NWT RESEARCH DATABASE

Tags: physical sciences, vegetation, climate change, environmental change, arctic ecosystems, biogeochemistry

Objective(s): To advance the understanding of how Canadian arctic tundra ecosystems function, and therefore how they are likely to be affected by perturbations including climate change, and resource development and extraction (e.g. mining and pipeline construction).

Project Description: The Arctic is undoubtedly experiencing several major perturbations including climate change, and resource development and extraction (e.g. mining and pipeline construction) that are very likely to substantially affect the structure and functioning of its ecosystems. As a terrestrial ecosystem ecologist, the long-term goal of this research over the next 15-20 years is to substantially advance the understanding of how Canadian arctic tundra ecosystems function, and therefore how they are likely to be affected by these perturbations. There are three major interconnecting themes that underlay this research program: A) The biogeochemical and ecological significance of the recent discovery that mesic tundra plant growth can be co-limited by nitrogen (N) and phosphorus (P), rather than by N alone as has widely been assumed; B) the likely direct and indirect impacts of climate change on tundra shrubs that will affect the structure, functioning and distribution of low arctic terrestrial ecosystems; and C) the fundamental biology of plant-microbial-soil interactions that have significant functional influences on ecosystem biogeochemistry. Below are the specific research questions associated with the themes above, and the methodologies that will be used to address them. Q1. Apart from birch, which other species within the mesic tundra plant community also have their growth co-limited by N and P? The research team will harvest whole plant communities to 10 cm soil depth from 1 m2 plots in the chronic NxP factorial addition experiments that have been maintained since 2004, and separate each species into its major growth components (current year’s shoot growth, older shoot biomass, belowground stems, and roots). Biomass, nutrient concentrations and pool sizes in each component will be compared across the treatments to test the hypothesis that growth of all major mesic tundra species is NP co-limited, but that the extent of P influence relative to N is largest for non-mycorrhizal species (e.g. Eriophorum), moderate for ecto- and ericoid mycorrhizal host plant species (e.g. Betula, Ledum and Vaccinium) and lowest in arbuscular mycorrhizal species (e.g. Rubus). In addition to P, the research team will assist with ICPMS analyses to quantify other plant macro- and micro-nutrients (Mg, K, S, Ca, Fe, Mn, Zn, B, Cu and Mo) in the control and greenhouse warmed plots. In all of the above sampling, only small samples (< 1 litre) of soil will be taken at each time using a serrated knife, and so damage to the ecosystem should be negligible. Q2. How extensive is birch growth co-limitation by N and P across the Arctic? The research team will address this question by quantifying regional variation in birch leaf N:P ratio, and determining whether that ratio is indicative of variation in soil nutrient availability. The research team has had colleagues collect 118 samples of birch leaf tissue and soil directly beneath the sampled shrubs from across North America and will make the permit arrangements necessary to bring in samples from the Scandinavian and Russian Arctic. Leaf N:P concentration ratio is a good indicator of relative N and P limitation in wetlands, but was not related to soil soluble nutrient pools in tundra, probably because the latter is a short term highly dynamic pool that does not well reflect cumulative differences in nutrient availability over one or more growing seasons. Furthermore, the use of foliar N:P ratios to determine N and P limitation of growth assumes that luxury consumption of whichever of these nutrients is non-limiting will occur, and/or that the capacity of the plant to increase growth will match the rate of increased supply of the limiting nutrient. The research team will include leaf samples from the Daring Lake long-term fertilization studies to test this hypothesis, and to determine that foliar ratios can be used to infer the relative importance of these nutrients in limiting shrub growth. The research team will use the pan-Arctic leaf and soil data to test the hypothesis that birch leaf N:P varies significantly across the Arctic in correlation with variation in soil total N and P pools. In all of the above sampling, only small samples (< 400 g fresh weight) of leaves will be taken at each time, and so damage to the ecosystem should be negligible. Q3. What are the implications of NP co-limitation of plant growth for predicting mesic tundra ecosystem responses to climate warming? The greenhouse warming treatment increased dwarf birch new apical stem growth by a factor of 2.5, while the lack of response to the low N addition treatment indicates that moderate increases in N alone (that might realistically be expected under climate warming) are unlikely to stimulate shrub growth. The strong greenhouse response but the absence of a low N addition response may be consistent with the factorial high N and P experiment results indicating that birch shoot growth is co-limited. To address this question, the research team first needs to test the hypothesis that soil warming enhances microbial release of organic and inorganic elemental forms of N and P from soil organic matter during its decomposition. The research team will repeatedly measure soil soluble organic, inorganic, and microbial N and P pools in our greenhouse treatment, which raises both mean summer air and soil temperatures by ~2.5 °C. Since soil P occurs in many different forms, the team will need to first develop a sequential extraction procedure. The team will also measure the activities of key enzymes (e.g. phosphatases that cleave P; N-acetyl glucosaminidase that degrades fungal chitin), and relate them to the relative abundances of bacterial and fungal phylotypes (Theme C) using multivariate statistics. Finally, P in particular is readily mobilized during soil wet up events that often occur late in the growing season in continental low arctic sites, resulting in potential differences in the timing of N and P supply into the soil solution. The research team will test the hypothesis that even if N and P availabilities are enhanced in a warmer climate, timing mismatches in supply could significantly constrain the growth responses of species that are co-limited. In all of the above sampling, only small samples (< 1 litre) of soil will be taken at each time using a serrated knife, and so damage to the ecosystem should be negligible. Enzyme analysis and other chemical determinations will be done after shipping the soils from Daring Lake to our lab at Queen’s. Q4. Is growth of mesic tundra plant communities enhanced by low level N and P additions? Chronic high level N and P additions are useful to determining the relative importance of availability of these nutrients in limiting growth. However, their use in inferring plant community responses to increased nutrient availability associated with warmer soils is suspect because the increases are likely to be much lower than the fertilizer addition levels. Even high projections for increases in soil fertility with climate warming would happen gradually and may elicit quite different plant responses than from the instantaneously high soil fertility generated by chronic large nutrient addition manipulations. To emulate realistic warming-induced nutrient enhancement levels, the research team have been running a low level N addition experiment since 2004 that is having little impact on birch growth. As a result of the recent NP co-limitation study, the team will start a new experiment using factorial low level N and P additions to test the hypotheses that: a) the lack of response to low N was because the plants there were P-limited; and b) that community responses to low level N and P additions correspond in trajectory to the responses to greenhouse warming and therefore represent a better basis for predicting impacts of climate-enhanced nutrient availabilities on tundra vegetation. This experimental manipulation, like the others mentioned earlier will be conducted on five 5 m x 7m plots in the research valley near the Daring Lake research station. Given the very small size of the plots on the landscape, the research team do not think that either locals or animals will be significantly affected. Sampling of soils and plants will be as described above. Q5. What is the mechanistic nature of the NP co-limitation? Recent reviews indicate that interpreting the mechanisms underlying co-limitation by N and P is complex. For example, at first, the results seem contradictory in that although birch shoot growth did not respond to the low N addition, there was a considerable increase in the high N (only) addition, even though one would anticipate that P co-limitation would have constrained growth in the latter treatment. The research team hypothesize that where there is sufficient increase in N availability, particular soil enzymes (all of which have substantial N) may be upregulated to enhance P acquisition (e.g. by phosphatases), thereby alleviating incipient P limitation and resulting in a growth response. The research team will test this by comparing N and P allocation to soil phosphatase, urease and protease enzymes across each of the factorial fertilizer treatments. Sampling will be as described for Questions 1 and 3 above. Q6. What is the outcome of tundra plant-soil microbial competition for climatically realistic slow and moderate increases in nutrient availability over the 5-10 year time scale? When nitrogen is added to tundra, soil microbes tend to accumulate most of it within days to weeks. Over five year and longer time scales, continued large nutrient additions result in enhanced plant uptake by deciduous shrubs in particular, presumably because microbes are ‘saturated’ and no longer competing. Although these studies have been interpreted as indicators of likely vegetation change as nutrient availability increases due to climate warming actual rates of nutrient increase are likely to be much lower. If so, will plants gain access to the enhanced N, or for how long will they be outcompeted by ‘hungry’ (i.e. unsaturated) microbes? Furthermore, will deciduous shrubs be as successful in competing for nutrients that are enhanced moderately and slowly? The recent pulse N addition study suggests that evergreen shrubs may in fact be stronger long term competitors under such conditions, and preliminary analyses of the 6 years of point-frame plant community data indicate an evergreen-dominated response in the greenhouses and a deciduous-dominated response in the fertilized plots. The research team will test the hypothesis that moderate increases in tundra soil N supply expected due to climate warming could be largely immobilized by microbes, resulting in slower and more evergreen-dominated plant community responses than are predicted from long term, annually repeated, high-level fertilizer addition studies. In the snow-shrub 15N study at Daring Lake that the research team completed in 2010, (and that is now accepted for publication pending minor revisions) the team left additional plots for later sampling. The research team will compare plant-microbial partitioning of N in those plots and in the single large N addition plots, and in the low level N addition plots. Sampling will be as described in Questions 1 and 3 above. The principal investigator is genuinely interested in directing that component of the research related to climate change in the Canadian North toward addressing the needs and concerns of indigenous peoples in the region. There is clearly an urgent need for Northern science that has indigenous values, knowledge and concerns right at the core. The research team expects the challenge over the next decade will be to develop the infrastructure to promote genuine two-way information exchange between a cohort of multidisciplinary scientists and the indigenous communities of the North. In the context of this research at Daring Lake in the NWT, the research team has supported local aboriginal students to assist with summer field work, and the team hopes to continue this practice when it is feasible and appropriate. Second, the research team has given talks in Yellowknife on the research that is aimed at public audiences. Third, the research team has had formal agreements with groups such as the North Slave Metis Alliance (NSMA) who were part of large IPY grant. Through this collaboration, locals from the NSMA collected samples on the team’s behalf at various locations in the Coppermine River basin. Fourth, the research team have a continuing collaboration with Joachim Obst who is an ecological consultant living in Yellowknife to examine patterns of vegetation change in the Daring Lake region. Finally, various locals and aboriginals working as or with GNWT officials visit the field site during summer, and the research team make a particular effort to spend time discussing what is being done, and getting their perspectives on the work and what are the most important ecological questions. Realistically, the research team has little interaction with aboriginals or other locals by means other than through the above practices because the nearest community (Wekweètì) is more than 150 km away from the research site. The proposed studies will lead to significantly improved understanding of the biological controls and interactions that govern Arctic terrestrial ecosystem functioning, while providing cutting edge training for students. Ultimately, the hope is that this research will lead to better land management in the North, as well as greater public appreciation of the ecological value of the Arctic, and therefore to more effective and sustainable environmental policies and practices for Canada as a whole. The research team will continue to communicate the results through several different mechanisms: 1) Media interviews with radio and newspapers, 2) work closely with any local assistants that we recruit and spend time explaining the intricacies of the work and its significance for the North, 3) research students actively participate in teaching about the work during the Daring Lake Science Camp which is held for ~18 local NWT high school students each summer, 4) publish the work in international peer-reviewed science journals so that it is accessible to everyone, including the NWT communities, 5) upload copies of relevant publications, theses and powerpoint talks each year as part of the Aurora Research Permit report process, and 6) continue to take advantage of opportunities to give public science talks in Yellowknife. The fieldwork for this study will be conducted from April 1, 2014 to September 30, 2014.