Sap‐feeding insects on forest trees along latitudinal gradients in northern Europe: a climate‐driven patterns

Knowledge of the latitudinal patterns in biotic interactions, and especially in herbivory, is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. We used sap‐feeding insects as a model group to test the hypotheses that the strength of plant–herbivore interactions in boreal forests decreases with latitude and that this latitudinal pattern is driven primarily by midsummer temperatures. We used a replicated sampling design and quantitatively collected and identified all sap‐feeding insects from four species of forest trees along five latitudinal gradients (750–1300 km in length, ten sites in each gradient) in northern Europe (59 to 70°N and 10 to 60°E) during 2008–2011. Similar decreases in diversity of sap‐feeding insects with latitude were observed in all gradients during all study years. The sap‐feeder load (i.e. insect biomass per unit of foliar biomass) decreased with latitude in typical summers, but increased in an exceptionally hot summer and was independent of latitude during a warm summer. Analysis of combined data from all sites and years revealed dome‐shaped relationships between the loads of sap‐feeders and midsummer temperatures, peaking at 17 °C in Picea abies, at 19.5 °C in Pinus sylvestris and Betula pubescens and at 22 °C in B. pendula. From these relationships, we predict that the losses of forest trees to sap‐feeders will increase by 0–45% of the current level in southern boreal forests and by 65–210% in subarctic forests with a 1 °C increase in summer temperatures. The observed relationships between temperatures and the loads of sap‐feeders differ between the coniferous and deciduous tree species. We conclude that climate warming will not only increase plant losses to sap‐feeding insects, especially in subarctic forests, but can also alter plant‐plant interactions, thereby affecting both the productivity and the structure of future forest ecosystems.     

Contrasting acclimation abilities of two dominant boreal conifers to elevated CO2 and temperature

High latitude forests will experience large changes in temperature and CO₂ concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO₂ (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO₂ assimilation rates (A_net), and acclimated dark respiration (R_dark). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of A_net decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of R_dark was weak. Elevated CO₂ tended to exacerbate these effects in warm‐grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO₂ treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism.     

Influence of insecticide, wax and biofungicide treatments, applied to Pinus sylvestris and Picea abies, on the olfactory orientation of the large pine weevil, Hylobius abietis L. (Coleoptera: Curculionidae)

1 A four-arm olfactometer was used to test the effect of permethrin ‘Gori 920^®’ and the wax treatment ‘CereNat^® E30’ on the response of mature adult Hylobius abietis to olfactory stimuli from seedlings of Pinus sylvestris and Picea abies. 2 Also investigated in the olfactometer was the effect of Rotstop^®, a biofungicide for the control of Fomes root and butt rot, Heterobasidion annosum, on the responses of mature adult female H. abietis to stump material of both P. sylvestris and P. abies. 3 Hylobius abietis was significantly more attracted to untreated P. sylvestris seedlings compared to those treated with CereNat^® E30 or permethrin Gori 920^®. No differences in response to P. abies seedlings were found. 4 With Rotstop^®, no changes in the attractiveness of P. sylvestris stump material were found, but that of P. abies was significantly increased compared to untreated material.     

Feeding and oviposition preferences of Monochamus galloprovincialis for certain conifers under laboratory conditions

Feeding and oviposition preferences of Monochamus galloprovincialis (Olivier) (Coleoptera: Cerambycidae: Monochamini), the vector of the pine wilt nematode (PWN), Bursaphelenchus xylophilus (Steiner and Bührer) Nickle (Nematoda: Aphelenchoididae), in Portugal, was studied in three laboratory experiments: an adult feeding preference trial between branches of five pines, a no‐choice oviposition experiment on seven conifer bolts, and an oviposition choice trial between maritime pine, Pinus pinaster Aiton, and four other pines, P. pinea L., P. sylvestris L., P. halepensis Miller, or P. radiata D. Don. Scots pine (P. sylvestris) was the pine with the largest bark feeding area, while P. radiata was the least chosen to feed upon. Female M. galloprovincialis laid eggs on P. sylvestris, P. halepensis, P. pinaster, P. radiata, P. pinea, and Pseudotsuga menziesii (Mirbel) Franco, but larvae successfully completed development only on the first four pines. Beetles emerging from maritime pine bolts were slightly bigger than the others. No preference for oviposition was detected when beetles were given the choice between P. pinaster and P. sylvestris or P. halepensis. Inversely, greater oviposition occurred on P. pinaster when compared with P. pinea and P. radiata. Although PWN currently affects only P. pinaster in Portugal, the results on host suitability for M. galloprovincialis suggest that P. sylvestris and P. halepensis are adequate hosts for the insect's feeding and oviposition, and thus may also be PWN hosts in Europe. The systematically high feeding and oviposition attraction for P. sylvestris exhibited by the beetles means that the eventual spread of the PWN to areas where both M. galloprovincialis and P. sylvestris occur may have a significant impact on this widespread and economically important pine, considered to be highly susceptible to B. xylophilus.     

Relationship between respiratory depletion of sugars and loss of cold hardiness in coniferous seedlings over-wintering at raised temperatures: indications of different sensitivities of spruce and pine

Long-term effects of elevated winter temperatures on cold hardiness were investigated for Norway spruce (Picea abies L. Karst.), lodgepole pine (Pinus contorta Dougl.) and Scots pine (Pinus sylvestris L.). Two-year-old seedlings with the same pre-history of growth and cold hardening in the field were maintained from early December to late March at two field sites in northern Sweden and in a cold room. The temperatures at these locations averaged –13·5, –8·9 and 5·5°C, respectively. Following treatments, carbohydrate contents and cold tolerances were assessed. Needle respiration was also analysed during the 5·5°C treatment. Cold tolerance of lodgepole pine and Scots pine was much reduced following the 5·5°C treatment. Cold tolerance was somewhat reduced in lodgepole pine following the –8·9 °C treatment, but was essentially maintained in spruce throughout all treatments. The cold tolerance of needles was strongly correlated with their soluble sugar contents. Spruce maintained cold hardiness by having larger reserves of sugars and lower rates of respiration which decreased more rapidly as sugars were depleted. Tolerance of lodgepole pine to frost desiccation was also much reduced following the 5·5°C treatment.     

Interrelations between respiration and dormancy in buds of three hardwood species with different chilling requirements for dormancy release

 Respiration in vegetative buds of mature Betula pendula, Alnus glutinosa and Prunus padus trees was measured monthly at 15°C from mid-October 1996 to natural outdoor budburst in April 1997. In B. pendula the effect of bud water content on respiration was also estimated (December–April) by artificial imbibition of buds for 24 h prior to measurement of respiration. For estimation of corresponding bud dormancy status, batches of twigs were forced at identical monthly intervals at 15°C in long days (24 h), and budburst recorded. In all species dormancy was deepest when the leaves were shed in October, and dormancy was first alleviated in P. padus followed by B. pendula and A. glutinosa. However, bud respiration capacity was not related to dormancy release as it decreased in all species from October to November and displayed no notable increase until February in P. padus, March in B. pendula and April in A. glutinosa, after completion of dormancy release. Rather, increase in respiration coincided with growth resumption prior to budburst. Artificial imbibition of B. pendula buds increased the water content by approximately 10% (FW) and induced a doubling of the respiration rate (December–February). Moreover, the seasonal variation in bud water content (October–April) explained 94% of the variation in respiration in B. pendula and P. padus, and 84% in A. glutinosa. These observations suggest an important role of water content for respiration. During a cold period from mid-December to mid-January with mean temperature of –9.7°C dormancy release was arrested in P. padus, and to some degree in A. glutinosa, whereas dormancy release progressed normally in B. pendula. This indicates species differences in lower critical temperatures for dormancy release. Received: 30 June 1997 / Acceped: 1 October 1997     

Formation and growth of the ectomycorrhiza of Cantharellus cibarius

New data on the physiology of Cantharellus cibarius mycorrhiza formation has resulted in a new aseptic routine method for in vitro formation. The advances are short formation time, healthy plants and reliable colonization. A high glucose demand and a good gas exchange with additional carbon dioxide are important factors in the mycorrhiza formation. Mycorrhiza was observed after 8 weeks, but strong colonization occurred after 10–12 weeks, when mycorrhiza was established to the depth of 5 cm. A C. cibarius strain connected to Picea abies in nature successfully colonized Pinus sylvestris in vitro, but not Betula pendula. Mycorrhizal plants have been successfully transferred to unsterile environments in greenhouses. The mycorrhizae continued to colonize new roots and the unsterile peat soil for 10 months. However, C. cibarius mycorrhiza is highly sensitive to flooding. With PCR and RFLP, fruit bodies, isolated mycelia and artificially formed mycorrhizae have been compared to prove that C. cibarius was used. Climatic changes did not induce primordia formation but factors behind fruit body formation are discussed.     

Woody plants of Yakutia and low-temperature stress

Physiological and biochemical features of woody plants (Pinus sylvestris L. and Betula platyphylla Sukacz.) during transition from vegetative to frost-resistant state under conditions of extremely severe climate of Yakutia were studied. In P. sylvestris such transition was accompanied by a decrease in the content of chlorophylls long before first frosts and by an increase in the proportion of Xanth and simultaneous decrease in the content of β-carotene in needles during the first and second phases of hardening. In the period of preparation to dormancy, overwintering organs of both P. sylvestris (needles) and B. platyphylla (buds) accumulated the two groups of major dehydrins, with low mol wts of 15–21 kD and middle mol wts of 66–141 kD. Simultaneously, low temperature led to a great increase in the content of polyunsaturated fatty acids (FAs) in lipids of P. sylvestris needles and B. platyphylla buds, primarily linoleic acid and also eicosenoic FAs differing in the extent of desaturation. Observed qualitative and quantitative changes in pigments, total proteins, dehydrins, and FAs during autumn hardening of P. sylvestris and B. platyphylla plants presume their important role in the development of resistance of these tree species to low-temperature stress (down to −60°C) in the cryolithic zone of Yakutia.     

Stem-growth response of Pinus sylvestris and Picea abies to nitrogen fertilization as related to needle nitrogen concentration

 Responses of stem-volume growth to N application were evaluated in relation to foliar N concentrations. Data from N-fertilization experiments in 28 Pinus sylvestris stands and 21 Picea abies stands were used. Relative stem-growth responses were negatively related to concentrations of N in current-year needles of unfertilized trees. There appeared to be a threshold value of 15–16 mg (g DM)^–1 N in current-year needles, above which N-application is unlikely to stimulate growth. However, relations were non-significant between N concentrations in current-year needles and the absolute stem-growth response [dm³ ha^–1 (5 years)^–1]. The indicated threshold values are discussed in relation to other variables reflecting the N richness of the environment.--> Received: 20 December 1996 / Accepted: 30 September 1997     

The growth and nutrients status of conifers on ash-treated cutaway peatland

Key message

Use of wood ash or a mixture of wood and oil shale ashes increases the concentrations of P and K in the assimilation organs of conifers and stimulates tree growth.

Abstract

The effect of fertilization with wood ash (10 and 15 t ha^?1) and a mixture of wood ash (10 t ha^?1) and oil shale ash (8 t ha^?1) on the growth (height, root collar diameter, biomass, biomass production) and nutrient concentrations in subsoil and needles of young Pinus sylvestris and Picea abies plants on the Puhatu (Northeast Estonia) cutaway peatland in the first 2 years were studied. After the second growing year differences in the average height growth of P. abies and P. sylvestris were statistically significantly higher on ash-treated plots than on the control plots (p < 0.05), being respectively 1.4–1.6 and 1.5–1.7 times greater than height growth of the control trees. The best results on root collar diameter were observed on mixture ash treatments: the root collars were 1.9 (P. abies) and 2.2 (P. sylvestris) times larger than of the control trees. The biomass of the two conifer species and the biomass production of P. sylvestris in 2012 was the greatest on the mixture ash treatments. Five months after fertilization with ashes the concentrations of P, K, Ca and Mg were higher on the treated plots than on the control plot. Although the concentrations of P and K in P. sylvestris needles rose after the treatment with ash, seedlings suffered from P and K deficiency. The concentrations of P and K in P. abies needles were on optimum. The P/N and the K/N ratios in needles were also improved compared to control trees needles.     

Effects of forest fire ash on germination and early growth of four pinus species

Fire modifies the germination of seeds of numerous species. One of the fire factors that cause these modifications is the ash. This study analysed the germination of seeds of Pinus sylvestris L., Pinus nigra Arn. Pinus radiata D. Don and Pinus pinaster Aiton, subjected to different ash treatments, and the development of seedlings that grew in these conditions for 14 weeks. We obtained the ash by completely burning leaves and small twigs from the most abundant woody species in populations of pines. The ash treatments applied were as follows: Control (without ash), Low (half of the amount registered in a fire), Medium (equal to the amount registered in a fire) and High (double the amount registered in a fire). Germination took place on paper inside Petri dishes and in soil, and growth was only analysed for the plants that grew in the soil. For all species, and especially P. nigra, germination rate decreased as the amount of ash applied increased. The negative effects of the ash were more apparent following treatments in Petri dishes than in the soil. In the dishes, the average germination time varies little between the four species of pine. In soil, the average germination time is more prolonged, and only some differences were observed between treatments in P. sylvestris and P. nigra. In these species the high treatment significantly increased the average germination time. We found a greater sensitivity of P. sylvestris and P. nigra to the addition of ash, showing a higher seedling mortality rate. However, it appears that seedling development (length and weight) was not affected in any of the species through the addition of ash. These tests allow us to deduce that, in P. sylvestris, P. nigra, P. radiata and P. pinaster, the ash produced by forest fires has an inhibiting effect on germination and little effect on the development of seedlings in the first months of life. This revised version was published online in June 2006 with corrections to the Cover Date.     

Litter decomposition rate is dependent on litter Mn concentrations

A statistically significant linear relationship was found between annual mass loss of foliar litter in the late stages of decomposition and Mn concentration in the litter. We used existing decomposition data on needle and leaf decomposition of Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta var. contorta), Norway spruce (Picea abies (L.) Karst.), silver birch (Betula pendula L.), and grey alder (Alnus incana L.) from Sweden and Aleppo pine (Pinus halepensis Mill.) from Libya, to represent boreal, temperate, and Mediterranean climates. The later the decomposition stage as indicated by higher sulfuric-acid lignin concentrations, the better were the linear relationships between litter mass loss and Mn concentrations. We conclude that Mn concentrations in litter have an influence on litter mass-loss rates in very late decomposition stages (up to 5 years), provided that the litter has high enough Mn concentration. The relationship may be dependent on species as the relationship is stronger with species that take up high enough amounts of Mn.     

Combination of the fungus Beauveria bassiana and pheromone in an attract‐and‐kill strategy against the banana weevil,Cosmopolites sordidus

An attract‐and‐kill approach based on pellets from soybean or palm stearin fats blended with the entomopathogenic fungus Beauveria bassiana (Bals.) Vuill. sensu lato and the aggregation pheromone sordidin (Cosmolure^®) was tested against the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae). The viability of B. bassiana conidia, blended with hydrogenated oil and exposed for up to 150 min to heating at 50 °C, was not affected and the aggregation pheromone did not undergo any decomposition. Conidial viability in pellets decreased by 50% after an average of 15.1 and 9.1 days at 25 and 40 °C, respectively, when packaged in polypropylene bags. Active packaging (hermetic bag + O₂/moisture‐absorbing sachet) increased the shelf lives almost 10 and 6 times at 25 and 40 °C, respectively. In olfactometer bioassays, fat pellets amended with pheromone (sordidin, 1% wt/vol) were highly attractive to C. sordidus adults for up to 15 days, after which the pheromone release rate had decreased by about 90% and pellets were no longer attractive. Pellets with pheromone and conidia were as attractive to C. sordidus as banana rhizomes, and considerably more attractive than pieces of pseudostem. In no‐choice experiments conducted in boxes, survival of insects exposed to fungus‐impregnated pellets was affected by fat type (soybean fat vs. palm stearin) and bioassay temperature (25 vs. 30 °C), with results favoring soybean fat pellets at the higher temperature (96.9% of mortality after 18 days and ST₅₀ of 7.7 days). However, mortality levels were low (21.7% for soybean fat pellets) or very low (1–5% for palm stearin pellets) in choice experiments carried out at 25 °C when fungus‐impregnated pellets were applied before or after exposure of pseudostem residues to insects, respectively. The potential of this delivery system to manage C. sordidus populations and other insect pests (including those with cryptic habits) is discussed.     

Impact of global warming on the tree species composition of boreal forests in Finland and effects on emissions of isoprenoids

This study aims to identify how climate change may influence total emissions of monoterpene and isoprene from boreal forest canopies. The whole of Finland is assumed to experience an annual mean temperature (T) increase of 4 °C and a precipitation increase of 10% by the year 2100. This will increase forest resources throughout the country. At the same time, the proportions of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in southern Finland (60°≤ latitude < 65°N) will be reduced from the current 40–50% to less than 10–20%, with increased dominance of birches (Betula pendula and Betula pubescens). In northern Finland (65°≤ latitude < 70°N), the proportions of Norway spruce and Scots pine will be balanced at a level of about 40% as the result of an increase in Norway spruce from the current 21% to 37% and a concurrent reduction in Scots pine from 63% to 40%. The proportion of birches is predicted to increase from the current 17% to 23%, but these will become the dominant species only on the most fertile sites. Total mean emissions of monoterpene by Scots pine will be reduced by 80% in southern Finland, but will increase by 62% in the north. Emissions from Norway spruce canopies will increase by 4% in the south but by 428% in the north, while those from birch canopies will increase by about 300% and 113%, respectively. Overall emissions of monoterpene over the whole country amount to about 950 kg km^?2 y^?1 under current temperature conditions and will increase by 17% to 1100 kg km^?2 y^?1 with elevated temperature and precipitation, mainly because of an increase at northern latitudes. Under current conditions, emissions of isoprene follow the spatial distribution of spruce canopies (the only isoprene‐emitting tree species that forms forests in Finland) with four times higher emissions in the south than in the north. The elevated temperature and the changes in the areal distribution of Norway spruce will result in increases in isoprene emissions of about 37% in southern Finland and 435% in northern Finland. Annual mean isoprene emissions from Norway spruce canopies over the whole country will increase by about 60% up to the year 2100.     

Effects of an enhanced ultraviolet-B irradiation on photosynthetic apparatus of several forest coniferous tree species from different locations

The effect of UV-B on the photosynthetic apparatus of coniferous trees: Picea abies (L.) Karst., Picea pungens (Engelm.), Pinus sylvestris (L.), Pinus cembra (L.) and Abies alba (Mill.) was investigated. Three and four-year-old plantlets coming from different latitudes, longitudes and altitudes were used. The experiment was carried out in greenhouse. Two doses of ultraviolet-B irradiation were applied: control=0, low dose=11.32 and high dose=22.64 kJ·m⁻²·d⁻¹ UV-B_BE (biologically effective irradiance of UV-B). Measurements of chlorophyll fluorescence, gas exchange, chlorophyll and flavonoids content were carried out. Response of forest trees to an increased UV-B radiation depends on species, location of place of pantalets collecting and UV-B dose. Pinus cembra, Picea abies and Pinus sylvestris from high altitude (1000 m a.s.l.) were less sensitive to UV-B than these from plain location. The altitude determined adaptation of forest coniferous trees to an enhanced UV-B radiation much more than the latitudinal gradient. Permanent discoloration was observed only on the young needles of the fir plantlets that were grown in light limiting conditions. Photosynthetic parameters were affected by the UV-B radiation. Both maximal and the steady state fluorescence of chlorophyll were reduced as a consequence of elevated UV-B in case of some species. The chlorophyll content was enhanced, increased or was not affected according to species and to locations. The flavonoids content in the needles increased with chlorophyll content at both UV-B treatments. An opposite trend was found in the control. The increased content of screening pigments in the needles of all the tested coniferous trees was detected. Picea abies and Picea pungens photosynthesis response curves to the light and to the intercellular CO₂ concentration did not change significantly under increased UV-B because of higher concentration in screening pigments in leaves. The increased concentration of flavonoids in forest litter may lead to changes in the biogeochemical cycle in the forest ecosystem.     

Antennal and behavioural responses of the spruce seed moth,Cydia strobilella,to floral volatiles of Norway spruce,Picea abies,and temporal variation in emission of active compounds

We investigated whether spruce seed moth, Cydia strobilella L. (Lepidoptera: Tortricidae, Grapholitini), one of the most damaging seed predators on Norway spruce, Picea abies (L.) H. Karst (Pinaceae), uses olfactory cues during host search. Analyses with coupled gas chromatography and electroantennography revealed that antennae of both sexes of moths responded consistently to three compounds in the headspace from female spruce flowers, i.e., α‐pinene, β‐pinene, and myrcene, but not to limonene as has been previously reported for this species. The amounts of these active volatiles released from flowers and cones of P. abies were quantified, and their diurnal and seasonal variation was monitored. The total release of the active volatiles correlated well with the diurnal and seasonal flight activity of C. strobilella as revealed by catches of males in pheromone‐baited traps. In field trapping experiments, where baits were loaded with proportions and enantiomeric ratios of α‐pinene, β‐pinene, and myrcene matching those of the female P. abies floral headspace, substantial catches of male C. strobilella were achieved, whereas few females were captured. These surprising results suggest that male C. strobilella make use of host volatiles to aid them in their search for females.     

Belowground interspecific competition in mixed boreal forests: fine root and ectomycorrhiza characteristics along stand developmental stage and soil fertility gradients

We studied fine roots and ectomycorrhizas in relation to aboveground tree and stand characteristics in five mixed Betula pendula Roth, Picea abies (L.) H. Karst., and Pinus sylvestris L. stands in Southern Finland. The stands formed gradients of developmental stage (15-, 30-, and 50-year-old stands) in the stands of medium fertility, and of site fertility in the young stands (30-year-old fertile, medium fertile, and least fertile stands). The biomass of the external hyphae of ectomycorrhizas (ECM) was the highest, and the diversity of the fungal community the lowest, in the most fertile stand. The vertical distributions of fine roots of the three tree species were mostly overlapping, indicating high inter-specific belowground competition in the stands. We did not find any clear trends in the fine root biomass (FRB) or length across the stand developmental stages. The FRB of the conifers varied with site fertility, whereas in B. pendula it was almost constant. In contrast to the conifers, the specific root length (SRL) of B. pendula clearly increased from the most fertile to the least fertile stand. This indicates differences in the primary nutrient acquisition strategy between conifers and B. pendula.     

Variation in needle-trace diameter in respect of needle morphology in five conifer species

Needle traces have been used for revealing historical changes in needle longevity of Pinus sylvestris L. and Picea abies (L.) H. Karst. In this paper we compared the needle-trace diameter of five conifer species [Abies sibirica Ledeb., Picea abies, Pinus sylvestris, Pseudotsuga menziesii (Mirb.) Franco and Taxus baccata L.] in respect of needle size. In case of P. sylvestris, we studied how needle-trace diameter varied among different growing sites and among different shoots within a site, and related that variation to needle morphology and structure. Pinus sylvestris had the greatest diameter of needle traces (mean±SD =233±30 m) followed by T. baccata (141±19 m) and P. menziesii (121±30 m). The smallest needle-trace diameter was in A. sibirica (85±21 m) and P. abies (80±18 m). Among species, the needle-trace diameter was strongly correlated with needle length (r =0.93, P <0.05). Within P. sylvestris, the needle-trace diameter was significantly affected by location (P <0.001) and varied significantly among shoots within a location (P <0.001). The shoots of P. sylvestris which had greater average diameter of needle traces, tended to have longer (r =0.36, P <0.05), wider (r =0.55, P <0.05) and heavier (r =0.64, P <0.05) needles with greater stomatal density (r =0.55, P <0.05). Although our results do not allow the use of needle-trace diameter to predict properties of needle structure at the present stage, they nevertheless indicate general trends and point to a need for further studies.     

The charcoal effect in Boreal forests: mechanisms and ecological consequences

Wildfire is the principal disturbance regime in northern Boreal forests, where it has important rejuvenating effects on soil properties and encourages tree seedling regeneration and growth. One possible agent of this rejuvenation is fire-produced charcoal, which adsorbs secondary metabolites such as humus phenolics produced by ericaceous vegetation in the absence of fire, which retard nutrient cycling and tree seedling growth. We investigated short-term ecological effects of charcoal on the Boreal forest plant-soil system in a glasshouse experiment by planting seedlings of Betula pendula and Pinus sylvestris in each of three humus substrates with and without charcoal, and with and without phenol-rich Vaccinium myrtillus litter. These three substrates were from: (1) a high-productivity site with herbaceous ground vegetation; (2) a site of intermediate productivity dominated by ericaceous ground vegetation; and (3) an unproductive site dominated by Cladina spp. Growth of B. pendula was stimulated by charcoal addition and retarded by litter addition in the ericaceous substrate (but not in the other two), presumably because of the high levels of phenolics present in that substrate. Growth of P. sylvestris, which was less sensitive to substrate origin than was B. pendula, was unresponsive to charcoal. Charcoal addition enhanced seedling shoot to root ratios of both tree species, but again only for the ericaceous substrate. This response is indicative of greater N uptake and greater efficiency of nutrient uptake (and presumably less binding of nutrients by phenolics) in the presence of charcoal. These effects were especially pronounced for B. pendula, which took up 6.22 times more nitrogen when charcoal was added. Charcoal had no effect on the competitive balance between B. pendula and P. sylvestris, probably due to the low intensity of competition present. Juvenile mosses and ferns growing in the pots were extremely responsive to charcoal for all sites; fern prothalli were entirely absent in the ericaceous substrate unless charcoal was also present. Charcoal stimulated active soil microbial biomass in some instances, and also exerted significant although idiosyncratic effects on decomposition of the added litter. Our results provide clear evidence that immediately after wildfire fresh charcoal can have important effects in Boreal forest ecosystems dominated by ericaceous dwarf shrubs, and this is likely to provide a major contribution to the rejuvenating effects of wildfire on forest ecosystems. Received: 23 May 1997 / Accepted: 24 February 1998