Boreal Forest Understory Research Articles

Current Perspective Concerning the Potential Value of Chloroplast Lipidome in Assessing Moss Response to Abiotic Stress During Boreal Forest Regeneration

Mosses play important roles in the regulation of environmental or metabolic conditions in boreal forest ecosystems. Sphagnum and feathermoss are the two main bryophytes found in boreal forest understory. Clearcut harvesting (common method of boreal forest regeneration) can expose understory vegetation to water and light stress. Water and light stress can significantly impact moss growth during boreal forest regeneration. Analysis of the membrane lipidome, photosynthetic parameters and pigments can be very effective in assessing moss response to abiotic stress following clearcut harvesting. Although lipidomics is commonly used in environmental stress assessment of plants, application to assess moss lipidome and stress response is very limited. Bryophytes may alter or remodel their membrane lipid composition to acclimate or adapt to environmental stressors. Thus, this perspective provides insights into how moss lipids may serve as useful biomarkers of moss stress response or adaptation to environmental stress during boreal forest regeneration following clearcut harvesting.

Effects of plant functional group removal on CO2 fluxes and belowground C stocks across contrasting ecosystems.

Changes in plant communities can have large effects on ecosystem carbon (C) dynamics and long‐term C stocks. However, how these effects are mediated by environmental context or vary among ecosystems is not well understood. To study this, we used a long‐term plant removal experiment set up across 30 forested lake islands in northern Sweden that collectively represent a strong gradient of soil fertility and ecosystem productivity. We measured forest floor CO2 exchange and aboveground and belowground C stocks for a 22‐yr experiment involving factorial removal of the two dominant functional groups of the boreal forest understory, namely ericaceous dwarf shrubs and feather mosses, on each of the 30 islands. We found that long‐term shrub and moss removal increased forest floor net CO2 loss and decreased belowground C stocks consistently across the islands irrespective of their productivity or soil fertility. However, we did see context‐dependent responses of respiration to shrub removals because removals only increased respiration on islands of intermediate productivity. Both CO2 exchange and C stocks responded more strongly to shrub removal than to moss removal. Shrub removal reduced gross primary productivity of the forest floor consistently across the island gradient, but it had no effect on respiration, which suggests that loss of belowground C caused by the removals was driven by reduced litter inputs. Across the island gradient, shrub removal consistently depleted C stocks in the soil organic horizon by 0.8 kg C/m2. Our results show that the effect of plant functional group diversity on C dynamics can be relatively consistent across contrasting ecosystems that vary greatly in productivity and soil fertility. These findings underline the key role of understory vegetation in forest C cycling, and suggest that global change leading to changes in the relative abundance of both shrubs and mosses could impact on the capacity of boreal forests to store C.

Patterns of bryophyte succession in a 160-year chronosequence in deciduous and coniferous forests of boreal Alaska

Bryophytes are dominant components of boreal forest understories and play a large role in regulating soil microclimate and nutrient cycling. Therefore, shifts in bryophyte communities have the potential to affect boreal forests’ ecosystem processes. We investigated how bryophyte communities varied in 83 forest stands in interior Alaska that ranged in age (since fire) from 8 to 163 years and had canopies dominated by deciduous broadleaf (Populus tremuloides Michx. or Betula neoalaskana Sarg.) or coniferous trees (Picea mariana Mill B.S.P.). In each stand, we measured bryophyte community composition, along with environmental variables (e.g., organic layer depth, leaf litter cover, moisture). Bryophyte communities were initially similar in deciduous vs. coniferous forests but diverged in older stands in association with changes in organic layer depth and leaf litter cover. Our data suggest two tipping points in bryophyte succession: one at the disappearance of early colonizing taxa 20 years after fire and another at 40 years after fire, which corresponds to canopy closure and differential leaf litter inputs in mature deciduous and coniferous canopies. Our results enhance understanding of the processes that shape compositional patterns and ecosystem services of bryophytes in relation to stand age, canopy composition, and changing disturbances such as fire that may trigger changes in canopy composition.

Does CWD mediate microclimate for epixylic vegetation in boreal forest understories? A test of the moisture-capacitor hypothesis

Coarse woody debris (CWD) is an important microhabitat in forests, particularly for bryophytes and lichens. Many have suggested that this is because CWD provides a uniquely stable and humid surface microclimate, which facilitates epixylic flora, although few have adequately tested this hypothesis. Consequently, it is uncertain whether the apparent relationship between CWD and epixylic vegetation is because CWD regulates moisture, or because moisture is simply correlated with other important CWD traits. This study tested the moisture-capacitor hypothesis of epixylic flora-CWD association by measuring the surface moisture availability directly on CWD under thinned and unthinned forest canopies, and (1) comparing (a) cumulative moisture levels, and (b) variation in moisture levels with ambient and adjacent forest floor microclimates, (2) testing whether associations with log and canopy properties are stronger through direct or indirect pathways, and (3) testing whether desiccation-sensitive functional groups respond more strongly to surface moisture than other groups. Path models were built using canopy cover and log physical traits (log covering, size, acidity, and hardness) as predictors, surface moisture indices as mediators, and epixylic functional group cover as outcome variables. Forest floor and CWD microclimates were equally stable and humid, regardless of canopy cover. Log and canopy properties were more strongly related to epixylic group cover directly than indirectly (through surface moisture). Synthesis: The CWD moisture-capacitor hypothesis is rejected: CWD traits can influence surface moisture availability, but (1) log surface moisture does not differ from the forest floor, and (2) log and canopy properties are more strongly associated with epixylic taxa directly than as mediated through microclimate.

Early trajectories of forest understory development on reclamation sites: influence of forest floor placement and a cover crop

We tested whether direct placement of forest floor material (FFM: litter, fibric, humus layers and surface mineral horizons) and sowing of a cover crop (Melilotus officinalis) could facilitate the establishment of native forest understory species at a reclaimed coal mine in Alberta, Canada. FFM was salvaged at two depths (15 and 40 cm) from a recently harvested native aspen forest and immediately placed at the same depths on the reclamation site. Total richness (approximately 61 species in 96 subplots) was similar in each of 3 years post‐placement; total richness for all 3 years combined was 87 including 34 typical boreal forest understory species plus 30 other natives. The deeper treatment reduced cover of all species, native and non‐native species in year 1. In year 3, the deeper treatment still had lower cover of non‐native species but had higher cover of forest understory species in years 2 and 3. The deeper treatment also resulted in lower species richness per plot, but only in year 1. In year 2 (when the biennial clover was at its tall stage), the cover crop treatment was associated with lower cover of non‐native species but did not affect the cover of native forest understory species. Direct placement of FFM can help facilitate establishment of a diverse native boreal forest understory in a reclaimed landscape. Although richness and cover may be initially higher with shallower salvage and placement, deeper salvage may ultimately be better for encouraging establishment of native forest understory species.

Mechanisms for Success after Long-term Nutrient Enrichment in a Boreal Forest Understory

Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Many plant communities respond strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that was initiated in 1990 has been investigating the effects of long-term nutrient enrichment (fertilizer added annually) on a boreal forest understory community. We used this experiment to investigate why some species increase in abundance under nutrient enrichment whereas others decline. We focused on four species that differed in their responses to fertilization: Mertensia paniculata and Epilobium angustifolium increased in abundance, Achillea millefolium remained relatively constant and Festuca altaica declined. We hypothesized that the two species that were successful in the new high-nutrient, light-limited environment would be taller, have higher specific leaf area, change phenology by growing earlier in the season and be more morphologically plastic than their less successful counterparts. We compared plant height, specific leaf area, growth spurt date and allocation to leaves in plants grown in control and fertilized plots. We demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; M. paniculata has the highest specific leaf area of the four species whereas E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaves. These results indicate that rather than one strategy determining success when nutrients become available, a variety of traits and responses may contribute to a species' ability to persist in a nutrient-enriched boreal forest understory.

The Effects of Fertilization and Herbivory on the Phenology of the Understory Vegetation of the Boreal Forest in Northwestern Canada

Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring temperatures, and increased soil mineralization, have resulted in shifts in the timing of plant phenological events. We assessed the effects of fertilizer and herbivory on the phenology of 7 plant species from a boreal forest understory using long-term experimental plots in the southwest Yukon. Fertilizer and fencing treatments were initiated in 1990, and discontinued in half of each plot in 2000. There were few effects on phenology of either fertilizer or fencing. In some species, fertilizer affected the final phenological stage reached, but the presence and direction of the effect was species-dependent. Epilobium angustifolium was the only species where the timing of phenological stages responded to fertilization; early phenological stages were advanced with fertilizer. First leaf expansion for Arctostaphylos uva-ursi occurred earlier in fenced plots, although this effect disappeared in plots where the fencing treatment had been discontinued. We conclude that previously observed changes in species abundance with fertilizer treatments are likely not caused by changes in the phenology of these species, and are more likely due to fertilization imposed changes in vegetative growth.

Seasonal reflectance dynamics of common understory types in a northern European boreal forest

The influence of the seasonal cycle of boreal forest understory has been noticed in global remote sensing of vegetation, especially in remote sensing of biophysical properties (e.g. leaf area index) of the tree-layer in a forest. A general problem in the validation of operationally produced global biophysical vegetation products is the lack of ground reference data on the seasonal variability of different land surface types. Currently, little is known about the spectral properties of the understory layers of boreal forests, and even less is known about the seasonal dynamics of the spectra. In this paper, we report seasonal trajectories of understory reflectance spectra measured in a European boreal forest. Four study sites representing different forest fertility site types were selected from central Finland. The understory composition was recorded and its spectra measured with an ASD FieldSpec Hand-Held UV/VNIR Spectroradiometer ten times during the growing period (from May to September) in 2010. Our results show that the spectral differences between and within understory types are the largest at the peak of the growing season in early July whereas in the beginning and end of the growing season (i.e. early May and late September, respectively) the differences between the understory types are marginal. In general, the fertile sites had the brightest NIR spectra throughout the growing season whereas infertile types appeared darker in NIR. Our results also indicated that a mismatch in the seasonal development of understory and tree layers does not occur in boreal forests: the understory and tree layer vegetation develop at a similar pace in the spring (i.e. there are no or only few spring ephemerals present), and the forests with the strongest seasonal dynamics in tree canopy structure (LAI) have also the strongest dynamics in understory spectra.

Voles Are Attracted to Fertilizer in Field Experiments

We estimated the abundance of voles, or vole nests, in four different experiments in the Boreal forest understory in the southwestern Yukon to test if voles were attracted to fertilizer. In one experiment we counted vole nests in plots that had been fertilized or not fertilized for 4 years; in a second we counted trapped voles in plots that has been fertilized, or not, for 13 years; and in two others we counted the number of vole visits to Longworth traps baited with fertilizer. There were significantly more over-winter nests in the fertilized plots. In a June trapping session, more voles were captured in fertilized plots than unfertilized plots and voles entered locked-open traps and removed fertilizer. The results demonstrate a relationship between fertilizer and voles. Whether voles are attracted to the fertilizer alone or the higher quality feed it produces remains unclear. A plausible, yet untested, hypothesis is that the voles were consuming the fertilizer as a source of micro- or macronutrients.

Contrasts Among Mycorrhizal Plant Guilds in Foliar Nitrogen Concentration and δ15N Along Productivity Gradients of a Boreal Forest

The distribution of plant species in boreal forest understories is hypothesized to reflect mycorrhizal guilds and associated adaptations for organic nitrogen (N) acquisition. In this study of a natural edaphic gradient, where supply rates of inorganic N increase with site productivity, we noted a decline in understory ectomycorrhizal, ericoid, and arbutoid plant communities on productive sites, in contrast to a positive response by most arbuscular species. We then assessed the rate of change in foliar N concentration (Nconc) and abundance of 15N (δ15N) of select plants from these mycorrhizal guilds. Two arbuscular plant species (Rubus parviflorus and Viburnum edule) had the sharpest increases in foliar Nconc with enhanced supplies of NH4 + and NO3 −, but with no differences in foliar δ15N. An ectomycorrhizal species, Abies lasiocarpa, and ericoid species, Vaccinium membranaceum, had parallel increases in both Nconc and δ15N with soil N supply. The foliar δ15N of two arbutoid plants (Orthilia secunda and Pyrola asarifolia) were as enriched as ectomycorrhizal sporocarps, likely indicating N transfer from mycorrhizal networks. The depletion of foliar δ15N by ectomycorrhizal and ericoid plants on poorer sites likely reflected a high degree of N retention and photosynthate demand by fungi, whereas arbuscular plants may have had a less significant δ15N response because of a more passive role by fungi in scavenging organic N. The results suggest differences in how mycorrhiza exploit diverse soil N supplies (recalcitrant and labile organic N, NH4 +, NO3 −, and parasitized N) could be an important factor in boreal plant community composition.

Dominant Species and Diversity: Linking Relative Abundance to Controls of Species Establishment

Ecological theories make divergent predictions about whether extant species inhibit or promote the establishment of new species and which aspects of community composition determine these interactions; diversity, individual dominant species, and neutral interactions have all been argued to be most important. We experimentally tested these predictions by removing plant biomass (0%, 7%, 100%) from boreal forest understory communities. The 7% removals were restricted to the numerically dominant species, the second most dominant species, or many low-abundance species, thereby separating the effects of species composition from those of biomass. We tested the effects of all removal treatments on seedling establishment. Competitive effects were driven by one dominant species and were inconsistent with resource complementarity, neutral, or competition-colonization models. Facilitative effects were apparent only following removal of all vegetation, of which the most dominant species comprised more than 80%. Our results indicate that numerically dominant species in a community can influence the establishment of new species more than species diversity, but the direction of interaction can shift from facilitative to competitive as community density increases.

Spectral reflectance of multispecies herbaceous and moss canopies in the boreal forest understory and open field

The reflectance signal from the forest tree canopy is influenced by the optical properties of the background formed by understory vegetation. Our study shows that the herb-moss layer in the forest tends to be brighter in the visible wavelength when the canopy above is more closed because of the specific properties of plants grown in low light. When leaf-area-based chlorophyll content falls below approximately 150 mg·m–2, reflectance in the red region of the spectrum increases compared with that of the background. The best descriptors of the herb-moss layer for deriving optical parameters are herb layer dry mass for the visible wavelength range and total aboveground water mass for near-infrared (NIR) reflectance. In addition, chlorophyll content per leaf area considerably improves the red reflectance estimate.

Do Nutrient Availability and Competition Limit Plant Growth of Herbaceous Species in the Boreal Forest Understory?

An experiment was conducted to investigate whether nutrient availability and competition limit aboveground growth of five herbaceous species in the boreal forest understory. The experiment involved adding fertilizer and removing neighbors of selected target individuals of five different species. Aboveground biomass and leaf number were measured as the response variables, and treatment effects were tested using ANOVA and ANCOVA. The five species showed variable responses to fertilizer addition with Achillea millefolium, Festuca altaica, and Mertensia paniculata showing significant increases. Lupinus arcticus had no response and there was a strong negative effect on survival of Anemone parviflora. Achillea was the only species that showed some growth limitation due to neighboring individuals for both biomass and leaf number; other species showed no response.

Do Nutrient Availability and Competition Limit Plant Growth of Herbaceous Species in the Boreal Forest Understory?

Do Nutrient Availability and Competition Limit Plant Growth of Herbaceous Species in the Boreal Forest Understory?