The impact of planting density on forest monospecific plantations: An overview

•This review demonstrates a growing trend in research about planting density in the last 20 years.•Most studies were developed in countries with a well-established forest industry.•Studies focus on species with great commercial interests, as Pinus and Eucalyptus species.•Morphological characteristics are the most used to evaluate how planting density affects plant growth instead of physiological characteristics.•Individual growth tends to negatively correlate with planting density while stand productivity positively correlates with it. Knowledge of the morphological and physiological responses of trees to planting density is important for adjusting plantation establishment practices to attain the desired size and characteristics of wood products. In this review, we provide an overview of how planting density is managed as a silvicultural practice and summarize recent advances, trends, and gaps to guide future research in this field. We applied a PRISMA methodology to select papers published between 1981 and 2020 that tested how monospecies planting density affects the morphological and physiological characteristics of planted trees; we observed an increasing trend in the number of relevant publications over this period. These studies focused mainly on species used for timber production (Eucalyptus and Pinus), and research was concentrated in the United States, China, and Brazil. In general, the 255 selected studies identified that planting density has a major impact on stem diameter and volume, resulting in greater stand productivity of high-density stands, especially for trees having a smaller individual volume. Studies evaluating the physiological response of trees to planting density also showed that species ecology, solar radiation, water requirements, and site-specific characteristics have a greater influence on plant growth than planting density. Considering the physiological aspects in high-density plantations, water relations based on transpiration and water use efficiency are the main aspect guiding plants growth rate. Despite major advances in intensive silviculture around the world, there remains a knowledge gap for tree species grown for their leaves, fruits, or seeds and a lack of data related to the physiological response of trees to planting density. The results of this literature survey can improve forest management decisions in regard to forest stand use and develop novel study avenues for silviculture.