Browsing by Subject "dendrochronology"

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  • Kulha, Niko; Pasanen, Leena; Aakala, Tuomas (2018)
    Time series of repeat aerial photographs currently span decades in many regions. However, the lack of calibration data limits their use in forest change analysis. We propose an approach where we combine repeat aerial photography, tree-ring reconstructions, and Bayesian inference to study changes in forests. Using stereopairs of aerial photographs from five boreal forest landscapes, we visually interpreted canopy cover in contiguous 0.1-ha cells at three time points during 1959-2011. We used tree-ring measurements to produce calibration data for the interpretation, and to quantify the bias and error associated with the interpretation. Then, we discerned credible canopy cover changes from the interpretation error noise using Bayesian inference. We underestimated canopy cover using the historical low-quality photographs, and overestimated it using the recent high-quality photographs. Further, due to differences in tree species composition and canopy cover in the cells, the interpretation bias varied between the landscapes. In addition, the random interpretation error varied between and within the landscapes. Due to the varying bias and error, the magnitude of credibly detectable canopy cover change in the 0.1-ha cells depended on the studied time interval and landscape, ranging from -10 to -18 percentage points (decrease), and from +10 to +19 percentage points (increase). Hence, changes occurring at stand scales were detectable, but smaller scale changes could not be separated from the error noise. Besides the abrupt changes, also slow continuous canopy cover changes could be detected with the proposed approach. Given the wide availability of historical aerial photographs, the proposed approach can be applied for forest change analysis in biomes where tree-rings form, while accounting for the bias and error in aerial photo interpretation.
  • Arnkil, Nora (Helsingin yliopisto, 2015)
    The boreal forest is one of the largest biomes in the world, maintaining natural disturbances such as forest fires and insect outbreaks, still occurring widely at their full scale, frequencies and patterns. However, the knowledge of natural forest dynamics, disturbance factors other than fire and post-disturbance development is still inadequate; this is partly due to the lack of accurate, repetitive measurements with adequate temporal resolution. The aim of this study was to examine the structural change and development of natural, Abies balsamea (L.) Mill. dominated forest stands following an insect outbreak of late-1970s to early 1980s. The focus was on annual tree basal area and species composition change at the stand level during the recent decades. The post-disturbance stand development was studied to see whether the stands were following the development model of steady state and quasi-equilibrium. Additionally, the size and age structure of the stands were studied. The objectives were achieved by using dendrochronological methods with tree-ring analyses, in which the forest characteristics were reconstructed at an annual resolution. The study was carried out in the province of Quebec in Eastern Canada, in the North Shore region (Côte-Nord) of St. Lawrence River. Nine sites of a size of 32 m x 32 m were chosen for data collection. The results showed that the tree species composition of the studied stands had clearly changed from the pre-episode to the current state: the composition of A.balsamea, Picea mariana (Mill.) BSP and Betula papyrifera Marsh. in 1975 had changed towards the abundance of A. balsamea, with notably less of P. mariana and B. papyrifera in 2013. Particularly B. papyrifera seems to disappearing from the studied stands. On average, the shape of the live tree diameter distribution for the whole study area was close to a reverse-J, whereas the dead tree diameter distribution resembled rotated sigmoid, with a plateau in the middle. The trees were of all age, mean age for live A. balsamea, P. mariana and B. papyrifera trees was 91 years (SD ± 32 years), 135 years (SD ± 48 years) and 180 years (SD ± 24 years), respectively. The examination of the development of total basal area showed apparent changes during the four decades from 1975 to 2013. The average stand development of total basal area for the whole study area was modest decline after a dramatic post-outbreak drop: the basal area was 33.8 m2 ha-1 (SD ± 4.5 m2 ha-1) in 1975 and 20.7 m2 ha-1 (SD ± 6.0 m2 ha-1) in 2013. Stands showed different types of development: for some of the stands basal area had dropped throughout the observation period, for some the total basal area had started to recover after a decline, and some stands had fairly stable development throughout the observation period. The development of basal area in the past fifteen years has been negative in over half of the studied stands; it seems that the stands are not following the expected post-disturbance development, where the biomass of the forest recovers to the pre-outbreak level and over it, at these time scales of 30-years of observation. Results suggest that the studied forest stands have reached a state where the basal area is yet to be recovered from the decline following the spruce budworm outbreak in the late-1970s to early 1980s. There is a new, on-going defoliation of spruce budworm – that already has heavily affected particularly A. balsamea – in the study area, and therefore the basal area of the forest stands might be expected to furthermore decline in the future.
  • Kilpinen, Satu (Helsingin yliopisto, 2018)
    Increasing human impact has radically changed the forest structure and depleted the area of natural forests in many regions. In spite of active natural forest research during the last decades, the current knowledge on the southern boreal region, deciduous and mixed forests fall short. The knowledge on natural forests can be utilized for planning sustainable forestry, conservation measures or in environmental impact assessments regarding human influence on forests. The natural disturbance dynamics can be used as a model in silviculture, for example in the planning of restoration activities or commercial forest cuttings. The underlying presumption is that emulating natural forest dynamics can help to preserve such values as the forest biodiversity or resilience. In this study we, investigated the historical variability of disturbances in four different sites in southern boreal forests of Russian Karelia. The target was to determine the disturbance frequency, intensity and range in different spatial scales (plot, site/stand, regional). Also the connection between disturbance quality and forest characteristics was explored. To gather field data and extract increment cores from canopy trees, we established altogether 14 sample plots in four different sites (3-5 per site). We used standard dendroecological methods to record growth releases and gap recruitment from individual trees, indicating past disturbance. With this information, we reconstructed the disturbance chronologies for all plots and sites from the past two or three centuries. From the studied forest sites, two were spruce-dominated stands and one pine-dominated stand. One stand represented mixed-forests with quite an even mix of pine, larch, spruce, and birch trees. The age structure of the forests was uneven-aged in three of the sites. As an exception was the pine-dominated site with relatively even-aged trees (80-100 years). The disturbance chronologies showed significant variation in spatial and temporal scales between sites and sample plots. During the last three centuries (22-26 decades of observation), all the sites had low (0-20%) to moderate (20-40%) or low to high severity (>40%) disturbances. All the sites had gone through a stand-scale disturbance, but no evidence on regional scale disturbance was found. There was no clear connection between forest characteristics and disturbance quality, albeit the data was too small to detect any strong interdependence. The disturbance dynamics showed remarkable variability in disturbance frequency, intensity, and range. The results emphasize the natural heterogeneity and variance related to forest structure, composition and disturbance history. These results back up the prior understanding of the disturbance regime in the Karelian region.