Annamari Markkola

SEVERE DEFOLIATION OF SCOTS PINE REDUCES REPRODUCTIVE INVESTMENT BY ECTOMYCORRHIZAL SYMBIONTS

Reduction in the photosynthetic capacity of plants is presumed to negatively affect their fungal symbionts. To test this hypothesis under natural conditions, we artificially removed 100% of previous year needles in two successive years on Scots pine trees (Pinus sylvestris L.) to simulate pine sawfly attack. Despite a decline in the shoot growth of defoliated trees, root biomass did not differ from control trees. The ergosterol (fungal biomass) and starch concentration of fine roots, however, slightly declined in defoliated trees. Percent ectomycorrhizal colonization of fine root tips remained high in both defoliated and control trees. The dominant tubercle morphotypes were slightly more abundant in the control than in defoliated trees. In contrast to the relatively weak effects on vegetative ectomycorrhizae, reproduction declined near the defoliated pines. Average sporocarp numbers and, consequently, the relative fungal investment to reproduction of the estimated total fungal biomass were more than three times higher near controls than defoliated trees in the first treatment year. Defoliation also reduced the diversity of ectomycorrhizal species producing sporocarps. Mutualistic fungal symbionts may thus alter their reproductive investment in response to restrictions on host resources. Because fungal biomass in the roots as well as colonization percentage remained unchanged, Scots pine evidently continues to invest in the maintenance of the symbiosis despite the reduced photosynthetic capacity due to defoliation.

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of Betula pubescens

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.

Breakdown and macroinvertebrate and fungal colonization of alder, birch, and willow leaves in a boreal forest stream

We examined the decomposition of 3 riparian leaf species (alder, birch, and willow) in a boreal woodland stream during winter. Based on initial leaf quality, we expected willow leaves to decompose at a slower rate than alder and birch leaves. We also expected alder and birch leaves to be colonized by fungi and macroinvertebrates more rapidly and in higher numbers and/or biomass than willow leaves. We also assessed how closely shredder life cycles are related to leaf litter availability in this stream. Alder and birch had fast processing rates (k = 0.019 and 0.011/d, respectively), whereas willow leaves were processed slower (k = 0.005/d). Ergosterol concentrations increased rapidly in all leaf species during the first 4 wk, after which concentrations remained nearly constant in alder and birch, but increased slowly in willow. Birch supported the highest overall fungal biomass, whereas fungal biomass in alder and willow did not differ on most sample dates. Macroinvertebrate abundances increased rapidly in the early phases of the experiment for all leaf species (maximum values of 297 and 198 individuals/mesh bag in November for alder and willow, respectively, and 84 ind./mesh bag in January for birch). Alder supported significantly more invertebrates than did birch and willow. Shredder abundance for each leaf species was maximum in November, with a secondary peak in numbers (all leaf species) and biomass (only birch) at the end of the experiment in April. The 3 leaf species studied here formed a range of processing rates, translating into a continuum of availability to shredding invertebrates. In late spring, when alder leaves had practically disappeared, the shredding stonefly Amphinemura borealis entered an active growth period prior to emergence. Late spring clearly represents a potential bottleneck for shredder populations in boreal streams. Spring–summer shredders may survive this bottleneck by shifting to seasonally abundant food sources, especially fine particulate detritus.

Root endophytes along a primary succession gradient in northern Finland

Primary successional vegetation gradients are characterized by changes in the soil microbial communities. However, information on possible shifts of the root endophytes along these gradients is scarce. The objective of the current study was to identify root endophytic fungi from a primary successional gradient on land uplift seashore of a geographically isolated island area. We applied a sequencing approach by amplifying the ITS region with fungal specific primers. We used mainly an isolate-based method, and to compare the abundance of culturable and unculturable endophytes, direct sequencing of one representative plant specimen Deschampsia flexuosa was also carried out. A total of 38 cultured endophytic strains were sequenced from Empetrum nigrum (Empetraceae), Vaccinium vitis-idaea (Ericaceae) and Deschampsia flexuosa (Poaceae). Out of these, 27 were identified as Phialocephala fortinii, three as Mollisia minutella, four as Phialophora sp., one as Ascomycetes sp. and three remained unidentified. The strains clustered into five clades in the phylogram, mostly irrespective of the successional stages and hosts from which they had been isolated. The early successional seashore dune ridge plants however, seemed to host a distinct fungal taxon, Phialophora sp. Culture-independent methods were applied on a root sample of a mid-successional Deschampsia flexuosa specimen and a total of 16 clones were randomly selected and sequenced. Out of 16 sequences, 13 were identified as unculturable strains and three showed closest similarity to a basidiomycete Cortinarius callisteus. The unculturable sequences were grouped into two main clades and were different from any culturable isolate in this study. Our results suggest that (i) P. fortinii dominates the isolate data at mid to late successional stages, (ii) roots of the ericaceous plants and the grass Deschampsia flexuosa are colonized by the same endophytic fungi in this ecosystem, and (iii) unculturable endophytes are common and potentially more abundant than the culturables. To our knowledge, this is the first report of the molecular phylogenies of the DSE in the mid-boreal zone and also the first report of the unculturable root endophytes of D. flexuosa.

Defoliation-induced responses in peroxidases, phenolics, and polyamines in scots pine (Pinus sylvestris L.) needles

Effects of artificial defoliation on defensive needle chemistry in Scots pine (Pinus sylvestris L.) were evaluated with particular emphasis on peroxidases, phenolic compounds, soluble sugars, polyamines, and foliar nitrogen levels. The study was carried out on a nutrient-poor Scots pine stand with 8- to 25-year-old trees. Defoliation treatment consisted of repeated defoliation in two successive years with respective control trees. Defoliation was done before needle flushing by removing all mature needles. Guaiacol peroxidase activity increased in the needles after the first defoliation. The difference between treatments diminished towards autumn, and disappeared before the second defoliation in the next summer. After the second defoliation, the activities showed a similar trend. Apparently, peroxidases are involved in inducible chemical changes and recovery reactions that occur in the intact needles shortly after defoliation. After the second defoliation, total nitrogen concentration in the current year needles was about 20% lower, and free putrescine (a polyamine) concentration was 40% lower in the defoliated trees than in control needles. These changes indicate a loss of nitrogen due to defoliation. Specific phenolic compounds such as quercitrin, (+)-catechin, and two catechin derivatives increased in current year needles in response to defoliation. Accumulation of starch and sucrose in the current year needles of repeatedly defoliated trees may imply decreased assimilate transport. The results are indicative that changes in needle phytochemistry in response to defoliation accompany changes in needle nitrogen metabolism.

Urban polluted forest soils induce elevated root peroxidase activity in Scots pine (Pinus sylvestris l.) seedlings

Plant biomass. mycorrhizal status and root peroxidase activity were measured in ectomycorrhizal Scots pine (Pinus sylvestris L.) seedlings grown in urban polluted and native, non-polluted forest soils with added ammonium or potassium sulphates simulating N and S deposition of urban areas. Peroxidase activity in the fine roots of seedlings planted in polluted forest soils was higher than in those planted in non-polluted soils and correlated positively with the activities measured in an earlier study in the roots of mature Scots pines growing at the sites from where the soils were collected. Growth of seedlings and mycorrhizal status were not affected by the origin of soil. Exposing the seedlings to winter acclimation conditions for 6 weeks elevated peroxidase activity in the roots. The addition of ammonium or potassium sulphate to non-polluted soils did not induce elevated root peroxidase activity, although at the levels of 0.5 and 1.0 g of ammonium sulphate a slight increasing trend was observed. We suggest, that indirect biotic factors, i.e. changes in the community structure of soil fungi, early stages of recognition, and defence reactions of pine roots against saprophytic and pathogenic fungi may be participating in the elicitation of peroxidase (POD) activity, although the possible role of heavy metals cannot be excluded.

Decomposer communities in human‐impacted streams: species dominance rather than richness affects leaf decomposition

Summary There is compelling evidence that anthropogenic disturbance can decrease biodiversity and impair ecological functioning. A major challenge to biodiversity–ecosystem function research is to disentangle the effects of biodiversity loss on ecosystem functions from the direct effects of human disturbance. We studied the influence of human disturbance (acidification and eutrophication) and a natural stressor (low pH due to bedrock geology) on leaf-shredding macroinvertebrates, fungal decomposers and leaf decomposition rates in boreal streams. We used pyrosequencing techniques to determine fungal richness and assemblage structure. Decomposition rates were higher in anthropogenically disturbed than in circumneutral reference or naturally acidic sites, but did not differ between the latter two groups. Macroinvertebrate richness was higher in circumneutral than in human-impacted or naturally acidic sites, and shredder evenness was highest in circumneutral sites. Fungal evenness was also lower in human-disturbed than in reference sites, whereas fungal richness did not vary among site groups. Decomposition rate in fine-mesh bags was related positively to current velocity and fungal dominance, while in coarse-mesh bags, it was related positively to total phosphorus. In anthropogenically disturbed streams, the effects of low pH were overridden by eutrophication, and increased decomposition rates resulted from disturbance-induced increase in species dominance rather than richness. Furthermore, decomposition rates were positively correlated with abundances of dominant taxa, suggesting that ecosystem processes may be driven by a few key species. Synthesis and applications. Our results suggest that leaf decomposition rates are insensitive to natural background variation, supporting the use of decomposition assays, preferably accompanied by molecular analysis of fungal assemblages, to assess stream ecosystem health. Instead of focusing solely on diversity, however, more emphasis should be placed at changes in dominance patterns, particularly if management aims are to improve stream ecosystem functioning.

Induced accumulation of phenolics and sawfly performance in Scots pine in response to previous defoliation

Phenolic compounds often accumulate in foliar tissues of deciduous woody plants in response to previous insect defoliation, but similar responses have been observed infrequently in evergreen conifers. We studied the effects of defoliation on the foliar chemistry of Scots pine (Pinus sylvestris L.) and cocoon mass, and survival of the pine sawfly (Diprion pini L.). In two successive years, needles were excised early in the season leaving only the current-year shoot intact (defoliated trees); untreated entire shoots served as controls (control trees). A year after the second defoliation, pine sawfly larvae were transferred to the trees. Delayed induced resistance in Scots pine in response to defoliation was indicated by (1) reduced cocoon mass in defoliated trees and (2) increased concentrations of phenolics and soluble condensed tannins in the foliage of defoliated trees compared with controls. Myricetin-3-galactoside, which showed the strongest induced response (104% and 71% increase in current-year (C) and previous-year (C+1) needles) of the compounds analyzed, also entered the regression model explaining variation in sawfly performance. Other compounds that entered the model, e.g., (+)-catechin, showed weaker responses to defoliation than myricetin-3-galactoside. Hyperin, condensed tannins and quercitrin showed strong induced responses in C or C+1 needles, or both, but these compounds did not explain the variation in sawfly performance. Accumulation of phenolics is sometimes associated with the reduced foliage nitrogen (N) concentrations in deciduous trees, and our results suggest that this may also be the case in evergreen conifers. Based on the earlier findings that defoliation reduces needle N concentration and N deficiency results in the accumulation of the same phenolic compounds, i.e., myricetin and quercetin glycosides, and soluble condensed tannins, we suggest that the accumulation of phenolics in defoliated trees occurred in response to the reduced foliar N concentration.

Moth herbivory enhances resource turnover in subarctic mountain birch forests

Massive moth outbreaks cause large-scale damage in subarctic mountain birch forests with a concomitant decrease in carbon flux to mycorrhizal fungi and an increased deposition of dissolved carbon and nutrients as moth frass into soil. We investigated impacts of moth herbivory along three replicated gradients with three levels of moth herbivory (undamaged, once damaged, repeatedly damaged) on soil nutrient levels and biological parameters. We found an increase in soil nutrients and in the biomass of enchytraeid worms, which are key faunal decomposers. Fungi bacteria ratio and C:N ratio decreased in humus with increasing severity of herbivory. Our findings suggest enhanced resource turnover in mountain birch forests due to massive moth herbivory. This may provide a shortcut for carbon and nutrient input to subarctic soils, which largely bypasses the main routes of carbon from plants to soil via mycorrhizal and litter-decomposing fungi. Moreover, a temporal shift occurs in carbon allocation to soil, providing decomposers an opportunity to use an early-season peak in resource availability. Our results suggest a hitherto unappreciated role of massive insect herbivore attacks on resource dynamics in subarctic ecosystems.

Impact of wet deposited nickel on the cation content of a mat-forming lichen Cladina stellaris

The impact of experimentally sprayed aqueous nickel solution on the concentrations of potassium, calcium, magnesium and nickel in three horizontal strata (top, 0-20 mm; middle, 20-40 mm; and base, 40-60 mm) of the cushion-forming lichen Cladina stellaris was investigated. The experimental nickel deposition range used corresponded with that from the pristine forests of the Finnish border to polluted industrial sites of Russian Kola Peninsula (0-1000 mg Ni(2+) m(-2) year(-1)). The lichen mat retained ca. 31-66% of the nickel deposited during two growing seasons and the relative retention efficiency was highest at the low deposition end. The concentrations of cations in lichen thalli were significantly reduced only after the highest nickel deposition. Furthermore, the separate horizontal strata responded differently to nickel exposure indicating that the cation exchange sites of the top stratum were not completely saturated by nickel even after the most severe treatment. However, nickel deposited in high doses caused considerable reduction in potassium concentration indicating damage to cell membranes. Episodically deposited high concentrations of nickel can probably affect membrane integrity before detectable changes in total concentrations of cations in the lichen thallus take place. Thus, ratios of total concentrations of cations in the lichen thallus are fairly insensitive to nickel deposition, which reduces the risk of compounding effects when the ratios are used to indicate long-term acid deposition in areas with multiple pollution problems such as Kola Peninsula.