Leaf area and biomass of Rhododendron understory in a stand of Scots pine

Crown structure and biomass of a Rhododendron ponticum L. understory in a sparse Scots pine (Pinus sylvestris L.) forest were studied in two stands in the Belgian Campine region. Dry mass of leaves, shoots, and flower buds, leaf area, shrub height, length, and diameter of branches were measured at the shrub level by the "cloud" technique based on some destructive sampling. Shrub height was linearly related to branch basal diameter, dry mass of leaves, shoots, and buds, as well as to leaf area index. R. ponticum is a typically rather dense shrub without a pronounced main stem and with a complicated branching structure growing in the compact parcels with gaps between them. A three-step scaling procedure (from individual stem to stand area) was applied: (1) approximation of the shrub biomass per stand area in the more or less homogeneous parcels in model plots with a closed canopy; (2) assessment of gaps and stand cover by shrubs of different height in three other model plots with discontinuous canopy; and (3) up-scaling from those three model plots to the whole stand. Considering the entire stand, LAI of the shrub understory was 1.25, which represented about 42% of the LAI of the overstory trees (when both were measured by the destructive method). Leaf dry mass of the Rhododendron understory (around 2.4 t ha(-1)) was about one-third of its total dry mass and represented about 38% of the total needle dry mass of overstory pine. This illustrates the importance of the leaf biomass and leaf area of Rhododendron shrubs for the mass and energy exchanges of the entire pine forest. (C) 2003 Elsevier B.V. All rights reserved.