Analysing twenty-six years of phosphorus return via litterfall in two contrasting Pinus sylvestris forests under different thinning intensities (western Pyrenees, Spain)

Introduction

Phosphorus (P) is frequently a limiting nutrient in temperate forest ecosystems, yet accurate long-term estimates of P fluxes remain rare because they require uninterrupted field measurement across many years. Sustainable management of temperate forests should ideally account for these cycles, since harvesting decisions can either conserve or progressively deplete site P capital.

Methods

This study draws on a long-term litterfall monitoring programme in two even-aged *Pinus sylvestris* L. forests that represent the extremes of a site-quality gradient in the western Pyrenees (Navarre, Spain): Aspurz, a productive Mediterranean site (625m, mean annual temperature 12°C, 912 mm precipitation), and Garde, a lower-productivity continental site (1335m, 8.2 °C, 1268 mm). At each site, a randomized block design compares three treatments; an unthinned control, moderate thinning (20% of basal area removed) and heavy thinning (30%). Since 2000, litterfall has been collected over regular intervals using circular litter traps, separated into needles, other pine organs and miscellaneous fractions, oven-dried and analysed for P, allowing annual P return (kg P ha⁻¹ yr⁻¹) to be quantified for each year from 2000 to 2025.

Expected Results

To analyse long-term trends, we will use non-parametric trend analysis (Mann–Kendall test with Sen's slope estimator) together with time-series decomposition to analyse the dataset, with the aim of separating real trends from interannual climatic variability. We will further use machine-learning (e.g. random-forest models with variable-importance analysis) to disentangle the relative influence of climatic versus management drivers on the magnitude and direction of P-return trends.

Conclusions

We expect to identify site and treatment dependent trends in litterfall P return that reveal how thinning intensity shapes long-term phosphorus cycling and inform sustainable management of temperate Scots pine forests.