Litter-Fall In a Pinus radiata Forest: The Effect of Irrigation and Fertilizer Treatments

Abstract

(1) A 15 year-old Pinus radiata plantation near Sale, Victoria, Australia, was irrigated with waste-water for 3 years. A litter-fall of 400 g m-2 year-1 was recorded in irrigated plots compared with 320 g m-2 year-1 in non-irrigated plots. The increase in litter-fall resulted from an increase in the production of new needles and greater shedding of older needles. Periods of soil water deficit caused increased shedding of needles in both irrigated and non-irrigated plots. (2) Irrigation with waste-water increased the concentration and amounts of most elements in the litter-fall which were present in the waste-water. Whilst the effect of irrigation on tree growth and the rate of turnover in organic matter was beneficial during the period of study, increases in pH and alkalinity in the standing litter and soil could eventually reduce the availability of iron and other trace elements. Superphosphate fertilizer decreased litter-fall slightly and the concentration of most elements except phosphorus. (3) Distinct seasonal patterns were observed in the concentrations of nutrients in litterfall and the following hypothesis has been proposed to explain these seasonal changes: increasing stress during spring and summer caused abscission of mature needles to occur more rapidly and higher in the crown, resulting in higher concentrations of N, P and K in falling litter. Male strobili contained very low concentrations of Ca and a sharp reduction in the concentration of this element occurred in litter-fall when strobili were shed in spring. Seasonal patterns observed in the concentrations of N, P and K in litter-fall were the reverse of those found in living foliage. The efficiency of nutrient withdrawal thus appeared to reach maximum values in winter and minimum values in summer.