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We measured the vertical distribution and seasonal patterns of fine-root production and mortality using minirhizotrons in a cool-temperate forest in northern Japan mainly dominated by Mongolian oak (Quercus crispula) and covered with a dense understory of dwarf bamboo (Sasa senanensis). We also investigated the vertical distribution of the fine-root biomass using soil coring. We also measured environmental factors such as air and soil temperature, soil moisture and leaf area indices (LAI) of trees and the understory Sasa canopy for comparison with the fine-root dynamics. Fine-root biomass to a depth of 60 cm in September 2003 totaled 774 g m-², of which 71% was accounted for by Sasa and 60% was concentrated in the surface soil layer (0-15 cm), indicating that understory Sasa was an important component of the fine-root biomass in this ecosystem. Fine-root production increased in late summer (August) when soil temperatures were high, suggesting that temperature partially controls the seasonality of fine-root production. In addition, monthly fine-root production was significantly related to Sasa LAI (P<0.001), suggesting that fine-root production was also affected by the specific phenology of Sasa. Fine-root mortality was relatively constant throughout the year. Fine-root production, mortality, and turnover rates were highest in the surface soil (0-15 cm) and decreased with increasing soil depth. Turnover rates of production and mortality in the surface soil were 1.7 year-¹ and 1.1 year-¹, respectively.

To investigate the effect of reduced snow cover on fine root dynamics in a cool-temperate forest in northern Japan because of decreases in snowfall at high latitudes due to global warming, we monitored root length, production, and mortality before and after snow removal with an in-ground root scanner. We measured root dynamics of both overstory deciduous oak (Quercus crispula) and understory evergreen dwarf bamboo (Sasa nipponica), the two major species in the forest. Snow removal advanced the timing of peak root production by a month both in total and in Sasa, but not in oak. There was a significant interaction between snow removal and plant form on root production; this indicates that enhanced Sasa root production following snow removal might increase its ability to compete with oak. In contrast, snow removal did not enhance root mortality, suggesting that the roots of these species tolerate soil freezing. The earlier snow disappearance in the snow removal plot expanded the growing season in Sasa. We speculate that this change in the understory environment would advance the timing of root production by Sasa by extending the photosynthetic period in spring. We propose that different responses of root production to reduced snow cover between the two species would change the competitive interactions of overstory and understory vegetation, influencing net primary production and biogeochemistry (e.g., carbon and nitrogen cycles) in the forest ecosystem.

Key messageExtremely high fine root mortalities were observed under drought in 2018, increasing fine root mortality in young trees could be explained by differences in root distribution compared to old trees.Juvenile trees in floodplains are prone to high mortality leading to a low success in reforestation in these habitats. One of the reasons contributing to high mortality of juvenile trees could be limited water and nutrient uptake due to a high level of fine root mortality, especially during summer droughts on higher terraces of floodplains. Strategies of different tree species in hardwood floodplain forests (HFFs) on keeping fine roots alive are still poorly understood. During the record-breaking summer drought of 2018, we examined the relationship between tree age and fine root dynamics of Quercus robur and Ulmus laevis along the Elbe River in Germany. Root area index (RAI), live root density (LRD) and relative root mortality (RRM) of young and old Q. robur and U. laevis were analysed by taking soil cores three times during the progression of a summer drought. Old oaks had a lower RAI in the upper soil than young oaks, while RAI of elms did not differ between young and old trees. RRM was very high during summer reaching more than 100% on average. RRM of young trees of both species increased with increasing drought during summer, while RRM of old trees did not change. We argue that differences in the response of RAI between oaks and elms reflect the sink competition between growing deep roots and shallow roots, which is pronounced in oaks due to their characteristic tap root system. Differences in root distribution patterns and the ability to perform hydraulic redistribution may explain the differences in RRM between trees of different species and ages.

Air warming (TC: control; TW: +3 °C) and precipitation manipulation (PC: control; PD: −30%; PI: +30%) were established to examine effects of these treatments on fine root production (FRP), fine root mortality (FRM), and total root (coarse and fine root) biomass in 33- to 59-month-old Pinus densiflora Sieb. et Zucc. seedlings for two years. We hypothesized that warming and altered precipitation would affect the growth, death, and biomass of fine roots by changing soil temperature and soil water availability. Mean annual FRP and total root biomass were significantly altered by only precipitation manipulation: they were 29.3% (during the two-year period) and 69.0% (after the entire two years) higher, respectively, in PD plots than in PC plots, respectively. In contrast, only warming had a significant effect on mean annual FRM, being 13.2% lower in TW plots than TC plots during the two-year period. Meanwhile, fine root biomass was affected negatively and simultaneously by both soil temperature and soil moisture. It seemed that fine root dynamics have changed so that they maintain their systems in response to the altered soil temperature and moisture. The current study adds significant knowledge for understanding the fine root dynamics of P. densiflora seedlings under altered temperature and precipitation regimes.

Low temperatures are crucial for the formation of the alpine treeline worldwide. Since soil temperature in the shade of tree canopies is lower than in open sites, it was assumed that self-shading may impair the trees' root growth performance. While experiments with tree saplings demonstrate root growth impairment at soil temperatures below 5-7°C, field studies exploring the soil temperature - root growth relationship at the treeline are missing. We recorded soil temperature and fine root abundance and dynamics in shaded and sun-exposed areas under canopies of isolated trees at the alpine treeline. In contrast to the mentioned assumption, we found more fine root biomass and higher fine root growth in colder than in warmer soil areas. Moreover, colder areas showed higher fine root turnover and thus lower root lifespan than warmer places. We conclude that balances enhanced fine root mortality in cold soils with higher fine root activity and by maintaining higher fine root biomass, most likely as a response to shortage in soil resource supply. The results from our study highlight the importance of measurements on mature trees to understand the fine root response and carbon allocation pattern to the thermal growth conditions at the alpine treeline.

Key message Fine root ingrowth and mortality of European beech are related to evapotranspiration, cumulative forest floor precipitation, soil temperature and water content, which are affected by forest management and gap creation. The ingrowth and mortality of European beech ( Fagus sylvatica L.) fine roots (diameters <2 mm) were studied in relation to environmental variables describing temperature and water availability at four sites, covering a range in environmental conditions likely to be encountered in Slovenian beech forests. Minirhizotron images were used to determine fine root dynamics in a stand and gap in each of the sites for 12 periods during the 2007–2009 growing seasons. The environmental variables included air and soil temperatures, precipitation, forest floor precipitation, evapotranspiration and soil water contents. For data analysis, the daily mean values for each period for all variables were used. Fine root ingrowth and mortality were higher in the managed stand and gap compared to the old-growth stand and gap, but only significantly correlated with each other in the case of the managed stand. Forest floor precipitation and soil temperature were significant in explaining fine root ingrowth, whereas maximal evapotranspiration, soil temperature and soil water content were more important for fine root mortality. However, the correlations were weak and inconsistent among the four sites. By including site as predictor as well as environmental variables, R 2 values of 0.49 and 0.55 for ingrowth and mortality, respectively, were achieved. Despite this, the relationships between the fine root dynamics and selected environmental factors appeared relatively weak and complex, especially for fine root ingrowth and might be partially related also to differences in successional stages of the forests under study.

Growth and death of fine roots represent an important carbon sink in forests. Our understanding of the patterns of fine root turnover is limited, in particular in tropical forests, despite its acknowledged importance in the global carbon cycle. We used the minirhizotron technique for studying the changes in fine root longevity and turnover along a 2000-m-elevational transect in the tropical mountain forests of South Ecuador. Fine root growth and loss rates were monitored during a 5-mo period at intervals of four weeks with each 10 minirhizotron tubes in three stands at 1050, 1890, and 3060 m asl. Average root loss rate decreased from 1.07 to 0.72 g/g/yr from 1050 to 1890 m, indicating an increase in mean root longevity with increasing elevation. However average root loss rate increased again toward the uppermost stand at 3060 m (1.30 g/g/yr). Thus, root longevity increased from lower montane to mid-montane elevation as would be expected from an effect of low temperature on root turnover, but it decreased further upslope despite colder temperatures. We suggest that adverse soil conditions may reduce root longevity at high elevations in South Ecuador, and are thus additional factors besides temperature that control root dynamics in tropical mountain forests.

Water table fluctuation in arid land regions may alter tree fine-root growth and mortality, thereby affecting leaf growth. To reveal the effects of water table fluctuation on fine-root growth and mortality and their relation to leaf growth, we exposed P. alba L. cuttings to various fluctuating water table depths. 1-year-old rooted cuttings were grown individually in pots containing sandy soil in a greenhouse in three water table depth treatments for 45 days: constant depth at 45 cm from the soil surface, fluctuating depths between 45 and 30 cm, and fluctuating depths between 45 and 15 cm. Fine-root biomass and mortality, biomass partitioning among plant parts, and whole-tree growth responses were determined in cuttings harvested every 15 days. Fluctuation of water tables increased the mortality of fine roots at the layers near the soil surface. Fine-root mortality increased during the shallower water table depth period. At the whole-root system level, although fine-root mortality increased when the water table was shallower, fine-root biomass was similar among the treatments, suggesting that P. alba cuttings would sustains its standing fine-root biomass under fluctuating water table depth conditions. Our structural equation modeling showed the fine-root proportion affects leaf morphological changes, suggesting that there would be a parallel relationship of morphological changes between roots and leaves with fluctuating water tables.

The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology. A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71 ± 8 g m⁻² yr⁻¹. Mean annual production for the period analysed was 260+11 g m⁻² yr⁻¹. Mortality was similar to production, with a mean value of 253±3 g m⁻² yr⁻¹. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10-20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0-10 cm and the 50-60 cm depth intervals, both production and mortality were lower.

Effects of nitrogen and sulphur (ammonium sulphate [NS] application), nitrogen, other nutrients and water (liquid fertilization; LF), and N-free fertilizer (N-free) on fine root dynamics (production, mortality and longevity) were studied in a Norway spruce stand, using minirhizotrons. Data were collected and analysed during a two-year period at depths of 0-20 cm, 21-40 cm and 41-85 cm, five to six years after the start of fertilizer application. Ammonium sulphate application resulted in lower fine root length production at all depths compared with other treatments. Fine root length mortality in the NS treatment was higher than the control (C) and N-free fertilizer plots. Root length production in liquid fertilization was lower compared with control plots at depth 0-20 cm and higher at depth 41-85 cm compared with other treatments. N-free fertilisers lowered fine root mortality relative to control and ammonium sulphate application. N-free fertilizer extended the longevity of fine roots while ammonium sulphate application and liquid fertilization (N treatments) shortened fine root longevity. Median life span of fine roots in N-free, LF, C and NS plots was 350, 280, 280 and 250 days, respectively. It was concluded that a high input of nitrogen increased fine root mortality and decreased production and longevity whilst addition of nitrogen-free fertilizer extended fine root longevity.