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.

Ecosystem-based forest management associated with partial harvesting (PH) is intended to balance ecological and economic values of sustainable forest management. The potential for delayed growth response and elevated mortality of advance regeneration following PH remains a critical concern, and may present a barrier to more widespread implementation of this approach. We used 835 permanent continuous forest inventory plots to examine the rate and time course of species-specific regeneration growth and mortality of eight tree species in the first fifteen years following operational partial harvests in the mixed-species forests of Maine, United States. We aimed to provide a quantitative understanding on how regeneration of different species responded to PH in terms of growth and mortality. In addition, we evaluated how the patterns and magnitudes of growth and mortality responses developed over time, if these responses occur gradually or suddenly, and if the patterns of the responses were persistent. We found that the response magnitude, temporal trajectories of responses, and the length of initial lag-period largely varied across species, PH treatments, and the variables examined. For sapling diameter growth, paper birch (Betula papyrifera Marshall) and red maple (Acer rubrum L.) showed immediate responses to high-intensity PH, while a five-year lag-period was observed in balsam fir (Abies balsamea (L.) Mill.), American beech (Fagus grandifolia Ehrh.), red spruce (Picea rubens Sarg.) and eastern hemlock (Tsuga canadensis (L.) Carrière) and a 10-year lag period in northern white-cedar (Thuja occidentalis L.). The initial increase in sapling mortality was observed in balsam fir, American beech, red maple and northern white-cedar, but not in other species. Sapling survival reached a stable state irrespective of species after the initial five-years following harvests. In partially harvested stands, identifying preharvest conditions related to postharvest density, growth, and mortality was complex and interacted with time since harvest. Our results suggest that broad application of PH only results in species-specific gains, losses, and delays in regeneration responses within mixed-species stands. Future research should consider PH in combination with other treatments to initiate immediate responses to a wider range of species.