The chemical composition of rainwater in relation to cycling of nutrients in mature eucalyptus forest

Abstract

The alteration of the ionic composition of rainwater by vegetation has been attributed in the literature both to foliar leaching (representing circulation of elements within an ecosystem) and to the washing from leaves of particulate matter (an addition of elements to the ecosystem). The purpose of this study was to estimate the magnitudes of these components in a matureEucalyptus obliqua forest on the Great Dividing Range, Australia. Rainwater samples collected both within the forest and from an opening devoid of trees at regular intervals during a two year period were analyzed for sodium, potassium, calcium, magnesium, and phosphorus. The ionic composition of rainwater sampled at the open area fits an expected geographical distribution pattern, and the origin of the ions is considered to be mainly oceanic and partly terrestrial. The concentration of ions in rainwater collected both at the open area and from within the forest is related, inversely and exponentially, to the intensity of rainfall during a collection period. Ionic concentrations in rainwater collected beneath the forest canopy were greater than concentrations in rainwater collected at the open area. Considerations of ionic ratios lead to the conclusion that this increase is principally the result of foliar leaching. Furthermore the data for this mature forest conform closely to the results, reported in the literature of leaching experiments carried out under controlled conditions with small, individual plants.