly but significantly due to fertilization and evergreen dwarf shrubs showed no response to fertilization. However, the use of functional growth forms for predicting the responses of nutrient enrichment and grazing must be questioned, as responses to fertilization as well as preferences by herbivores were shown to be species‐specific rather than uniform within functional groups based on plant growth forms.     

Consumption of leaf detritus by a stream shredder: Influence of tree species and nutrient status

Four species of riparian vegetation (alder, birch, willow and poplar) were fertilized with nitrogen, phosphorus, nitrogen + phosphorus, or no fertilizer (control). The resulting leaf detritus (leached but not microbially colonized) was offered to a stream shredder, Hydatophylax variabilis (Trichoptera: Limnephilidae). In one experiment, shredder consumption of leaf detritus from different nutrient treatments (within tree species) was compared, and in a second experiment, consumption of different tree species (within nutrient treatments) was compared. Larvae preferred leaf detritus from nitrogen + phosphorus treatments (except in poplar where nitrogen treatment was preferred). Alder was preferred over other tree species for all treatments. Chemical and physical analyses of leaf litter showed differences between tree species and nutrient treatments in nutrient content, tannins and leaf toughness. Leaf consumption by larvae was positively associated with nitrogen content and negatively associated with condensed tannin content. Species composition and nutrient status of riparian vegetation may strongly influence detrital food webs in streams.     

Restoring Aristida stricta to Pinus palustris Ecosystems on the Atlantic Coastal Plain, U.S.A.

Aristida stricta (wiregrass), a perennial bunchgrass, quickly accumulates dead leaves, which along with the shed needles of Pinus palustris (longleaf pine) provide the fuel for frequent surface fires. Thus, historically, wiregrass played a key role in many longleaf communities where it significantly influenced the natural fire regime and thereby the composition of the plant community. Reestablishment of wiregrass is, therefore, critical to restoring the native understory of Atlantic Coastal Plain longleaf pine ecosystems. This study measured the effects of different site preparations and fertilizer application on the survival and growth of wiregrass seedlings. Two-month–old seedlings were underplanted in existing longleaf pine stands on dry Lakeland soils at the Savannah River Site in South Carolina. Survival was acceptable at 51% after four years, although reduced owing to drought and small seedling size. Survival and growth could both be increased by using older seedlings with an initial height of at least 6 cm. Wiregrass leaves grew quite rapidly and attained an average length of 48 cm in four years on control plots. Basal area growth rate was greater than expected, averaging 40% on control treatments and 55% on cultivated and fertilized plots. If growth rates during the first four seasons continue, wiregrass will attain mature size on cultivated and fertilized plots at six years, while non-fertilized control plots will take eight years. A planting density of one seedling per m² is recommended to provide sufficient wiregrass foliar cover to influence fire regimes in a reasonable length of time (i.e., 5–7 years).     

Chemical defense production in Lotus corniculatus L. II. Trade-offs among growth,reproduction and defense

Summary Ecological trade-offs between growth, reproduction and both condensed tannins and cyanogenic glycosides were examined in Lotus corniculatus by correlating shoot (leaves and stem) size and reproductive output with chemical concentrations. We found that cyanide concentration was not related to shoot size, but that condensed tannin concentrations were positively correlated with shoot size; larger plants contained higher tannin concentrations. Both tannin and cyanide concentrations were depressed when plants produced fruits. Defense costs change as plants mature and begin to reproduce. These trade-offs indicate that cost of defense chemical production cannot be predicted merely on the basis of molecular size, composition or concentration.     

Biomass and nutrient dynamics of Chinese fir seedlings under conventional and exponential fertilization regimes

Containerized Chinese fir (Cunninghamia lanceolata (Lamb) Hook) were reared from seed at four fertilizer levels (0, 15, 45, 75 mg N seedling-1 season-1) and two topdressing schedules (conventional or exponential) for a 22-week greenhouse rotation to assess growth, nutrition and nutrient loading capacity of seedlings. Extra P supplemented high fertilization (or nutrient loading) treatments to test for induced deficiency of this element. The schedule and rate of fertilization significantly affected growth and nutrient dynamics of the seedlings. Steady-state nutrition and superior growth performance were achieved by seedlings fertilized exponentially at the operational dose (15 mg N), yielding 23, 72 and 52% more in respective biomass, N uptake and P uptake than seedlings fertilized conventionally at the equivalent dose. The improved growth and fertilizer efficiency were attributed to close synchronization of exponential nutrient supply with exponential growth and nutrient demand of plants. High dose exponential fertilization (45 and 75 mg N) induced steady state-nutrition late in the season, increasing seedling N and P uptake by 72–83% and 50–96% compared to low dose exponential fertilization, demonstrating effective nutrient loading of plants without changing biomass. The extra P stimulated P uptake without altering growth or N uptake, thus P was probably not limiting during the greenhouse culture despite high N additions.     

Effects of repeated damage and fertilization on palatability of Vaccinium myrtillus to grey sided voles, Clethrionomys rufocanus

In this study we examined the responses of the dominant understorey plant species, Vaccinium myrtillus, in a Swedish boreal forest to nitrogen applications and repeated damage by clipping. Four years of clipping V. myrtillus reduced its abundance, regardless of whether the clipping was combined with fertilization or not. The treatments also induced changes in growth form and concentration of phenolic compounds in the shoots. Repeated damage to the shrub caused reductions in both the length and diameter of the shoots, while fertilization alone increased their diameter. Fertilization also decreased the concentration of condensed tannins in shoots of V. myrtillus, while clipping had no significant effect in this respect. Condensed tannin concentrations were higher in shoots given the combined fertilization and clipping treatment than in shoots that were fertilized but not clipped. The effect on tannins is in accordance with the predictions of the CNB‐hypothesis. Among the seven individual phenolic compounds analysed only one, a cinnamic acid derivate, showed a significant effect of the treatments. Repeated damage resulted in decreased concentration of this phenolic acid. In addition there was a tendency towards treatment effects on both nitrogen and carbon concentration of the V. myrtillus shoots, but none of these effects were statistically significant. The treatment‐induced changes in V. myrtillus also affected the food preferences of grey‐sided voles (Clethrionomys rufocanus), resulting in the following order of preference among the treatments: 1) fertilization and clipping, 2) fertilization, 3) control and 4) clipping. Not only biochemical changes, but also changes in growth form were found to influence the preferences, as the voles avoided the smallest shoots. This size‐dependent feeding may partly explain the observed differences in their preferences. Thus, induced changes in growth form need to be considered when conclusions about changes in herbivores’ preference are made.