Abstract:

Fine root life-span and turnover play an important role in carbon allocation and nutrient cycling in forest ecosystems because of high levels of fine root production and consequent mortality and decomposition. Fine roots typically are defined as having the following characteristics: less than 1 or 2 mm in diameter; short life-span; and greater efficiency in accessing belowground resources than large diameter roots. However, when categorizing roots by diameter size, the position of an individual root on the complex lateral branching pattern has often been ignored, and our knowledge about relationships between branching order and root function is limited. More recently, studies on root order have found that first-order fine roots at the distal end of a root system, which has the primary function of nutrient uptake rather than storage and transport, are thinner in root diameter and have higher tissue nitrogen (N) concentrations, higher maintenance respiration rates, and lower total nonstructural carbohydrate (TNC) concentrations. Thus, the smaller diameter roots have a shorter life-span in contrast to higher order roots (which are coarser roots with larger diameter). Although either approach (diameter or root order) is reasonable to use in fine root studies, estimates of fine root life-span or turnover are much more variable when using root diameters because of the large variation in diameter sizes. The objectives of this study were (1) to examine variations of fine root diameter and diameter range (minimum-maximum) as a function of branching order from first order to fifth order; and (2) to determine the relationship between fine root diameter and root branching order and soil resource availability using two tree species, Manchurian ash (Fraxinus mandshurica) and Dahurian larch (Larix gmelinii).
This research was conducted in Maoershan Forest Research Station (45°21'-45°25'N, 127°30'-127°34' E) owned by Northeast Forestry University in Harbin, Heilongjiang, China. Both ash and larch forests were planted in 1986. In each plantation, we established three 20 m×30 m plots at an elevation of 506 m. On May 15, July 15 and September 15 of 2003, three small intact segments of the fine root system were excavated carefully at a random location in each plot. Soil blocks (20 cm×20 cm×10 cm) were excavated from the sties at depth of 0~10 cm and 11~20 cm. All intact root segments were collected from each block. Once excavated, the intact segments were put into plastic bags with ice and stored at a temperature of 0~2 ℃. In the laboratory, each individual root was dissected by branching order beginning with the distal end of the root system (labeled as the first-order) increasing sequentially with each branch from the first to higher order roots. After the dissection, length, diameter and dry weight of a given order were determined.
The results showed that mean diameters of fine roots were significantly different (p<0.001) among orders with the diameter increasing regularly from first-order to fifth-order branches in both species. The mean diameter of first-order roots was 0.26 mm for ash and 0.34 mm for larch, and fifth-order roots had average diameters of 1.54 mm and 1.70 mm for ash and larch, respectively. If fine roots were defined as having a diameter less than 1-2 mm, five orders of ash and four orders of larch would be defined as being fine roots. If the diameter of fine roots was defined as being smaller than 0.5 mm, the first three orders of ash roots and the first two orders of larch roots would be included in the fine root population. Within the same root order, there was variation within fine root diameters and there were differences between the two species. The diameter ranges of the fine roots from first order to fifth order were 0.15-0.58, 0.18-0.70, 0.26-1.05, 0.36-1.43, and 0.71-2.96 mm for ash, and 0.17-0.76, 0.23-1.02, 0.26-1.10, 0.38-1.77, and 0.84-2.80 mm for larch. The mean coefficient of variation in first-order roots was less than 10%, second- and third-order was 10%-20%, fourth- and fifth-order was 20%-30%. Thus variation in root diameter also increased with increasing root order.
These results suggest that “fine roots”, which are traditionally defined as an arbitrary diameter class (i.e., <2 mm in diameter), may be too large a size class when compared to the finest roots. The finest roots have much shorter life-spans than larger diameter roots; however, the larger roots are still considered a component of the fine root system. Root order also is important to root life-span, because variation in diameters among roots within the same order is large and diameters varied from <0.2 mm, to 0.2-0.5 mm and to >0.5 mm even in the first-order roots. Differences in the life-span between root diameter and root order affect estimates of root turnover. Therefore, based on this study, both diameter and order should be considered when estimating fine root life-span and turnover.

Key words: Fine root diameter, Root order, Life-span, Turnover, Fraxinus mandshurica, Larix gmelinii