Appropriate forestry practices for the management of austrian pine (Pinus nigra Arn.) plantations with understory of manna ash (Fraxinus ornus L.) in Sofia surrounding mountains

The object of the current study are Austrian pine ( Pinus nigra Arn.) plantations with an understory of Manna ash ( Fraxinus ornus L.), located in the mountain areas near Sofia, Bulgaria on altitude from 700 to 850 m a.s.l. The aim is to study the natural regeneration processes taking place in the plantations and to provide guidelines for their management. It has been established that their transformation into deciduous stands is a natural process that is currently ongoing. The new generation of trees consists mainly of native oak species and their natural companions. However, the Manna ash understory is overgrowing and silencing the seedlings. The amount of seedlings depends on the forest site and increases as growing conditions improve. With the increase of canopy density, understory density decreases rapidly, while the amount of seedlings does not change significantly. With the increase of stand age, the amount of seedlings increases at the expense of the understory. Increasing the height of the understory results in significant loss of seedlings. In order to create conditions for successful transformation of the pine plantations with economically valuable autochthonous species, it is recommended to manage them at a canopy density above 0.6 as long as possible. Usually, a sufficient amount of seedlings is available after the stand age of 65, when the felling for their transformation can also begin. Forest sites with better conditions have seedlings present in the entire area, while in forest sites with worse conditions their distribution is uneven. In the first years of the felling the number of required thinnings depends on the height of the understory, which should not be allowed to exceed 120 cm until the new stand forms a canopy.