Folia Geographica 10. sējums - Latvijas Universitāte

NATURE RESEARCHcodominant species of mixed deciduous forests mainly in the transitional region between amaritime air masses dependant area (“Europe’s peninsula”) and the inner continental territory:in the former the prevailing role belongs to beech (Fagus sylvatica) and oaks (Quercus robur,Q.petraea); in the latter the nemoral type forests are made up by species with a broaderdistribution limits (mostly Quercus robur).This transitional region is located on the Europe’s submeridional axis, which traverses thecontinent from the east coast of the Baltic Sea to the Black Sea next to the mouth of the Danube.In its physico-geographical aspect, it is particularly significant in at least three ways:(1) the boreo-nemoral ecotone (transitional link between nemoral and boreal zones) reachesits greatest width here;(2) a considerable part of the Europe’s submeridional axe southern sector can be regarded asa conditional boundary between nemoral zone and forest steppe;(3) it is the borderline between two distinctive natural zone systems.However, the hornbeam’s area toward the north (to approximately 57° N) spreads furtherat the western side of submeridional axis, i.e., in Scandinavia – on the border betweensuboceanic and subcontinental climatic regions. Here, as well as on the other side of the BalticSea, it reaches into boreonemoral ecotone. The northern edge of hornbeam in Latvia lies almostat the same longitude as in Scandinavia, although its distance from the southern boundary ofboreonemoral ecotone is approximately 250 km, while in Scandinavia it is less than 50 km. Inthat respect the landscapes of Sventāja and Lukna might properly have more northern (boreal)characteristics than its analogues in Scandinavia.It is likely that along the hornbeam’s northern and northwestern boundary the mostdominant climatic factor that might limit its further spread is the lack of warmth in summers. InLatvia’s case the length of vegetation period (>5°C, from April 16 to October 29 – 195 days)and the duration of active temperatures period (>10°C, from May 12 to September 29 –139 days) are close to the lower numerical values of these indicators in the European hornbeamdistribution area (Table 2).On the other hand, winter cold might not be the limiting ecological factor that determinesthe northern edge of the hornbeam along the Baltic Sea eastern coastline. In the researchedlandscapes hornbeam is not dependant on the substantially greater influence of a winter freeze,compared with Scandinavia or even the nemoral zone at the Europe’s submeridional axis, wherea notable inclusion of the hornbeam in natural forest stands can be observed. Less injurythrough frost in small forest patches is observed in Byelorussia, which lies further to the east,where in the hornbeam’s distribution area the period of winter freeze is longer by one monthand the winter cold is more rigorous [Юркевич, Тютюнов 1985].The total precipitation (717 mm/year) corresponds rather to the mean than the highest orlowest numerical values in the hornbeam’s area of distribution. On the other hand, one mighthave to consider the significant irregularity of seasonal precipitation: maximum at the end ofsummer (86 mm in August); increased precipitation in autumn and winter, which ischaracteristic of the maritime climate (64 % of the annual amount from July to December);minimum at the end of winter (33 mm in March).In both model areas the biogeocenoses with the inclusion of the hornbeam form “patches”or short intermittent “corridors” in the “matrix” of forests composed of other tree species oragricultural lands. These stands are the remains of formerly more widespread hornbeam foreststhat 130 years ago covered approximately 750 hectares in Southwest Kurzeme. The sharpenlargement of agricultural lands and increased forest utilization, as well as coniferous forestdevelopment for profit, led to a decrease in hornbeam forest. However in the natural stands, too,the presence of hornbeam had decreased due to the ecological competition with spruce. As aconsequence in 1920 hornbeam stands covered only 130 hectares [Kiršteins, Eiche 1933], andnowadays the total area is estimated at approximately 50 hectares (Figure 4).In the Lukna’s model area hornbeam grows on the wavy ground moraine plain mainly inthe spruce (Picea abies) and secondary birch (Betula pendula) stands in the lower tree strataaccompanied by lime (Tilia cordata) as well as some other species; however, it also formsmixed stands with spruce, lime and maple in the upper tree layer (Figure 5). The sapling and45