Short-term tundra plant-community nutrient responses to herbivory and warming: New insights from Near infrared-reflectance spectroscopy methodology

In the long-term, herbivores and climate warming have been shown to alter nutrient levels in tundra plant communities by changing the functional composition of the vegetation. Here, I asked the extent to which they affect tundra plant-community nutrient levels in the short-term by directly modifying the chemistry of plants. I first developed a time- and cost-effective method that allowed me to account for the high variability in nutrient-related plant traits among plant individuals, and further scale up to the plant-community level (Paper I). Then, I applied such methodology to investigate short-term (one/two-year) plant-community nutrient-level responses to herbivores in sub-Arctic/alpine tundra-grasslands (Finnmark) and to herbivory and warming across different habitats in a high-Arctic ecosystem (Svalbard). Herbivory and warming were key, short-term modifiers of tundra plant-community nutrient levels, thus affecting plant-community nutrient dynamics (Paper II), herbivore forage quality (Papers III, V) and the amount of nutrients available to herbivores in summer (Paper V), and the biogeochemistry of the ecosystem (Paper IV). This thesis provides clear evidence that herbivores and climate warming cause immediate changes in tundra plant-community nutrient levels, and that these changes are happening at a much shorter time-scale than previously revealed. Considerable short-term changes in plant-community nutrient levels, as those detected in this work, are likely to have strong implications for the immediate functioning of tundra ecosystems and the trophic interactions established therein

Forage quality in tundra grasslands under herbivory: Silicon-based defences, nutrients and their ratios in grasses

1. Herbivore-induced changes in both leaf silicon-based defence and nutrient levels are potential mechanisms through which grazers alter the quality of their own grass supply. In tundra grasslands, herbivores have been shown to increase nutrient contents of grasses; yet, it is an open question whether they also increase grass silicon-based defence levels. Here, we asked if, and to what extent, herbivores affect silicon content and silicon:nutrient ratios of grasses found in tundra grasslands. 2. We performed an herbivore-interaction field-experiment spanning four tundragrassland sites. At each site, we established reindeer-open and reindeer-exclusion plots in tundra-patches that had been disturbed or not by small rodents during the previous winter, for a total of 96 plots. We randomly collected over 1,150 leaf samples of inherently silicon-rich and silicon-poor grass species throughout a growing season and analysed silicon, nitrogen and phosphorus contents of each leaf. 3. Small-rodent winter disturbance did not affect grass silicon content, but increased grass quality (i.e. lowered silicon:nutrient ratios) by enhancing nutrient levels of both silicon-rich (+20%–22%) and silicon-poor (+26%–34%) grasses. Reindeer summer herbivory increased the quality of silicon-rich grasses by decreasing their silicon content (−7%). However, the two herbivores together offset both these quality increments in silicon-rich grasses, thus reducing their quality towards the level of those found in the absence of herbivores and further enhancing their silicon:nutrient ratios (+13%–22%) relative to silicon-poor grasses. 4. Synthesis. We provide the first community-level, field-based assessment of how herbivory-driven changes in both leaf silicon-based defence and nutrient levels alter grass-forage quality in tundra grasslands. Herbivores did not promote a net silicon accumulation in grasses, but rather enhanced their overall quality. Yet, the magnitude of these quality increments varied depending on the herbivore(s

Long-term herbivore removal experiments reveal how geese and reindeer shape vegetation and ecosystem CO2-fluxes in high-Arctic tundra

1. Given the current rates of climate change, with associated shifts in herbivore population densities, understanding the role of different herbivores in ecosystem functioning is critical for predicting ecosystem responses. Here, we examined how migratory geese and resident, non-migratory reindeer—two dominating yet functionally contrasting herbivores—control vegetation and ecosystem processes in rapidly warming Arctic tundra. 2. We collected vegetation and ecosystem carbon (C) flux data at peak plant growing season in the two longest running, fully replicated herbivore removal experiments found in high-Arctic Svalbard. Experiments had been set up independently in wet habitat utilised by barnacle geese Branta leucopsis in summer and in moist-to-dry habitat utilised by wild reindeer Rangifer tarandus platyrhynchus year-round. 3. Excluding geese induced vegetation state transitions from heavily grazed, mossdominated (only 4 g m−2 of live above-ground vascular plant biomass) to ungrazed, graminoid-dominated (60 g m−2 after 4-year exclusion) and horsetail-dominated (150 g m−2 after 15-year exclusion) tundra. This caused large increases in vegetation C and nitrogen (N) pools, dead biomass and moss-layer depth. Alterations in plant N concentration and CN ratio suggest overall slower plant community nutrient dynamics in the short-term (4-year) absence of geese. Long-term (15-year) goose removal quadrupled net ecosystem C sequestration (NEE) by increasing ecosystem photosynthesis more than ecosystem respiration (ER). 4. Excluding reindeer for 21 years also produced detectable increases in live aboveground vascular plant biomass (from 50 to 80 g m−2; without promoting vegetation state shifts), as well as in vegetation C and N pools, dead biomass, moss-layer depth and ER. Yet, reindeer removal did not alter the chemistry of plants and soil or NEE. 5. Synthesis. Although both herbivores were key drivers of ecosystem structure and function, the control exerted by geese in their main habitat (wet tundra) was much more pronounced than that exerted by reindeer in their main habitat (moist-todry tundra). Importantly, these herbivore effects are scale dependent, because geese are more spatially concentrated and thereby affect a smaller portion of the tundra landscape compared to reindeer. Our results highlight the substantial heterogeneity in how herbivores shape tundra vegetation and ecosystem processes, with implications for ongoing environmental change

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km² resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e., offset) between in-situ soil temperature measurements, based on time series from over 1200 1-km² pixels (summarized from 8500 unique temperature sensors) across all the world’s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in-situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.publishedVersio

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world\u27s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (−0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km² resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e., offset) between in-situ soil temperature measurements, based on time series from over 1200 1-km² pixels (summarized from 8500 unique temperature sensors) across all the world’s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in-situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Global maps of soil temperature.

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0-5 and 5-15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Short-term tundra plant-community nutrient responses to herbivory and warming: New insights from Near infrared-reflectance spectroscopy methodology

In the long-term, herbivores and climate warming have been shown to alter nutrient levels in tundra plant communities by changing the functional composition of the vegetation. Here, I asked the extent to which they affect tundra plant-community nutrient levels in the short-term by directly modifying the chemistry of plants. I first developed a time- and cost-effective method that allowed me to account for the high variability in nutrient-related plant traits among plant individuals, and further scale up to the plant-community level (Paper I). Then, I applied such methodology to investigate short-term (one/two-year) plant-community nutrient-level responses to herbivores in sub-Arctic/alpine tundra-grasslands (Finnmark) and to herbivory and warming across different habitats in a high-Arctic ecosystem (Svalbard). Herbivory and warming were key, short-term modifiers of tundra plant-community nutrient levels, thus affecting plant-community nutrient dynamics (Paper II), herbivore forage quality (Papers III, V) and the amount of nutrients available to herbivores in summer (Paper V), and the biogeochemistry of the ecosystem (Paper IV). This thesis provides clear evidence that herbivores and climate warming cause immediate changes in tundra plant-community nutrient levels, and that these changes are happening at a much shorter time-scale than previously revealed. Considerable short-term changes in plant-community nutrient levels, as those detected in this work, are likely to have strong implications for the immediate functioning of tundra ecosystems and the trophic interactions established therein

One leaf for all: Chemical traits of single leaves measured at the leaf surface using near-infrared reflectance spectroscopy

1. The leaf is an essential unit for measures of plant ecological traits. Yet, measures of plant chemical traits are often achieved by merging several leaves, masking potential foliar variation within and among plant individuals. This is also the case with cost‐effective measures derived using near‐infrared reflectance spectroscopy (NIRS). The calibration models developed for converting NIRS spectral information to chemical traits are typically based on spectra from merged and milled leaves. In this study, we ask whether such calibration models can be applied to spectra derived from whole leaves, providing measures of chemical traits of single leaves. 2. We sampled cohorts of single leaves from different biogeographic regions, growth forms, species and phenological stages to include variation in leaf and chemical traits. For each cohort, we first sampled NIRS spectra from each whole, single leaf, including leaf sizes down to Ø 4 mm (the minimum area of our NIRS application). Next, we merged, milled and tableted the leaves and sampled spectra from the cohort as a tablet. We applied arctic–alpine calibration models to all spectra and derived chemical traits. Finally, we evaluated the performance of the models in predicting chemical traits of whole, single leaves by comparing the traits derived at the level of leaves to that of the tablets. 3. We found that the arctic–alpine calibration models can successfully be applied to single, whole leaves for measures of nitrogen (R2 = 0.88, RMSE = 0.824), phosphorus (R2 = 0.65, RMSE = 0.081) and carbon (R2 = 0.78, RMSE = 2.199) content. For silicon content, we found the method acceptable when applied to silicon‐rich growth forms (R2 = 0.67, RMSE = 0.677). We found a considerable variation in chemical trait values among leaves within the cohorts. 4. This time‐ and cost‐efficient NIRS application provides non‐destructive measures of a set of chemical traits in single, whole leaves, including leaves of small sizes. The application can facilitate research into the scales of variability of chemical traits and include intra‐individual variation. Potential trade‐offs among chemical traits and other traits within the leaf unit can be identified and be related to ecological processes. In sum, this NIRS application can facilitate further ecological understanding of the role of leaf chemical traits