Browsing by Subject "tree mortality"

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  • Cailleret, Maxime; Dakos, Vasilis; Jansen, Steven; Robert, Elisabeth M.R.; Aakala, Tuomas; Amoroso, Mariano M.; Antos, Joe A.; Bigler, Christof; Bugmann, Harald; Caccianaga, Marco; Camarero, Jesus-Julio; Cherubini, Paolo; Goeya, Marie R.; Cufar, Katarina; Das, Adrian J.; Davi, Hendrik; Gea-Izquierdo, Guillermo; Gillner, Sten; Haavik, Laurel J.; Hartmann, Henrik; Heres, Ana-Maria; Hultine, Kevin R.; Janda, Pavel; Kane, Jeffrey M.; Kharuk, Vlachelsav I.; Kitzberger, Thomas; Klein, Tamir; Levanic, Tom; Linares, Juan-Carlos; Lombardi, Fabio; Mäkinen, Harri; Meszaros, Ilona; Metsaranta, Juha M.; Oberhuber, Walter; Papadopoulos, Andreas; Petritan, Any Mary; Rohner, Brigitte; Sanguesa-Barreda, Gabriel; Smith, Jeremy M.; Stan, Amanda B.; Stojanovic, Dejan B.; Laura Suarez, Maria; Svoboda, Miroslav; Trotsiuk, Volodymyr; Villalba, Ricardo; Westwood, Alana R.; Wyckoff, Peter H.; Martinez-Vilalta, Jordi (2019)
    Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter- annual growth variability and a decrease in growth synchrony in the last similar to 20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.
  • Kinnunen, Aleksi (Helsingin yliopisto, 2021)
    Trees face an increasing variety of health threats. The overall effects of climate change on trees and forests are difficult to predict. As a result of the warming climate, the growing season is lengthening, improving the growth of the trees, but at the same time drought and insect damages may become more common and the risk of storm damage increases. There are many benefits to monitoring tree mortality. It can be used to assess the health status of forests, productivity, carbon sequestration and the ecological impacts of dead trees on forest ecosystems. Causes leading to tree death can include biological, climatic or human related factors. Monitoring can increase understanding of the causes of death and potentially help to protect forests better. Tree-related mortality is a spatially and temporally irregular process that is difficult to monitor using traditional inventory methods. Remote sensing makes it possible to map and monitor tree mortality more effectively. The purpose of this thesis was to find out how remote sensing data can be utilized in monitoring tree mortality. The aim was to find out how tree mortality has varied regionally and quantitatively in the Central Park of Helsinki and how accurately dead trees can be identified from aerial imagery. The study period was 2005–2019, during which orthophotos of seven different years were examined. Reference data of 14 212 trees were collected from the aerial time series covering a 15-year period by visual image interpretation. The data included healthy, weakened and dead trees. Heatmap time series were created from the locations of weakened and dead trees to examine quantitative and regional variability in mortality. The average temperatures over the years as well as the rainfall were compared with the dead tree numbers and the correlations between the observations were examined. The collected reference data was also utilized in health status classifications, which were implemented using semi-automatic machine learning methods. The object of the classifications was to identify healthy, weakened and dead trees as well as possible from each other. The canopies of individual trees were delimited by canopy segments obtained from laser scanning data. From the pixels contained in the delimited canopies, image features describing individual trees were calculated. Considerable changes in tree mortality were observed. The number of dead trees at the beginning of the study period increased significantly from year 2005 to year 2009. An exceptionally dry summer in 2006 was identified as a possible reason. In the following years, the situation remained moderate, but in quantitative and regional terms, mortality was at its highest in 2017. Overall, there was an upward trend in mortality during the study period, and average annual temperatures were found correlating strongly with the number of dead trees (r=0.73). The classification accuracies of tree health status varied annually between 89–96%. The seven-year average accuracy was 93.6% with a kappa value of 0.88. The most important features in the classification were the features calculated from the blue channel, such as the maximum value of the channel (B_max), the difference between the maximum and minimum of the channel (B_range) and the skewness of the distribution (B_skew). The results of the thesis showed that tree mortality can be monitored using remote sensing data. Clear changes in the number of dead trees were observed using the time series and possible causes were identified. By identifying the causes behind rising mortality, the effects of climate change can also be better understood. Tree health status classification accuracies were at a good level and dead trees can be mapped from aerial imagery by semi-automatic methods. Based on the thesis, it can be rightly stated that changes in tree mortality can be observed with aerial imagery time series. In addition, the semi-automatic identification of dead trees from aerial imagery can be said to be accurate enough for large-scale use.
  • Kosunen, Maiju; Lyytikaeinen-Saarenmaa, Paeivi; Ojanen, Paavo; Blomqvist, Minna; Starr, Mike (2019)
    Disturbances such as storm events and bark beetle outbreaks can have a major influence on forest soil carbon (C) cycling. Both autotrophic and heterotrophic soil respiration may be affected by the increase in tree mortality. We studied the effect of a storm in 2010 followed by an outbreak of the European spruce bark beetle (Ips typographus L.) on the soil surface respiration (respiration by soil and ground vegetation) at two Norway spruce (Picea abies L.) dominated sites in southeastern Finland. Soil surface respiration, soil temperature, and soil moisture were measured in three types of plotsliving trees (undisturbed), storm-felled trees, and standing dead trees killed by I. typographusduring the summer-autumn period for three years (2015-2017). Measurements at storm-felled tree plots were separated into dead tree detritus-covered (under storm-felled trees) and open-vegetated (on open areas) microsites. The soil surface total respiration for 2017 was separated into its autotrophic and heterotrophic components using trenching. The soil surface total respiration rates at the disturbed plots were 64%-82% of those at the living tree plots at one site and were due to a decrease in autotrophic respiration, but there was no clear difference in soil surface total respiration between the plots at the other site, due to shifts in either autotrophic or heterotrophic respiration. The soil surface respiration rates were related to plot basal area (living and all trees), as well as to soil temperature and soil moisture. As storm and bark beetle disturbances are predicted to become more common in the future, their effects on forest ecosystem C cycling and CO2 fluxes will therefore become increasingly important.
  • da Costa, Antonio C. L.; Rowland, Lucy; Oliveira, Rafael S.; Oliveira, Alex A. R.; Binks, Oliver J.; Salmon, Yann; Vasconcelos, Steel S.; Junior, João A. S.; Ferreira, Leandro V.; Poyatos, Rafael; Mencuccini, Maurizio; Meir, Patrick (2018)
    Transpiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function with climate change can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. We report the only study of forest transpiration following a long-term (>10 year) experimental drought treatment in Amazonian forest. After 15 years of receiving half the normal rainfall, drought-related tree mortality caused total forest transpiration to decrease by 30%. However, the surviving droughted trees maintained or increased transpiration because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent nondroughted forest. If these drought conditions were accompanied by a modest increase in temperature (e.g., 1.5°C), water demand would exceed supply, making the forest more prone to increased tree mortality.
  • Piha, Aura (Helsingfors universitet, 2011)
    Fire is an important driver of the boreal forest ecosystem, and a useful tool for the restoration of degraded forests. However, we lack knowledge on the ecological processes initiated by prescribed fires, and whether they bring about the desired restoration effects. The purpose of this study was to investigate the impacts of low-intensity experimental prescribed fires on four ecological processes in young commercial Scots pine (Pinus sylvestris) stands eight years after the burning. The processes of interest were tree mortality, dead wood creation, regeneration and fire scar formation. These were inventoried in twelve study plots, which were 30 m x 30 m in size. The plots belonged to two different stand age classes: 30-35 years or 45 years old at the time of burning. The study was partly a follow-up of study plots researched by Sidoroff et al. (2007) one year after burning in 2003. Tree mortality increased from 183 stems ha-1 in 2003 to 259 stems ha-1 in 2010, corresponding to 15 % and 21 % of stem number respectively. Most mortality was experienced in the stands of the younger age class, in smaller diameter classes and among species other than Scots pine. By 2010, the average mortality of Scots pine per plot was 18%, but varied greatly ranging from 0% to 63% of stem number. Delayed mortality, i.e. mortality that occurred between 2 and 8 years after fire, seemed to become more important with increasing diameter. The input of dead wood also varied greatly between plots, from none to 72 m3 ha-1, averaging at 12 m3 ha-1. The amount of fire scarred trees per plot ranged from none to 20 %. Four out of twelve plots (43 %) did not have any fire scars. Scars were on average small: 95% of scars were less than 4 cm in width, and 75% less than 40 cm in length. Owing to the light nature of the fire, the remaining overstorey and thick organic layer, regeneration was poor overall. The abundance of pine and other seedlings indicated a viable seed source existed, but the seedlings failed to establish under dense canopy. The number of saplings ranged from 0 to 12 333 stems ha-1. The results of this study indicate that a low intensity fire does not necessarily initiate the ecological processes of tree mortality, dead wood creation and regeneration in the desired scale. Fire scars, which form the basis of fire dating in fire history studies, did not form in all cases.