Suchergebnisse

Abstract
The research objectives were: 1) to determine healthy young ash supply with nutrients and their concentrations in natural growth conditions; 2) to estimate nutrient balance and inter-correlations within a system "soil-plants"; and 3) to determine the potential effect of environmental conditions (forest type, plant phytosociological group, soil group, etc.) on nutrient accumulation in soil and leaves of young ash. The investigation was conducted in 28 different forest sites with young ash in Latvia. The results demonstrated that young ash grows well on a wide range of site types in terms of soil composition, forest type and phytosociological tree group. Although highly heterogeneous, soil in ash stands in Latvia can provide a sufficient supply of nutrients. Therefore, the nutrient status of healthy young ash leaves can be characterized as sufficient, although low levels of P, Zn, and K in leaves was found for most sites. Significant impact of soil group and forest type was found on nutrient composition of ash soil, while leaf nutrient concentrations were more dependent on the forest type and phytosociological tree group. The obtained results confirmed the ability of ash to accumulate nutrients within a certain range from soils of different fertility, organic matter content and soil reaction in Latvia.

Abstract
In Latvia, during the last 15 years (2000–2015), the area of common ash Fraxinus excelsior forest stands has decreased by 40.6%. The dieback was predominantly caused by the fungal pathogen Hymenoscyphus fraxineus. Mostly young stands (up to 40 years old) were affected, accounting for 77.3% of the area of dieback. In this paper, we analysed the dynamics of young ash stand area within nature regions in Latvia to attempt to determine patterns of spread and the major migration routes of H. fraxineus. As suggested by the available data, the invasion of the fungal pathogen began in the southern part of Latvia, and then gradually dispersed across the country. The largest decline of young ash stands, during the period 2000–2010, occurred in lowlands. According to our estimate, the average rate of dispersal in Latvia was 40 km per year. At the scale of Latvia, the dispersal routes of fungal pathogen H. fraxineus largely coincide with the major migration corridors of biota and are related to macro-relief forms and their configuration.

Abstract
The chemical composition of pine bark was used for the determination of environmental quality in Rīga. Bark samples were collected at 54 sites in Rīga differing in vegetation and building area characteristics (urban forests, parks and cemeteries, dwelling houses, and streets and railway zones) and at 52 sites from rural areas in Latvia (background level). Concentrations of Ca, Mg, Na, K, Fe, Mn, Zn, Ni, Cu, Cd, Pb, and pH was determined in the samples. The results showed significantly higher concentrations of chemical elements in pine bark collected in Rīga, compared to background levels: Fe and Cu concentrations three times higher, and other elements - up to 1.8 times. The highest element concentrations in Rīga were near streets (Sarkandaugava, Jaunmīlgrāvis, Daugavgrīva, Čiekurkalns, Imanta area, etc.). Element concentration and pH was significantly dependent on the distance from the closest street. The lowest element concentrations were found in urban forests (Jugla, Biķernieki, Beberbeķi) and parks and cemeteries (I Forest Cemetery, Jaunciema Cemetery). Mežaparks (forest), Sarkandaugava, Jaunmīlgrāvis, and Vecmīlgrāvis together formed the most polluted area in Rīga due to the close location near and downwind from the harbour.

Abstract
The effect of climatic factors on tree-ring width (TRW) of European ash (Fraxinus excelsior L.) with healthy and damaged crowns growing in the central and eastern part of Latvia was assessed by dendrochronological techniques. Pointer year indices were calculated to assess the variability of TRW. Positive pointer years were more frequent than negative, similarly for damaged and healthy trees, suggesting similar limiting factors. The relationships between TRW and climatic factors differed regionally and locally. In the eastern part of Latvia, TRW was affected by temperature in spring of current and preceding years. In the central part of Latvia, weather conditions in current and preceding summer mainly affected TRW; temperature and precipitation had positive effect suggesting occurrence of water deficit, but the daily range of temperature had a negative effect. Hence, regional diversification of radial growth patterns might be expected. Some differences in sensitivity to climate were observed between the damaged and healthy trees. In the central part of Latvia, the damaged trees appeared additionally sensitive to temperature in summer. In the eastern part of Latvia, damaged trees were additionally sensitive to temperature in May. Nevertheless, some additional factors were significant for the healthy trees.

Abstract
Intense dieback of Fraxinus excelsior L. has been causing rapid changes in advance growth of trees and understory shrub growth of the affected stands. In this study, changes in composition and density of understory were studied in 15 permanent plots (each 235.6 m2), repeatedly sampled in 2005, 2010, and 2015. Within each plot, the number and average height of understory individuals were determined. The successional changes in understory were assessed by Detrended Correspondence Analysis. In total, 11 advance growth and 20 undergrowth species were recorded. A significant increase in the density of understory was observed only in 2015, mainly due to understorey growth of Corylus avellana L., Padus avium Mill., and Lonicera xylosteum L. Regarding advanced growth, the highest density was observed for Ulmus glabra Huds., F. excelsior and Acer platanoides L.; the density of A. platanoides and F. excelsior increased particularly in the period from 2010–2015. The observed successional changes suggested individuality of development of the affected stands according to the composition of the remaining and neighbouring canopy trees.

Abstract
Robinia pseudoacacia is an alien tree species that has wide distribution in green areas of Rīga and Kaunas. In recent years, the spread and invasion of this species was observed. The aim of this study was to determine the relative importance of invasiveness of R. pseudoacacia in Rīga and Kaunas. The degree of species invasion was estimated by applying the Pest Plant Prioritization Process, which is based on the Analytic Hierarchy Process method. The results showed that the invasive degree of R. pseudoacacia was near medium (0.4); the present compared to potential distribution rating was medium (0.57) and the social, environmental and economic impact score was very low (0.17). The Final Pest Plant Score for R. pseudoacacia was near medium (0.426). The obtained estimates indicated that black locust was medium invasive, and that well-lit conditions favour its establishment.

Abstract
Investigation of species composition, soil physical and chemical properties, as well as forest productivity of oligomesic dry pine forests (Vacciniosa and Myrtillosa forest types) was carried out in six dry land zones differing in age of the Lake Engure catchment area (sediment zones of the drained lakebed, Mia, Limnea, Litorina Sea, Baltic Ice Lake and glaciofluvial sediment zone in Northern Kursa Upland). Higher species diversity in the tree layer and a more typical podzolisation process was found in the older dry land zones (sediments of the Baltic Ice Lake, Northern Kursa Upland), while higher species diversity in the field layer (higher proportion of grasses and sword grasses), more intensive gleying process in soils, as well as considerably higher stand productivity was observed in the younger dry land zones (the drained lakebed, Mia, Limnea and Littorina Sea stages). Characteristic species of the Vaccinio-Piceetea class were dominant in the pine forests of older dry land zones, whereas species of the Pulsatillo-Pinetea class were typical in the sea coastal pine forests.

Alien flora in the Lake Engure Nature Park
This paper describes the alien vascular plant flora in the Lake Engure Nature Park, one of the most species-richest and diverse areas in Latvia. The review was based on the floristic inventories conducted in 1983-1989 and a field survey carried out in 2010. In total, 68 alien species (7.6% of the vascular plant flora in the nature park) were recorded, of which four had established in the period between both surveys. 22 species were recognised as adventive species and 46 as escaped species. Most of the alien species area concentrated in coastal villages, around human settlements, roadsides, coastal dunes and along streams discharging in the Lake Engure, thus mostly in transformed habitats. They are absent in most of the natural and semi-natural habitats. In comparison to the overall situation in Latvia, the Lake Engure Nature Park area is relatively little affected by plant invasions, most probably because large areas of natural and semi-natural habitats remain, since high naturalness of vegetation plays a significant role as a natural barrier hindering alien invasions.

Environmental quality assessment of the drainage basin of Lake Engure using Scots pine as a bioindicatorEnvironmental quality assessment of the Lake Engure drainage area, which is the LT(S)ER region of the Latvian National Long-term Ecological Research network (Latvia LTER), was conducted using three bioindication methods based on Scots pinePinus sylvestrisL.: unspecific bioindication by pine needle tip necrosis, ground level ozone assessment by pine needle chlorotic mottling, and chemical analysis of pine bark. Samples were collected from 40 sites of the region in November 2010. Extent of needle tip necroses did not exceed class 4 (maximum possible value 6). The highest value of index of needle damage by ozone was Idam= 1.62 (maximum possible value 6). Multiple regression analysis of variables describing bark chemistry and needle damage in relation to distance from the sea, nearest roads and villages was performed. Pine bark acidity pH(KCl) and concentration of Mg and Fe decreased significantly (βpH(KCl)= -0.672,P< 0.001; βMg= -0.676, P< 0.001;βFe= -0.514, P< 0.001) with distance of sample site from gravel roads. Electric conductivity (EC) of pine bark and the extent of ozone damage of the first year needles Idamsignificantly decreased with distance from the sea(βozone= -0.507, P< 0.01;βEC= -0.453, P< 0.01). PCA of the pine bark chemistry data showed dust pollution from gravel roads to be the main factor responsible for the 33% variation of data. No statistically significant correlations were found between different bioindicator characteristics, except between first and second year ozone damage (ρ = 0.589, P = 0.01).

Abstract
The article discusses the results of the national project aimed at elaboration of a conceptual integrated model of the Engure LT(S)ER (Long-Term Socio-Ecological Research) platform of Latvia. The platform represents the drainage basin of costal Lake Engure (644 km2) together with the coastal marine zone of the Gulf of Rîga. The core zone of the ecoregion is the Lake Engure Nature Park (LENP), which is a Ramsar site. The conceptual model is a slightly modified version based on the DPSIR (Drivers-Pressures-States-Impacts-Responses) concept. The socioecological system was spatially demarcated and drivers were subdivided in two groups - external and local ones. The Engure ecoregion was subdivided into seven zones or sub-regions mostly demarcated by natural geological and geographical barriers. Each zone has a specific set of drivers and pressures as well as a specific ecosystem structure and elements of biodiversity. Analysis of the governing drivers and pressures was performed separately for each sub-region during three time periods: 19th century - beginning of 20th century, period of Soviet occupation (1940-1991), and period after restoration of independence of Latvia (1991 - up to now). Characteristics of the state of ecosystems and biodiversity are given. Responses of the socioeconomic component of the socio-ecological system are represented mainly by external factors to the ecoregion, including environmental legislation and funding necessary for research and ecological management. Two alternative scenarios of the development of the Engure ecoregion are discussed: (i) depopulation and land abandonment, and (ii) intensification of agriculture, small-scale industry and building construction. In both cases the present state of ecosystems and the structure of species diversity would be subjected to significant change. Sustainable development of the ecoregion can be provided only by implementation of certain environmental management measures accompanied by long-term socio-ecological research and ecological monitoring.