A System for Acquisition, Processing and Visualization of Image Time Series from Multiple Camera Networks

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Spatial variation and seasonal dynamics of leaf-area index in the arctic tundra-implications for linking ground observations and satellite images

Vegetation in the arctic tundra typically consists of a small-scale mosaic of plant communities, with species differing in growth forms, seasonality, and biogeochemical properties. Characterization of this variation is essential for understanding and modeling the functioning of the arctic tundra in global carbon cycling, as well as for evaluating the resolution requirements for remote sensing. Our objective was to quantify the seasonal development of the leaf-area index (LAI) and its variation among plant communities in the arctic tundra near Tiksi, coastal Siberia, consisting of graminoid, dwarf shrub, moss, and lichen vegetation. We measured the LAI in the field and used two very-high-spatial resolution multispectral satellite images (QuickBird and WorldView-2), acquired at different phenological stages, to predict landscape-scale patterns. We used the empirical relationships between the plant community-specific LAI and degree-day accumulation (0 degrees C threshold) and quantified the relationship between the LAI and satellite NDVI (normalized difference vegetation index). Due to the temporal difference between the field data and satellite images, the LAI was approximated for the imagery dates, using the empirical model. LAI explained variation in the NDVI values well (R-adj.(2) 0.42-0.92). Of the plant functional types, the graminoid LAI showed the largest seasonal amplitudes and was the main cause of the varying spatial patterns of the NDVI and the related LAI between the two images. Our results illustrate how the short growing season, rapid development of the LAI, yearly climatic variation, and timing of the satellite data should be accounted for in matching imagery and field verification data in the Arctic region.Peer reviewe

Digital photography for assessing the link between vegetation phenology and CO2 exchange in two contrasting northern ecosystems

Digital repeat photography has become a widely used tool for assessing the annual course of vegetation phenology of different ecosystems. By using the green chromatic coordinate (GCC) as a greenness measure, we examined the feasibility of digital repeat photography for assessing the vegetation phenology in two contrasting high-latitude ecosystems. Ecosystem-atmosphere CO2 fluxes and various meteorological variables were continuously measured at both sites. While the seasonal changes in GCC were more obvious for the ecosystem that is dominated by annual plants (open wetland), clear seasonal patterns were also observed for the evergreen ecosystem (coniferous forest). Daily and seasonal time periods with sufficient solar radiation were determined based on images of a grey reference plate. The variability in cloudiness had only a minor effect on GCC, and GCC did not depend on the sun angle and direction either. The daily GCC of wetland correlated well with the daily photosynthetic capacity estimated from the CO2 flux measurements. At the forest site, the correlation was high in 2015 but there were discernible deviations during the course of the summer of 2014. The year-to-year differences were most likely generated by meteorological conditions, with higher temperatures coinciding with higher GCCs. In addition to depicting the seasonal course of ecosystem functioning, GCC was shown to respond to environmental changes on a timescale of days. Overall, monitoring of phenological variations with digital images provides a powerful tool for linking gross primary production and phenology.Peer reviewe

Variation in CO2 and CH4 fluxes among land cover types in heterogeneous Arctic tundra in northeastern Siberia

Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem-atmosphere exchange of carbon (C). However, the estimates of regional carbon dioxide (CO2) and methane (CH4) budgets are highly uncertain. We measured CO2 and CH4 fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71 degrees N, 128 degrees E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined the net ecosystem exchange (NEE) of CO2, ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg), and CH4 flux during the growing season. We applied a previously developed high-spatial-resolution land cover map over an area of 35.8 km(2) for spatial extrapolation. Among the land cover types varying from barren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at the photosynthetically active photon flux density of 800 mu mol m(-2) h(-1) (NEE800 and Pg(800)) were greatest in the graminoid-dominated habitats, i.e., streamside meadow and fens, with NEE800 and Pg(800) of up to -21 (uptake) and 28 mmol M-2 h(-1), respectively. Vascular LAI was a robust predictor of both NEE800 and Pg(800) and, on a landscape scale, the fens were disproportionately important for the summertime CO2 sequestration. Dry tun- dra, including the dwarf-shrub and lichen tundra, had smaller CO2 exchange rates. The fens were the largest source of CH4, while the dry mineral soil tundra consumed atmospheric CH4, which on a landscape scale amounted to -9 % of the total CH(4 )balance during the growing season. The largest seasonal mean CH4 consumption rate of 0.02 mmol m(-2) h(-1) occurred in sand- and stone-covered barren areas. The high consumption rate agrees with the estimate based on the eddy covariance measurements at the same site. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land cover type. This study highlights the need to distinguish different land cover types including the dry tundra habitats to account for their different CO2 and CH4 flux patterns, especially the consumption of atmospheric CH4, when estimating tundra C exchange on a larger spatial scale.Peer reviewe

Networked web-cameras monitor congruent seasonal development of birches with phenological field observations

Ecosystems' potential to provide services, e.g. to sequester carbon, is largely driven by the phonological cycle of vegetation. Timing of phenological events is required for understanding and predicting the influence of climate change on ecosystems and to support analyses of ecosystem functioning. Analyses of conventional camera time series mounted near vegetation has been suggested as a means of monitoring phenological events and supporting wider monitoring of phenological cycle of biomes that is frequently done with satellite earth observation (EO). Especially in the boreal biome, sparsely scattered deciduous trees amongst conifer-dominant forests pose a problem for EO techniques as species phenological signal mix, and render EO data difficult to interpret. Therefore, deriving phonological information from on the ground measurements would provide valuable reference data for earth observed phonology products in a larger scale. Keeping this in mind, we established a network of digital cameras for automated monitoring of phenological activity of vegetation in the boreal ecosystems of Finland. Cameras were mounted at 14 sites, each site having 1-3 cameras. In this study, we used data from 12 sites to investigate how well networked cameras can detect the phenological development of birches (Betula spp.) along a latitudinal gradient. Birches typically appear in small quantities within the dominant species. We tested whether the small, scattered birch image elements allow a reliable extraction of colour indices and the temporal changes therein. We compared automatically derived phenological dates from these birch image elements both to visually determined dates from the same image time series and to independent observations recorded in the phenological monitoring network covering the same region, Automatically extracted season start dates, which were based on the change of green colour fraction in spring, corresponded well with the visually interpreted start of the season, and also to the budburst dates observed in the field. Red colour fraction turned out to be superior to the green colour-based indices in predicting leaf yellowing and fall. The latitudinal gradients derived using automated phenological date extraction corresponded well with the gradients estimated from the phenological field observations. We conclude that small and scattered birch image elements allow reliable extraction of key phonological dates for the season start and end of deciduous species studied here, thus providing important species-specific data for model validation and for explaining the temporal variation in EO phenology products.Peer reviewe

Refining the role of phenology in regulating gross ecosystem productivity across European peatlands

Abstract The role of plant phenology as regulator for gross ecosystem productivity (GEP) in peatlands is empirically not well constrained. This is because proxies to track vegetation development with daily coverage at the ecosystem scale have only recently become available and the lack of such data has hampered the disentangling of biotic and abiotic effects. This study aimed at unraveling the mechanisms that regulate the seasonal variation in GEP across a network of eight European peatlands. Therefore, we described phenology with canopy greenness derived from digital repeat photography and disentangled the effects of radiation, temperature and phenology on GEP with commonality analysis and structural equation modeling. The resulting relational network could not only delineate direct effects but also accounted for possible effect combinations such as interdependencies (mediation) and interactions (moderation). We found that peatland GEP was controlled by the same mechanisms across all sites: phenology constituted a key predictor for the seasonal variation in GEP and further acted as distinct mediator for temperature and radiation effects on GEP. In particular, the effect of air temperature on GEP was fully mediated through phenology, implying that direct temperature effects representing the thermoregulation of photosynthesis were negligible. The tight coupling between temperature, phenology and GEP applied especially to high latitude and high altitude peatlands and during phenological transition phases. Our study highlights the importance of phenological effects when evaluating the future response of peatland GEP to climate change. Climate change will affect peatland GEP especially through changing temperature patterns during plant-phenologically sensitive phases in high latitude and high altitude regions.Peer reviewe

Set-up and instrumentation of the greenhouse gas measurements on experimental sites of continuous cover forestry

A set of experimental study sites was established to monitor greenhouse gas (GHG) emissions from drained peatland forests under different harvesting regimes in Finland. The purpose of these experimental sites is to study the effects of continuous cover forestry (CCF) and clear-cutting (CC) on ecosystem processes including GHG emissions and stand development on drained peatland forests. The sites represent fertile Norway spruce dominated peatland forests, where soil GHG emissions are high due to drainage that has exposed peat to decomposition in aerobic conditions. Two “flagship” sites for greenhouse gas (GHG) monitoring have been established and instrumented by the Natural Resources Institute Finland (Luke), University of Helsinki (UH) and the Finnish Meteorological Institute (FMI). The sites host continuous GHG monitoring with Eddy Covariance (EC) towers and with automatic chambers. In addition, greenhouse gas (CO2, CH4, and N2O) emissions are monitored with manually operated chambers at four sites, where effects of selection (CCF) harvests are studied with replicated treatments. These data will be used to calculate the ecosystem and soil GHG balances of the sites by using methodologies standardized earlier and compatible with the IPCC guidelines. On all experimental sites, ground water table (WT), tree growth and regeneration are monitored in different management trials. These data will form the basic data needed for designing and demonstrating optimal harvesting cycles and evaluating and generalizing the climate impacts. The results including the biological drainage capacity (evapotranspiration) of different-sized tree stands as well as the soil GHG balance of different tree stand – WT combinations will be incorporated into existing models that can be used to estimate the mitigation obtained with different management options and in different site and climatic conditions. The study sites are actively used for training and demonstration of alternative peatland management practices by host projects and by multiple stakeholders. The host projects and organizations also promote further extensions for the measurements and all complementary research activities are welcome to these study sites

Insights into the above- and belowground CO2 fluxes on pristine and managed peatlands: tools for examining ecosystem carbon cycle

Climate change and land-use changes modify the conditions and characteristics of northern peatlands and thus affect their carbon (C) balance. In this thesis the above- and belowground carbon dioxide (CO2) fluxes of peatland ecosystems were examined with process-specific methods. These methods provide additional information and support the standardized measurements. Phenology cameras were used for observing the seasonal development and greenness of vegetation at different ecosystems (pristine peatlands and a forest site). This vegetation phenology data was combined with CO2 flux measurements, meteorological data and satellite-derived data. Also, the soil CO2 fluxes and priming effect (PE), i.e., the change in decomposition of soil after addition of fresh C, were studied in laboratory experiments with isotope techniques. The peat for these experiments was collected from two forestry-drained peatlands with different nutrient status. The phenology cameras were found to be a solid method for tracking changes in ecosystem greenness. Variations in vegetation dynamics were observed between different study years as well as between different ecosystems and plant communities within a peatland. These differences in vegetation greenness were linked to nutrient availability and vegetation composition of the site or the region of interest. The peatland vegetation also compensated for the late growing season start: during a summer with the latest growing season start the vegetation was capable to reach the maximum GCC and maximal photosynthetic activity over the measurement years. The effect of drought on greenness and maximal photosynthetic activity was found to depend on the hydrological features of the site. The camera- and satellite-derived greenness index showed similar seasonal cycle, but the poorer temporal resolution of the satellite data affected the reliable determination of the timing of different phenological phases. The results of the laboratory studies indicated that the nutrient-rich peat had higher basal peat respiration rate and also higher peat respiration in the presence of plants. The results are in accordance with the observed differences in the CO2 balances at the sites from which the peat samples were collected: the nutrient-rich peat soil has been observed to lose C and the nutrient-poor peat accumulate C. No priming or negative priming was found in these studies. Thus, it can be concluded that the addition of fresh C (in the form of seedling roots or glucose) did not increase, or it even decreased, the old peat decomposition in these peat soils. The negative priming was stronger in the peat soil with less nutrients available.Muuttuva ilmasto ja maankäytön muutokset muokkaavat pohjoisten turvemaiden olosuhteita ja ominaispiirteitä, ja siten vaikuttavat niiden hiilitaseeseen. Tässä väitöskirjassa tutkittiin maanpäällisiä ja maanalaisia hiilidioksidivirtoja prosessikohtaisilla menetelmillä. Näillä menetelmillä tutkituista prosesseista voidaan tuottaa lisätietoa, jota voidaan käyttää standardoitujen mittauksien tukena. Erilaisten ekosysteemien kasvillisuuden kehitystä ja vihreyttä havainnoitiin fenologiakameroiden avulla. Tätä aineistoa yhdistettiin hiilidioksidimittausten, meteorologisten mittausten ja satelliittiaineistojen kanssa. Lisäksi turpeen hiilidioksidivuota ja priming effectiä tarkasteltiin laboratoriokokeissa isotooppimenetelmien avulla. Priming effectillä tarkoitetaan maaperän vanhan orgaanisen aineen hajotuksessa tapahtuvaa muutosta, kun maahan lisätään helposti hajotettavaa tuoretta orgaanista ainesta. Turve näitä laboratoriokokeita varten kerättiin kahdelta metsäojitetulta suolta. Fenologiakamerat osoittautuivat luotettavaksi menetelmäksi ekosysteemin vihreyden ja sen kehityksen havainnointiin. Sekä erilaisten ekosysteemien, kasvillisuusryhmien, että vuosien välillä havaittiin eroavaisuuksia. Näiden eroavaisuuksien todettiin liittyvän tutkittavien kohteiden ravinteiden saatavuuteen ja kasvillisuuden ominaisuuksiin. Suokasvillisuus oli myös kykenevä kompensoimaan kasvuaan myöhemmin kesän aikana huolimatta kasvukauden alun viivästymisestä. Kuivuuden vaikutus kasvillisuuden vihreyteen ja fotosynteettiseen aktiivisuuteen riippui suon hydrologisista ominaispiirteistä. Kameroista ja satelliitista johdetuilla vihreysindekseillä oli samanlainen kausittainen vaihtelu, mutta huonomman ajallisen resoluution vuoksi erilaisten fenologisten vaiheiden määrittäminen satelliittidatasta oli epäluotettavampaa. Laboratoriokokeiden tulokset osoittivat, että hajotus oli suurempaa ravinteikkaammassa turpeessa. Nämä tulokset ovat yhdenmukaisia aiempien havaintojen kanssa, joiden mukaan ravinteikkaamman metsäojitetun suon maaperä on hiilen lähde ja vähäravinteisen metsäojitetun suon maaperä hiilen nielu. Näissä tutkimuksissa ei havaittu priming effectiä tai se oli negatiivista. Siten voidaan päätellä, että tuoreen hiilen lisäys ei lisännyt, vaan jopa vähensi, vanhan hiilen hajotusta näillä turvemailla. Negatiivinen priming effect oli lisäksi voimakkaampaa vähäravinteisessa turpeessa

Stronger negative priming effect and lower basal respiration rates in nutrient-poor as compared to nutrient-rich forestry-drained peatland

Peer reviewe