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• Temperate forest ecosystems are exposed to a higher frequency, duration and severity of drought. To promote forest longevity in a changing climate, we require a better understanding of the long-term impacts of repetitive drought events on fine-root dynamics in mature forests. • Using minirhizotron methods, we investigated the effect of seasonal drought on fine-root dynamics in single-species and mixed-species arrangements of Fagus sylvatica (European beech) and Picea abies (Norway spruce) by means of a 4-yr-long throughfall-exclusion experiment. • Fine-root production of both species decreased under drought. However, this reduction was not evident for P. abies when grown intermixed with F. sylvatica. Throughfall-exclusion prolonged the lifespan of P. abies roots but did not change the lifespan of F. sylvatica roots, except in 2016. Fagus sylvatica responded to drought by reducing fine-root production at specific depths and during roof closure. • This is the first study to examine long-term trends in mature forest fine-root dynamics under repetitive drought events. Species-specific fine-root responses to drought have implications for the rate and depth of root-derived organic matter supply to soil. From a root dynamics perspective, intermixing tree species is not beneficial to all species but dampens drought impacts on the belowground productivity of P. abies.

Lucidophyllous (evergreen broad-leaved) forests are the dominant forests in human-dominated subtropical/warm-temperate regions in East Asia. Biometric-based estimates of net primary production (NPP) were conducted in a secondary lucidophyllous forest on Mt. Kinka (35°26′ N, 136°47′ E) near the northern limit of their distribution in central Japan for three years, including the masting event. The forest stand mainly consists of Castanopsis cuspidata (Thunb.) Schottky and Cleyera japonica Thunb. in the canopy and subtree layers, respectively. In 2018, the total NPP of the masting year was 14.53 ± 2.03 ton ha−1 yr−1, including woody NPP (above: 2.63 ± 0.35 ton ha−1 yr−1; below: 0.57 ± 0.08 ton ha−1 yr−1), foliage NPP (4.07 ± 0.23 ton ha−1 yr−1), reproductive NPP (4.81 ± 0.77 ton ha−1 yr−1), and fine root production (Pfr) (2.46 ± 1.84 ton ha−1 yr−1). Pfr and belowground production comprised 16.9% and 20.9%, respectively, of the total NPP. The nut production of C. cuspidata in 2018 (4.31 ± 0.75 ton ha−1 yr−1) was significantly higher than that in 2017 (0.77 ± 0.13 ton ha−1 yr−1) and 2019 (0.23 ± 0.06 ton ha−1 yr−1). No significant change was observed for the three years of foliage NPP and total NPP without Pfr. However, the woody NPP in 2018 (3.20 ± 0.43) was lower than in 2017 (5.37 ± 0.33 ton ha−1 yr−1) and 2019 (4.71 ± 0.38 ton ha−1 yr−1). This suggests that nut production in the masting years compensated by decreasing woody production in the Castanopsis forest.

Information on the capacity of organic soils to capture and store carbon in old‐growth forests in the hemiboreal forest zone is scarce and fragmented. However, fine‐root data can provide valuable insights into soil carbon fluxes. Thus, the aim of the current study was to provide estimates of the fine‐root biomass (FRB), fine‐root production (FRP), and fine‐root turnover (FRT) rate by tree species and other functional groups in old‐growth (stand age 131–179 years) forests on mesotrophic organic soils dominated by Scots pine (Pinus sylvestris L.), with and without the effects of forest drainage. The sequential soil coring method was used to estimate the FRB and FRP. The total FRB was significantly higher in the undrained sites (6.8 ± 0.3 t ha−1) than in the drained sites (3.97 ± 0.1 t ha−1). The FRB of Scots pine in the undrained forest was significantly higher (1.7 ± 0.1 t ha−1) than in the drained forest (0.5 ± 0.1 t ha−1), supporting an extensive foraging strategy. The significantly higher mean FRB of Norway spruce (Picea abies [L.] Karst.) (1.4 ± 0.1 t ha−1) in the drained sites than the undrained sites (0.7 ± 0.2 t ha−1) can be explained by there being a higher proportion of spruce in the stand compositions, thus a higher standing volume of this species and an increased FRB. The FRB of dwarf shrubs (2.43 ± 0.2 t ha−1) formed the largest part of the total FRB in the undrained sites and the second largest (1.16 ± 0.1 t ha−1), following Norway spruce, in the drained sites. The total FRP was similar between the undrained (2.05 ± 0.31 t ha−1 year−1) and drained (1.82 ± 0.26 t ha−1 year−1) stands. However, considerable variability in the FRP was observed between different sites of the same forest site type. The FRT rate of Scots pine was twice as high in the drained sites than the undrained sites, suggesting faster nutrient and carbon input into the drained soil compared with the undrained soil. Estimates of FRB, FRP, and FRT rates for different functional groups can be used in carbon‐cycle modeling and in further calculations to estimate the carbon budget (balance) in forests on organic soils.

• Fine roots are the key link for plant water and nutrient uptake, soil carbon (C) input and soil microbial activity in forest ecosystems, and play a critical role in regulating ecosystem C balance and its response to global change. • Red maple (Acer rubrum) and sugar maple (Acer saccharum) seedlings were grown for four growing seasons in open-top chambers and exposed to ambient or elevated carbon dioxide concentration [ CO2] in combination with ambient or elevated temperature. Fine-root production and mortality were monitored using minirhizotrons, and root biomass was determined from soil cores. • Both elevated [ CO2] and temperature significantly enhanced production and mortality of fine roots during spring and summer of 1996. At the end of the experiment in September 1997, fine root biomass was significantly lower in elevated temperature chambers, but there were no effects of elevated [ CO2] or the interactions between elevated [ CO2] and temperature. • Deciduous trees have dynamic root systems, and their activity can be enhanced by CO2 enrichment and climatic warming. Static measures of root response, such as soil core data, obscure the dynamic nature, which is critical for understanding the response of forest C cycling to global change.

• Precipitation is one of the most important factors that determine productivity of terrestrial ecosystems. Precipitation across the globe is predicted to change more intensively under future climate change scenarios, but the resulting impact on plant roots remains unclear. • Based on 154 observations from experiments in which precipitation was manipulated in the field and root biomass was measured, we investigated responses in fine-root biomass of herbaceous and woody plants to alterations in precipitation. • We found that root biomass of herbaceous and woody plants responded differently to precipitation change. In particular, precipitation increase consistently enhanced fine-root biomass of woody plants but had variable effects on herb roots in arid and semi-arid ecosystems. In contrast, precipitation decrease reduced root biomass of herbaceous plants but not woody plants. In addition, with precipitation alteration, the magnitude of root responses was greater in dry areas than in wet areas. • Together, these results indicate that herbaceous and woody plants have different rooting strategies to cope with altered precipitation regimes, particularly in water-limited ecosystems. These findings suggest that root responses to precipitation change may critically influence root productivity and soil carbon dynamics under future climate change scenarios.

CONTENTS: 34 I. 35 II. 35 III. 41 IV. 43 V. 49 VI. 50 VII. 51 VIII. 52 53 References 53 SUMMARY: Plant roots play a critical role in ecosystem function in arctic tundra, but root dynamics in these ecosystems are poorly understood. To address this knowledge gap, we synthesized available literature on tundra roots, including their distribution, dynamics and contribution to ecosystem carbon and nutrient fluxes, and highlighted key aspects of their representation in terrestrial biosphere models. Across all tundra ecosystems, belowground plant biomass exceeded aboveground biomass, with the exception of polar desert tundra. Roots were shallowly distributed in the thin layer of soil that thaws annually, and were often found in surface organic soil horizons. Root traits – including distribution, chemistry, anatomy and resource partitioning – play an important role in controlling plant species competition, and therefore ecosystem carbon and nutrient fluxes, under changing climatic conditions, but have only been quantified for a small fraction of tundra plants. Further, the annual production and mortality of fine roots are key components of ecosystem processes in tundra, but extant data are sparse. Tundra root traits and dynamics should be the focus of future research efforts. Better representation of the dynamics and characteristics of tundra roots will improve the utility of models for the evaluation of the responses of tundra ecosystems to changing environmental conditions.

Aim: Root production and turnover play a key role in regulating carbon (C) flow in terrestrial ecosystems. However, a general pattern reflecting the responses of roots to increasing nitrogen (N) input has yet to be described. Location: Global terrestrial ecosystems. Methods: We conducted a meta-analysis to assess the central tendencies of root production, turnover rate and standing crop with respect to the experimental addition of N. We evaluated the effect of the form of N, root diameter and climatic (mean annual temperature, MAT; mean annual precipitation, MAP), biotic (ecosystem type, plant type and forest stand age) and forcing factors (experimental duration, N addition rate and cumulative amount of N) on the variations in root response. Results: Globally, the addition of N significantly decreased root production and turnover rate but had only a minor impact on root standing crop. In different ecosystems, the three root variables exhibited heterogeneous responses to N enrichment. Additionally, root production and turnover rate responded distinctly to diverse forms of N. The responses of root production and turnover rate to the addition of N were generally positively correlated with MAT and MAP but negatively related to forest stand age and experimental duration. The response pattern of root standing crop was negatively affected by MAT, MAP and forest stand age. However, none of the three root parameters had any obvious correlations with N addition rate or cumulative amount of N. Main conclusions: Our results demonstrate that, on aggregate, the addition of N decreased root production and turnover rate at the global scale. These root response patterns and the regulatory factors can be incorporated into earth system models to improve the prediction of belowground C dynamics.

1. Optimal resource partitioning theory predicts that plants should increase the ratio between water absorbing and transpiring surfaces under short water supply. An increase in fine root mass and surface area relative to leaf area has frequently been found in herbaceous plants, but supporting evidence from mature trees is scarce and several results are contradictory. 2. In 12 mature Fagus sylvatica forests across a precipitation gradient (820-540 mm yr⁻¹), we tested several predictions of the theory by analysing the dependence of standing fine root biomass, fine root production and fine root morphology on mean annual precipitation (MAP), the precipitation of the study year, and stand structural and edaphic variables. The water storage capacity of the soil (WSC) was included as a covariable by comparing pairs of stands on sandy (lower WSC) and loam-richer soils (higher WSC). 3. Fine root biomass, total fine root surface area, fine root production and the fine root : leaf biomass production ratio markedly increased with reduced MAP and precipitation in the study year, while WSC was only a secondary factor and stand structure had no effect. 4. The precipitation effect on fine root biomass and production was more pronounced in stands on sandy soil with lower WSC, which had, at equal precipitation, a higher fine root biomass and productivity than stands on loam-richer soil. 5. The high degree of allocational plasticity in mature F. sylvatica trees contrasts with a low morphological plasticity of the fine roots. On the more extreme sandy soils, a significant decrease in mean fine root diameter and increase in specific root area with decreasing precipitation were found; a similar effect was absent on the loam-richer soils. 6. Synthesis. In support of optimal partitioning theory, mature Fagus sylvatica trees showed a remarkable allocational plasticity as a long-term response to significant precipitation reduction with a large increase in the size and productivity of the fine root system, while only minor adaptive modifications occurred in root morphology. More severe summer droughts in a future warmer climate may substantially alter the above-/below-ground C partitioning of this tree species with major implications for the forest C cycle.

There is compelling evidence from experiments and observations that climate warming prolongs the growing season in arctic regions. Until now, the start, peak, and end of the growing season, which are used to model influences of vegetation on biogeochemical cycles, were commonly quantified using above‐ground phenological data. Yet, over 80% of the plant biomass in arctic regions can be below ground, and the timing of root growth affects biogeochemical processes by influencing plant water and nutrient uptake, soil carbon input and microbial activity. We measured timing of above‐ and below‐ground production in three plant communities along an arctic elevation gradient over two growing seasons. Below‐ground production peaked later in the season and was more temporally uniform than above‐ground production. Most importantly, the growing season continued c. 50% longer below than above ground. Our results strongly suggest that traditional above‐ground estimates of phenology in arctic regions, including remotely sensed information, are not as complete a representation of whole‐plant production intensity or duration, as studies that include root phenology. We therefore argue for explicit consideration of root phenology in studies of carbon and nutrient cycling, in terrestrial biosphere models, and scenarios of how arctic ecosystems will respond to climate warming.

Here, soil CO₂ efflux, minirhizotron fine root production (FRP), and estimated total below-ground carbon allocation (TBCA) were examined along an elevation and hybridization gradient between two cottonwood species. FRP was 72% greater under high-elevation Populus angustifolia, but soil CO₂ efflux and TBCA were 62% and 94% greater, respectively, under low-elevation stands dominated by Populus fremontii, with a hybrid stand showing intermediate values. Differences between the responses of FRP, soil CO₂ efflux and TBCA may potentially be explained in terms of genetic controls; while plant species and hybridization explained variance in carbon flux, we found only weak correlations of FRP and TBCA with soil moisture, and no correlations with soil temperature or nitrogen availability. Soil CO₂ efflux and TBCA were uncorrelated with FRP, suggesting that, although below-ground carbon fluxes may change along environmental and genetic gradients, major components of below-ground carbon flux may be decoupled.