Erosion av restaurerade lekbottnar för öring i två bäckar i norra Sverige

Many rivers and streams in northern Sweden have been channelized due to timber floating. This has severely degraded the spawning habitats for salmon and trout, and therefore a common action when restoring channelized rivers is to establish new spawning beds by adding gravel to the streambed. However, little is known about the longevity of these spawning beds, and erosion caused by both water discharge and spawning fish may move gravel/pebbles out the spawning bed, gradually decreasing their functionality. In this thesis, I studied erosion of eight spawning beds (approx. 5 x 2 m), constructed in two different streams, Storkvarnbäcken and Mattjokkbäcken, in northern Sweden. In total, 643 pebbles were tagged with half-duplex passive integrated transponder tags and placed on newly constructed spawning beds, which made it possible to study their movement over multiple years by scanning the spawning beds with PIT-tag reader/antennas. The overall objectives for this study were to quantify and describe erosion process of constructed spawning beds. More specifically, I investigated the effect of streambed slope and pebble size on the distance substrate were moved by erosion processes, and on the likelihood of substrate remaining on the spawning bed. Further, I evaluated different methods used to position PIT-tagged pebbles in boreal streams (conventional GPS, High Precision GPS, and Laser distance meter), and specifically investigated precision and accuracy of commonly used GPS-systems integrated in PIT-tag reader units. Overall, recovery of PIT-tagged pebbles by scanning spawning beds with PIT-tag antennas were high despite multiple years between the deployment of pebbles and the scanning. 85 % of pebbles were found in Storkvarnbäcken 8 years after deployment, and 60 % were found in Mattjokkbäcken 6 years after deployment. There was a big difference between the two study streams in the proportion of recovered pebbles that had remained on the spawning beds (82% remaining in Storkvarnbäcken, and 45% in Mattjokkbäcken), and the probability of pebbles being found outside the spawning bed were significantly higher in Mattjokkbäcken. The average distance pebbles had been moved by erosion was 1m in Storkvarnbäcken, translating into 0.13 m / year, and 6.21 m in Mattjokkbäcken (1,04 m /year). However, there was a considerable variation in erosion rate over time, and in Storkvarnbäcken the average distance moved was 0.48m the first month after deployment, after which erosion-rate decreased to less than 0.08/year, suggesting that average annual erosion rates should be used carefully. There was a positive relationship between the slope of the spawning bed and the distance pebbles were moved by erosion, and pebbles located on spawning beds with a steeper slope had moved longer distances on average. Within the range of 0-3% slope, average distance increased from 7 m. Erosion caused small pebbles (45-55 mm) to move a significantly longer distance than medium sized pebbles (65-75 mm), however no difference in distance moved could be found between small and large size (85-95 mm) pebbles. In average, GPS integrated in PIT-tag reader units had very poor accuracy compare to high-precision GPS (Mean Absolute Error (MAE) 4.7m), and to laser-distance meter (MAE 2.5m). Also, the precision of pebble distances calculated using integrated GPS were low, (±2.87m for mean distances in Storkvarnbäcken, ±1.34m for Mattjokk). The MAE for the high precision GPS was somewhat higher than when using the laser, which indicates that the high precision GPS is closer to the true value. Results are discussed based on the many interacting factors underlying erosion rates and spawning bed longevity, and the way the results could be used as guidelines for construction of trout spawning beds

Är hyggesfritt en mossig framgång?

Det moderna trakthyggesbruket har inneburit bruten kontinuitet samt homogenisering av de boreala skogarna, vilket haft en negativ inverkan på biodiversiteten. Under senare år har hyggesfria metoder, såsom blädning, ökat i intresse eftersom ett kontinuerligt trädskikt kan bibehålla de livsmiljöer som många skogslevande arter behöver. Kunskapen kring hur blädning påverkar markvegetationen är dock begränsad. Många marklevande mossarter är beroende av skugga och hög luftighet vilket ett slutet krontak ger. Vi ville därför undersöka hur mossor påverkas av blädning. För att undersöka detta användes abundans av mossor för att estimera funktionell förändring av markvegetationen och artrikedom för att estimera påverkan på biodiversitet. Nio lokaler i Norrland och norra Svealand, vardera försedda med en blädnings- och en kontrollyta, inventerades före och efter blädning för insamling av mossdata. Huvudslutsatserna är (i) att abundansen och den totala artrikedomen av mossor inte påverkas av blädning, (ii) endast artrikedomen för bladmossor visade på en signifikant skillnad mellan kontroll- och blädningsytorna, (iii) en minskad artrikedom har setts på kontrollytorna men inte på blädningsytorna och (iv) signalarterna samt de rödlistade arterna tycks inte ha påverkats av blädning i någon större utsträckning. Den extrema värme och torka som präglade sommaren 2018 kan ha orsakat de förändringar som observerats. Resultatet indikerar att blädning kan vara ett bra komplement till det traditionella skogsbruket eftersom blädningsytorna inte drabbats hårdare än kontrollytorna i avseende på abundans och artrikedom hos mossor, vilket tyder på att blädning inte reducerar ett bestånds motståndskraft mot torka på kort sikt. För att dra slutsatser kring långtidseffekterna av återkommande blädning krävs fler studier som sträcker sig över en längre tidshorisont.Mordern forestry has resulted in broken continuity and homogenization of the boreal forests, which has lead to a negative impact on biodiversity. The interest regarding continuous-cover forestry methods, like selective thinning, has increased over the years since a continuous tree layer can maintain the habitats that many forest-living species need. However, the knowledge about how the vegetation on the forest floor is affected by selective thinning is limited. Many ground-living bryophyte species are dependent on a shady and humid climate provided by a closed canopy. We therefore wanted to investigate how bryophytes are affected by selective thinning. To study this, abundance of mosses was used to estimate functional change in the forest floor vegetation and species richness in order to estimate the impact on biodiversity. Nine stands in central boreal Sweden, each including one plotwith selective thinning and one control plot, were inventoried before and after treatment for the collection of bryophyte data. The main results are (i) that the abundance and the total species richness of mosses are not affected by selective thinning, (ii) only species diversity of mosses showed a significant difference between the control sites and sites with selective thinning and (iv) signal species and red-list species do not appear to be affected by selective thinning. The heat and drought of the summer 2018 may have affected the results. The result indicates that selective thinning can be a good complement to modern forestry. Since, plots treated with selective thinning are not more affected than the control plots. Selective thinning can therefore retain the short term resistance against drought. More studies are required to investigate the long-term effects of continuous selective thinning