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Forests play an important role in both regional and global C cycles. However, the spatial patterns of biomass C density and underlying factors in Northeast Asia remain unclear. Here, we characterized spatial patterns and important drivers of biomass C density for Northeast Asia, based on multisource data from in situ forest inventories, as well as remote sensing, bioclimatic, topographic, and human footprint data. We derived, for the first time, high-resolution (1 km × 1 km) maps of the current and future forest biomass C density for this region. Based on these maps, we estimated that current biomass C stock in northeastern China, the Democratic People's Republic of Korea, and Republic of Korea to be 2.53, 0.40, and 0.35 Pg C, respectively. Biomass C stock in Northeast Asia has increased by 20%-46% over the past 20 years, of which 40%-76% was contributed by planted forests. We estimated the biomass C stock in 2080 to be 6.13 and 6.50 Pg C under RCP4.5 and RCP8.5 scenarios, respectively, which exceeded the present region-wide C stock value by 2.85-3.22 Pg C, and were 8%-14% higher than the baseline C stock value (5.70 Pg C). The spatial patterns of biomass C densities were found to vary greatly across the Northeast Asia, and largely decided by mean diameter at breast height, dominant height, elevation, and human footprint. Our results suggest that reforestation and forest conservation in Northeast Asia have effectively expanded the size of the carbon sink in the region, and sustainable forest management practices such as precision forestry and close forest monitoring for fire and insect outbreaks would be important to maintain and improve this critical carbon sink for Northeast Asia.

This study investigated the effects of thinning intensities on tree water use and growth of 50-year-old Korean pine forests for four years. Thinning was conducted with two intensities (light thinning, heavy thinning, and control) in March 2012. Tree water use was estimated from sap flux density using Granier-type thermal dissipation sensors on 84 of total 103 trees. Tree diameter growth was measured in 97 trees with a dendrometer and converted to aboveground biomass increment using an allometric equation. Sap flux density increased initially in both thinning groups, but the effects of light thinning decreased over time. In the heavy thinning plot, sap flux density was stimulated for the entire study period. While light-thinning affected little on tree growth, tree diameter growth significantly increased from second year and this enhancement increased with time in the heavy-thinning plot. The tree diameter and growth rate were positively correlated in the light-thinning plot and negatively correlated in the heavy-thinning plot. The enhancement of sap flux density and diameter growth caused higher tree water use and tree level aboveground net primary productivity in heavy-thinning plot. Due to higher water use in the heavy thinning plot, increase in tree-level water use efficiency was observed in the high growth year only. Our results indicate that physiological responses of trees differed depending on thinning intensity and heavy thinning is an appropriate management practice for mature pine plantations.

It is difficult to scale up measurements of the sap flux density (JS) for the characterization of tree or stand transpiration (E) due to spatial variations in JS and their temporal changes. To assess spatial variations in the sap flux density of Korean pine (Pinus koraiensis) and their effects on E estimates, we measured the JS using Granier-type sensors. Within trees, the JS decreased exponentially with the radial depth, and the JS of the east aspects were higher than those of the west aspects. Among trees, there was a positive relationship between JS and the tree diameter at breast height, and this positive relationship became stronger as the transpiration demand increased. The spatial variations that caused large errors in E estimates (i.e., up to 110.8 % when radial variation was ignored) had varied systematically with environmental factors systematic characteristics in relation to environmental factors. However, changes in these variations did not generate substantial errors in the E estimates. For our study periods, the differences in the daily E (ED) calculated by ignoring radial, azimuthal and tree-to-tree variations and the measured ED were fairly constant, especially when the daily vapor pressure deficit (D_D) was higher than 0.6 kPa. These results imply that the effect of spatial variations changes on sap flow can be a minor source of error compared with spatial variations (radial, azimuthal and tree-to-tree variations) when considering E estimates.

Accurate estimates of stand transpiration (E) require the consideration of three types of variation in sap flux density (J S): radial, azimuthal, and tree-to-tree variation. In this study, the J S variation of 50-year-old Korean pine (Pinus koraiensis) trees and its effect on E estimates was evaluated using Granier-type heat dissipation sensors. The value of J S decreased exponentially with the radial depth from cambium to pith, and the coefficient of variation (CV) for radial variation was 124.3 %. Regarding the azimuthal variation, the value of J S differed significantly among aspects and the average CV was 23.6 %. The average CV for tree-to-tree variation was 34.0 %, and the daily CV increased with increasing vapor pressure deficit (D). The error in the E estimates caused by ignoring the radial variation was the largest (109.2 %), followed by those caused by ignoring the tree-to-tree and azimuthal variations (24.3 and 12.6 %, respectively). While the contribution of the azimuthal variation to the E estimates was minimal in comparison to the other variations, the azimuthal variation among aspects was significant, and the usage of the north aspect measurement did not generate substantial error in the E estimates (0.6 %). Our results suggest that the variation, particularly the species- and site-specific radial variation, must be considered when accurately calculating E estimates.