Ecological consequences of the expansion of N2-fixing plants in cold biomes

Research in warm-climate biomes has shown that invasion by symbiotic dinitrogen (N2)-fixing plants can transform ecosystems in ways analogous to the transformations observed as a consequence of anthropogenic, atmospheric nitrogen (N) deposition: declines in biodiversity, soil acidification, and alterations to carbon and nutrient cycling, including increased N losses through nitrate leaching and emissions of the powerful greenhouse gas nitrous oxide (N2O). Here, we used literature review and case study approaches to assess the evidence for similar transformations in cold-climate ecosystems of the boreal, subarctic and upper montane-temperate life zones. Our assessment focuses on the plant genera Lupinus and Alnus, which have become invasive largely as a consequence of deliberate introductions and/or reduced land management. These cold biomes are commonly located in remote areas with low anthropogenic N inputs, and the environmental impacts of N2-fixer invasion appear to be as severe as those from anthropogenic N deposition in highly N polluted areas. Hence, inputs of N from N2 fixation can affect ecosystems as dramatically or even more strongly than N inputs from atmospheric deposition, and biomes in cold climates represent no exception with regard to the risk of being invaded by N2-fixing species. In particular, the cold biomes studied here show both a strong potential to be transformed by N2-fixing plants and a rapid subsequent saturation in the ecosystem's capacity to retain N. Therefore, analogous to increases in N deposition, N2-fixing plant invasions must be deemed significant threats to biodiversity and to environmental quality

Ecological consequences of the expansion of N₂‑fixing plants in cold biomes

Research in warm-climate biomes has shown that invasion by symbiotic dinitrogen (N₂)-fixing plants can transform ecosystems in ways analogous to the transformations observed as a consequence of anthropogenic, atmospheric nitrogen (N) deposition: declines in biodiversity, soil acidification, and alterations to carbon and nutrient cycling, including increased N losses through nitrate leaching and emissions of the powerful greenhouse gas nitrous oxide (N₂O). Here, we used literature review and case study approaches to assess the evidence for similar transformations in cold-climate ecosystems of the boreal, subarctic and upper montane-temperate life zones. Our assessment focuses on the plant genera Lupinus and Alnus, which have become invasive largely as a consequence of deliberate introductions and/or reduced land management. These cold biomes are commonly located in remote areas with low anthropogenic N inputs, and the environmental impacts of N₂-fixer invasion appear to be as severe as those from anthropogenic N deposition in highly N polluted areas. Hence, inputs of N from N₂ fixation can affect ecosystems as dramatically or even more strongly than N inputs from atmospheric deposition, and biomes in cold climates represent no exception with regard to the risk of being invaded by N₂-fixing species. In particular, the cold biomes studied here show both a strong potential to be transformed by N₂-fixing plants and a rapid subsequent saturation in the ecosystem’s capacity to retain N. Therefore, analogous to increases in N deposition, N₂-fixing plant invasions must be deemed significant threats to biodiversity and to environmental quality.Keywords: Carbon, Lupinus, Biodiversity, Invasive, Nitrogen, Alnu

Vegetation and soil disturbances in bogs traversed by power line corridors in Manitoba

Effects of power line construction and vegetation management on bog vegetation and organic matter breakdown, were examined along two right-of-ways (ROWs) extending from 50-56 oN in Manitoba. Vegetation, peat and soil water were sampled and collected from the ROW and adjacent undisturbed forest at sixteen sites. Organic matter decomposition estimates were derived from measurements of seasonal soil respiration rates in the ROW and forest at three sites. Erosion of exposed peat was estimated in the ROW at three sites. Effects of the herbicides picloram and 2,4-D on growth of Sphagnum fuscum were studied at two sites; and in the laboratory the effects of the herbicides on respiration rate (CO2 production) in S.fuscum peat cores were examined. Decorana ordination and cluster anlysis showed similar patterns in the vegetation. Major groupings were formed along moisture and disturbance gradients. Canonical correlation analysis revealed that variation in bare ground, peat humification and soil water chemistry correlated strongly with the vegetation pattern. Six (among them S.fuscum) of the fifty most abundant plant species were significantly reduced in abundance in the ROW compared to the forest, while only, Polytrichum strictum had significantly higher abundance in the ROW than the forest community. Annual decomposition of soil organic matter was estimated to be of the order 350-670 g dwt m-2 over the latitudinal range of the study area. It was not evident that disturbances in the ROW had affected rates of organic matter decomposition. Organic matter losses due to erosion of surface peat in the ROW were very low in comparison to decompositional losses. Applications of picloram on Sphagnum fuscum growing in hummocks were found to permanently destroy most of the shoots of the moss and suppress its regeneration for several years. Cyclic use of picloram in the type of bogs studied is considered to lead to a gradual destruction of the hummock habitat and its associated community. Picloram and 2,4-D applications caused initial stimulation of respiration rates in peat cores, which were minimal within 30 days of applications. The herbicides were not considered to cause long term changes in peat decomposition rates

Bryophytes of Surtsey, Iceland: Latest developments and a glimpse of the future

Surtsey island was formed in a volcanic eruption south of Iceland in 1963 – 1967 and has since then been protected and monitored by scientists. It is the youngest island in the Vestmannaeyjar archipelago. The archipelago is of volcanic origin, but the other islands are ca. 5 000 to 40 000 yrs old. The first two moss species were found on Surtsey as early as 1967 and several new bryophyte species were discovered every year until 1973 when regular sampling ended. Systematic bryophyte inventories in a grid of 100 m × 100 m quadrats were made in 1970 – 1972 and 2008. Here we report results from an inventory in 2018, when the same quadrats of the grid system as in 2008 were searched for bryophytes. In addition, we surveyed the bryophyte flora of Elliðaey – a ca. 5 000 yrs old island at the more sheltered north-eastern end of the archipelago.On Surtsey, distributional expansion and contraction of earlier colonists was revealed as well as presence of new colonists. Total number of taxa increased from 43 to 59 between 2008 and 2018. The average species richness increased from 4.5 taxa/quadrat in 2008 to 6.6 taxa/quadrat in 2018 (empty quadrats omitted): 32 quadrats showed an increase in species richness; three quadrats showed no change; ten quadrats showed a slight decrease of 1 – 2 taxa, while one quadrat showed a considerable decrease of 7 taxa – that quadrat was within the lush grassland of the gull colony where bryophytes were outcompeted by the graminoids. Quadrats with the strongest increase in species richness were also within areas affected by seabirds but had not been as overgrown with grassland. On Elliðaey, the predominant habitat was grassland, like the one at the centre of the gull colony on Surtsey. On the island, we registered 22 taxa; 13 were also found on Surtsey in 2018, 4 have been found on Surtsey in earlier surveys and 4 species have never been found on Surtsey. We predict the species richness on Surtsey will continue to grow but level off before starting to decrease as the lava fields disappear and grassland becomes more dominant. Continued monitoring, without long breaks, is essential to evaluate how fast the bryophyte vegetation develops in the years to come

Biomass and composition of understory vegetation and the forest floor carbon stock across Siberian larch and mountain birch chronosequences in Iceland

International audienceChanges in understory biomass, forest floor carbon (C) stock and vegetation composition were studied in six age-classes of Siberian larch (Larix sibirica) and two age-classes of native birch (Betula pubescens) in Iceland. The ground vegetation was less in the larch during the thicket stage and in the old-growth birch compared to a treeless pasture. Understory biomass was strongly related to canopy gap fraction across forest stands (P < 0.001), but not to soil pH or soil C/N ratio. Increased mass of dead wood and alterations in vegetation composition increased the forest floor C-stock of older forests. The forest floor had reached as high C-stock as the pasture's ground vegetation in ca. 50 years in the managed larch plantations and in ca. hundred years in the unmanaged birch forest. This study clearly shows the importance of which time-step is used when changes in forest floor C-stocks are computed for afforestation areas.Biomasse et composition de la végétation de sous-bois, et stock de carbone du sol dans une chronoséquence de mélèze de Sibérie et de bouleau pubescent en Islande. Nous avons étudié en Islande les modifications de biomasse, de composition floristique, et de stock de carbone en sous bois dans des peuplements de mélèze de Sibérie correspondant à 6 classes d'âge, et dans des peuplements de bouleau pubescent correspondant à deux classes d'âge. En comparaison avec une pâture non boisée, la végétation au sol était moins développée dans le peuplement de mélèze au stade gaulis et dans celui de bouleau mature. La biomasse au sol était fortement corrélée à la fraction de trouées dans l'ensemble des peuplements (p < 0,001) mais ni au pH du sol ou au rapport C/N. Une quantité croissante de bois mort et des changements dans la composition floristique étaient à l'origine de la croissance des stocks de carbone au sol dans les peuplement âgés. La surface du sol était aussi riche en carbone que la végétation d'une pâture dès l'âge de 50 ans dans les plantations gérées de mélèze atteignant 50 ans environ et dans des peuplements spontanés de bouleau à 100 ans. Cette étude montre clairement l'importance du choix du pas de temps pour l'estimation des stocks de carbone au sol de peuplements forestiers

Icelandic grasslands as long-term C sinks under elevated organic N inputs

About 10% of the anthropogenic CO2 emissions have been absorbed by northern terrestrial ecosystems during the past decades. It has been hypothesized that part of this increasing carbon (C) sink is caused by the alleviation of nitrogen (N) limitation by increasing anthropogenic N inputs. However, little is known about this N-dependent C sink. Here, we studied the effect of chronic seabird-derived N inputs (47–67 kg N ha−1 year−1) on the net soil organic C (SOC) storage rate of unmanaged Icelandic grasslands on the volcanic Vestmannaeyjar archipelago by using a stock change approach in combination with soil dating. We studied both early developmental (young) soils that had been receiving increased N inputs over a decadal timescale since an eruption in 1963, and well-developed soils, that had been receiving N inputs over a millennial timescale. For the latter, however, the effects on both decadal (topsoil; 40 years) and millennial (total soil profile; 1600 years) SOC storage could be studied, as the age of topsoil and the total soil profile could be determined from volcanic ash layers deposited in 1973 and 395 AD. We found that enhanced N availability—either from accumulation over time, or seabird derived—increased the net SOC storage rate. Under low N inputs, early developmental soils were weak decadal C sinks (0.018 ton SOC ha−1 year−1), but this increased quickly under ca. 30 years of elevated N inputs to 0.29 ton SOC ha−1 year−1, thereby equalling the decadal SOC storage rate of the unfertilized well-developed soils. Furthermore, for the well-developed soils, chronically elevated N inputs not only stimulated the decadal SOC storage rate in the topsoil, but also the total millennial SOC storage was consistently higher. Hence, our study suggests that Icelandic grasslands, if not disturbed, can remain C sinks for many centuries under current climatic conditions and that chronically elevated N inputs can induce a permanent strengthening of this sink