Influence of site characteristics and thinning intensity on litterfall production in two Pinus sylvestris L. forests in the western Pyrenees

Litterfall production was studied for 30 months in two contrasting Pinus sylvestris L. stands in the western Pyrenees managed under two low thinning intensities: Aspurz (625 m a.s.l., Mediterranean climate) and Garde (1335 m a.s.l., continental climate). Nine 0.12 ha plots per site were set up with three treatments three times repeated: removal of 0% (not thinned, P0), 20% (P20) and 30% (P30) of initial basal area. Total litterfall was significantly higher in Aspurz (5533 kg ha −1 year −1) than in Garde (3986 kg ha −1 year −1). Maximum annual needle fall values were in September (Aspurz) or October (Aspurz and Garde), and second order peaks were in June or July at both sites. In both sites, cones and other pine organs (e.g., seeds, inflorescences) showed the same seasonal patterns, while bark and branches, affected by sporadic occurrence of storms or snowfalls, followed no clear pattern. Low thinning effects were site-dependent (e.g., needles; Aspurz, P0 > (P20 = P30); Garde, P0 > P20 > P30) due to between site differences in stand structure. Thus, more dominant and codominant trees (i.e., high litterfall producers) were felled in P30 than in P20 in Garde relative to Aspurz. Needle fall in Aspurz showed unique significant positive correlations with days of dryness and moisture deficit, probably reflecting its Mediterranean climate. However, relative to Aspurz, Garde exhibited significant retarded responses of total litterfall and needle fall to changes in air temperature, soil temperature and soil moisture suggesting in part that Garde's needles are more resistant to climatic stress. Furthermore, litterfall was better predicted in Aspurz than in Garde, partly due to the greater influence of sporadic windstorms and snowstorms in Garde. It appears that in these Pyrenean P. sylvestris stands, site-dependent climatic variables have an important control on litterfall. At the local scale, however, stand structure influences effects of thinning on litterfall.