Fine Root Production Research Articles

Contrasting synchrony in root and leaf phenology across multiple sub‐Arctic plant communities

Summary Roots account for > 50% of net primary production in many ecosystems and are widely accepted as playing a fundamental role in ecosystem carbon and nutrient cycling. Despite this, the timing and controls of root production and the relationships between root and leaf phenology are still poorly understood in many plant communities, making it challenging to elucidate broad scale patterns and to predict ecosystem responses to a changing climate. Here, we investigate and compare the extent of synchrony of community‐level fine root and leaf production in 10 widespread sub‐Arctic plant communities, including deciduous forest, evergreen shrub, deciduous shrub and sedge‐dominated communities. We show that across contrasting plant communities, timing of leaf production does not vary substantially, but there are considerable differences in timing of root production between woody plant‐ and sedge‐dominated vegetation. In sedge communities, root production occurs early in the growing season, soon after the peak of leaf production, whereas in woody plant communities, leaf and root production are highly asynchronous, with the majority of root production occurring late in the growing season. Seasonal dynamics of root growth were not consistently correlated with soil temperature, suggesting that differences in timing of production relate to inherent differences between plant functional types. This raises important questions as to the drivers of root production, the functional reasons underlying these contrasting plant strategies, as well as the consequences for microbial communities and nutrient cycling of such contrasting timing of carbon inputs to soil. Synthesis. We demonstrate that major differences exist in the synchrony of temporal dynamics of leaf and root production between woody plant and sedge‐dominated sub‐Arctic vegetation. The substantially greater asynchrony in root and leaf production in woody plants compared with sedges suggests that vegetation change associated with climate warming will result in significant shifts in timing of carbon fluxes to soil, and highlights the need for separate leaf and root components in ecosystem carbon and nutrient cycle models.

Seasonal Patterns of Fine Root Production and Turnover in a Mature Rubber Tree (Hevea brasiliensis Müll. Arg.) Stand- Differentiation with Soil Depth and Implications for Soil Carbon Stocks.

Fine root dynamics is a main driver of soil carbon stocks, particularly in tropical forests, yet major uncertainties still surround estimates of fine root production and turnover. This lack of knowledge is largely due to the fact that studying root dynamics in situ, particularly deep in the soil, remains highly challenging. We explored the interactions between fine root dynamics, soil depth, and rainfall in mature rubber trees (Hevea brasiliensis Müll. Arg.) exposed to sub-optimal edaphic and climatic conditions. A root observation access well was installed in northern Thailand to monitor root dynamics along a 4.5 m deep soil profile. Image-based measurements of root elongation and lifespan of individual roots were carried out at monthly intervals over 3 years. Soil depth was found to have a significant effect on root turnover. Surprisingly, root turnover increased with soil depth and root half-life was 16, 6–8, and only 4 months at 0.5, 1.0, 2.5, and 3.0 m deep, respectively (with the exception of roots at 4.5 m which had a half-life similar to that found between depths of 1.0 and 2.5 m). Within the first two meters of the soil profile, the highest rates of root emergence occurred about 3 months after the onset of the rainy season, while deeper in the soil, root emergence was not linked to the rainfall pattern. Root emergence was limited during leaf flushing (between March and May), particularly within the first two meters of the profile. Between soil depths of 0.5 and 2.0 m, root mortality appeared independent of variations in root emergence, but below 2.0 m, peaks in root emergence and death were synchronized. Shallow parts of the root system were more responsive to rainfall than their deeper counterparts. Increased root emergence in deep soil toward the onset of the dry season could correspond to a drought acclimation mechanism, with the relative importance of deep water capture increasing once rainfall ceased. The considerable soil depth regularly explored by fine roots, even though significantly less than in surface layers in terms of root length density and biomass, will impact strongly the evaluation of soil carbon stocks.

Fine root growth and contribution to soil carbon in a mixed mature Pinus koraiensis forest

Forest fine roots contribute substantially to carbon cycling, and old growth forests have an important role in the global carbon budget. We, hence, studied temporal variation in fine root production and turnover in an old forest. Fine root dynamics were investigated, using sequential soil cores for 6 years, and related to rainfall and air temperature in a mixed mature broad-leaved Pinus koraiensis forest in northeastern China. The mass, production, mortality, and disappearance of fine roots showed intra-annual variation. Monthly live and dead fine root mass were significantly positively correlated with meteorological conditions of the previous month. Monthly fine root production was associated with air temperature of the current month. The intra-annual pattern of fine root production was similar to that of net primary production and opposite to that of leaf litterfall. Total fine root production accounted for 25 % of net primary production, and the fine root input to soil carbon was 1.2 times larger than the leaf litterfall. Our results indicate tight linkages between fine roots and meteorological conditions. The important role of fine roots in forest carbon budget suggests that forest carbon flux estimations need to take fine root dynamics into account.

Fine root biomass, productivity and turnover in two contrasting aspects in natural Chir Pine (Pinus roxburghii Sarg.) forests of Central Himalaya

Fine roots (≥1 mm) production in Chir Pine (Pinus roxburghii Sarg.) forests was estimated across three different seasons for one year at four sites located between 1600–1850 meters elevation on two contrasting aspects. At each site three different slope positions (ridge slope, mid hill slope and hill base) were selected for the study. The tree density across the study sites ranged between 390 to 1910 tree/ha–1. The fine root biomass and productivity was estimated upto 40 cm soil depth. The total fine root biomass ranged between 3.34 and 3.72 t ha–1 across southern aspect while it ranged between 2.70 and 2.94 t ha–1 across northern aspect. Across the studied forest the NPP ranged between 1.02 and 2.2 t ha–1yr–1 and total turnover ranged between 0.52 and 0.96 yr–1.The fine root biomass declined as the soil depth increased and varied significantly (P < 0.001) across the soil depths and seasons. The fine root net primary production was positively correlated (P < 0.001) with soil organic carbon (t ha–1) and showed an inverse relationship with soil moisture percent and soil bulk density (g/cc).

Applicability of the net sheet method for estimating fine root production in forest ecosystems

Key message The estimates of fine root production by net sheet method were not affected by the net material or the aperture size, but flexible nets are preferable to avoid an underestimation.

Root-zone management practices impact above and belowground growth in Cabernet Franc grapevines

Background and Aims The objective of this study was to assess the effect of root pruning (RP) and annual under-trellis cover crops (UTCC) on vegetative growth and physiological processes in Cabernet Franc (Vitis vinifera L.) grapevines grafted on 3309C rootstock. Methods and Results Five root-zone management treatments, namely under-trellis vegetation-free strip control (C), RP and three distinct annual UTCC [annual ryegrass (AR), buckwheat, (BW) and turnip (TP)] were evaluated and compared over three seasons in a vigorous vineyard in the Northeastern USA. Root pruning had the greatest impact in reducing grapevine size; AR reduced the duration of shoot growth 2 out of 3 years. Competition between grapevine and AR and BW fine roots resulted in a lower grapevine fine root production in shallow soil layers and in a decrease in fine root lifespan. Conclusions Root pruning was effective in reducing grapevine size for 3 years; RP effectiveness, however, may vary greatly depending on site conditions. Annual UTCC hold potential for reducing grapevine size in vigorous plantings in regions with ample precipitation. Appropriate choice of cover crop species, however, remains unclear. Significance of the Study This work demonstrates that root-zone management practices impact grapevine vegetative growth and induce changes in root morphological traits.

Fine root dynamics in lodgepole pine and white spruce stands along productivity gradients in reclaimed oil sands sites.

Open-pit mining activities in the oil sands region of Alberta, Canada, create disturbed lands that, by law, must be reclaimed to a land capability equivalent to that existed before the disturbance. Re-establishment of forest cover will be affected by the production and turnover rate of fine roots. However, the relationship between fine root dynamics and tree growth has not been studied in reclaimed oil sands sites. Fine root properties (root length density, mean surface area, total root biomass, and rates of root production, turnover, and decomposition) were assessed from May to October 2011 and 2012 using sequential coring and ingrowth core methods in lodgepole pine (Pinus contorta Dougl.) and white spruce (Picea glauca (Moench.) Voss) stands. The pine and spruce stands were planted on peat mineral soil mix placed over tailings sand and overburden substrates, respectively, in reclaimed oil sands sites in Alberta. We selected stands that form a productivity gradient (low, medium, and high productivities) of each tree species based on differences in tree height and diameter at breast height (DBH) increments. In lodgepole pine stands, fine root length density and fine root production, and turnover rates were in the order of high > medium > low productivity sites and were positively correlated with tree height and DBH and negatively correlated with soil salinity (P<0.05). In white spruce stands, fine root surface area was the only parameter that increased along the productivity gradient and was negatively correlated with soil compaction. In conclusion, fine root dynamics along the stand productivity gradients were closely linked to stand productivity and were affected by limiting soil properties related to the specific substrate used for reconstructing the reclaimed soil. Understanding the impact of soil properties on fine root dynamics and overall stand productivity will help improve land reclamation outcomes.

Response of the fine root production, phenology, and turnover rate of six shrub species from a subtropical forest to a soil moisture gradient and shading

Knowledge of the fine root dynamics of different life forms in forest ecosystems is critical to understanding how the overall belowground carbon cycling is affected by climate change. However, our current knowledge regarding how endogenous or exogenous factors regulate the root dynamics of understory vegetation is limited. The aims of this study were to test the effects of soil moisture gradient and shading on the fine root production, phenology, and turnover rate of six shrub species from a subtropical forest. We selected a suite of study sites representing different habitats with gradients of soil moisture and solar radiation (shading or no shading). We assessed the fine root production phenology, the total fine root production, and the turnover among six understory shrub species in a subtropical climate, and examined the responses of the fine root dynamics to gradients in the soil moisture and solar radiation. The shrubs included three evergreen species, Loropetalum chinense, Vaccinium bracteatum, and Adinandra millettii, and three deciduous species, Serissa serissoides, Rubus corchorifolius, and Lespedeza davidii. Results We observed that variations in the annual fine root production and turnover among species were significant in the deciduous group but not in the evergreen group. Notably, V. bracteatum and S. serissoides presented the greatest responses in terms of root phenology to gradients in the soil moisture and shading: high-moisture habitat led to a decrease and shade led to an increase in fine root production during spring. Species with smaller fine roots of the 1st+2nd-order diameter presented more sensitive responses in terms of fine root phenology to a soil moisture gradient. Species with a lower fine root carbon- to nitrogen ratio exhibited more sensitive responses in terms of fine root annual production to shading. Soil moisture and shading did not change the annual fine root production as much as the turnover rate. The fine root dynamics of some understory shrubs varied significantly with soil moisture and solar radiation status and may be different from tree species. Our results emphasize the need to study the understory fine root dynamics in the achievement of a complete understanding of the overall belowground carbon cycling in a forest ecosystem, particularly ecosystems in which the understory fine root highly contributes to the belowground biomass.

Belowground carbon allocation patterns as determined by the in-growth soil core 13C technique across different ecosystem types

Belowground carbon inputs, in particular rhizodeposition, are a key component of the global carbon cycle and yet their accurate quantification remains a major challenge. In the present paper, the in-growth soil cores-13C method was used to quantify net root carbon input (root-derived C). Four different ecosystem types (forest, alpine grassland, apple orchard and vineyard) in northern Italy, characterized by C3 vegetation with a broad range of aboveground net primary production (ANPP; 155–770gCm−2y−1) were investigated. Cores, filled with soil of a known C4 isotopic signature were inserted at each site for twelve months. After extraction, root-derived C was quantified by applying a mass balance equation. Gross primary production (GPP) was determined by eddy covariance whereas ANPP was quantified using a biometric approach.NPP partitioning among sites differed, with fruit production dominating at agricultural sites. At these sites, belowground C inputs were dominated by rhizodeposits, likely due to relatively high root turnover. In natural ecosystems (forest and grassland) fine root production dominated belowground net primary production (BNPP) likely due to higher root growth determined by low phosphorus availability. Root derived C represented a significant contribution to BNPP varying from 40 to 60%. Our results underline the fact that failure to account for rhizodeposits may lead to a significant underestimation of BNPP.

Fine root turnover of Japanese white birch (Betula platyphylla var. japonica) grown under elevated CO2 in northern Japan

Elevated CO 2 reduced fine root dynamics (production and turnover) of white birch seedlings, especially grown in volcanic ash soil compared with brown forest soil. Increased atmospheric CO2 usually enhances photosynthetic ability and growth of trees. To understand how increased CO2 affects below-ground part of trees under varied soil condition, we investigated the responses of the fine root (diameter <2 mm) dynamics of Japanese white birch (Betula platyphylla var. japonica) which was planted in 2010. The three-year-old birch seedlings were grown in four experimental treatments comprising two levels of CO2, i.e., ambient: 380–390 and elevated: 500 μmol mol−1, in combination with two kinds of soil: brown forest (BF) soil and volcanic ash (VA) soil which has few nutrients. The growth and turnover of fine roots were measured for 3 years (2011–2013) using the Mini-rhizotron. In the first observation year, live fine root length (standing crop) in BF soil was not affected by CO2 treatment, but it was reduced by the elevated CO2 from the second observation year. In VA soil, live fine root length was reduced by elevated CO2 for all 3 years. Fine root turnover tended to decrease under elevated CO2 compared with ambient in both soil types during the first and second observation years. Turnover of fine root production and mortality was also affected by the two factors, elevated CO2 and different soil types. Median longevity of fine root increased under elevated CO2, especially in VA soil at the beginning, and a shorter fine root lifespan appeared after 2 years of observation (2011–2012). These results suggest that elevated CO2 does not consistently stimulate fine root turnover, particularly during the plant seedlings stage, as it may depend on the costs and benefits of constructing and retaining roots. Therefore, despite the other uncontrollable environment factors, carbon sequestration to the root system may be varied by CO2 treatment period, soil type and plant age.

Methods for estimating root biomass and production in forest and woodland ecosystem carbon studies: A review

Fine and coarse roots are key contributors to belowground net primary productivity, and play critical roles in the biogeochemical cycling of forest and woodland ecosystems. Despite their critical roles, roots have been understudied mainly due to methodological challenges. There is currently no consensus on which methods are most suitable to accurately study root biomass and production. Critical evaluation of the assumptions, strengths and inherent limitations of methods to study root biomass and production are necessary to help investigators decide which method is best for their purposes. This synthesis compares existing methods for root biomass and production estimation based on relevant criteria that include cost, labor requirements, time efficiency and accuracy and, also compares fine- and coarse-root biomass and production estimates from different methods measured at the same sites. Root excavation and soil-pit methods are commonly used to estimate coarse-root biomass, despite the high cost and labor required. Ground-Penetrating Radar is a very promising indirect approach to estimate coarse-root biomass, but may not be suitable for ecosystems with dense understory and soils with high organic matter and ion contents. Soil-core remains the most preferred method to estimate fine-root biomass. Empirical models are accepted as fast and cost-effective indirect approach to predict fine- and coarse-root biomass and production. Fine-root production is usually estimated with the (mini) rhizotrons, sequential-coring and ingrowth-core methods. Coarse-root biomass estimates were not significantly different between soil-pit and soil-core methods. There was a significant positive correlation (r2=0.91, p<0.0001) between fine-root biomass estimates obtained from soil-pit and soil-core methods. Fine-root production estimates were lower in the ingrowth-core (2.06±0.23Mgha−1year−1) compared to the (mini) rhizotrons (3.81±0.46Mgha−1year−1) and sequential-coring (3.84±0.93Mgha−1year−1) methods. Based on the reviewed literature and comparative analysis we propose that (mini) rhizotrons should be preferred over the others in estimating fine-root production. In situations where cost and site conditions preclude their use, the sequential-coring and ingrowth-core methods are suitable. The ingrowth-core should be used with caution in sites where root growth is slow and root biomass may be influenced by strong seasonal fluctuations. Multiple methods are still recommended for yielding realistic estimates of fine- and coarse-root production, and more comparative studies of different methods should be conducted on the same sites.

A modified soil coring method for measuring fine root production, mortality and decomposition in forests

Fine root (diameter < 2 mm) production, mortality and decomposition have been poorly estimated at ecosystem scales due to technical limitations. The soil coring method can accurately assess fine root biomass and necromass, but the concurrent growth, death and decomposition processes were not reasonably assessed during the sampling period, leading to greatly biased rate estimates. We developed a dynamic-flow method with two variations to address these processes by combining the soil coring method with an improved decomposition experiment. For a certain interval i (1 ≤ i) in the growing season, the dead fine roots were classified into fine roots dying before the start of interval i (GⅠ-i) and those dying during interval i (GⅡ-i). The decompositions of GⅠ-i and GⅡ-i were separately quantified and integrated into a modified mass balance model to estimate the production, mortality and decomposition. An example study conducted in a secondary Mongolian oak (Quercus mongolica Fischer ex Ledebour) forest showed that fine root production, mortality and decomposition were greatly underestimated by conventional soil coring methods failing to address the simultaneous growth, death and decomposition processes but overestimated by the method in which the decompositions of GⅠ-i and GⅡ-i were not separately determined and the decomposition rate was assumed to be constant. The dynamic-flow method greatly improved the accuracy of fine root estimates and can be widely applied to forests.

The influence of boreal tree species mixtures on ecosystem carbon storage and fluxes

Plant species mixtures are often seen as being able to achieve higher productivity and carbon (C) sequestration than their single-species counterparts, but it is unclear whether this is true in natural forests. Here, we investigated whether naturally-regenerated mixtures of common North American boreal tree species were more productive and stored more C than single-species stands. We also examined how closely the different C pools and fluxes were interrelated and whether these relationships varied with species composition. Single- and mixed-species stands of trembling aspen, black spruce and jack pine on mesic sites were selected in two regions of the Canadian boreal forest to assess aboveground and belowground productivity and C storage. Although previous studies conducted in these stands found synergistic effects of tree species mixtures on specific C pools and fluxes, such as higher organic layer C stocks and higher fine root productivity in some mixtures, no effects were detected on combined C pools or fluxes at the ecosystem level in the current study. Aspen abundance was linked with higher aboveground tree productivity, higher aboveground living biomass and higher soil heterotrophic respiration, indicating that aspen acts as a key driver of ecosystem C storage and fluxes in these natural forest ecosystems, more so than species richness. However, our results do not rule out the possibility of increased productivity and C storage in mixed stands under environmental conditions or stand developmental stages that are different from the ones studied here. Furthermore, when the entire forest ecosystem is considered (not only tree parts), synergistic effects of tree species mixtures may be more difficult to observe because the beneficial effect of species mixing on one specific C pool may be counterbalanced by a negative effect on another pool.

Fine root production in three zones of secondary mangrove forest in eastern Thailand

High root productions, especially in the fine roots, estimated by ingrowth cores were confirmed in mangrove forests. The zonal variation in root production was caused by inundation regime and soil temperature. Mangrove forests have high net primary productivity (NPP), and it is well known that these trees allocate high amounts of biomass to their root systems. In particular, fine root production (FRP) comprises a large component of the NPP. However, information on root production remains scarce. We studied FRP in three zones (Avicennia, Rhizophora, and Xylocarpus) of a mangrove forest in eastern Thailand using ingrowth cores (0–30 cm of soil depth). The root biomass and necromass were periodically harvested from the cores and weighed during the one-year study. The FRP was determined by summation of the fine root biomass (FRB) and root necromass. The results showed that the FRB clearly increased in the wet and cool dry seasons. Magnitude of FRB in the Rhizophora and Xylocarpus zones was 1171.07 and 764.23 g/m2/30 cm, respectively. The lowest FRB (292.74 g/m2/30 cm) was recorded in the Avicennia zone locating on the river edge where there is a greater frequency of inundation than the other zones. Root necromass was high in the Rhizophora and Xylocarpus zones, and accumulated noticeably when soil temperatures rapidly declined during the middle of the wet season to cool dry season. However, root necromass in the Avicennia zone varied within a small range. We attributed the small accumulation of root necromass in the Avicennia zone to the relative high soil temperature that likely caused a high root decomposition rate. The average FRP (3.403–4.079 ton/ha/year) accounted for 74.4, 81.5, and 92.4 % of the total root production in the Avicennia, Rhizophora, and Xylocarpus zone, respectively. The root production and causative factors (i.e., soil temperature and inundation regime) are discussed in relation to the carbon cycle of a mangrove forest.

Erratum to: Dwarf pine invasion in an alpine tundra of discontinuous permafrost area: effects on fine root and soil carbon dynamics

Recent invasion of Pinus pumila , a highly productive shrub-like tree species, into alpine tundra does not significantly modify the dynamics of fine root and soil carbon in the tundra. Climate warming may directly and indirectly affect the large carbon stock in discontinuous permafrost soil at high latitudes. In recent decades, Siberian dwarf pine [Pinus pumila (Pall.) Regel] has been invading dry heath alpine tundra in the northern Amur region of Far East Russia. Siberian dwarf pine is known to have high aboveground productivity, comparable to that of tall coniferous trees. We hypothesised that the invasion of Siberian dwarf pine into alpine tundra could increase soil carbon stocks via an increase in fine roots. Contrary to our expectations, the invasion of dwarf pine did not significantly increase the fine root biomass and productivity of the tundra, probably due to the belowground competitive exclusion between the dwarf pine and alpine tundra plants. Furthermore, the invasion of the dwarf pine did not affect soil carbon in the alpine tundra ecosystem. These results show that the recent invasion of Siberian dwarf pine into tundra did not influence the fine root dynamics or the soil carbon stock in the study site. Together, these results implied that (1) it takes a long time for pine invasion to change the belowground ecosystem properties of tundra vegetation to that of pine thickets and therefore (2) the lack of an increase in soil carbon from recent tree invasion should be taken into account when modelling future carbon dynamics in alpine tundra.

A new approach for estimating fine root production in forests: a combination of ingrowth core and scanner

A method of combination of ingrowth core and scanner provides better estimation of fine root production. Estimating fine root (diameter ≤2 mm) production has been difficult. The best method remains unknown because each method has potential biases associated with the estimation methods and field techniques applied, leading to over- or underestimation. In this study, a new method combining ingrowth core and scanner named as “combined method” was described, and it was applied to a Quercus serrata plantation in Tsukuba, Japan. Comparing accuracy of estimation of fine root production by decision matrix and continuous inflow method using ingrowth core with that by “combined method” was conducted. Closed-box protected scanners and ingrowth cores were buried in a field on December 6, 2012, and fine root images were nondestructively scanned biweekly for growth measurement. In addition, thermometers were used to record temperature automatically at 1 m from the ground and at a depth of 10 cm in soil at 12 a.m. daily to evaluate the effect of temperature on fine root production. The results indicated that the combined method gave 19 and 16 % higher estimation of fine root production than decision matrix and continuous inflow method, respectively. In the combined method, using scanned images at different time intervals gave different estimates. Generally, longer time-interval images gave lower estimates: biweekly-interval images yielded an estimate that was 16 % higher than that yielded using 2-month-interval images. A 1-month time lag relationship (R 2 = 0.84, p < 0.01) between soil temperature and fine root production was found at the study site. This relationship implied that higher fine root production occurred in summer. It was concluded that the combined method gave a more accurate estimation of fine root production in forests among the three methods compared.

Fine root dynamics of trees and understorey vegetation in a chronosequence of Betula platyphylla stands

Despite the recognized importance of fine roots in influencing forest carbon and nutrient cycling, knowledge of how fine root biomass, production and turnover rates change with stand age is still limited. In this study, we applied both sequential soil coring and ingrowth cores techniques to assess the influence of stand age on fine root biomass, live/dead ratio, production and turnover rates of trees and understorey vegetation in 20-, 36-, 57- and 82-year-old Betula platyphylla stands in northeastern China. A total of 1440 sequential cores and 288 ingrowth cores were collected from three soil depths (0–10, 10–20, and 20–30cm) during 2009–2010. Fine root biomass of trees increased from 20- to 36-year-old stands, and decreased thereafter in 57- and 82-year-old stands, while the distribution of understorey fine roots differed from that of tree fine roots. Fine root biomass of the understorey vegetation accounted for 49%, 12%, 31% and 60% of total fine root biomass in 20-, 36-, 57- and 82-year-old stands, respectively. Total biomass of fine roots (trees and understorey vegetation) increased with stand age. The live/dead ratio decreased with stand age, indicating that the older the stands, the greater the proportion of fine root necromass. Furthermore, our results showed that the mean tree basal area could explain 67% of the variation in tree fine root biomass and 53% of that in the total fine root biomass at the tree level. These tree-level analyses may be of great value in predicting fine root biomass values in B.platyphylla stands at the stand and/or tree level. Fine root production and turnover rates based on both sequential soil coring and ingrowth cores techniques exhibited similar age-related patterns, strengthening the results that fine root production and turnover rates increased with stand age in the B.platyphylla chronosequence we studied.

Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere processes.

Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.

Fine root productivity varies along nitrogen and phosphorus gradients in high-rainfall mangrove forests of Micronesia

Belowground biomass is thought to account for much of the total biomass in mangrove forests and may be related to soil fertility. The Yela River and the Sapwalap River, Federated States of Micronesia, contain a natural soil resource gradient defined by total phosphorus (P) density ranging from 0.05 to 0.42 mg cm−3 in different hydrogeomorphic settings. We used this fertility gradient to test the hypothesis that edaphic conditions constrain mangrove productivity through differential allocation of biomass to belowground roots. We removed sequential cores and implanted root ingrowth bags to measure in situ biomass and productivity, respectively. Belowground root biomass values ranged among sites from 0.448 ± 0.096 to 2.641 ± 0.534 kg m−2. Root productivity (roots ≤20 mm) did not vary significantly along the gradient (P = 0.3355) or with P fertilization after 6 months (P = 0.2968). Fine root productivity (roots ≤2 mm), however, did vary significantly among sites (P = 0.0363) and ranged from 45.88 ± 21.37 to 118.66 ± 38.05 g m−2 year−1. The distribution of total standing root biomass and fine root productivity followed patterns of N:P ratios as hypothesized, with larger root mass generally associated with lower relative P concentrations. Many of the processes of nutrient acquisition reported from nutrient-limited mangrove forests may also occur in forests of greater biomass and productivity when growing along soil nutrient gradients.

The role of environmental filters and functional traits in predicting the root biomass and productivity in savannas and tropical seasonal forests

Accurate measures of plant biomass and productivity are important to predict the impacts caused by current anthropogenic changes in the carbon pool. Changes in the carbon pool may be decisive whether plant communities act as sinks or sources for carbon dioxide. However, there are not accurate assessments of savanna and seasonal forest biomass, particularly belowground, which is essential to evaluate their carbon stock. We tested whether we could use soil variables, fire frequency, topography, and functional traits to build simple models to predict the belowground system in savanna and seasonal forest. In central Brazil, we collected root biomass up to 100cm deep and annual fine root productivity in the top 40cm of soil with an ingrowth donut, in 100 plots in savanna and 20 plots in seasonal forest. We used increasing complexity general linear modeling to find the models predicting the root biomass and productivity. We found significant models in all cases, even though the explanatory power for the savanna was low. The main ecological forces related to the root biomass and productivity were soils poor in nutrients, foraging for potassium in the savanna and for nitrogen in the forest, drought, resistance to disturbance, and niche complementarity. Reliable estimates of root biomass might be used to replace direct but laborious excavation methods. The carbon stock of savanna and seasonal forest are large and should not be neglected when estimating the impacts caused by climate and land-use changes.