Nitrogen fixation in mixed Hylocomium splendens moss communities.

The pleurocarpus feather moss, Hylocomium splendens, is one of two co-dominant moss species in boreal forest ecosystems and one of the most common mosses on earth, yet little is known regarding its capacity to host cyanobacterial associates and thus contribute total ecosystem N. In these studies, we evaluated the N-fixation potential of the H. splendens-cyanobacteria association and contrasted the N-fixation activity with that of the putative N-fixing moss-cyanobacteria association of Pleurozium schreberi. Studies were conducted to: quantify N-fixation in H. splendens and P. schreberi in sites ranging from southern to northern Fennoscandia; assess N and P availability as drivers of N-fixation rates; contrast season-long N-fixation rates for both mosses; and characterize the cyanobacteria that colonize shoots of H. splendens. Nitrogen-fixation rates were generally low at southern latitudes and higher at northern latitudes (64-69 degrees N) potentially related to anthropogenic N deposition across this gradient. Nitrogen fixation in H. splendens appeared to be less sensitive to N deposition than P. schreberi. The season-long assessment of N-fixation rates at a mixed feather moss site in northern Sweden showed that H. splendens fixed a substantial quantity of N, but about 50% less total N compared to the contribution from P. schreberi. In total, both species provided 1.6 kg fixed N ha(-1) year(-1). Interestingly, H. splendens demonstrated somewhat higher N-fixation rates at high fertility sites compared to P. schreberi. Nostoc spp. and Stigonema spp. were the primary cyanobacteria found to colonize H. splendens and P. schreberi. These results suggest that H. splendens with associated Nostoc or Stigonema communities contributes a significant quantity of N to boreal forest ecosystems, but the contribution is subordinate to that of P. schreberi at northern latitudes. Epiphytic cyanobacteria are likely a key factor determining the co-dominant presence of these two feather mosses across the boreal biome.

Nitrogen mineralization and phenol accumulation along a fire chronosequence in northern Sweden.

Scots pine (Pinus sylvestris L.) forests of northern Sweden are often considered to be N limited. This limitation may have been exacerbated by the elimination of wildfire as a natural disturbance factor in these boreal forests. Phenolic inhibition of N mineralization and nitrification (due to litter and exudates of ericaceous shrubs) has been proposed as a mechanism for N limitation of these forests, but this hypothesis remains largely untested. N mineralization rates, nitrification rates, and sorption of free phenolic compounds were assessed along a fire-induced chronosequence in northern Sweden. A total of 34 forest stands varying in age since the last fire were identified and characterized. Overstorey and understorey vegetative composition and depth of humus were analysed in replicated plots at all 34 sites. Eight of the forest stands aged 3-352 years since the last fire were selected for intensive investigation in which ten replicate ionic resin capsules (used to assess net N mineralization and nitrification) and non-ionic carbonaceous resin capsules (used to assess free phenolic compounds) were installed at the interface of humus and mineral soil. A highly significant correlation was observed between site age and net sorption of inorganic N to resin capsules. Net accumulation of NH4+ and NO3- on resin capsules followed a linear decrease (R 2=0.61, P<0.01) with time perhaps as a result of increased N immobilization with successional C loading. NO3- sorption to resin capsules followed a logarithmic decrease (R 2=0.80, P<0.01) that may be related to a logarithmic increase in dwarf shrub cover and decreased soil charcoal sorption potential along this chronosequence. A replicated field study was conducted at one of the late successional field sites to assess the influence of charcoal and an added labile N source on N turnover. Three rates of charcoal (0, 100, and 1,000 g M-2) and two rates of glycine (0 and 50 g N as glycine M-2) were applied in a factorial design to microplots in a randomized complete block pattern. Net ammonification (as assessed by NH4+ sorption to resins) was readily increased by the addition of a labile N source, but this increase in NH4+ did not stimulate nitrification. Nitrification was stimulated slightly by the addition of charcoal resulting in similar levels of resin-sorbed NO3- as those found in early successional sites. Resin-sorbed polyphenol concentrations were decreased with charcoal amendments, but were actually increased with N amendments (likely due to decomposition of polyphenols). Net N mineralization appears to be limited by rapid NH4+ immobilization whereas nitrification is limited by the lack of an appropriate environment or by the presence of inhibitory compounds in late successional forests of northern Sweden.

Characterisation of the differential interference effects of two boreal dwarf shrub species.

Our purpose was to characterize the relative competitive and phytotoxic potential of two closely related dwarf-shrub species, Empetrum nigrum and E. hermaphroditum, which form clones in a mosaic pattern in post-fire successions of the boreal forest of northern Sweden. We determined morphological and growth parameters of both species, performed bioassays and chemical analysis, and established field experiments to explore possible differing interference effects on trees by the two species. Both Empetrum species had very similar morphological and growth characteristics. E. hermaphroditum exerted considerably greater negative effects than E. nigrum against Pinus sylvestris and Populus tremula seed and seedlings. These negative effects were related to the different substitution of a bibenzyl in the two species. The effect on seed germination of the two bibenzyls isolated from E. nigrum and E. hermaphroditum was compared with that of other simple phenolics; the latter were found to be inactive, indicating a specific phytotoxic component in the bibenzyls. P. sylvestris seeds planted into clones of both species in the field revealed that E. hermaphroditum had much stronger inhibitory effects than did E. nigrum and the addition of activated carbon partially reversed these effects. Seed germination, biomass and survival of P. sylvestris after four seasons were significantly lower in E. hermaphroditum- than in E. nigrum-dominated plots. We conclude that while both Empetrum species are superficially very similar morphologically, they have vastly different effects on tree seed germination, seedling establishment and growth. These effects appear to be due, at least in part, to the different chemical profile of the two species.

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.

Effects of bilberry (Vaccinium myrtillus L.) litter on seed germination and early seedling growth of four boreal tree species.

Laboratory and greenhouse bioassays were used to test for inhibitory effects of senescent and decomposed leaves and aqueous extract from bilberry (Vaccinium myrtillus L.) against seed germination and seedling growth of aspen (Populus tremula L.), birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), and Norway spruce [Picea abies (L.) Karst.]. Aqueous extracts from bilberry leaves were inhibitory to aspen seed germination and seedling growth and also induced root damage and growth abnormalities. Addition of activated carbon removed the inhibitory effects of extracts. Senescent leaves reduced pine and spruce seed germination, but rinsing of seeds reversed this inhibition. Senescent leaves were more inhibitory than decomposed leaf litter, suggesting that the inhibitory compounds in bilberry leaves are relatively soluble and released at early stages during decomposition. Spruce was generally less negatively affected by litter and aqueous extracts than the other tested species. This study indicates that chemical effects of bilberry litter have the potential to inhibit tree seedling recruitment, but these effects were not consistently strong. Phytotoxicity is unlikely to be of critical importance in determining success for spruce seedling establishment.

Seasonal variation in phytotoxicity of bracken (Pteridium aquilinum L. Kuhn).

Laboratory bioassays were used to test for the phytotoxicity of volatile compounds, fresh plant material as a seed bed, and water extracts from bracken [Pteridium aquilinum (L.) Kuhn] pinnules to germination and seedling growth of aspen (Populus tremula L.) and Scots pine (Pinus sylvestris L.). Fronds were sampled from two bracken populations, one in the south and one in the north of Sweden. All three bioassays showed inhibitory effects, and these varied seasonally with the most inhibitory effects occurring in May, June, and September. The peak of inhibition in May and June coincides with the start of the growing season when bracken still is immature and vulnerable to interference from other species. The increase in inhibitory effects in September appears to be due to transformation of natural products or an accumulation of inhibitory compounds that are released during decomposition following frond death. Addition of activated carbon did not remove the inhibitory effects.

Inhibition of Scots pine seedling establishment byEmpetrum hermaphroditum.

Poor establishment and reduced seedling growth of Scots pine (Pinus silvestris L.) in northern Sweden is related to an allelopathic inhibition by the dwarf shrubEmpetrum hermaphroditum Hagerup. Indoor bioassays with green and brown leaves ofEmpetrum have strong negative effects on rooting ability, radicle elongation, and growth of Scots pine seedlings. Bioassays with soil samples show that phytotoxic substances leached fromEmpetrum foliage accumulate in the soil. Field experiments reveal that chemical inhibition byEmpetrum, causing high mortality and slow growth of pine seedlings, can be reduced by adding activated carbon to the soil.