Overstory Effects on the Understory of Aleppo Pine Plantations—Implications for Ecosystem Restoration

At the end of the 19th century and along the first half of the 20th century, public policiesin Mediterranean countries and elsewhere in Europe strongly promoted pine afforestation for landreclamation and wood production. In many cases, the transition to native forests, more resilient andmore diverse, was also foreseen. This study aims to find the overstory characteristics that are bestrelated to the understory of Aleppo pine (Pinus halepensis) plantations, to assist ecosystem restorationgoals. We installed 33 circular plots in mature Aleppo pine plantations located in the southeasternSalento peninsula, Apulia, Italy. We measured overstory characteristics and the correspondingunderstory on these plots. We assessed the effects of overstory variables on understory development(plant density, plant height, and the Magini regeneration index) and diversity (species richness and theShannon–Wiener index) using linear mixed models (LMM). Understory development and diversitywere positively correlated with the Hart–Becking spacing index and negatively correlated with basalarea and canopy cover, the three overstory variables that best explained variance. We used polynomialfitting and the subsequent derivation of these functions to determine the values of the Hart–Beckingindex and of the canopy cover that corresponded to the maximum development (33.7% and 84.6%,respectively) and diversity (32.6% and 86.5%, respectively) of the understory. Redundancy analysis(RDA) showed that late-successional species, includingQuercus coccifera, were associated with higherlevels of understory development and stand spacing. These results may assist in the restoration ofnative ecosystems in Aleppo pine plantations installed in similar ecological conditions.