Characterizing the use of carbon (C) reserves in trees is important for understanding regional and global C cycles, stress responses, asynchrony between photosynthetic activity and growth demand, and isotopic exchanges in studies of tree physiology and ecosystem C cycling. Using an inadvertent, whole-ecosystem radiocarbon ((14)C) release in a temperate deciduous oak forest and numerical modeling, we estimated that the mean age of stored C used to grow both leaf buds and new roots is 0.7 years and about 55% of new-root growth annually comes from stored C. Therefore, the calculated mean age of C used to grow new-root tissue is similar to 0.4 years. In short, new roots contain a lot of stored C but it is young in age. Additionally, the type of structure used to model stored C input is important. Model structures that did not include storage, or that assumed stored and new C mixed well (within root or shoot tissues) before being used for root growth, did not fit the data nearly as well as when a distinct storage pool was used. Consistent with these whole-ecosystem labeling results, the mean age of C in new-root tissues determined using 'bomb-(14)C' in three additional forest sites in North America and Europe (one deciduous, two coniferous) was less than 1-2 years. The effect of stored reserves on estimated ages of fine roots is unlikely to be large in most natural abundance isotope studies. However, models of root C dynamics should take stored reserves into account, particularly for pulse-labeling studies and fast-cycling roots (< 1 years).

In the study reported here we examined the short-term effects (1-3 years) of slash retention (SR) and the long-term effects (13-15 years) of wood-ash application (A) on fine roots and mycorrhizae in a 40-year-old Norway spruce forest in southwest Sweden. Soil cores were used to obtain estimates of the biomass (g m(-2)) of roots in three diameter classes (< 0.5, 0.5-1 and 1-2 mm), root length density (RLD), specific root length (SRL) and mycorrhizal root tip density (RTD). Fine root (< 1 mm) length production and mortality, and mycelium production, were estimated using minirhizotron and mesh bag techniques, respectively. Compared with the control plots (C), the biomass of fine roots in diameter classes < 0.5 mm and 0.5-1 mm was significantly higher in A plots, but lower in SR plots. In addition, RLD was significantly lower in the humus layer of SR plots than in the humus layers of C and A plots, but not in the other layers. None of the treatments affected the SRL. In all soil layers, the SR treatment resulted in significant reductions in the number of ectomycorrhizal root tips, and the mycelia production of fungi in mesh bags, relative to the C treatment, but the C and A treatments induced no significant changes in these variables. Fine root length production in the C, A and SR plots amounted to 94, 87 and 70 turn tube(-1) during the 2003 growing season, respectively. Fine root mortality in treated plots did not change over the course of the study. We suggest that leaving logging residues on fertile sites may result in nitrogen mineralisation, which may in turn induce reductions in root biomass, and both root and mycelium production, and consequently affect nutrient uptake and the accumulation of organic carbon in soil derived from roots and mycorrhizae. (C) 2008 Elsevier B.V. All rights reserved.

We assessed the influence of stand age on fine root biomass and morphology of trees and understory vegetation in 10-, 30-, 60- and 120-year-old Norway spruce stands growing in sandy soil in southeast Norway. Fine root (< 1, 1–2 and 2–5 mm in diameter) biomass of trees and understory vegetation (< 2 mm in diameter) was sampled by soil coring to a depth of 60 cm. Fine root morphological characteristics, such as specific root length (SRL), root length density (RLD), root surface area (RSA), root tip number and branching frequency (per unit root length or mass), were determined based on digitized root data. Fine root biomass and morphological characteristics related to biomass (RLD and RSA) followed the same tendency with chronosequence and were significantly higher in the 30-year-old stand and lower in the 10-year-old stand than in the other stands. Among stands, mean fine root (< 2 mm) biomass ranged from 49 to 398 g m–2, SLR from 13.4 to 19.8 m g–1, RLD from 980 to 11,650 m m–3 and RSA from 2.4 to 35.4 m2 m–3. Most fine root biomass of trees was concentrated in the upper 20 cm of the mineral soil and in the humus layer (0–5 cm) in all stands. Understory fine roots accounted for 67 and 25% of total fine root biomass in the 10- and 120-year-old stands, respectively. Stand age had no affect on root tip number or branching frequency, but both parameters changed with soil depth, with increasing number of root tips and decreasing branching frequency with increasing soil depth for root fractions < 2 mm in diameter. Specific (mass based) root tip number and branching density were highest for the finest roots (< 1 mm) in the humus layer. Season (spring or fall) had no effect on tree fine root biomass, but there was a small and significant increase in understory fine root biomass in fall relative to spring. All morphological characteristics showed strong seasonal variation, especially the finest root fraction, with consistently and significantly higher values in spring than in fall. We conclude that fine root biomass, especially in the finest fraction (< 1 mm in diameter), is strongly dependent on stand age. Among stands, carbon concentration in fine root biomass was highest in the 30-year-old stand, and appeared to be associated with the high tree and canopy density during the early stage of stand development. Values of RLD and RSA, morphological features indicative of stand nutrient-uptake efficiency, were higher in the 30-year-old stand than in the other stands.

• Effects of warming on root morphology, root mass distribution and microbialactivity were studied in organic and mineral soil layers in two alpine ecosystems over > 10 yr, using open-top chambers, in Swedish Lapland.

• Root mass was estimated using soil cores. Washed roots were scanned and sortedinto four diameter classes, for which variables including root mass (g dry matter(g DM) m –2 ), root length density (RLD; cm cm –3 soil), specific root length (SRL; m gDM –1 ), specific root area (SRA; m 2 kg DM –1 ), and number of root tips m –2 weredetermined. Nitrification (NEA) and denitrification enzyme activity (DEA) in the top10 cm of soil were measured.

• Soil warming shifted the rooting zone towards the upper soil organic layer in bothplant communities. In the dry heath, warming increased SRL and SRA of the finestroots in both soil layers, whereas the dry meadow was unaffected. Neither NEA norDEA exhibited differences attributable to warming.

• Tundra plants may respond to climate change by altering their root morphologyand mass while microbial activity may be unaffected. This suggests that carbon maybe incorporated in tundra soils partly as a result of increases in the mass of the finerroots if temperatures rise.

Mass loss, degradation of lignin and the qualitative change of the organic C structures of spruce root litter (2-5 mm in diameter) in O-horizon were studied for a period of 6 years (1995-2001) in a Norway spruce stand with a current deposition of 13 kg N and 12 kg S ha(-1) yr(-1). The stand was fertilized annually by addition of 100 kg N and 114 kg S ha(-1) (NS). Litterbags, acid detergent lignin (ADL), CuO-oxidation as well as C-13-NMR were used for measurements of mass loss, lignin concentration, degradation of lignin and changes of the organic C structures, respectively. The roots originating from the NS-treated plots lost 20% of their mass in the first year while in control (CON) plots the corresponding value was 10%. After 1879 days of decomposition the fertilized roots had a cumulative mass loss of 54% compared with the CON roots of 44%. The C/N ratios were significantly lower in the NS roots (35) than in the CON roots (59) after 1879 days of decomposition. The initial concentrations of ADL were 34.7 and 36.6 in CON and NS roots and increased to 50 and 56%, respectively, after 1879 days. Using CuO-oxidation method the degree of lignin degradation was significantly higher in the NS than CON roots after 853 days while C-13 NMR method showed no change. Our results indicate that CuO-oxidation and solid-state C-13 NMR methods give a qualitative measure of lignin decomposition, while the litterbag and ADL methods allow us to quantify mass loss and lignin concentration, respectively. It is concluded that the mass loss of root litter in fertilized plots is higher than needle litter decomposition in the same stand and the higher nitrogen concentration increases the lignin degradation.

The turnover of fine roots in northern coniferous forests has conventionally been assumed to be rapid, in line with results from sequential coring in the late 1970s in a Swedish Scots pine stand (SWECON project) where a rate of 7.4 year(-1) was estimated. New quantifications of the root respiration in other stands motivated a recalculation of the SWECON data; an indirect estimation of the turnover rate was much slower, about 2.1 year(-1). As a consequence, fine-root production is considered to be much lower than in previous estimates. Furthermore, direct observations of Norway spruce fine roots (< 1 mm) from minirhizotrons in Sweden, including a site close to the SWECON site, indicated a slower estimate, with fine-root turnover rate of 0.9 year

Specific root length (SRL, m g(-1)) is probably the most frequently measured morphological parameter of fine roots. It is believed to characterize economic aspects of the root system and to be indicative of environmental changes. The main objectives of this paper were to review and summarize the published SRL data for different tree species throughout Europe and to assess SRL under varying environmental conditions. Meta-analysis was used to summarize the response of SRL to the following manipulated environmental conditions: fertilization, irrigation, elevated temperature, elevated CO(2), Al-stress, reduced light, heavy metal stress and physical disturbance of soil. SRL was found to be strongly dependent on the fine root classes, i.e. on the ectomycorrhizal short roots (ECM), and on the roots < 0.5 mm, < 1 mm, < 2 mm and 1-2 mm in diameter SRL was largest for ECM and decreased with increasing diameter. Changes in soil factors influenced most strongly the SRL of ECM and roots < 0.5 mm. The variation in the SRL components, root diameter and root tissue density, and their impact on the SRL value were computed. Meta-analyses showed that SRL decreased significantly under fertilization and Al-stress; it responded negatively to reduced light, elevated temperature and CO(2). We suggest that SRL can be used successfully as an indicator of nutrient availability to trees in experimental conditions.

Fine roots (< 2 mm) are very dynamic and play a key role in forest ecosystem carbon and nutrient cycling and accumulation. We reviewed root biomass data of three main European tree species European beech, (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.), in order to identify the differences between species, and within and between vegetation zones, and to show the relationships between root biomass and the climatic, site and stand factors. The collected literature consisted of data from 36 beech, 71 spruce and 43 pine stands. The mean fine root biomass of beech was 389 g m(-2), and that of spruce and pine 297 g m(-2) and 277 g m(-2), respectively. Data from pine stands supported the hypothesis that: root biomass is higher in the temperate than in the boreal zone. The results indicated that the root biomass of deciduous trees is higher than that of conifers. The correlations between root biomass and site fertility characteristics seemed to be species specific. There was no correlation between soil acidity and root biomass. Beech fine root. biomass decreased with stand age whereas pine root biomass increased with stand age. Fine root biomass at tree level. correlated better than stand level root biomass with stand characteristics. The results showed that there exists a strong relationship between the fine root biomass and the above-ground biomass.