The assessment of forest productivity at early stages of stand development may help to define the most appropriate silviculture treatment to be applied for each stand. Site index (dominant height at a reference age) is a useful tool for forest productivity estimation. The aim of this study was to develop a model to predict site index for Scots pine (Pinus sylvestris L.) plantations in northern Spain acidic plateau by using soil (physical, chemical and biochemical), climatic and physiographic parameters. To meet this objective, data from 35 stands classified into three different site quality classes and 63 soil, climatic and physiographic parameters were examined in order to develop a discriminant model. After selecting 12 discriminant models which were biologically consistent and presented the higher cross-validated rate of correct classification, a model including four parameters (latitude, inorganic Al, porosity and microbial biomass carbon) as predictors was chosen. The discriminant model classified 71% of cases correctly and no inferior-quality stands were misassigned to the highest quality class. Soil and physiographic parameters included in the above model are easily obtainable in the field or by simple laboratory analysis, thus our results can be easily integrated in operational forestry to determine site quality.

& Key message Stand density has a positive effect on C, K and Mg concentration in needle litterfall and a negative one on C, N, Ca, K, Mg, P, S, Zn, and Cu release from needle litter. Consequently, forest management practices such as thinning decrease nutrient concentration in needle litterfall and accelerate nutrient release from decomposing needles in Pinus halepensis plantations in Spain. & Context Silvicultural practices usually include stand density reduction resulting in changes in litterfall and litter decomposition rates. Little is known about the effect on nutrient concentrations in litterfall and nutrient release during decomposition even when this is the main path of nutrient return to soils. & Aims The aims of the study are to evaluate the seasonal pattern of nutrient concentration in litterfall, to study how nutrients are released from needle litterfall during decomposition, and to assess whether local basal area of the stand affects nutrient concentration of litterfall and nutrient release during litter decomposition. & Methods Eight plots were established on each of four stands covering the widest range in local basal area. A littertrap and 15 litterbags were placed on each plot. Periodically, needle litterfall and litter contained in the litterbags were analyzed for C,

& Key message Water availability and soil pH seem to be major constraints for enzyme activities in calcareous soils under Pinus halepensis and acidic soils under Pinus sylvestris plantations respectively. Proposals for improving enzyme activities may include the promotion of broadleaf species to increase soil pH and the modulation of stand density or the implementation of soil preparation techniques to facilitate water infiltration. & Context Soil enzymes play a key role in nutrient turnover in forest ecosystems, as they are responsible for the transformation of organic matter into available nutrients for plants. Enzyme activities are commonly influenced by temperature, humidity, nutrient availability, pH, and organic matter content. & Aims To assess the differences between enzyme activities in calcareous soils below Pinus halepensis and acidic soils below Pinus sylvestris plantations in Spain and to trace those differences back to edapho-climatic parameters to answer the questions: Which environmental factors drive enzyme activities in these soils? How can forest management improve them? & Methods The differences in climatic, soil physical, chemical, and biochemical parameters and the correlations between these parameters and enzyme activities in soils were assessed. & Results Low pH and high level of phenols in acidic soils under Pinus sylvestris and water deficit in calcareous soils under Pinus halepensis plantations appeared to be the most limiting factors for enzyme activities.

& Key message Pinus sylvestris L. productivity in Spanish plantations is driven by P availability, which, in turn, is determined by the activity of soil microorganisms, responsible for inorganic P solubilization; Fe and Al contents, responsible for P retention; and organic matter, which is source of organic P, inhibits its precipitation as insoluble compounds, and reduces P retention. & Context Phosphorus is often a limiting nutrient in forest ecosystems mainly due to the low solubility of P compounds and the sorption processes occurring in soils. & Aims The main aims of this work were to evaluate soil P availability, to assess which soil properties are driving P availability, and to study whether soil P availability is determining forest productivity in Pinus sylvestris L. plantations in Northern Spain. & Methods Soil properties and forest productivity were studied in 34 plots located in monospecific P. sylvestris plantations. Tiessen and Moir (Canadian Society of Soil Science 75-86, 1993) sequential fractionation method was carried out to determine different forms of soil P and to provide a comprehensive assessment of available P in soils. To explore the relationships between these variables, canonical correlation analyses and Pearson's correlations were studied. & Results Significant correlations were found between P fractions and soil properties related to Fe and Al contents, organic matter, and microbial biomass. Besides, significant correlations were found between site index and the studied P fractions except for P extracted with anion exchange membrane (P AEM) and the recalcitrant P fraction. & Conclusion In the studied soils, P availability is low and the predominant fractions of P are the recalcitrant forms. Aluminum and iron contents in the soils studied play an important role in sorption processes related to the highly and moderately labile P This article is part of the topical collection on Mediterranean pines Handling Editor: Andreas Bolte Contribution of the co-authors Teresa Bueis designed the experiment, carried out the field and laboratory work except P fractionation, run the data analysis, discussed the results, and wrote the paper. Felipe Bravo designed the experiment, coordinated the research project, and corrected the manuscript. Valentín Pando supported the statistical analysis and corrected the manuscript. Yaovi-Abel Kissi carried out part of the laboratory work (P fractionation) and corrected the manuscript. María-Belén Turrión designed the experiment, supervised the laboratory analysis, and corrected the manuscript.

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