Woody vegetation structure of xeric forest stands under different edaphic site conditions and disturbance histories in the Biosphere Reserve 'Parque Costers del Sur', Argentina

We studied stand structures and soil properties in an old-growth forest and two 30-yr-old second-growth stands. In the old-growth forest, the total density and basal area average 1566 trees >1.25 m height ha(-1) and 46.73 m(2) ha(-1). The dominant trees are Scutia buxifolia and Celtis tala. Using multivariate techniques we distinguished three stands: PS1, dominated by S, buxifolia, is 1000 m far from the river. Its soil is shallow and loamy. PS2 and PS3, co-dominated by S. buxifolia and C, tala, are over 1200 m distant from the river. There the soil is deeper and has thicker texture and higher phosphorus and calcium concentrations than the near-the-river forest soil. Scutia buxifolia shows reverse J-shaped size-distributions and has morphological features of stress-tolerant species. Celtis tala shows oscillating decay size-curves that suggest recruitment pulses related to small gaps and it has morphological features of competitive species. Celtis tala was selectively cut in the past in the second-growth stands SNRD and SRD. The stand SNRD, 1000 m Ear from the river, is dominated by S. buxifolia. The second species is Schinus polygamus which presents the bell-shaped size-structure of the pioneer species. SNRD does not differ from its old-growth counterpart PS1 in total tree density, basal area and tree branching. The stand SRD, over 1200 m distant from the river, is co-dominated by S. buxifolia and by C, tala trees regenerated from stumps. SRD does not differ from its old-growth counterparts PS2 and PS3 in total tree density and basal area. As to tree branching, it does not differ from PS3, but differs from PS2. All the stands are being invaded by the exotic tree Ligustrum lucidum. The differences between the old-growth stands seem to be related to the gradients of soil texture and nutrient concentrations raisin edaphic stress towards the river. In SNRD, the stress, the stress-tolerance of S. buxifolia and the aptitude of S. polygamus to recruit in disturbed habitats seem to have prevented the post-logging recruitment of C. tala. In SRD, C, tala regenerated possibly due to a better competitive performance ill a more favorable site. Under protection the second-growth stands recovered the old-growth quantitative features. We recommend the restoration of the qualitative features and the control of L. lucidum.