Influence of site factors on Salix growth with emphasis on nitrogen response under different soil conditions

This thesis summarises studies of the impact of site related factors and nitrogen fertilisation regimes on production in willow coppice stands. Above-ground, woody biomass production in different clones of Salix viminalis L. and S. dasyclados Wimm. was measured during several years in willow stands growing on different soil types. One study was carried out in eight commercially grown Salix viminalis stands, in three different geographical regions in south Sweden. Cumulative stem production during five years ranged from 16 to 54 tonnes DM ha-1. Generally, production was found to increase with increasing content of clay in the soil. This was interpreted to be due to the favourable soil conditions provided by clay soils, implying the presence of a fair amount of stored plant-available water at the beginning of the growing seasons. Effects of nitrogen fertilisation on cumulative stem production during four years in a S. viminalis stand were evaluated in a field experiment. Nitrogen was applied annually according to a response surface, experimental design. Fertilisation performed in the second growing season from planting was most efficient in terms of enhanced yield per amount of nitrogen supplied. Effects of nitrogen fertilisation on stem production in different clones of S. viminalis and S. dasyclados, in five different field experiments were evaluated. Generally, nitrogen fertilization enhance yields considerably during the four years studied. Site conditions influenced mainly the average production levels. However, the ability of the clones to utilise applied N efficiently, turned out to be strongly affected by site conditions. Effects of pre-harvest nitrogen fertilisation on willow coppice regrowth during three years were evaluated in one field experiment. Cumulative stem production during the period was negatively affected by pre-harvest treatment. Leaf nitrogen concentration was negatively correlated to pre-harvest fertilisation towards the need of the period. These negatively correlated to pre-harvest fertilisation towards the end of the period. These results implied that plant-available nitrogen was lower if high amounts of fertilisers had been applied during the period before harvest. Apparently, nitrogen had been lost from the system, indicating that the possibility to accumulate nitrogen in recyclable pools, under certain conditions, may be more difficult than previously assumed.