TRANSPIRATION AND DROUGHT RESISTANCE OF DOUGLAS-FIR SEEDLINGS EXPOSEDTO EXCESS AMMONIUM

In a pot trial growth and transpiration of 3-year-old Douglas-fir seed lings on an acid, sandy soil was examined at a deficient (30 kg N ha(- 1) year(-1)) and an excessive level (120 kg N ha(-1) year(-1)) of NH4 application. Dissolved ammonium sulphate was applied to the pots weekl y for two growing seasons. In half of the pots a complete set of other nutrients was applied in optimal proportions to the applied nitrogen. Water supply was optimal and transpiration was recorded. At the end o f the second treatment season irrigation was stopped for 2 weeks durin g dry and sunny weather. Both high application of NH4 and additional n utrients increased shoot growth and transpiration demand in the first treatment year. The root system was smaller at higher N level and this reduced water uptake accordingly. In the second year the combination of high NH4+ and additional nutrients affected root functioning predom inantly due to salinity effects and this seriously decreased water upt ake capacity and shoot water potentials, finally resulting in tree dea th. Without addition of other nutrients the high NH4+ application resu lted in a high degree of soil acidification, which damaged the roots, that showed a decrease in water uptake capacity. At the low NH4 supply level soil acidification was lower, and root functioning was not affe cted, and the trees recovered quickly from the imposed drought. Higher needle K and P status depressed transpiration rates at the low NH4 ap plication rate.