Climate, site conditions, and stand characteristics influence maximum size-density relationships in Korean red pine (Pinus densiflora) and Mongolian oak (Quercus mongolica) stands, South Korea

Abstract

This study examines the effects of climate, site index and structural diversity on maximum size-density relationships (MSDRs) for Korean red pine (Pinus densiflora Siebold & Zucc.) and Mongolian oak (Quercus mongolica Fisch. ex Ledeb.) which are widely distributed and abundant across South Korea. We used data collected between 2006 and 2015 from the Korean National Forest Inventory and stochastic frontier function regression to develop static self-thinning lines (STLs) for pure pine, pure oak, and pine-oak mixed-species stands. Climate variables, site condition (e.g., site index), and stand characteristics (e.g., structural diversity and pine proportion) were applied to the STLs to examine the factors affecting MSDRs and the effects of climate on MSDRs. Our results indicate that the slope and intercept values differed significantly between the three stand types, with the pine STL being steeper than other STLs. Degree days above 18 °C in spring (DD18_sp), degree days above 5 °C in spring (DD5_sp), and annual heat moisture index (AHM) mainly influenced the position of STLs. Increases in these climate variables (e.g., warmer and drier conditions) reduce maximum densities owing to increased evapotranspiration and water stress in stands. Increasing site index (SI) was associated with an upward shift in the position of the STL for pine, and the result shows that better site quality can improve carrying capacity and increase stand stocking. However, high stand stocking can encourage tree competition with increasing dryness and reduce maximum stocking. Higher stand structural diversity caused an upward shift in all STLs, and the effect of climate on the STLs of different structural diversities varied with stand type. Structurally diverse stands can stock more trees in their stands owing to efficient space use, especially with respect to shade tolerant species. Nonetheless, under drier conditions, trees in dense stands experience severe competition, and their carrying capacity decreases. Our results suggest that it is important to include stand density control in forest management plans to prepare for global warming in South Korea.