Studies of the Development of Woodland Conditions Under Different Trees: I. Soils pH

Abstract

The establishment of tree species frequently brings about important changes in those site factors which will ultimately determine the growth of succeeding plantations. Since many of the areas available for tree planting are poor marginal lands, the forester should anticipate the changes which will result from the establishment of a tree crop and aim at conserving and improving soil fertility by selecting the most suitable species for planting. These changes vary not only with the tree species planted but also with the inherent site factors, and are difficult to follow progressively in the same plot because of the time needed for a tree crop to reach maturity. Some indication of the trend and rate of change can be obtained by comparing conditions in adjacent plots, unplanted and planted with different crops. The advantage of this comparative method is that variations may be studied in a fairly homogeneous environment and differences between plots assessed more easily. In the present investigation five sites have been selected, and in each of these there are a considerable number of contiguous plots within a small area, the boundaries between the plots being straight lines and not following any obvious site differences. The individual forest plots have been planted for at least twenty years and are in the main occupied by a single species which has closed canopy. Except for the plots on the Powis estate, the sites are all managed by the Forestry Commission and are situated in the state forests of Thetford Chase, Forest of Dean and Bedgebury (P1. 2). There is considerable variation in the amount of organic matter accumulated at the surface of the mineral soil under different tree species and, in order that corresponding horizons between plots could be compared, all soil depths were measured from the zero line separating the surface humus layers from the underlying mineral soil. This proved to be fairly easy to recognize in the field. In addition, the surface organic matter was divided into an upper zone of slightly decomposed material (L) and an underlying zone in which decomposition is more advanced (F + H) (Romell & Heiberg, 1931; Heiberg & Chandler, 1941). Generally this division could be readily applied to the forest floor of the conifers, but little formation of an H layer occurred in the relatively thin layer of unincorporated organic matter under the hardwoods.