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The influence of thinning on growth, development, and yield of a spruce understory and a pine overstory was examined based on an experiment including five stands in southern Finland. Two thinning intensities (regular and heavy) were applied to the pine overstory. They were combined with four treatments in the spruce understory: total removal, retention of 800 and 1600 stems ha−1, and untreated. The plots were remeasured at 5-year intervals during the monitoring period of 10–15 years. The results indicated that thinning of the pine overstory significantly promoted the diameter growth of the spruce understory trees throughout the 10–15-year study period without notable delay, and height growth with a substantial lag during the first 5-year period. The influence was swifter and greater with heavier thinning. Thinning of the understory did not influence the height growth of the dominant understory spruces or the growth of the overstory. Complete removal of the understory resulted did not consistently influence growth of the overstory. It was concluded that a spruce understory seemed to thrive and grow relatively well under an overstory of pine, but repeated thinnings in the overstory were required to sustain understory vitality and growth in a prolonged two-storied situation. [ABSTRACT FROM AUTHOR]

Inherent variability in the spectral properties of boreal forests complicates the retrieval of canopy properties such as canopy leaf area index from satellite images. Understanding the drivers of this variability could help provide better estimates of desired canopy cover properties. Field plot data from the Finnish National Forest Inventory and Landsat thematic mapper (TM) images were used to investigate the variation in canopy and understory reflectance during stand development in coniferous boreal forests. Spectral data for each plot were obtained from the Landsat pixel within which the plot center coordinates fell. Nonlinear unmixing was used to estimate the bidirectional reflectance factors (BRFs) of the 'sunlit understory' and 'canopy and shaded ground' components by site fertility and stand development classes. A forest albedo model was used to estimate the contribution of diffuse radiation reflected downwards from the canopy to the sunlit understory component. The sunlit understory BRF in the near-infrared spectral band decreased as the site fertility decreased and the forest matured, whereas the sunlit understory BRFs in the red and shortwave-infrared spectral bands concurrently increased. The BRFs of the canopy and shaded ground component decreased slightly during stand development, mostly in the near-infrared spectral band. Adding the diffuse contribution to the sunlit understory component changed the estimated component BRFs only a little (0.1%-1.7%) compared with those obtained using a linear mixing assumption. This effect was largest in the near-infrared spectral band and smallest in the red spectral band. For Norway spruce plots, the measured and estimated forest variables were well correlated with the BRFs in all of the studied spectral bands, but for the Scots pine plots, the correlations were notably weaker. Results show a greater importance of the fraction of visible sunlit understory on forest reflectance in Scots pine than in Norway spruce forests. [ABSTRACT FROM AUTHOR]

This study presents empirical data on regeneration and growth of understory trees and constructs simple models for predicting these characteristics at various stand structure and post-thinning standing volume levels. The field experiment was established on a grass/herb mineral soil site in central Finland. Regeneration and understory tree growth data were collected from 24 Norway spruce ( Picea abies L.)-dominated mixed substands. Seedlings with heights from 5 to 130 cm were measured in 1996 and 2007. In addition, the annual height growth of Norway spruce seedlings was measured in 2007. The spatial pattern of the substands varied from clustered to regular with a decreasing standing volume. Stand complexity was uniform across the range of standing volume. In the 2007 survey, the amount of Norway spruce regeneration increased from approximately 400 to 5,000 seedlings ha when the post-thinning standing volume level was reduced from 230 to 90 m ha, respectively. Nearly no seedlings were found when the standing volume was over 300 m ha. The annual diameter increment in Norway spruce understory (dbh < 5 cm) trees decreased, on the average, from 2.3 to 0.3 mm with an increase in the standing volume level from 90 to 340 m ha, respectively; their height growth showed also a decreasing trend when the standing volume increased. The results indicated that a post-thinning standing volume lower than 150 m ha with a regular overstory spatial structure provides a suitable environment for regeneration and growth of Norway spruce understory trees in the studied forest type. [ABSTRACT FROM AUTHOR]

Abstract: The spectral and directional reflection properties of pine forest understory in Suonenjoki, Finland were measured using a newly developed transportable field goniospectrometer under direct sunlight or plant lamp. The samples represent the most typical types in Finnish forests. Large differences between species were found. Wax-leaved shrubs such as lingonberry and blueberry proved to be strong forward scatterers, whereas lichen and soft-leaved dwarf shrubs such as heather were strong backscatterers. The measured moss showed both forward and backscattering features. There were variations among the samples of the same species, but many typical features appeared consistent and reproducible. Both “pure” and mixed samples were measured, the latter showing smoother behavior than the former, that is, the strongest forward and backward features are downscaled. The results provide a starting point for an empirical understory model and a basis for development and validation of a theoretical model. [Copyright &y& Elsevier]

In open forest canopies, such as in boreal forests, forest floor can contribute significantly to the observed top of canopy reflectance. In order to retrieve the biophysical properties of the tree layer, correcting for forest floor is essential. Traditionally, the algorithms for retrieval of forest floor reflectance depend on tree layer information such as leaf area index, canopy cover, and site fertility. To overcome these circular dependencies, we propose an algorithm that can be applied only using airborne remote sensing data. We acquired airborne hyperspectral imagery over the Hyytiälä forest research station (61°50′N, 24°17′E) in central Finland on July 3rd in 2015 using a hyperspectral pushbroom line scanner. The image data had a spectral resolution of 4.6 nm, and the spatial resolution was 0.6 m. We developed a linear spectral unmixing algorithm, which is based on the definition of the reflectance factor, taking into account the variation of incident irradiance inside the canopy. The weights of the mixture can be computed from tree canopy gap fractions, a tree species insensitive leaf albedo, and average tree stand reflectance. Canopy gap fractions were retrieved with empirical methods available in scientific literature. The forest floor reflectance in the near-infrared increased with site fertility in agreement with the forest floor field measurements. Moreover, we found that in near infrared, the reflectance of moderately rich and moist upland forests was significantly different from all other fertility classes. Finally, we tested the reflectance decomposition on the photochemical reflectance index (PRI) known to be heavily affected by understory reflectance and canopy structure, and the forest PRI to be decoupled from the PRI of the over- and understory. • Airborne hyperspectral imagery used to estimate boreal forest floor reflectance. • Separation of forest floor and overstory reflectance without field stand variables. • Boreal forest floor reflectance in near-infrared increases with site fertility. • Calculated forest floor reflectance matches moist upland spectra. • Forest floor reflectance in NIR significantly different among fertility classes. [ABSTRACT FROM AUTHOR]