Size–Density Trajectory in Regenerated Maritime Pine Stands after Fire

Abstract

Research Highlights: This study bridges a gap of knowledge about the maximum size-density trajectory for juvenile stands of maritime pine. The continuity of the trajectory along the development stages to maturity is assured with a straightforward approach providing support to determine optimum density along all the revolution periods for the species. Background and Objectives: Forest fire is a significant threat to forests in the Mediterranean regions, but also a natural disturbance that plays a vital role in the perpetuation of forest stands. In recent decades, there has been an increase of burnt area in maritime forests in Portugal, followed by an increased interest in managing the natural and usually abundant regeneration occurring after the fires. The gap in the knowledge of growth dynamics for juvenile stages, for these forest systems, currently constrains their correct management, for forest planning, particularly in determining the optimal densities. The study aims to identify the maximum attainable density trajectory at the early stages of development of the species that could support a non-empirical definition of silvicultural prescriptions and thinning decisions, along the revolution. Materials and Methods: A representative data set collected in stands regenerated after fire supports the analysis of the maximum size-density trajectory for the species. Results: The maximum size-density trajectory for the juvenile stands deviates from the expected trajectory defined in the self-thinning line published for the species. Significant deviation occurs at the lower end of the line, indicating the need for a reevaluation of the existing self-thinning line. We propose a new self-thinning model for the species that explicitly considers the behavior of size-density for juvenile stands. The new model assures a logical continuity for the trajectory from the young stages of development to maturity. Conclusions: The proposed model based on the maximum attainable size-density trajectory provides ecological-based support to define silvicultural guidelines for management of the species.

Management of Maritime Pine: Energetic Potential with Alternative Silvicultural Guidelines

“…These scenarios were specifically designed by the authors for areas with a high density of natural regeneration after fire. Both initialize with a density of 40,000 plants per hectare at the age of 8, which are supported by Almeida et al [21] and corroborated by Enes et al's [19] findings on the size-density trajectory in regenerated pine stands after a fire. Scenario 3 considers a harvest age of 45 years as typically occurs in traditional management.…”

“…In areas naturally regenerated post-fire, density at the beginning of stand establishment can reach a far greater number than the figures of stand density registered in the traditional silvicultural models with artificial regeneration. In datasets used by Enes et al [19] to identify the maximum attainable density trajectory at the early stages of development of the species, maximum values ranging from 7500 to 90,000 plants are reported in juvenile maritime pine stands (age less than 20 years) in Portugal. The availability of naturally regenerated forest promotes harnessing the natural regeneration from seeds as an option that enables saving in site preparation and plantation costs.…”

Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources: An Overview

“…Maritime pine is managed in high-forest stands [2] (see Figures 2 and 4). The structure is most frequently even-aged, whether from natural [41][42][43][44] (Figure 3) or artificial [2,40,45,46] regeneration. Traditionally, maritime pine is managed in pure stands.…”

“…Traditionally, maritime pine is managed in pure stands. The preference for even-aged stands is related to easier management, promotion of wood quantity and quality [2,40,[47][48][49][50][51], and disturbances, mainly fire or harvest events that usually result in one regeneration cohort shortly after disturbance, if seed is available [41][42][43][44].…”

“…Density regulation based on SDI relies on the assumption that in monospecific even-aged populations of trees experiencing complete crown closure, mortality is density-dependent. The natural trajectory of the number of trees per tree size was defined by Luis and Fonseca [82] and revised by Enes et al [44]. The use of relative values of SDI is suitable for management purposes, as it provides information on the appropriate number of living trees for given tree size, according to the management aims (e.g., optimum growth-density interval, maximization of stand volume, or maximization of mean tree size).…”