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Few studies describe root distributions at the species level in diverse forests, although belowground species interactions and traits are often assumed to affect fine-root biomass (FRB). We used molecular barcoding to study how FRB of trees relates to soil characteristics, species identity, root diversity, and root traits, and how these relationships are affected by proximity to ecotones in a temperate forest landscape. We found that soil patch root biomass increased in response to soil resources across all species, and there was little belowground vertical or horizontal spatial segregation among species. Root traits and species relative abundance did not explain significant variation in FRB after correcting for soil fertility. A positive relationship between phylogenetic diversity and FRB indicated significant belowground overyielding attributable to local root diversity. Finally, variation in FRB explained by soil fertility and diversity was reduced near ecotones, but only because of a reduction in biomass in periodically anoxic areas. These results suggest that symmetric responses to soil properties are coupled with complementary species traits and interactions to explain variation in FRB among soil patches. In addition, landscape-level dispersal among habitats and across ecotones helps explain variation in the strength of these relationships in complex landscapes.

Aim Rising air temperature and changing precipitation patterns already strongly influence forest ecosystems, yet large‐scale patterns of belowground root trait variation and their underlying drivers are poorly understood. Here, we investigated general patterns of root tip adjustments within fine‐root systems and the potential ecological implications of these patterns. Location Global. Methods We synthesize key fine‐root traits related to resource acquisition and determined their responses along climate and edaphic gradients. We specifically identified patterns of root tip abundance (number of root tips per dry biomass of fine roots ≤2 mm in diameter), and root tip density (number of root tips per soil volume) among angiosperm and gymnosperm trees to climate, edaphic gradients and stand properties. Results We found that angiosperm trees, which were more common in warmer, sometimes drier climates with more fertile soil, formed more root tips (higher root tip abundance, root tip density and higher slope of root tip density vs. fine‐root biomass) than gymnosperm trees, which lived in cooler, wetter climates with poor soil. Angiosperm and gymnosperm trees exhibited opposing trends in response to gradients in climate as gymnosperm trees tended to decrease root tip abundance and root tip density but alternatively increase mycorrhizal mycelial biomass with increasing MAT/MAP (ratio of mean annual temperature to mean annual precipitation), while angiosperm trees tended to increase root tip abundance and root tip density with increasing MAT/MAP. However, the individual trends of root tip abundance and root tip density for angiosperm and gymnosperm trees to MAT or MAP were more similar and often non‐significant. Main conclusions These results suggest disparate carbon or biomass adjustment strategies within gymnosperm and angiosperm tree fine‐root systems along climate gradients. Differences in angiosperm and gymnosperm tree adjustments in their fine‐root systems to changing environments have implications for how these plant groups are likely to perform in different environments and how their responses to future climate change should be modelled.

Competition or complementarity between associated plants due to belowground interactions has been observed in alley-cropping systems (ACs), but the initialization of these processes remains poorly investigated. Here, we used the core-break and soil coring method to quantify the vertical tree and wheat fine root abundance and biomass down to 120 cm in a 4-year-old temperate AC. Fine roots were measured at 2 m from a reference tree (hornbeam, wild cherry or willow) in tree–wheat AC, pure-forest associated with ryegrass (FC) and wheat sole-crop (CC) plots at the Ramecourt experimental site. The mean wheat fine root abundance (WFRA) was twice as high in the CC plot (874 ± 152 m−2) as in the AC plot (437 ± 47 m−2). It was significantly higher for wheat associated with hornbeam than for willow, particularly at the 10 cm depth. Tree fine root abundance (TFRA) was linearly correlated with tree fine root biomass (TFRB) for hornbeam (R2 = 0.79***), willow (R2 = 0.77***) and wild cherry (R2 = 0.54***). Using TFRA, the van Noordwijk's equation gave a better prediction of the TFRB for willow and wild cherry than for hornbeam. The mean value of the TFRA was seven times higher in the FC plot (1116 ± 97 m−2) than in the AC plot (146 ± 24 m−2) for all soil depths and all tree species due to the lack of nutrients from the absence of fertilization. At 4 years old, willow and hornbeam fine roots cohabited with wheat in the upper soil layer, whereas wild cherry had already developed deep fine roots under the crop rooting zone.

We investigated the effects of seasonal changes in soil moisture on the morphological and growth traits of fine roots (<2 mm in diameter) in a mature Turkey-oak stand ( Quercus cerris L.) in the Southern Apennines of Italy. Root samples (diameter: <0.5, 0.5–1.0, 1.0–1.5, and 1.5–2.0 mm) were collected with the Auger method. Mean annual fine-root mass and length on site was 443 g m −2 (oak fine roots 321 g m −2 ; other species 122 g m −2 ) and 3.18 km m −2 (oak fine roots 1.14 km m −2 ; other species 2.04 km m −2 ), respectively. Mean specific root length was 8.3 m g −1 . All fine-root traits displayed a complex pattern that was significantly related to season. In the four diameter classes, both fine-root biomass and length peaked in summer when soil water content was the lowest and air temperature the highest of the season. Moreover, both fine-root biomass and length were inversely related with soil moisture ( p  < 0.001). The finest roots (<0.5 mm in diameter) constituted an important fraction of total fine-root length (79 %), but only 21 % of biomass. Only in this root class, consequent to change in mean diameter, specific root length peaked when soil water content was lowest showing an inverse relationship ( p  < 0.001). Furthermore, fine-root production and turnover decreased with increasing root diameter. These results suggest that changes in root length per unit mass, and pulses in root growth to exploit transient periods of low soil water content may enable trees to increase nutrient and water uptake under seasonal drought conditions.

Tropical forests store and sequester large amounts of carbon in above‐ and below‐ground plant biomass and soil organic matter (SOM), but how these are driven by abiotic and biotic factors remains poorly understood. Here, we test the effects of abiotic factors (light variation, caused by logging disturbance, and soil fertility) and biotic factors (species richness and functional trait composition) on biomass stocks (above‐ground biomass, fine root biomass), SOM and productivity in a relatively monodominant Guyanese tropical rainforest. This forest grows on nutrient‐poor soils and has few species that contribute most to total abundance. We, therefore, expected strong effects of soil fertility and species’ traits that determine resource acquisition and conservation, but not of diversity. We evaluated 6 years of data for 30 0.4‐ha plots and tested hypotheses using structural equation models. Disturbance increased productivity but decreased above‐ground biomass stocks. Soil phosphorus (P) enhanced above‐ground biomass and productivity, whereas soil nitrogen reduced fine root biomass. In contrast to expectations, trait values representing acquisitive strategies (e.g. high leaf nutrient concentration) increased biomass stocks, possibly because they indicate higher nutrient absorption and thus higher biomass build‐up. However, under harsh conditions where biomass increase is slow, acquisitive trait values may increase respiration and vulnerability to hazards and therefore increase biomass loss. As expected, species richness did not affect productivity. We conclude that light availability (through disturbance) and soil fertility—especially P—strongly limit forest biomass productivity and stocks in this Guyanese forest. Low P availability may cause strong environmental filtering, which in turn results in a small set of dominant species. As a result, community trait composition but not species richness determines productivity and stocks of biomass and SOM in tropical forest on poor soils. A plain language summary is available for this article. Plain Language Summary

I present a meta-analysis of plant responses to 48 nutrient addition experiments conducted with native species in naturally growing tropical forests, exclusive of mangrove forests. The added nutrients include nitrogen (N) in 36 experiments, phosphorus (P) in 33 experiments, calcium and potassium in one experiment each, and various mixtures of essential nutrients in the remaining experiments. I evaluate the hypotheses that nutrients limit tropical forest plants, nutrient limitation is stronger in successional than in old-growth forests, P but not N is limiting in lowland forests, and N is limiting in montane forests. Responses to the most complete nutrient mix used in each experiment were strong for plant functions that contribute to aboveground production (Hedges' g averages 0.87) and nonsignificant for fine root biomass. Responses to N addition and to P addition were strong for tissue concentrations of the added element (Hedges' g averages 0.75 and 1.4, respectively), moderate for fine litter production (0.64 and 0.65, respectively), moderate to weak for plant growth (0.46 and 0.37, respectively) and nonsignificant for fine root biomass. Growth responses were stronger in successional than in old-growth forests. All responses were unrelated to elevation. The 48 experiments included 30 factorial nitrogen-phosphorus experiments that enable additional direct tests of the widely cited hypotheses that P limitation is stronger than N limitation in lowland forests and vice versa in montane forests. Both hypotheses were rejected. The N × P interaction effect was nonsignificant across the factorial experiments. In conclusion, nutrients clearly limit tropical forest plants. Limitation by N is widespread in both lowland and montane forests, and the same is true for P. Single experiments identify limitation by calcium and potassium, and correlative studies suggest limitation by calcium, potassium, and magnesium. The available evidence is consistent with the possibility that most macronutrients limit tropical forest plants; however, experiments focus almost exclusively on N and P. The way forward will include taking fuller advantage of existing nutrient addition experiments, siting new experiments strategically, and developing cost-effective methods to assay responses to all of the essential nutrients soils supply to plants.

Aim: Positive relationships between plant species diversity and above-ground productivity have been observed across a wide range of terrestrial ecosystems. Despite a critical contribution of below-ground productivity to overall terrestrial productivity, no consensus exists about the nature of the relationship between species diversity and below-ground productivity. Location: Global. Methods: We collected data from published studies conducted in natural and planted forests and experimental grassland, crop and pot systems that were purposely implemented to isolate the effects of plant species diversity from other factors, such as soil conditions and topographic features. We conducted meta-analyses of 170 observations for root biomass and 23 observations for root production, derived from 48 published studies, using weighted linear modelling with bootstrap procedures to reconcile the effects of diversity on fine root productivity. Results: We found that species mixtures had, on average 28.4% higher fine root biomass and 44.8% higher annual production than monocultures. Higher fine root biomass in species mixtures than in monocultures was consistent across natural forests, planted grasslands, croplands and pot systems, except for young planted forests. Transgressive overyielding was only evident for planted grasslands. The log response ratio of fine root biomass in species mixtures to that in respective monocultures increased with species richness across all ecosystem types, and also increased with experiment age in grasslands. Main conclusions: Our meta-analysis reveals positive effects of species diversity on below-ground productivity. Despite profound differences in environments among terrestrial ecosystems, our analysis demonstrated that below-ground productivity responds similarly to variations in species richness. Furthermore, our study also reveals shifts in the effects of diversity over time in both forests and grasslands. Future efforts are needed to further understand below-ground productivity-diversity relationships.

Elevated atmospheric N deposition has been well documented to enhance fine root production in N-limited temperate forests, but how fine roots respond to N deposition in N-rich tropical and subtropical forests remains poorly understood. The sequential coring and minirhizotron methods were applied to quantify fine root biomass, production, and turnover of a N-rich but P-limited subtropical forest in southern China and to assess the responses of these root variables to a gradient of N additions (control (0), low-N (35), medium-N (70), and high-N (105 kg N ha−1 year−1)) during the first 3 years of experimentation. The high- and medium-N additions significantly reduced fine root diameter by about 30% but increased the specific root length by 20–105%, i.e., fine roots became thinner and longer under the experimental N addition. Both low- and medium-N additions generally stimulated fine root production (10–88%) and turnover (3–40%), whereas high-N suppressed them by 32–70% and 8–54%, respectively, varying with sampling season and estimation method. The stimulatory effects were presumably ascribed to the increased fine root growth for P acquisition, the suppressive effect, to the deleterious damage to the root health and micronutrient availability. Overall, the N effects were more pronounced in the surface (0–10 cm) than in the deeper (10–40 cm) soil layers and for the first-order than the higher-order fine roots. Our results indicate that lower-order absorptive fine roots are responsive to elevated N deposition, and complex responses could emerge due to the interactive influences of the N deposition rate, seasonality, and soil depth.

Aims It has been increasingly recognized that only distal lower order roots turn over actively within the <2 mm fine root system of trees. This study aimed to estimate fine root production and turnover rate based on lower order fine roots and their relations to soil variables in mangroves. Methods We conducted sequential coring in five natural mangrove forests at Dongzhai Bay, China. Annual fine root production and turnover rate were calculated based on the seasonal variations of the biomass and necromass of lower order roots or the whole fine root system. Results Annual fine root production and turnover rate ranged between 571 and 2838 g m⁻² and 1.46-5.96 yr⁻¹, respectively, estimated with lower order roots, and they were increased by 0-30 % and reduced by 13-48 %, respectively, estimated with the whole fine root system. Annual fine root production was 1-3.5 times higher than aboveground litter production and was positively related to soil carbon, nitrogen and phosphorus concentrations. Fine root turnover rate was negatively related to soil salinity. Conclusions Mangrove fine root turnover plays a more important role than aboveground litter production in soil C accumulation. Sites with higher soil nutrients and lower salinity favor fine root production and turnover, and thus favor soil C accumulation.

Global warming is predicted to impact high-latitude and high-altitude forests severely, jeopardizing their overall functioning and carbon storage, both of which depend on the warming response of tree fine root systems. This paper investigates the effect of soil warming on the biomass, morphology and colonizing ectomycorrhizal community of spruce fine and absorptive fine roots. We compare the responses of spruce roots growing at a man-made long-term soil warming (+ 4°C) experiment to results obtained from a geothermal soil temperature gradient (+ 1 to + 14°C) extending to the forest die-off edge, to shed light on the generalizability of the warming response and reveal any thresholds in acclimation ability. Trees in warmer soils formed longer and less-branched absorptive roots with higher specific root length and area, and lower root tissue density in both spruce stands, irrespective of warming method and location. Soil warming at the experimental warming site also supported the occurrence of a more varied EcM community and an increase in the abundance of Tomentella spp., indicating a shift in nutrient foraging. Fine and absorptive fine root biomass decreased toward warmer soil, with a sharp reduction occurring between + 4 and + 6°Cfrom the ambient and leading to the collapse of the fine root system at the geothermal gradient. At the experimental warming site, the applied + 4°C warming had no effect on fine and absorptive fine root biomass. The similar fine root responses at the two warming sites suggest that the observations possibly reflect general acclimation patterns in spruce forests to global warming.