Mycorrhizas on nursery and field seedlings of Quercus garryana

Oak woodland regeneration and restoration requires that seedlings develop mycorrhizas, yet the need for this mutualistic association is often overlooked. In this study, we asked whether Quercus garryana seedlings in nursery beds acquire mycorrhizas without artificial inoculation or access to a mycorrhizal network of other ectomycorrhizal hosts. We also assessed the relationship between mycorrhizal infection and seedling growth in a nursery. Further, we compared the mycorrhizal assemblage of oak nursery seedlings to that of conifer seedlings in the nursery and to that of oak seedlings in nearby oak woodlands. Seedlings were excavated and the roots washed and examined microscopically. Mycorrhizas were identified by DNA sequences of the internal transcribed spacer region and by morphotype. On oak nursery seedlings, predominant mycorrhizas were species of Laccaria and Tuber with single occurrences of Entoloma and Peziza. In adjacent beds, seedlings of Pseudotsuga menziesii were mycorrhizal with Hysterangium and a different species of Laccaria; seedlings of Pinus monticola were mycorrhizal with Geneabea, Tarzetta, and Thelephora. Height of Q. garryana seedlings correlated with root biomass and mycorrhizal abundance. Total mycorrhizal abundance and abundance of Laccaria mycorrhizas significantly predicted seedling height in the nursery. Native oak seedlings from nearby Q. garryana woodlands were mycorrhizal with 13 fungal symbionts, none of which occurred on the nursery seedlings. These results demonstrate the value of mycorrhizas to the growth of oak seedlings. Although seedlings in nursery beds developed mycorrhizas without intentional inoculation, their mycorrhizas differed from and were less species rich than those on native seedlings.     

Testing the effect of restoration‐focused silviculture on oak regeneration and groundlayer plant communities in urban–exurban oak woodlands

Throughout their global range, oak‐dominated ecosystems have undergone state changes in stand structure and composition. Land managers face an especially acute challenge in restoring oak ecosystems and promoting oak regeneration in urban–exurban areas, where high‐intensity silvicultural treatments are often not feasible. To investigate low‐intensity management alternatives which could be widely applied in urban–exurban forests, a large‐scale adaptive management experiment was implemented in Lake County, IL, in 2012. Five canopy manipulation treatments of varying intensity, timing, and spatial aggregation were replicated across three study areas and oak seedlings were under‐planted into treatment units following management. Responses of understory light environment, shrub and groundlayer plant communities, and survival and growth of underplanted oak seedlings were evaluated. Understory light availability, canopy openness, total groundlayer plant cover, and groundlayer species diversity all differed among treatments. However, although understory light availability was significantly increased by canopy manipulation, groundlayer communities and oak seedling survival and growth did not differ among treatments. High overall seedling survival rates suggest current conditions are amenable to oak regeneration, but long‐term monitoring will be needed to assess the potential for seedlings to transition to the sapling and canopy layers. Early results demonstrate that canopy‐focused silvicultural treatments can affect the understory light environment and, to some degree, groundlayer plant communities. However, underplanting of oak seedlings paired with subcanopy thinning may be sufficient to restore an oak seedling layer, and (when necessary or preferred) canopy manipulation could potentially be deferred until later in the restoration timeline to promote oak recruitment.     

Winter nitrogen fertilization of loblolly pine seedlings

Loblolly pine (Pinus taeda L.) seedlings were nitrogen fertilized during winter in a bare root forest tree nursery located in the coastal plain of the southeastern United States. Total application rates were 0, 50, 100, and 200 kg/N/ha applied in split applications 4 weeks apart in January and February. Seedlings were lifted and outplanted in March, 4 weeks after the second fertilization and measured at 3 and 6 months after outplanting. No seedling morphological differences were encountered at the time of lifting and outplanting although seedling shoot nitrogen content was 28% greater in the highest fertilization treatment compared to the check. Shoot nitrogen concentrations fell after outplanting regardless of treatment, decreasing from an average of 1.51% across all treatments at the time of planting to 0.64% at 6 months after planting. When measured at 6 months after outplanting, seedling dry weight and height growth after planting was shown to increase by 12% and 24%, respectively, for the high nitrogen treatment. This and other studies across a variety of sites have found positive post-outplanting seedling growth response after nutrient loading in the nursery.     

Post-Planting Treatments Increase Growth of Oregon White Oak (Quercus garryana Dougl. ex Hook.) Seedlings

The extent of Oregon white oak woodland and savanna ecosystems in the U.S. Pacific Northwest has diminished significantly during the past century due to land use changes and fire suppression. Planting Oregon white oak seedlings is often necessary when restoring these plant communities. Our objective was to determine the efficacy of post‐planting treatments for establishing Oregon white oak seedlings on sites characterized by low growing season precipitation and coarse‐textured soils. We evaluated the effects of control of competing vegetation, tree shelters, fertilization, irrigation, and planting date on growth of planted seedlings. Survival was generally high (90%), but growth rate varied substantially among treatments. Plastic mulch increased soil water content and increased annual seedling height growth by an average of 56% relative to one‐time manual removal of competing vegetation. Solid‐walled tree shelters reduced browse damage and increased mean annual height growth compared to mesh shelters and no shelter by averages of 7.5 and 10.9 cm, respectively. Controlled‐release fertilizer applied at planting did not consistently increase seedling growth. Weekly irrigation (3.8 L/seedling) increased first‐year seedling growth only where mulch also was applied. Seedlings planted by late February had greater root growth by summer than those planted in early April. Soil water management was necessary for best seedling growth, and the improved height growth in solid‐walled tree shelters allowed the terminal shoot to grow more quickly above the height of animal browse. Our results indicate effective methods for establishing Oregon white oak seedlings, but these results may also be applicable to establishment of other tree species on similarly droughty sites.     

EFFECTS OF SOIL MOISTURE CONTENT ON DRY NURSERY SEEDLING QUALITY OF LATE DOUBLE CROPPING RICE IN SOUTH CHINA

In the present paper, an experiment was conducted to study the effects of soil moisture content on dry nursery seedling quality in Guangzhou in 1995. Through comparing the difference of dry nursery seedlings and wet nursery seedlings, we found a close relationship between soil moisture content and seedling growth. The seedling emergence of dry nursery seedling was more even, tidy and faster, and the survival rate was higher than that of wet nursery seedling. Dry nursery seedlings had small plant stature, slow leaf stretching speed and low individual seedling dry weight, but had high dry/fresh weight ratio. This was abeneficial factor for seedlings to recover from transplanting shock more quickly. As com-pared with the wet nursery seedlings, dry nursery seedlings had poor rooting ability,but had more vigorous white roots and fewer rust roots. It was the possibly important reasonfor dry nursery seedlings to form strong“explosive force”.     

Mycorrhizal networks affect ectomycorrhizal fungal community similarity between conspecific trees and seedlings

Ectomycorrhizal (EM) networks (MN) are thought to be an important mode of EM fungal colonization of coniferous seedlings. How MNs affect EM communities on seedlings, and how this varies with biotic and abiotic factors, is integral to understanding their importance in seedling establishment. We examined EM fungal community similarity between mature trees and conspecific interior Douglas-fir (Pseudotsuga menziesii var. glauca) seedlings in two experiments where seed and nursery-grown seedlings originating from different locations were planted at various distances from trees along a climatic gradient. At harvest, trees shared 60% of their fungal taxa in common with outplanted seedlings and 77% with germinants, indicating potential for seedlings to join the network of residual trees. In both experiments, community similarity between trees and seedlings increased with drought. However, community similarity was lower among nursery seedlings growing at 2.5 m from trees when they were able to form an MN, suggesting MNs reduced seedling EM fungal richness. For field germinants, MNs resulted in lower community similarity in the driest climates. Distance from trees affected community similarity of nursery seedlings to trees, but there was no interaction of provenance with MNs in their effect on similarity in either nursery seedlings or field germinants as hypothesized. We conclude that MNs of trees influence EM colonization patterns of seedlings, and the strength of these effects increases with climatic drought.     

Love of Nurse Plants is Not Enough for Restoring Oak Forests in a Seasonally Dry Tropical Environment

The highest concentration of oak species in the world occurs in Mexico, but human activities have strongly degraded these oak forests. Mexican oaks have high economic, social, and cultural value, and restoring these forests is of paramount importance for the people of Mexico. Here, we propose a method for restoring oak forests using native shrubs that colonize degraded areas as nurse plants for oak seedlings. To test the viability of this proposal, seedling transplant experiments were performed in a degraded area near a protected oak forest relict. Two pioneer shrubs were identified as potential nurse species: Mimosa luisana and Senecio sp. The target oak species was Quercus castanea . Oak seedlings were located beneath the canopies of both shrubs and in the surrounding area without shrub cover. Water is a limiting resource for oak establishment in seasonally dry environments; therefore, we included irrigation systems in our experimental design to determine whether the combination of nurse plants plus watering led to higher rate of survival than the presence of nurse species alone. Seedling survival without watering was less than 20% both beneath nurse species and in the surrounding habitat. When water was supplied, survival rate beneath nurse species increased up to 58% while survival in the surrounding habitat did not differ from that observed in treatments without watering. Our results indicate that survival rate of oak seedlings is increased by the presence of nurse plants only when water is supplied. This suggests that restoration of oak forests in these degraded areas requires both nurse plants and watering.     

Seedling vigour and the early growth of transplanted rice (Oryza sativa)

Previous studies suggest that the positive response of transplanted rice (Oryza sativa L.) to nursery fertiliser application was due to increased seedling vigour or possibly to increased nutrient content. This paper presents results of two glasshouse experiments designed to test the hypothesis that seedling vigour was responsible for the response of transplanted seedlings to nursery treatments. The aim of the present study was to explore the concept of seedling vigour of transplanted rice and to determine what plant attributes conferred vigour on the seedlings. Seedling vigour treatments were established by subjecting seedlings to short-term submergence (0, 1 and 2 days/week) in one experiment and to leaf clipping or root pruning and water stress in another to determine their effect on plant growth after transplanting. Submerging seedlings increased plant height but depressed shoot and root dry matter and root:shoot ratio of the seedling at 28 days after sowing. After transplanting these seedlings, prior submergence depressed shoot dry matter at 40 days. Nursery nutrient application increased plant height, increased root and shoot dry matter, but generally decreased root:shoot ratio. Pruning up to 60% of the roots at transplanting decreased shoot and root dry matter, P concentration in leaves at panicle initiation (PI) and straw dry matter and grain yield at maturity. By contrast, pruning 30% of leaves depressed shoot and root dry matter by 30% at PI, and root dry matter and straw and grain yield by 20% at maturity. The combined effects of leaf clipping and root pruning on shoot, root and straw dry matter were largely additive. It is concluded that the response of rice yield to nursery treatments is largely due to increased seedling vigour and can be effected by a range of nutritional as well as non-nutritional treatments of seedlings that increase seedling dry matter, nutrient content, and nutrient concentration. Impairment of leaf growth and to a lesser extent root growth in the nursery depressed seedling vigour after transplanting. However, rather than increasing stress tolerance, seedling vigour was more beneficial when post transplant growth was not limited by nutrient or water stresses.     

Growth and mycorrhizal community structure of Pinus sylvestris seedlings following the addition of forest litter

We report the effects of pine and oak litter on species composition and diversity of mycorrhizal fungi colonizing 2-year-old Pinus sylvestris L. seedlings grown in a bare-root nursery in Lithuania. A layer of pine or oak litter was placed on the surface of the nursery bed soil to mimic natural litter cover. Oak litter amendment appeared to be most favorable for seedling survival, with a 73% survival rate, in contrast to the untreated mineral bed soil (44%). The concentrations of total N, P, K, Ca, and Mg were higher in oak growth medium than in pine growth medium. Relative to the control (pH 6.1), the pH was lower in pine growth medium (5.8) and higher in oak growth medium (6.3). There were also twofold and threefold increases in the C content of growth medium with the addition of pine and oak litter, respectively. Among seven mycorrhizal morphotypes, eight different mycorrhizal taxa were identified: Suillus luteus, Suillus variegatus, Wilcoxina mikolae, a Tuber sp., a Tomentella sp., Cenococcum geophilum, Amphinema byssoides, and one unidentified ectomycorrhizal symbiont. Forest litter addition affected the relative abundance of mycorrhizal symbionts more than their overall representation. This was more pronounced for pine litter than for oak litter, with 40% and 25% increases in the abundance of suilloid mycorrhizae, respectively. Our findings provide preliminary evidence that changes in the supply of organic matter through litter manipulation may have far-reaching effects on the chemistry of soil, thus influencing the growth and survival of Scots pine seedlings and their mycorrhizal communities.     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

Competitive suppression of Quercus douglasii (Fagaceae) seedling emergence and growth

Reduced recruitment of blue oak (Quercus douglasii) seedlings in California grasslands and woodlands may result from shifts in seasonal soil water availability coincident with replacement of the native perennial herbaceous community by Mediterranean annuals. We used a combination of container and field experiments to examine the interrelationships between soil water potential, herbaceous neighborhood composition, and blue oak seedling shoot emergence and growth. Neighborhoods of exotic annuals depleted soil moisture more rapidly than neighborhoods of a perennial grass or "no-neighbor" controls. Although effects of neighborhood composition on oak seedling root elongation were not statistically significant, seedling shoot emergence was significantly inhibited in the annual neighborhoods where soil water was rapidly depleted. Seedling water status directly reflected soil water potential, which also determined the extent and duration of oak seedling growth during the first year. End-of-season seedling height significantly influenced survival and growth in subsequent years. While growth and survival of blue oak seedlings may be initially constrained by competition with herbaceous species, subsequent competition with adult blue oak trees may further contribute to reduced sapling recruitment.     

Effects of rodent gnawing on the survival of current-year seedlings of Quercus crispula

Emergence, survival and mortality patterns of current-year oak (Quercus crispula Blume) seedlings were investigated for 4 consecutive years in a secondary oak forest in Hokkaido, northern Japan. Despite the emergence of a considerable number of oak seedlings in the years following masting, few current-year seedlings survived until the end of the growing season. Almost all of the seedlings died from damage to their stems caused by the gnawing of rodents. Rodent gnawing on transplanted oak seedlings was also observed in the year following masting but not in the year following a bad crop year. Cuttings of dwarf bamboo, Sasa, did not reduce the seedling mortality caused by gnawing. However, transplanted oak seedlings were gnawed more quickly when they were placed on the forest floor with a thicker Sasa covering. All rodents trapped in the vicinity of the study area were Apodemus speciosus Temminck. These results suggest that rodents strongly influence the recruitment of oak trees not only through the predation and dispersal of acorns but also through gnawing seedlings.     

Feedbacks between canopy composition and seedling regeneration in mixed conifer broad‐leaved forests

To address the role of canopy‐seedling feedbacks in the structure and dynamics of mixed conifer broad‐leaved forests in the eastern US, we monitored seedling regeneration patterns and environmental conditions in the understorey of stands dominated by either hemlock (Tsuga canadensis) or red oak (Quercus rubra) for three years. Hemlock seedlings were favoured over other species’ seedlings in hemlock stands (a true positive feedback), due to a combination of high seed inputs, high seedling emergence and relatively high seedling survival during the growing season, which allowed hemlock to remain dominant under its own canopy. Red oak stands favoured a suite of mid‐successional broad‐leaved species over hemlock. A more even age structure of broad‐leaved species in red oak stands revealed that high seedling survival in such stands were driving this feedback. Canopy‐mediated variations in both understorey light availability (1.5% for hemlock vs 3.5% for red oak) and soil pH (3.9 for hemlock vs 4.4 for red oak) were found to be the primary correlates of stand‐level differences in seedling regeneration dynamics. In mixed temperate forests in the eastern US, canopy‐seedling feedbacks could act to slow successional trajectories and contribute to the maintenance of a stable landscape structure over many generations.     

An Afforestation System for Restoring Bottomland Hardwood Forests: Biomass Accumulation of Nuttall Oak Seedlings Interplanted Beneath Eastern Cottonwood

Bottomland hardwood forests of the southeastern United States have declined in extent since European settlement. Forest restoration activities over the past decade, however, have driven recent changes in land use through an intensified afforestation effort on former agricultural land. This intense afforestation effort, particularly in the Lower Mississippi Alluvial Valley, has generated a demand for alternative afforestation systems that accommodate various landowner objectives through restoration of sustainable forests. We are currently studying an afforestation system that involves initial establishment of the rapidly growing native species eastern cottonwood (Populus deltoides Bartr. ex Marsh.), followed by enrichment of the plantation understory with Nuttall oak (Quercus nuttallii Palm.). In this article, we examine the growth and biomass accumulation by Nuttall oak seedlings to determine whether this species can be established and whether it will develop beneath the cottonwood overstory. After 3 years of growth beneath cottonwood canopies, Nuttall oak seedlings were similar in height (126 cm), but were 20% smaller in root‐collar diameter than seedlings established in open fields. Seedlings established in the open accumulated more than twice the biomass of seedlings growing beneath a cottonwood canopy. However, the relative distribution of accumulated biomass in seedlings did not differ in the two environments. Ten percent of total seedling biomass was maintained in leaf tissue, 42% was maintained in stem tissue, and 48% was maintained in root tissue on open‐grown seedlings and seedlings established in the understory of cottonwood plantations. Though establishment in the more shaded understory environment reduced Nuttall oak growth, seedling function was not limited enough to induce changes in plant morphology. Our results suggest that an afforestation system involving rapid establishment of forest cover with a quick‐growing plantation species, followed by understory enrichment with species of later succession, may provide an alternative method of forest restoration on bottomland hardwood sites and perhaps other sites degraded by agriculture throughout temperate regions.     

Survival and growth of konara oak (Quercus serrata Thunb.) seedlings in an abandoned coppice forest

Survival and growth of konara oak (Quercus serrata) seedlings were examined on the forest floor of a konara oak coppice stand in Mitaka, Tokyo, for 5 yr (1976–1980) after exclusion of human management. Attention was mainly focused on a large cohort derived from a bumper acorn crop in 1975. Seedlings began to branch in the second year, although some did not branch during the whole 5-yr period. Branched seedling showed higher survival and greater weight in 1980 than unbranched seedlings. Many of the branched seedlings had lost an orthotropic leader shoot and spread their branches laterally, suggesting a response for efficient light-capture. As for unbranched seedlings, a size relationship between leaf area and stem length developing every year was dependent on the shoot size. A small shoot gave priority to enlargement of leaf area, whereas a large shoot gave priority to stem elongation. Yearly variations in seedling mortality and stem elongation were related to the amount of precipitation. Both the initial height and leaf area of germinated seedlings were positively correlated with their survival and weight after five years. Five-year survival of seedlings showed a positive correlation with light intensity at the growth site but a negative correlation with the extent of leaf predation. Sprouts of shrub species grew vigorously and became much taller than the oak seedlings in the 5-yr observation period. The characteristics of forest-floor seedlings were discussed in relation to the regeneration of konara oak forests in general.     

Tree shelters facilitate brown oak seedling survival and establishment in a grazing‐dominant forest of Bhutan,Eastern Himalaya

Brown oak (Quercus semecarpifolia) forest is essential for ecological and socioeconomic functions, mainly grazing in the Himalayas. The tree has failed to regenerate naturally and is a threatened species. Restoration of brown oaks is crucial to ensure sustainability while maintaining livestock grazing in these habitats. Achieving this requires cost‐effective restoration techniques that are practicable and sympathetic to the multiple uses of the forest. We assessed the combined effect of grazing (control) and three tree shelters (Protex tubes, mesh wires, and wooden frames) on the field performance of oak seedlings in a forest with heavy grazing pressure. Seedling survival and morphological indicators, including seedling height, collar diameter, sturdiness quotient (SQ), and leaf mass per area (LMA) indices, were measured. More than 90% of control seedlings without protective shelters suffered severe browsing and demonstrated significantly lower survival rates compared to tree shelter seedlings, indicating that grazing was the primary factor governing regeneration success. Seedling survival in tree shelters was three times higher, while the height increase was two times higher than the control. Additionally, locally made mesh wire and wooden tree shelters were more effective than Protex and control in producing quality seedlings reflected by the SQ and LMA values. We suggest that tree shelter is a promising option to restore brown oaks due to its efficacy to defend grazing and support the local community's rights to forest grazing. Our finding is expected to support Bhutan's forest policy of incorporating grazing and tree regeneration into forest management.     

Effects of Unseeded Areas on Species Richness of Coal Mines Reclaimed with Municipal Biosolids

Land application of municipal biosolids on coal mine spoils can benefit vegetation establishment in mine reclamation. However, the application of biosolids leads to domination by early‐successional species, such as grasses, and low establishment of woody and volunteer species, thus reducing potential for forestry as a postmining land use. In this experiment, tree seedlings were planted in strips (0.6‐, 1‐, and 4‐m wide) that were not seeded with grasses, and the effects of unseeded strip width on seedling growth and species richness were assessed. Planted seedling mortality was high; therefore, the effect of unseeded strip width on seedling growth could not be determined. However, it was found that natural plant invasion and species richness were highest in the 4‐m unseeded strips. The practice of leaving 4‐m‐wide unseeded strips in mine reclamation with biosolids in the eastern United States, along with the improvement of tree seedling planting practices and planting stock, would help promote a more species‐rich plant community that could be utilized for forestry or a variety of other postmining land uses.     

Growth and nitrogen nutrition of Chinese fir seedlings exposed to nutrient loading and fertilization

Nutrient loaded and non-loaded Chinese fir (Cunninghamia lanceolata (Lamb) Hook) seedlings were transplanted in a pot trial to examine effects of exponential nutrient loading and fertilization treatments on first season growth and N nutrition. The treatments tested four rates of N (0, 30, 60, and 90 mg tree^-1) as a mixed NPK fertilizer applied before planting to create a soil fertility gradient, and two topdressings applied only to non-loaded seedlings later in the season. Nutrient loading alone consistently enhanced seedling growth on the four soil fertility classes, increasing respective biomass and N uptake 42, 45, 20 and 8%, and 65, 67, 29 and 18% more than non-loaded seedlings. The positive response was attributed to increased N retranslocation from higher nutrient reserves built up by loading during nursery culture. Net retranslocation from old shoots to new growth was highest soon after planting when nutrient stress was most severe. Pre-plant soil fertilization and post-plant topdressings were also effective in promoting seedling productivity, but equivalent additions yielded less biomass than that from nutrient loading alone. Implications are that exponential nutrient loading may be more efficient in improving early growth performance of Chinese fir seedlings than traditional field fertilization practices at plantation establishment, and may on competitive sites avoid problems of stimulating surrounding vegetation rather than trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of surrounding vegetation on insect herbivory: A matter of spatial scale and herbivore specialisation

The diversity of surrounding vegetation is thought to modify the interactions between a focal plant and its herbivores, disrupting (associational resistance) or enhancing (associational susceptibility) host plant location and colonisation. We compared the effects of host plant concentration on herbivory by generalist and specialist insects feeding on oak seedlings by increasing local concentration of seedlings. We also assessed the effects of the composition and structure of surrounding vegetation, both at stand and local levels. The damage caused by generalist leaf-feeding insects depended on the structure of plant communities at stand level, and increased with tree cover. By contrast, infestation by specialist leaf miners was affected by local understorey vegetation surrounding oak seedlings, and decreased with increasing shrub cover and stratification diversity. Leaf mine abundance was higher at higher oak seedling density, giving support to the host concentration hypothesis. However, the abundance of these specialist herbivores was also negatively correlated with damage caused by the generalist external leaf-feeders, suggesting competitive interactions.     

Autumn fertilization with nitrogen improves nutritional status,cold hardiness and the oxidative stress response of Holm oak (<Emphasis Type="Italic">Quercus ilex</Emphasis> ssp. <Emphasis Type="Italic">ballota</Emphasis> [Desf.] Samp) nursery seedlings

Holm oak (Quercus ilex ssp. ballota [Desf.] Samp) is one of the most important species in forest communities of the western Mediterranean Basin, but is very vulnerable to environmental stress during the first years of its life. In particular, summer drought and winter frosts limit the distribution, survival, and growth of Holm oak. These two stress factors can lead to plasmolysis and/or oxidative stress. We hypothesized that autumn fertilization with nitrogen (N) can improve plant tolerance to these abiotic stress factors and improve plant quality and therefore reforestation success. A 12-week autumn application of 30 or 60 mg N (plus 70 mg N applied to both treatments during the previous 28 weeks in the nursery, i.e. 100 and 130 mg N in total, respectively) improved overall growth, root growth capacity, frost tolerance, and nutritional status of plants relative to plants given 0.0 and 1.5 mg autumnal N, and had no negative effect on seedling response to water stress. A very small increment in N doses during the autumn (1.5 vs. 0.0 mg N) improved some morphological parameters, such as stem diameter (D) and shoot dry weight, and physiological parameters, such as total antioxidant activity. The highest autumnal N dose (60 mg) increased leaf nutrient content without decreasing the concentration, but had a tendency to reduce frost tolerance relative to seedlings given a lower autumnal N dose (30 mg).