When we discuss the multiple use of forests, we frequently treat medicinal plants as inexhaustible natural resource, sometimes uncritically and without valid database. The aim of this research was to estimate the biomass of medicinal plants in Northern mountain region of Republic of Srpska; where under biomass we understand the useful live substance amount expressed in kg per ha of air-dried parts of herbs [6].

As a representative of Northern Mountain region of Republic of Srpska, we took the Teslić forest management region. This region spreads over 48.000 ha of various forests and forest's lands within altitude 166 m and 1.383 m.

The medicinal herbs populations are in fact infinite in this area, so we used a sample in order to describe some of their characteristics and estimate their biomass. As a first step we divided above mentioned region in 12 less heterogeneous subsets or site types (Tab.1) on the basis of already known ecological-productive entities [4,7]. As the second step all stands in each ecological-productive entity have been numerated, and than some of them singled out, in a random manner as a sample, until at least 50 sample plots per hectare per each site type have been attained, with a few exceptions. As the third step, we spread the sample unit network at a 100 m distance in directions N-S and E-W. The sample unit was a circular surface of 10 m² (r = 1.86 m) for low-growing plants and 100 m² (r = 5.65 m) for trees and shrubs. On every sample unit we measured altitude, inclination and exposition and estimated the density of crown canopy so as to meet the site-specific relief and light conditions. The cover density ranged in decimal order from 1.0 (for very dense sites where no part of direct solar radiation reaches the ground) to 0.0 (for open sites, which receives the full amount of direct solar radiation). We counted individuals of each species of medicinal herbs. Finally, we collected and weighed their useful parts in fresh condition from each forest stand and than reduced their mass to air-dried condition using appropriate coefficient of dryness from literature [2. 5.]. In this way, data from 628 sample plots were gathered in 1999 [1,2,3]. Applying standard statistical tool [8], we arrived to the following results.

Basic characteristics of study area

Basic characteristics of study area could be assumed through mean and categorized relief and canopy attributes (Tab.2). These results are largely in agreement with those reported by literature [7]

Diversity

The eighty-six species of medicinal plants were found on the sample plots in this forest area which are, apart from being curative, at the same time attractive for market (previous demand for medicinal plants on ten market places all over the former Yugoslavia had been taken into consideration). They are: Achillea millefolium, Agrimonia eupatoria, Althaea officinalis, An- gelica silvestris, Arctium lappa, Artemisia spp., Atropa belladonna, Asarum europaeum, Asperula odorata, Bellis perennis, Betonica officinalis, Calamintha officinalis, Carlina aca-ulis, Centaurea cyanus, Cichorium intibus, Colchicum autumnale, Digitalis spp., Equisetum arvense, Erithraea centaurium, Eupatorium canabinum, Euphrasia officinallis, Fragaria spp., Filipendula spp., Geum urbanum, Glechoma hederacea, Hypericum perforatum, Lamium al- bum, Linaria vulgaris, Lycopus europaeus, Malva silvestris, Melittis melissophyllum, Mentha spp., Orchis spp., Origanum vulgare, Plantago spp., Pimpinela spp., Polygonatum officinale, Polypodium vulgare, Potentila tormentilla, Primula spp., Pulmonaria officinalis, Rubus ida-eus, Rubus spp. (R. hirtus, R. tomentosus, R. fruticosus et R. rusticans), Sanguisorba officina-lis, Sanicula europaea, Saponaria officinalis, Sedum spp., Symphytum officinale, Taraxacum officinale, Teucrium chamaedrys, Teucrium montanum, Thymus serpyllum, Thymus vulgaris, Trifolium spp., Tussilago farfara, Urtica dioica, Vaccinium myrtillus, Verbascum spp., Verbena officinalis, Veronica officinalis, Viola odorata, Viola tricolor, Betula pendula, Cerasus avium, Cerasus mahaleb, Cornus mas, Corylus spp., Crataegus spp., Hedera helix, Juglans spp., Juniperus communis, Malus silvestris, Ononis spinosa, Prunus spinosa, Pyrus communis, Rhamnus cathartica, Rhamnus frangula, Rosa spp (R. arvensis, R. canina, R. glutinosa, R. pendulina et R. spinosissima), Ruscus aculeatus, Salix alba, Sambucus nigra, Tilia grandifolia, Tilia parvifolia, Viscum album.

Biomass

Biomass of medicinal plants is a rather variable characteristic, which varies within and between species and site types.

Within a single sort of herb, biomass is mainly growing with increasing of accessible light intensity. For example, Fragariae herba is more than 25 times bigger in young coniferous stands with mean canopy of 0.7 than in beech and fir forests with mean canopy of 0.9, in spite of similar soils and parent rocks (Tab. 3 & 3.1).

If we compare total biomass between site types, we will come to the analogous conclusions. For instance, total biomass on strips under the power lines and by the forest roads amounted 328 kg per ha, while in high dense forests, such as first five site types (Tab.1) amounted less than 120 kg per ha. Meadows and pastures give the biggest biomas of about 690 kg per ha.

If we consider biomass of medicinal plants as a measure of habitat natural potentialities, the examined site types can be divided into three categories:

1. Site types with very limited natural potentialities of medicinal herbs where total biomass is less or near 70 kg per ha,

2. Site types with limited natural potentialities of medicinal herbs where total biomass varies between 100-200 kg per ha,

3. Site types with unlimited natural potentialities of medicinal herbs where total

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