Modelling the spatio-temporal pattern of primary dispersal in stone pine (Pinus pinea L.) stands in the Northern Plateau (Spain)

Abstract

Natural regeneration in stone pine (Pinus pinea L.) managed forests in the Spanish Northern Plateau is
not achieved successfully under current silviculture practices, constituting a main concern for forest
managers. We modelled spatio-temporal features of primary dispersal to test whether (a) present low
stand densities constrain natural regeneration success and (b) seed release is a climate-controlled process.
The present study is based on data collected from a 6 years seed trap experiment considering different
regeneration felling intensities. From a spatial perspective, we attempted alternate established kernels
under different data distribution assumptions to fit a spatial model able to predict P. pinea seed rain. Due
to P. pinea umbrella-like crown, models were adapted to account for crown effect through correction of
distances between potential seed arrival locations and seed sources. In addition, individual tree fecundity
was assessed independently from existing models, improving parameter estimation stability. Seed rain
simulation enabled to calculate seed dispersal indexes for diverse silvicultural regeneration treatments.
The selected spatial model of best fit (Weibull, Poisson assumption) predicted a highly clumped dispersal
pattern that resulted in a proportion of gaps where no seed arrival is expected (dispersal limitation)
between 0.25 and 0.30 for intermediate intensity regeneration fellings and over 0.50 for intense fellings.
To describe the temporal pattern, the proportion of seeds released during monthly intervals was modelled
as a function of climate variables – rainfall events – through a linear model that considered temporal
autocorrelation, whereas cone opening took place over a temperature threshold. Our findings suggest
the application of less intensive regeneration fellings, to be carried out after years of successful seedling
establishment and, seasonally, subsequent to the main rainfall period (late fall). This schedule would
avoid dispersal limitation and would allow for a complete seed release. These modifications in present
silviculture practices would produce a more efficient seed shadow in managed stands.