Structural root architecture of 5-year-old Pinus pinaster measured by 3D digitising and analysed with AMAPmod

Pinus pinaster (Ait.) is a high yielding forest tree, producing nearly a fourth of French marketed timber essentially from intensively managed stands located in southwestern France, in the Landes Forest. This species has generally a poor stem straightness, especially when it grows in poor sandy podzol of the Landes Forest, affected by summer droughts and winter floods. Above- and below-ground architecture and biomass as well as stem straightness were measured on twenty-nine 5-year-old planted trees uprooted by pulling with a lumbering crane. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitising device (Danjon et al., 1998; Sinoquet and Rivet, 1997). Data were analysed with AMAPmod, a database software designed to analyse plant topological structures (Godin et al., 1997). Several characteristics of root architecture were extracted by queries including root number, length, diameter, volume, spatial position, ramification order, branching angle and inter-laterals length. Differences between root systems originated from their dimensions, but also from the proportion of deep roots and the taproot size, which represented 8% of the total root volume. The proportion of root volume in the zone of rapid taper was negatively correlated with the proportion of root volume in the taproot indicating a compensation between taproot and main lateral root volume. Among all studied root characteristics the maximal rooting depth, the proportion of deep roots and the root partitioning coefficient were correlated with stem straightness. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterisation of structural tree root architecture using 3D digitising and AMAPmod software

A low-magnetic-field digitising device combined with AMAPmod, a software designed to analyse plant architecture, provided a very efficient method for measuring and studying the geometry and topology of the structural roots of trees. The digitising device measures co-ordinates in a 3D space. AMAPmod was used to assess several characteristics of the root architecture including spatial position, root lengths and volumes, branching order and branching pattern, and to reconstruct 3D images from the data to check for measurement errors. Structural root systems of three 20 to 28-year-old Quercus petraea and thirty 5-year-old Pinus pinaster were uprooted by using rapid mechanised techniques. Only roots with a diamter larger than 2 mm were measured. A fast and precise spatial localisation in combination with the topological characterisation of all root segments was carried out. Oak showed a stronger more oblique and vertical rooting, more branches, more forks and narrower branching angles than pine. Oak had only few small roots in the north west direction whereas half of pine root volume was located in the 10 cm upper soil volume. The contribution of this new method in the characterisation of structural root systems is discussed and other possible applications of this method in root studies are proposed. Since this method is precise and fairly rapid, it may be used for agronomic testing (i.e. comparing treatments) involving several dozen root systems. Almost all parameters needed for tree root system simulations can be estimated from such data. This revised version was published online in June 2006 with corrections to the Cover Date.     

Unusual Fine Root Distributions of Two Deciduous Tree Species in Southern France: What Consequences for Modelling of Tree Root Dynamics?

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees (Juglans regia×nigra cv. NG23) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.     

A density-based approach for the modelling of root architecture: application to Maritime pine (Pinus pinaster Ait.) root systems

Root morphology influences strongly plant/soil interactions. However, the complexity of root architecture is a major barrier when analysing many phenomena, e.g. anchorage, water or nutrient uptake. Therefore, we have developed a new approach for the representation and modelling of root architecture based on branching density. A general root branching density in a space of finite dimension was used and enabled us to consider various morphological properties. A root system model was then constructed which minimizes the difference between measured and simulated root systems, expressed with functions which map root density in the soil. The model was tested in 2D using data from Maritime pine Pinus pinaster Ait. structural roots as input. We showed that simulated and real root systems had similar root distributions in terms of radial distance, depth, branching angle and branching order. These results indicate that general density functions are not only a powerful basis for constructing models of architecture, but can also be used to represent such structures when considering root/soil interaction. These models are particularly useful in that they provide a local morphological characterization which is aggregated in a given unit of soil volume.     

黄土丘陵半干旱区密植枣林随树龄变化的根系空间分布特征

该文研究了黄土丘陵半干旱区密植枣( Ziziphus jujuba ‘Lizao’)林群体根系随树龄变化的空间分布特征。对1年生、4年生、8年生和11年生4种树龄的密植枣林采用剖面法, 获得0-1 m土壤剖面上直径>3 mm、1-3 mm及<1 mm的根系数量和空间位置信息。利用方差分析, 评价了山地密植枣林林分根系随树龄变化的水平和垂直分布特征。结果表明: 3种直径的根系数量均随着树龄的增长而增加, 直径< 1 mm的根系增长速度最快; 随着土层加深, 根系数量递减, 1年生枣林的根系主要聚集在0-40 cm土层中, 4年生及以上树龄的根系主要分布在0-60 cm土层中; 0-1 m土层内, 1年生枣林(株距1.2 m)及4年以上树龄(株距2 m), 同树龄枣林中直径<1 mm的根系水平分布无差异; 同一土层中(0-20 cm, 20-40 cm, 40-60 cm), 无论树龄大小及离树干的水平位置如何, 不同直径根系的数量都无差异。研究表明: 在有水肥管理措施的条件下, 枣林根系垂直方向形成浅层型的适应模式; 在密植环境下, 枣林细根形成根网型的适应模式。     

Localisation of mineral uptake by roots using Sr isotopes

To assess the contribution of deep soil horizons to the mineral supply of trees, we investigated the natural variation in the⁸⁷ Sr/⁸⁶Sr isotopic ratio of plant-available strontium with soil depth. In three sites of North-western Spain, this ratio increased with soil depth. The comparison of isotopic ratios of tree leaves and roots at different depths showed that most of the Sr accumulation in Eucalyptus globulus and Pinus pinaster growing on shallow and poor soils in this rainy climate originated from the upper soil layers. As Ca and Sr behave similarly in the soil-plant system, this conclusion can be applied to Ca. This superficial uptake is attributed to the low availability of Sr and Ca in the soil as well as to the shortness of the drought period as compared to the length of the growth period. This technique appears to offer a promising way of studying relative root distributions in soils and plant competition for nutrients.     

Quantifying the effects of root reinforcement of Persian Ironwood (Parrotia persica) on slope stability; a case study: Hillslope of Hyrcanian forests,northern Iran

Forest vegetation is known to enhance the stability of slopes by reinforcing soil and increasing its shear resistance through root system. The effects of root reinforcement depend on the morphological characteristics of the root system, the tensile strength of single roots, and the spatial distribution of the roots in soil. In the present study the results of research carried out in order to evaluate the biotechnical characteristics of the root system of Persian Ironwood (Parrotia persica), in northern Iran are presented. Profile trenching method was used to obtain root area ratio (RAR) values for uphill and downhill sides of the individual trees. For each species, single root specimens were sampled and tested for their tensile strength. It was found that root density generally decreases with depth according to an exponential law. Maximum RAR values were located within the first 0.1 m, with maximum rooting depth at about 0.65 m. RAR values ranged from 0.001% at lower depths to 1.39% near the surface, at upper 0.1 m depth. Significant differences of RAR values, rooting depth and root cohesion between uphill and downhill were observed, however, the differences were not significant for number of roots (ANCOVA). Downhill profiles had higher RAR values, rooting depth and root cohesion. In general, root tensile strength tends to decrease with diameter according to a power law, as observed by other researchers. Downhill roots were significantly stronger in tensile strength than uphill ones. Inter-species variation of tensile strength in downhill roots was also observed. The resulting data were used to evaluate the reinforcing effects in terms of increased shear strength of the soil, using Wu/Waldron Model. The root reinforcement provided by Persian Ironwood is about 46.0 kPa in the upper layers and 0.3 kPa in the deeper horizons. The results of Spearman test revealed a significant correlation between RAR and c_r and that best followed by a power law. The results presented in this paper contribute to expanding the knowledge on biotechnical characteristics of Persian Ironwood on slope reinforcement.     

Drip irrigation,soil characteristics and the root distribution and root activity of olive trees

A study was carried out on the root distribution and root activity of the olive tree (Olea Europaea, L., var. manzanillo) as influenced by drip irrigation and by several soil characteristics such as texture and depth. The experiments were conducted in two plots within a drip-irrigated grove of 20-year-old trees planted at 7×7 m spacing. One soil was a sandy loam, the other a clay-loam. Both cylinder and trench methods were used to determine root distribution. Labelling with ³²P was used to determine root activity. Under dryland conditions the adult tree adapted its rooting system, following the installation of a drip system, by concentrating the roots within the wet soil zones near the drippers. The highest root densities occur in those zones, down to a 0.6 m depth, the most abundant being the <0.5 mm diameter roots. The most intensive root activity was also found in that zone. For a given irrigation system, wet soil bulbs are more extensive and therefore root distribution expands to a larger soil volume when the soil is more clayey and with a hard calcareous pan present at about 0.8 m depth which prevents deep drainage.     

Rooting characteristics of two widely distributed woody plant species growing in different karst habitats of southwest China

A series of studies claimed that deep root development of plant established in karst regions was facilitated by fractured bedrock beneath the shallow soils; however, bedrock is not a uniform medium for root proliferation. We hypothesized plant species that survived in different karst habitats had some other rooting characteristics rather than deep penetration. To test the hypothesis, coarse root systems of two widely distributed woody species (one tree and one shrub) growing in three typical rocky karst habitats (shallow soil, loose rocky soil and exposed rock) were excavated in karst region of southwest China. Root systems were investigated based on four parameters: maximum rooting depth, maximum radial extent, root tapering pattern and root curvature. In all the three habitats, maximum rooting depths were no deeper than 120 and 40 cm for the tree and shrub species, respectively. Maximum radial extents were extremely large compared with maximum rooting depth, indicating that rooting characteristics were dominated by horizontal extension rather than deep penetration. Roots of both species growing in shallow soil habitat tapered gradually and curved slightly, which was consistent with the specific characteristics of this habitat. On the contrary, roots of the tree species growing in the other two habitats tapered rapidly but curved slightly, while roots of the shrub species tapered gradually but curved strongly. It was speculated that limited depths and rapid tapering rates of the tree roots were likely compensated by their utmost radial extensions, while the shrub species might benefit from its root curvature as the associated root tropisms may increase the ability of root to encounter more water and contribute to potentially high resource absorption efficiency. Our results highlight the importance of taking shallow-rooted species into account in understanding the distribution of natural plant communities and predicting future vegetation dynamics in karst regions.     

Rooting and acclimatization of micropropagated cuttings of Pinus pinaster and Pinus sylvestris are enhanced by the ectomycorrhizal fungus Hebeloma cylindrosporum

The effect of different strains of the ectomycorrhizal fungus Hebeloma cylindrosporum on rooting in vitro and acclimatization of micropropagated cuttings of Pinus pinaster and Pinus sylvestris was studied. Two clones of P. pinaster and one of P. sylvestris were unable to root in the absence of auxin, but were induced to root on a medium devoid of auxin by all the fungal strains. Wild-type and indoleacetic acid (IAA)-overproducing mutant strains of the fungus stimulated rooting of clones showing a good reactivity to auxin to the same extent. In contrast, with a clone of P. sylvestris that showed low reactivity to auxin, IAA-overproduction by the fungus was advantageous for the induction of rooting of cuttings. Adventitious roots formed in the presence of a fungal strain were completely surrounded by a loosely packed network of hyphae which formed mycorrhizas as soon as roots grew outside the agar medium. During acclimatization, fungal inoculation improved the survival of rooted cuttings. At the end of acclimatization, fungal mycelia could be easily detected in the culture substrate of cuttings inoculated with dikaryotic strains and most of the pines' short roots were mycorrhizal. Monokaryotic mycelia, which have a lower growth rate and a lower infectivity, displayed poor ability to colonize the substrate and to form mycorrhizas. Two months after the end of acclimatization, fungal inoculation frequently depressed the growth of acclimatized cuttings of the clone J of P. pinaster . No depressive effect was observed with clone 78 and growth stimulation could even be observed with the infective dikaryon D1 which formed numerous mycorrhizas. From these studies, it was concluded that ectomycorrhizal fungi could be a suitable tool for improving rooting in vitro and survival at acclimatization of micropropagated conifer cuttings.     

Root Morphology and Anchorage of Six Native Tree Species from a Tropical Montane Forest and an Elfin Forest in Ecuador

Root architecture of tree species was investigated at two different altitudes in tropical forests in Ecuador. Increasing altitude was accompanied by higher wind speeds and more shallow soils, while slope angles of both sites were comparable (20–50°). Three tree species typical for the montane forest at 1900 m (Graffenrieda emarginata (Ruiz & Pav.) Triana (Melastomataceae), Clethra revoluta (Ruiz & Pav.) Spreng. (Clethraceae), Vismia tomentosa Ruiz & Pav. (Clusiaceae)) and for the elfin forest at 3000 m (Weinmannia loxensis Harling (Cunoniaceae), Clusia spec. (Clusiacaea) Styrax foveolaria Perkins (Styraceae)) were examined. At 1900 m, 92% of the trees grew upright, in comparison to 52% at 3000 m. At 3000 m, 48% of the trees were inclined, lying or even partly uprooted. At this altitude, all trees with tap roots or with shoots connected by coarse rhizomes, 83% of the trees with stilt roots, and 50% of the trees in which stems or roots were supported by other trees grew upright, suggesting that these characteristics were relevant for tree stability. Root system morphology differed markedly between altitudes. In contrast to 1900 m, where 20% of structural roots originated in the deeper mineral soil, root origin at 3000 m was restricted to the forest floor. The mean ratio of root cross sectional area to tree height decreased significantly from 6.1 × 10⁻³ m² m⁻¹ at 1900 m to 3.2 × 10⁻³ m² m⁻¹ at 3000 m. The extent of root asymmetry increased significantly from 0.29 at 1900 m to 0.62 at 3000 m. This was accompanied by a significantly lower number of dominant roots at 3000 m (2.3 compared to 3.8 at 1900 m). In conclusion, native tree species growing in tropical montane and elfin forests show a variety of root traits that improve tree stability. Root system asymmetry is less important for tree stability where anchorage is provided by a deep and solid root–soil plate. When deep rooting is impeded, root traits improving the horizontal extension of the root–soil plate are more pronounced or occur more frequently. Furthermore, mutual mechanical support of roots and stems of neighboring trees seems to be an appropriate mechanism to provide anchorage in soils with low bulk density and in environments with high wind speeds.     

Different distribution patterns of woody species on a slope in relation to vertical root distribution and dynamics of soil moisture profiles

Distribution patterns along a slope and vertical root distribution were compared among seven major woody species in a secondary forest of the warm-temperate zone in central Japan in relation to differences in soil moisture profiles through a growing season among different positions along the slope. Pinus densiflora, Juniperus rigida, Ilex pedunculosa and Lyonia ovalifolia, growing mostly on the upper part of the slope with shallow soil depth had shallower roots. Quercus serrata and Quercus glauca, occurring mostly on the lower slope with deep soil showed deeper rooting. Styrax japonica, mainly restricted to the foot slope, had shallower roots in spite of growing on the deepest soil. These relations can be explained by the soil moisture profile under drought at each position on the slope. On the upper part of the slope and the foot slope, deep rooting brings little advantage in water uptake from the soil due to the total drying of the soil and no period of drying even in the shallow soil, respectively. However, deep rooting is useful on the lower slope where only the deep soil layer keeps moist. This was supported by better diameter growth of a deep-rooting species on deeper soil sites than on shallower soil sites, although a shallow-rooting species showed little difference between them.     

Clonal variability in root growth and drought resistance in tea (Camellia sinensis)

Summary An investigation was carried out to study whether differences in rooting depth, root weight and vertical distribution of roots in the soil were some of the factors responsible for clonal variation in drought resistance in tea. The results showed that of these factors only rooting depth influenced drought resistance. Shallow rooted clones were drought susceptible and deep rooted clones drought resistant. In shallow rooted clones drought resistance increased with rooting depth. However in deep rooted clones drought resistance was not related to rooting depth.     

Root development and configuration in intensively managed radiata pine plantations

Summary In south-east Australia, where radiata pine (Pinus radiata D. Don) is grown on sandy soils low in nutrients and short of water, early establishment, and rapid growth to canopy closure lead to increased productivity. At this stage demands for nutrients and water are high, and trees respond vigorously to silvicultural inputs.For several months after transplanting in winter roots are confined within a narrow planting wedge, low temperature restricts new root growth and slows recovery from water stress in plants. From spring, depending upon the configuration and vigour of the roots transplanted, lateral roots extend radially throughout the soil.Although there were small decreases in concentration of roots radially from the stems of very young trees, such spatial differences disappeared between ages 2 and 3, so that rooting density was independent of distance from the stem. The pattern of vertical distribution of lateral roots was not influenced by age and 80–90% of the lateral roots were within the top 30 cm soil. Roots developed rapidly as the trees grew towards canopy closure, but in general the rooting densities of these pines are among the lowest reported for plants. In rapidly growing trees approaching canopy closure, the secondary thickening of the lateral roots was sufficient to double the weight of roots without altering root length.Knowledge about root growth and root configuration during the early phase of plantation development will assist management decisions where intensive silviculture is practiced, and hence ensure the most efficient use of nutrients and water.     

陇中黄土高原主要造林树种细根生物量分布

以2 mm为粗、细根的划分界限,采用根钻法对黄土高原安家沟流域油松、白杨、山杏、刺槐、沙棘和柠条6个主要造林树种细根分布进行研究,并测定不同林地下土壤含水率和土壤理化性质.结果表明： 在水平方向上,油松细根生物量呈先增大后减小的二次多项式分布,其他5个树种细根生物量均呈对数分布,水平根系发达,细根主要分布在冠幅半径2~3倍的范围内,表明各植被通过水平扩展来获取更多的土壤水分.在垂直方向上,随着土层深度的增加,细根生物量均呈减小趋势.6种植被细根生物量与土壤水分、容重呈显著负相关,与有机质、全N含量呈显著正相关.     

Phenotypic plasticity of the coarse root system of Prosopis flexuosa, a phreatophyte tree, in the Monte Desert (Argentina)

Prosopis flexuosa trees in the Monte Desert grow in dune and inter-dune valleys, where the water table is located at 6–14 m depth. We asked whether trees in the dunes, which are less likely to access the water table, present a coarse surface root architecture that might favor the exploration / exploitation of dune resources, compensating for water table inaccessibility. We characterized the architecture of surface roots of valley and dune trees, together with the soil environment. The dune held 50 % less and deeper gravimetric soil water (along a 4 m profile), 3-times less organic matter, 2-times less available phosphorous, and a sharper contrast of ammonium and nitrate concentration between plant canopies and uncovered soil than the valley. Coarse surface roots of dune trees were highly branched and grew tortuously at 0.56?±?0.16 m depth before sinking downward near the tree crown, suggesting an intensive exploitation of the ephemeral, deep, and canopy-linked resources. In contrast, trees from the valley spread their profuse and less branched surface roots mainly horizontally at 0.26?±?0.08 m depth, several meters outside the crown probably exploring this resource-rich site. A model for the environmental control of root architecture together with potential ecological effects is discussed.     

Architectural Tradeoffs between Adventitious and Basal Roots for Phosphorus Acquisition

Adventitious rooting contributes to efficient phosphorus acquisition by enhancing topsoil foraging. However, metabolic investment in adventitious roots may retard the development of other root classes such as basal roots, which are also important for phosphorus acquisition. In this study we quantitatively assessed the potential effects of adventitious rooting on basal root growth and whole plant phosphorus acquisition in young bean plants. The geometric simulation model SimRoot was used to dynamically model root systems with varying architecture and C availability growing for 21 days at 3 planting depths in 3 soil types with contrasting nutrient mobility. Simulated root architectures, tradeoffs between adventitious and basal root growth, and phosphorus acquisition were validated with empirical measurements. Phosphorus acquisition and phosphorus acquisition efficiency (defined as mol phosphorus acquired per mol C allocated to roots) were estimated for plants growing in soil in which phosphorus availability was uniform with depth or was greatest in the topsoil, as occurs in most natural soils. Phosphorus acquisition and acquisition efficiency increased with increasing allocation to adventitious roots in stratified soil, due to increased phosphorus depletion of surface soil. In uniform soil, increased adventitious rooting decreased phosphorus acquisition by reducing the growth of lateral roots arising from the tap root and basal roots. The benefit of adventitious roots for phosphorus acquisition was dependent on the specific respiration rate of adventitious roots as well as on whether overall C allocation to root growth was increased, as occurs in plants under phosphorus stress, or was lower, as observed in unstressed plants. In stratified soil, adventitious rooting reduced the growth of tap and basal lateral roots, yet phosphorus acquisition increased by up to 10% when total C allocation to roots was high and adventitious root respiration was similar to that in basal roots. With C allocation to roots decreased by 38%, adventitious roots still increased phosphorus acquisition by 5%. Allocation to adventitious roots enhanced phosphorus acquisition and efficiency as long as the specific respiration of adventitious roots was similar to that of basal roots and less than twice that of tap roots. When adventitious roots were assigned greater specific respiration rates, increased adventitious rooting reduced phosphorus acquisition and efficiency by diverting carbohydrate from other root types. Varying the phosphorus diffusion coefficient to reflect varying mobilities in different soil types had little effect on the value of adventitious rooting for phosphorus acquisition. Adventitious roots benefited plants regardless of basal root growth angle. Seed planting depth only affected phosphorus uptake and efficiency when seed was planted below the high phosphorus surface stratum. Our results confirm the importance of root respiration in nutrient foraging strategies, and demonstrate functional tradeoffs among distinct components of the root system. These results will be useful in developing ideotypes for more nutrient efficient crops.     

岷江上游4个栽培树种细根功能性状垂直分布的差异性

细根(直径≤2 mm)功能性状及垂直分布格局是反映植物对土壤资源吸收策略和影响森林地下生态过程的关键。本研究以岷江上游4个人工林树种连香树(Cercidiphyllum japonicum)、白桦(Betula platyphylla)、华山松(Pinus armandii)和油松(P. tabuliformis)为对象,调查不同海拔树木细根功能性状及其在不同土层间的垂直分布格局,并分析细根功能性状分布与构型之间的相关关系。结果表明:阔叶树种比针叶树种有更大的根长密度、生物量、比根长和比表面积,而直径反之; 4个树种细根集中在0～20 cm土层,根长密度和生物量在较高海拔地段均显著大于较低海拔,且均随土壤深度增加而减少,但比根长、比表面积和直径无显著的海拔差异,随土层加深也无明显的垂直变化规律;针阔树种间的细根构型差异显著,但不受海拔差异的影响,阔叶树的细根分支强度与一级根数量显著大于针叶树种;一级根数和根尖密度与比根长以及分根比与根长密度和生物量均呈显著正相关,而分叉与几个细根功能参数均呈负相关;随着土层深度增加,细根总生长量明显减少,但细根资源利用效率和策略不变; 5个细根功能性状...     

Root dynamics influence tree–grass coexistence in an Australian savanna

Both resource and disturbance controls have been invoked to explain tree persistence among grasses in savannas. Here we determine the extent to which competition for available resources restricts the rooting depth of both grasses and trees, and how this may influence nutrient cycling under an infrequently burned savanna near Darwin, Australia. We sampled fine roots <2 mm in diameter from 24 soil pits under perennial as well as annual grasses and three levels of canopy cover. The relative proportion of C₃ (trees) and C₄ (grasses) derived carbon in a sample was determined using mass balance calculations. Our results show that regardless of the type of grass both tree and grass roots are concentrated in the top 20 cm of the soil. While trees have greater root production and contribute more fine root biomass grass roots contribute a disproportional amount of nitrogen and carbon to the soil relative to total root biomass. We postulate that grasses maintain soil nutrient pools and provide biomass for regular fires that prevent forest trees from establishing while savanna trees, are important for increasing soil N content, cycling and mineralization rates. We put forward our ideas as a hypothesis of resource‐regulated tree–grass coexistence in tropical savannas.