臺灣中部溪頭鳳凰山天然林及柳杉人工林之細根分布與動態

Abstract

Fine roots play critical roles in carbon and nutrient cycles in forest ecosystems. Thus, understanding fine root dynamics for characterizing carbon and nutrient cycles in forest ecosystems is important. The optical scanner method is useful for monitoring fine root dynamics. Few studies have applied this method in mountainous forests with various species and sloping conditions in Taiwan. In this study, we investigated the sampling position for scanned image acquisition and calculated the recovery time of fine root systems following soil disturbance caused by scanner box installation. This study was conducted 1) to clarify fine root distribution around single trees on slopes in a broad-leaved forest and in an adjacent Japanese cedar (Cryptomeria japonica) plantation and 2) to determine the differences in the fine root dynamics of the natural broad-leaved forest and the adjacent Japanese cedar plantation forest by using the optical scanner method. Moreover, 3) based on the derived data, the recovery time of fine root systems following soil disturbance was determined. To achieve the first objective, this study used the soil coring method for quantifying the distribution of fine root density (FRD) around tree trunks in the natural forest (number of target trees [n] = 10) and in the Japanese cedar plantation forest (n = 5). Both forest areas are part of the Experimental Forest of National Taiwan University located on Mount Fong-Huang in Xitou tract. Soil samples were collected from three positions around each sampled tree trunk along a slope (i.e., 1.0 m away in the upper, horizontal, and lower directions from the tree trunk). To achieve the second and third objectives, this study applied the optical scanner method beside the sample trees. Data were collected for two dominant tree species (n = 4 for each species) in the natural forest and Japanese cedar trees (n = 4) in the Japanese cedar plantation from May 2017 to April 2018. Using the temporal changes in fine root biomass (FRB) as indicated in the data from the first year after soil disturbance, this study estimated the recovery time of fine root systems in the forests. First, large variations were identified in the FRD around the tree trunks on the slopes. These variations were not correlated with the sampling positions in either the natural forest or the Japanese cedar plantation forest, suggesting that slope position was not a critical concern and that the sampling number was of more important for FRD estimates. The only difference in FRD between the two forests was the mean value for FRD, which was approximately twice as large for the natural forest (651.1 g m−2) as that for the Japanese cedar forest (309.6 g m−2). Second, fine root production was more active than decomposition in the study period. Fine root production was highly correlated with FRB, whereas fine root decomposition was not. This finding suggested that FRB dynamics were mainly determined by the fluctuation in fine root production during the study period. This study identified two temporal patterns for FRB in the two forests: linear increase (Pattern I) was observed in the FRB of the Japanese cedar trees and three broad-leaved trees, and a saturation-shaped-curve pattern (Pattern Ⅱ) was observed in the FRB of five broad-leaved trees. The patterns were associated with distinct temporal changes in fine root production. Trees with Pattern I exhibited active fine root production in the later part of the study period (autumn–winter), whereas trees characterized by Pattern II were inactive in fine root production at that time. Third, for the five broad-leaved trees that exhibited Pattern II, FRB reached a stable state 5–6 months after the soil disturbance caused by equipment installation. However, FRB did not reach a stable state in the Japanese cedar trees or the three broad-leaved trees characterized by Pattern I. These results suggested that the recovery time of fine root systems may vary among individual trees. One year may have been insufficient for some trees at the study sites to recover FRB after soil disturbance from installation of equipment for the optical scanner method.