Article

Habitat productivity influences root mass vertical distribution in grazed Mediterranean ecosystems

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Abstract

Herbivores are expected to influence grassland ecosystems by modifying root biomass and root spatial distribution of plant communities. Studies in perennial dominated grasslands suggest that grazing intensity and primary productivity may be strong determinants of the vertical distribution of subterranean biomass. However, no studies have addressed this question in annual dominated pastures. In this study we assess the effect of grazing and habitat productivity on the vertical distribution of root mass in an annual dominated Mediterranean pasture grazed by free-ranging sheep and wild rabbits. We evaluate the effects of grazing on total root mass and vertical root distribution (0-4, 4-8 and 8-12 cm depths) in two neighboring topographic sites (uplands and lowlands) with different productivity using a replicated fence experiment which excludes sheep and sheep plus rabbits. We found evidences that grazing affected root biomass and vertical distribution at lowlands (high productivity habitats), where places grazed by sheep plus rabbits exhibit more root mass and a higher concentration of it towards the soil surface than only rabbits and ungrazed places. In contrast, grazing did not affect root biomass and vertical distribution at uplands (low productivity habitats). We suggest that higher nitrogen and organic matter found in lowlands permit a plant adjustment for nitrogen acquisition by increasing biomass allocation to root production which would allow plant regrowth and the quick completion of the annual life cycle. Contrary, soil resources scarcity at uplands do not permit plants modify their root growth patterns in response to grazing. Our study emphasizes the importance of primary productivity in predicting grazing effect on belowground processes in Mediterranean environments dominated by annuals.

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... Despite its direct effect on aboveground biomass, defoliation can modify carbon allocation and root growth (Tomanek & Albertson 1957, Richards 1984, Bonachela 1996, Liu & Huang 2002, Patty et al. 2010, Rueda et al. 2010. It is generally assumed that repeated mowing and grazing cycles lead to a reduction of root biomass because assimilates are increasingly used for the regrowth of shoots (e.g., Speidel & Weiß 1972, Gass & Oertli 1980, Dawson et al. 2000. ...
... Defoliation greatly reduces the photosynthetic tissue and often modifies carbon allocation patterns and root growth activity (Tomanek & Albertson 1957, Gass & Oertli 1980, Richards 1984, Bonachela 1996, Liu & Huang 2002, Patty et al. 2010, Rueda et al. 2010. ...
... While most research on grazing effects in managed and natural grasslands has focused on aboveground biomass, the response of the root system to different grazing regimes is not well studied (Rueda et al. 2010). This gap of knowledge is unsatisfactory because belowground biomass is often larger than aboveground grassland biomass and a large fraction of annual carbon gain (up to > 70 %) is transferred to the roots in certain grasslands (Speidel & Weiß 1972, Jackson et al. 1996. ...
Article
In large areas of Central Europe, grassland management has shifted from extensive grazing and mowing to highly intensive systems during the past 50 years. Although effects on biodiversity have intensively been studied, little is known about the response of important ecosystem functions, in particular, water and nutrient cycling, to management intensification. We conducted a two-factorial grassland management experiment (GRASSMAN) with two cutting frequencies (one/three cuttings per year) and two fertilization levels (non-fertilized/N-fertilized) in a moderately species-rich temperate grassland to analyse the effects of management regimes on evapotranspiration (ET) and infiltration (I). Both were measured in the growing season 2009 with small, weighable lysimeters that contained undisturbed soil monoliths and vegetation. Aboveground biomass production (ANPP), belowground biomass, root length density, plant diversity, water use efficiency (WUE), and climatic factors were also measured. Fertilization with 180 kg N ha−1yr−1 increased aboveground biomass production by 50–70% and stand ET by 10–15% (or 20–40 mm), whereas I and ground water recharge decreased by about 50%. Consequently, fertilization increased the WUE of the grassland plants by 20–30%. However, increasing the mowing frequency from 1 to 3 had no significant effect. We found close relations between ANPP, ET and I and conclude that grassland management intensification influences the water balance primarily through fertilization effects on productivity. In areas of Central Europe with abundant grassland, ground water recharge must have significantly decreased with management intensification in the past 50 years. Copyright © 2011 John Wiley & Sons, Ltd.
... It is possible that the lack of grazing effects in these low-productivity areas could be also attributed to a lower herbivore grazing pressure at uplands compared to lowlands. However, a second study in the same area in 2002 found that grazing was more intense in the uplands: 47 % of the aboveground biomass was consumed at uplands versus 34 % at lowlands (Rueda et al. 2010). Interestingly, in our study, the effect of herbivores on green canopy cover and especially on vegetation height in uplands (Fig. 2a, e) did not impact on species richness. ...
... First, species richness remained high and unchanged over time in the plots grazed only by rabbits. This was not due to low levels of rabbit grazing; they consumed on average 37 % of the aboveground biomass versus 47 % that was consumed by rabbits plus sheep (Rueda et al. 2010). Second, a long history of grazing by rabbits could explain why this small mammal has no negative effect on species richness in these Mediterranean environments, whereas it adversely affects the richness in other semiarid environments outside of the wild rabbit's native range (Cooke et al. 2010). ...
... We found that in high-productivity lowlands, rabbits did not visit the small-herbivore grazing plots as frequently as expected. In fact, biomass consumption by rabbits in plots exposed only to small herbivores dropped from 37 % in uplands to 16 % in lowlands (Rueda et al. 2010). This could be related to the fact that higher soil moisture at lowlands limits rabbit burrowing. ...
Article
Vertebrate herbivores can be key determinants of grassland plant species richness, although the magnitude of their effects can largely depend on ecosystem and herbivore characteristics. It has been demonstrated that the combined effect of primary productivity and body size is critical when assessing the impact of herbivores on plant richness of perennial-dominated grasslands; however, the interaction of site productivity and herbivore size as determinants of plant richness in annual-dominated pastures remains unknown. We experimentally partitioned primary productivity and herbivore body size (sheep and wild rabbits) to study the effect of herbivores on the plant species richness of a Mediterranean semiarid annual plant community in central Spain over six years. We also analyzed the effect of grazing and productivity on the evenness and species composition of the plant community, and green cover, litter, and plant height. We found that plant richness was higher where the large herbivore was present at high-productivity sites but barely changed at low productivity. The small herbivore did not affect species richness at either productivity site despite its large effects on species composition. We propose that adaptations to resource scarcity and herbivory prevented plant richness changes at low-productivity sites, whereas litter accumulation in the absence of herbivores decreased plant richness at high productivity. Our results are consistent with predictions arising from a long history of grazing and highlight the importance of both large and small herbivores to the maintenance of plant diversity of Mediterranean annual-dominated pastures.
... We assessed soil nutrient dynamics as changes in C, N, P, and K, Ca, Mg cations right before (September 2014) and after the experiment (September 2015), in the first 10 cm of soil. This corresponds to the soil depth influenced by annual plant vegetation in Mediterranean ecosystems and contains 95% of the total community root biomass 42 . For chemical analyses, soils were dried in the lab at 30°C until constant weight, and sieved (2 mm) to eliminate stones and large roots. ...
Article
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Ecologists have long argued that higher functioning in diverse communities arises from the niche differences stabilizing species coexistence and from the fitness differences driving competitive dominance. However, rigorous tests are lacking. We couple field-parameterized models of competition between 10 annual plant species with a biodiversity-functioning experiment under two contrasting environmental conditions, to study how coexistence determinants link to biodiversity effects (selection and complementarity). We find that complementarity effects positively correlate with niche differences and selection effects differences correlate with fitness differences. However, niche differences also contribute to selection effects and fitness differences to complementarity effects. Despite this complexity, communities with an excess of niche differences (where niche differences exceeded those needed for coexistence) produce more biomass and have faster decomposition rates under drought, but do not take up nutrients more rapidly. We provide empirical evidence that the mechanisms determining coexistence correlate with those maximizing ecosystem functioning. It is unclear how biodiversity-ecosystem functioning and species coexistence mechanisms are linked. Here, Godoy and colleagues combine field-parameterised competition models with a BEF experiment to show that mechanisms leading to more stable species coexistence lead to greater productivity, but not necessarily to enhanced functions other than primary production.
... A remarkable characteristic of old-growth grasslands (that is, ancient, biodiverse grassy ecosystems; Veldman et al. 2015) is the high resilience of the plant community to endogenous disturbances such as fire and herbivory (Overbeck et al. 2005;Buisson et al. 2018). Most and especially the dominant species are able to resprout from below-ground gems (bud bank), allowing for quick vegetation recovery after above-growth biomass removal by disturbances such as fire (Overbeck et al. 2005;Fidelis & Blanco 2014) or grazing (Rueda et al. 2010). However, the soil seed bank (SSB) also is important in plant community assembly and vegetation recovery as it contributes to the recruitment of new individuals (Bakker et al. 1996). ...
Article
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Grazing is an important determinant for the composition and structure of grasslands; however, soil seed bank (SSB) response to grazing intensity is poorly investigated. We analyzed SSB richness and density in a subtropical grassland in southern Brazil with different forage offers (low, intermediate, high and very high), that is, contrasting grazing intensities. The SSB was evaluated by the seedling emergence method. We collected ten SSB samples at two layers (0-5 and 5-10 cm) in spring and autumn in each of grazing intensity treatments. We surveyed the established vegetation to assess its similarity with the SSB. Treatment effects were analyzed by Poisson regression while compositional differences were visualized by ordination. We found 103 species in the SSB, of which 71 were also found in established vegetation. We found a positive correlation between SSB density and grazing intensity. High grazing intensity influences patterns of composition and dominance in the SSB, while no strong differences were found among the other treatments. The SSB was characterized by low participation of dominant grasses in the vegetation and the dominance of ruderal species, indicating that recovery from the SSB after total removal of vegetation (severe disturbance) may be limited in grasslands in the region.
... The mechanism for this increase was the significant increase in aboveground plant biomass of grasses. However, contrary to aboveground plant biomass, plant root biomass decreased in the fenced plots, which was consistent with the results of other studies (Hafner et al. 2012;Rueda et al. 2010;Shi et al. 2013). We speculated this may possibly be due to the loss of forb species with a taproot system and/ or the decreased biomass allocation to roots for grass species. ...
Article
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Aims Fencing to prevent livestock grazing has widely been implemented to restore vegetation and soils of degraded alpine meadows on the Zoige Plateau in recent decades, but its efficacy is still unclear. This study was designed to investigate the responses of plant community and soil properties to long-term fencing. Methods We surveyed vegetation structure and production, soil physical and chemical properties in three sites, each having paired adjacent plots (i.e. one was fenced for 11–17 years and the other was allowed for regular grazing). Results Long-term fencing resulted in species loss at the community level and decreased plant species richness at the plot level. Fencing increased aboveground plant biomass and plant litter accumulation but reduced root biomass, and in particular it dramatically increased the aboveground biomass of grasses at the expense of legumes and sedges. Moreover, fencing decreased soil organic carbon and total N concentrations, and soil bulk density, but increased soil water infiltration rate, soil total P and soluble N concentrations, while soil soluble P concentration remained unchanged. Conclusions These results indicate that long-term (> 11 years) fencing is not beneficial to plant diversity and soil organic carbon sequestration of the Zoige alpine meadows.
... We assessed soil nutrient dynamics as changes in C, N, P, and K, Ca, Mg cations right before (September 2014) and after the experiment (September 2015), in the first 10 cm of soil. This corresponds to the soil depth influenced by annual plant vegetation in Mediterranean ecosystems and contains 95% of the total community root biomass 42 . For chemical analyses, soils were dried in the lab at 30°C until constant weight, and sieved (2 mm) to eliminate stones and large roots. ...
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With ongoing biodiversity loss, it is important to understand how the mechanisms that promote coexistence relate to those that increase functioning in diverse communities. Both coexistence and biodiversity functioning research have unified their mechanisms into two classes. However, despite seeming similarities, theory suggests that coexistence and biodiversity mechanisms do not necessarily map onto each other, yet direct empirical evidence for this prediction is lacking. We coupled field-parameterized models of competition between 10 plants with a biodiversity-functioning experiment measuring biomass production, litter decomposition, and soil nutrient content under contrasting environmental conditions. We related biodiversity mechanisms (complementarity and selection effects), to coexistence mechanisms (niche and fitness differences). As predicted by theory, complementarity effects were positively correlated with niche differences and differences in selection effects were correlated with fitness differences. However, we also found that niche differences contributed to selection effects and fitness differences to complementarity effects. Despite this complexity more stably coexisting communities (i.e. those in which niche differences offset fitness differences) produced more biomass, particularly under drought. This relationship was weaker for litter decomposition rates and soil nutrient acquisition, showing that the mechanisms promoting plant coexistence may differ from those promoting high levels of functions that are less directly related to plant performance. We provide the first empirical evidence that the mechanisms promoting stable coexistence correlate with those driving high biomass production. These findings establish a link between stable coexistence and functioning, which could allow better predictions of how diversity loss induced by global change translates to changes in ecosystem function.
... In the latter, the top 50 cm of soil contain over 90 % of the roots. These results are similar to those reported by several authors who found a highest vertical root distribution of below-ground root biomass towards the upper 50 cm of the soil profile (Pucheta et al., 2004;Rueda et al., 2010). Soil water availability is the important environmental parameter in TDF (Costa et al., 2014). ...
Article
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Root structure (distribution, biomass) was characterized along a successional chronosequence of secondary forests (2, 6, 12, and 30 years) and reference ecosystems (mature forest, woody savanna) in order to describe the recovery process of former agricultural land in south-western Madagascar. The distribution of root biomass as a function of depth fits well with a power law: B = aDb (B being the root biomass expressed in mg.dm-3 and D depth in cm). Root distribution was deeper in mature forest and young fallows and more superficial in the old fallows and woody savanna because of higher soil compaction. Root biomass increases with the age of the fallow until the 12th year and a decrease was registered in the 30-year-old fallow. Root biomass was 3.58 t.ha-1 in the 2-year-old fallow, 4.96 t.ha-1 in the 6-year-old fallow and 10.00 t.ha-1 in the 12-year-old fallow. Root biomass values in all fallows and woody savanna (7 t.ha-1) were lower than in the mature forest (18.5 t.ha-1). Thus, the environmental cost of deforestation in the study area corresponds to a loss of 16.9 t.ha-1 of root biomass after 30 years of abandonment.
... There is growing evidence that long-term grazing caused a higher BGB allocation and resulted in increased root/shoot ratios in a variety of ecosystems (Lindwall, Vowles, Ekblad, & Björk, 2013;Papatheodorou, Pantis, & Stamou, 1998;Veen, Vries, Bakker, Putten, & Olff, 2014). Nevertheless, the allocation of AGB and BGB of grazed grassland ecosystems depends on both grazing management and environmental conditions, such as temperature, precipitation, and soil nutrients (Gong, Fanselow, Dittert, Taube, & Lin, 2015;Patty, Halloy, Hiltbrunner, & Körner, 2010;Rueda, Rebollo, & Rodríguez, 2010). For instance, Gong et al. (2015) found that the responses of primary production to herbivory and resource additions were tightly linked to the biomass allocation patterns and that the precipitation and nitrogen statuses need to be considered when optimizing grazing practices in an annually rotating grazing semiarid grassland in Inner Mongolia. ...
Article
Understanding the impact of grazing patterns on grassland production is of fundamental importance for grassland conservation and management. The objective of this study is to obtain an understanding of the trade-offs between aboveground biomass (AGB) and belowground biomass (BGB), which are influenced by environmental factors in free grazing (FG) and grazing exclusion (GE) alpine grasslands on the Tibetan Plateau. We explored the relationships between the trade-off and environmental factors using correlation analysis, a generalized additive model and a structural equation model, and then found that the key factors that determine trade-off showed differences in FG and GE grasslands and that the final SEM result explained that 96% (path coefficient = 0.96) and 65% (path coefficient = 0.65) of the variations in the trade-off were due to FG or GE classifications, respectively. The results demonstrated that SOC, Soil C:N, and Soil AN affect the trade-off between above- and belowground biomass in FG grasslands more obviously than in GE grasslands. However, the effects of GST on the trade-off were insignificant, -0.218 and -0.181 in FG and GE grasslands, respectively. FG increased the soil bulk density, which resulted in an alteration in the soil pore size distribution and a greater resistance to root penetration. In addition, FG affected the level of soil nutrition, which will affect the nitrogen mineralization of decomposition and absorption, as well as the root biomass. Consequently, this study can provide guidance to improve the quality of grassland.
... Insect/pathogen outbreaks generally decreased above-ground biomass due to the biotic-induced defoliation or plant damage (Martin et al., 2009), causing the increase in litter fall in the first several months or longer and thereafter decreasing litter fall and litter mass (Fig. S1b in Appendix S4; Coupe & Cahill, 2003). Root production often depends on leaf photosynthetic input (Rueda et al., 2010). Leaf damage by insects or pathogens is likely to have reduced plant photosynthetic input and thus reduced root biomass, because plants spent more energy to repair the wounded tissues ( Fig. 2b; Grogan & Chapin, 2000). ...
Article
AimClimate change, especially the wider occurrence of extreme events, is likely to increase the intensity and frequency of insect/pathogen outbreaks (referred to as biotic disturbance), which may considerably affect plant ecophysiological traits and thus the ecosystem carbon (C) cycle. Little is known, however, about the ways in which biotic disturbance quantitatively affects ecosystem C processes, especially those that occur below ground. We reveal the general patterns of below-ground C responses to biotic disturbance from field manipulative experiments and opportunistic events.LocationGlobal.Method We carried out a meta-analysis examining the effects of biotic disturbance on 16 variables associated with below-ground C processes, based on 64 experimental studies.ResultsBiotic disturbance significantly decreased below-ground C pools with relatively long residence times (e.g. root biomass and soil organic carbon, SOC), but increased labile C pools (e.g. microbial biomass carbon, MBC; dissolved organic carbon, DOC), soil respiration (Rs) and its components, and microbial population sizes. Compared with the neutral or positive effects of other environmental changes on below-ground C pools and fluxes, biotic disturbance had a negative effect on plant biomass and SOC but a larger positive effect on MBC, DOC and Rs.Main conclusionsBiotic disturbance can have stronger impacts on below-ground C processes than other environmental changes, and the sensitive responses of soil labile C pools and C fluxes to biotic disturbance decrease long-term below-ground C sequestration. More research efforts are, however, needed to reduce the uncertainties in quantifying the effects of biotic disturbance and to improve forecasting of the feedback between the carbon cycle and climate.
Article
Livestock grazing can be a means to maintain biodiversity in grasslands, but the outcome for vegetation structure and species composition depends on livestock type and grazing regime. This study aims at disentangling the effects of plant functional‐group abundance and livestock type on the above‐ and below‐ground biomass and N allocation in temperate pastures.We investigated the effects of cattle, sheep and mixed stocking on above‐ground biomass (AGB) and belowground biomass (BGB) and plant N pools in a replicated grazing experiment in two pasture community types with different plant functional‐group abundance (diverse vs. grass‐dominated swards).In the six treatments, AGB was reduced up to 80% compared with an ungrazed control. Cattle reduced AGB to a larger extent than sheep in diverse pastures (80 vs 44% reduction) while sheep grazing tended to do so in grass‐dominated pastures (57 vs 46% reduction); mixed stocking led to intermediate values. Grazing reduced AGB more than the N pool in AGB, thus lowering the biomass C/N ratio relative to the ungrazed control. Neither BGB nor the N pool in BGB differed between the grazing treatments and the control plots.We conclude that livestock type and functional‐group abundance are interacting factors that influence plant biomass and N pools in swards of managed temperate pastures. The contrasting biomass removal rates of cattle and sheep could be used to increase the structural heterogeneity and total plant species pool of pastures by keeping different livestock species in neighbouring patches.
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Grazing exclusion has been proposed as a choice for restoring degraded grasslands on the Qinghai–Tibetan Plateau, but its effects on soil properties are not clear. The present study was designed to investigate whether various soil organic carbon (OC) and nitrogen (N) pools and enzymatic activities were changed through grazing exclusion. A paddock of grassland was fenced in May 2002 for exclusion of livestock grazing, while the surrounding grassland continued conventional grazing by yak (Bos grunniens) and sheep (Ovis aries). Eight years after grazing exclusion, besides a reduction in plant species, the root biomass and soil bulk density in the top 15-cm depth were reduced by 34% and 26%, respectively, compared to the grazed grassland. Grazing exclusion enhanced the C/N ratios of shoots and roots by 18–19%, indicating a quality reduction in the shoot and root litters compared with the non-exclusion. Grazing exclusion also lowered stocks of total soil OC and N, microbial biomass C and N, and acid-extracted carbohydrate C and soil enzymatic activities (per area) of β-glucosidase, urase, and phosphatase in the 0–15 cm soil layer. Under grazing exclusion, less C input from the root-associated sources and possibly greater C output through heterotrophic respiration might have reduced various soil OC storages. However, a significant increase in soil mineral N pool was found under no grazing compared to grazing, possibly due to less plant N demand and uptake and change in N mineralization and/or immobilization. In conclusion, grazing exclusion is not beneficial to soil OC sequestration on the northeastern Qinghai–Tibetan Plateau.
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Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.
Article
DM distribution in shoots and roots of vegetative plants depending on various environmental conditions and experimental interventions is discussed. Because of the difficulties in maintaining conditions at a sufficiently constant level for some time, functional equilibria are not likely to exist during prolonged periods of time. From the responses occurring after transferring plants including perennial ryegrass and maize from one condition to another or after disturbance of existing relationships, it is demonstrated once more that nutritional control, i.e. functional control, of distribution is still the most reasonable interpretation of the observed reaction patterns. (Abstract retrieved from CAB Abstracts by CABI’s permission)
Chapter
A knowledge of the pattern of root distribution in soil is critical to a number of areas of ecology. For example, our ability to model the interactions between climate and vegetation depends in part on our knowledge of the global pattern of distribution of belowground biomass at various soil depths, and how it will change as one vegetation type replaces another (Jackson et al. 1996). Similarly, our understanding of ecosystem processes is currently limited by poor understanding of the distribution, quantity and productivity of fine roots within a variety of ecosystems, even though, for example, the annual production of fine roots may be twice that of leaves in Northern American hardwood forests (Fahey and Hughes 1994), and up to 80 % of the biomass of some ecosystems is underground (Jackson et al. 1996). At a more local scale, we are limited in our ability to accurately model the processes involved in plant competition by our inadequate knowledge of the fine-scale distributions of roots of individual plants and of the distribution of their associated symbionts (Mou et al. 1995; Casper and Jackson 1997; Casper et al. 2000).
Chapter
Despite much recent attention over the past 30 years, belowground net primary production (BNPP) remains one of the poorest understood attributes of terrestrial ecosystems (Milchunas and Lauenroth 1992; Nadelhoffer and Raich 1992). Estimates of the ratio of BNPP to net primary production (NPP) across ecosystem types range from less than 0.20 to more than 0.80 (Bray 1963, Coleman 1976). Part of this range can be explained by differences among ecosystems, but a portion of it is the result of differences among methods. For instance, Aber et al. (1985) reported ratios of fine root production by two different methods for the same sites ranging from close to 1 to more than 10. In addition to a wide range of estimates of the importance of BNPP across ecosystem types, estimates within ecosystems also vary widely. For instance, Sims and Singh (1978) found that BNPP accounted for 24 to 87% of NPP over a range of grassland ecosystems and Vogt (1991) reported that fine root production accounted for 7 to 76% of NPP over a range of forest ecosystems. How much of this variability is the result of differences among ecosystems and how much is the result of differences among methods? While there is no agreement on the answer to this question, there is abundant evidence that all of the current methods have important strengths and limitations. The objective of this chapter is to describe the most frequently used methods to estimate BNPP and their strengths and weaknesses. The first section provides definitions of terms and descriptions of concepts. The next section describes the methods, comments on their strengths and weaknesses, and provides references in which they have been applied. The last section evaluates the degree to which uncertainty (variability) in input data influences the uncertainty of the resulting estimate of BNPP.
Article
The objective of this study was to evaluate for an 8-yr period the ecosystem-level impacts of no grazing vs. sustained moderate and heavy cattle grazing in terms of: (1) plant species basal cover, density, and composition; (2) aboveground net primary production (ANPP), N content of ANPP (ANPP-N), belowground net primary production (BNPP), and N content of BNPP (BNPP-N); (3) litter and root decomposition and N loss; and (4) soil C, total soil N, and net in situ soil N mineralization. Moderate and heavy grazing treatments were designed to achieve an end-of-the-grazing-season residual vegetation of 50% and 10%, respectively, of the long-term average ANPP of comparable ungrazed sites. The main factor affecting the vegetation response was the increase in precipitation after the drought of 1988; few differences were due to grazing intensity. The total absolute basal cover of grasses increased steadily in all treatments, from an average of 4% during the drought of 1988 to 14% in 1993. Forb density and diversity ...
Article
The more grazing-tolerant Agropyron desertorum consistently produced more regrowth in the absence of photosynthesis than A. spicatum but severe preclipping treatment (which has been shown to reduce carbohydrate reserves by >40%) did not significantly reduce etiolated regrowth in either species. Meristematic limitations appear to be the dominant control on the amount of etiolated regrowth produced. These limiations also appear to be of prime importance in determining regrowth in the light and for grazing tolerance of plants. -from Authors
Article
1 The interaction between natural and experimental gradients of productivity on competition intensity was tested by neighbour removal experiments. Water is the main limiting factor in this system and experimental gradients of productivity were obtained using a series of watering treatments. 2 Competition was determined as the total effect of neighbours on per-capita seed production of Stipa capensis. Both absolute and relative competition intensity were calculated for three types of habitats, during two successive years, and under different watering treatments in each habitat-year combination. 3 Both measures were positively correlated with productivity, but absolute competition intensity was more sensitive to changes in productivity than relative competition intensity. Natural gradients of productivity appeared to have stronger effects on competition intensity than experimental gradients, but this was largely due to their wider range. In those cases where the ranges of the two types of gradients were similar, experimental gradients had a stronger effect on competition intensity than natural gradients. 4 Patterns of spatial variation in competition intensity were correlated with standing crop under all watering conditions. However, slopes of the regression equations obtained for the various watering treatments were not homogeneous. This indicates that per-gram effects of standing crop on competition intensity may fluctuate from year to year, depending on rainfall conditions. Regression models constructed to test the relationships between standing crop and competition intensity over different years, habitat types and watering treatments accounted for 88% of the variation in absolute competition intensity and 83% of the variation in relative competition intensity. 5 The overall results of this study are consistent with the hypothesis that plant competition increases along productivity gradients. The results also indicate that patterns of variation in competition intensity along productivity gradients may be influenced by the type of the gradient along which competition is measured (natural vs. experimental), its range, and the way competition intensity is defined.
Article
Knowledge of root response, as well as shoot response, to defoliation is needed to manage grasslands in environments where water and/or nutrients are limiting. The objective of this study was to document the response of sand bluestem (Andropogon hallii Hack.) roots and shoots to different times and frequencies of defoliation. Individual sand bluestem plants were grown in 15 × 100-cm polyvinyl chloride (PVC) containers which were placed in the plants' natural setting. Twelve plants (replications) were clipped to a 7-cm stubble height during mid-month for each of the following defoliation schedules: 1) June, July, and August; 2) June and August; 3) June; 4) July; 5) August; and 6) October. The October defoliation, after shoot senescence, served as the control. Multiple defoliations reduced (P < 0.05) root weight, root area, root length, and weight of total nonstructural carbohydrates (TNC) in roots by an average of 33, 42, 43, and 34%, respectively, compared to control plants. A single defoliation in June only reduced root weight, root area, root length, and weight of TNC in roots by 14, 19, 16, and 13%, respectively, compared to control plants. Defoliating plants during the growing season did not affect (P > 0.05) number of tillers, weight per tiller, above-ground weight, number of buds, weight of rhizomes, or weight of TNC in rhizomes. Grazing sand bluestem more than once during the growing season may reduce root growth and diminish its ability to compete for water and nutrients. Grazing during the dormant season or once during the early part of the growing season should be least detrimental to sand bluestem.
Article
1 The response of plants to herbivory usually varies with the grazing regime experienced. We investigated (i) if the timing and frequency of grazing affected plant growth, (ii) if faeces deposition by herbivores stimulated plant growth, and (iii) if grazing affected the total nonstructural carbohydrate (TNC) reserves in the below-ground vegetation of two arctic graminoids, Dupontia fisheri and Eriophorum scheuchzeri. 2 This study was conducted in polygon fens exposed to intense summer grazing by greater snow geese (Chen caerulescens atlantica) on Bylot Island (73⚬N) in the Canadian High Arctic. We manipulated the frequency (once or three times) and the timing (early, mid or late in the season) of grazing and faeces deposition in controlled grazing trials using captive goslings. 3 Although ungrazed plants were taller than grazed ones at the end of the season, data on cumulative tiller elongation (net above-ground height production) showed that plants grazed once or three times produced new foliage after each defoliation in both species. However, neither grazing (presence or absence) nor its frequency affected the net above-ground primary production (NAPP) or the number of tillers at the end of the summer. Nitrogen concentration was highest in plants grazed three times, intermediate in those grazed once, and lowest in ungrazed plants. 4 Timing of grazing and presence of goose faeces with or without grazing had no effect on plant growth. 5 Eriophorum plants grazed three times had less TNC in their below-ground tissues than ungrazed plants, and the trend was similar in Dupontia. 6 Dupontia and Eriophorum were able to compensate for leaves lost to grazing and to maintain production at a level similar to ungrazed plants, but at some cost (reduced below-ground reserves). The absence of an effect of faeces on plant growth may explain the absence of a positive effect of grazing on NAPP (i.e. overcompensation) in this ecosystem.
Article
Kyllinga nervosa (Cyperaceae) and Digitaria macroblephara (Poaceae) were obtained from the Serengeti National Park of Tanzania. Kyllinga is most abundant on high pH, high Ca, low P, carbonatic ash-derived soils; Digitaria is more abundant on neutral, lower Ca, higher P soils. Both species come from intensely grazed grasslands. Clipping and P concentration of the growth medium both had major effects upon yield of all plant components. Total yield of Kyllinga was highest in unclipped plants at high levels of P supply; yields under all other conditions were lower and indistinguishable from one another. Yield of Digitaria, collected in areas with soils of higher P availability, was unaffected by defoliation but was almost 21/2 times greater when plants were grown at the higher P level. Root growth of Digitaria was stimulated by defoliation when P was abundant. Maximum yield to grazers from Kyllinga would be achieved if grazers foraged only at the end of the period; maximum yield of Digitaria to grazers would be achieved if grazers defoliated plants at frequent intervals. A variety of morphological changes associated with maintaining similar laef areas whether clipped or unclipped were associated with Digitaria's ability to compensate for simulated grazing. Yield to producers (the residual live biomass present at harvest) and yield to grazers were positively correlated, ie conditions promoting plant yield caused a proportional increase in yield to higher trophic levels.-from Authors
Article
The horizontal and vertical distribution of plant biomass was examined on shortgrass steppe communities in N-central Colorado that were heavily grazed or protected from grazing for 47 yr. Uplands and swales were sampled along the gently rolling topography. Long-term grazing had no effect on total biomass of surface crowns and only small effects on total biomass of roots down to 20 cm depth. The effect of grazing on the vertical distribution of crown and root biomass was also smaller than the difference between topographical positions. Grazing had a large influence on the horizontal distributions of all vertical components of the plant community of producing smoother more uniform horizontal distributions, most evident for the more heavily grazed swale communities. The grazing-lawn concept is extended to the belowground plant community and discussed in terms of possible herbivore mediated plant-plant interactions rather than as an aboveground grazing avoidance mechanism. -from Authors
Article
In tallgrass prairie, belowground processes are even more important than in forested systems because aboveground biomass and standing dead litter are periodically removed by frequent fires or grazers. Thus, studies that address factors regulating belowground processes are especially relevant for tallgrass prairie. We predicted that effects of grazing and burning differ belowground and that changes in root productivity caused by burning or grazing provide feedback that affects ecosystem fluxes of C and N. These differences in belowground response should be driven largely by changes in N dynamics and the degree to which burning and grazing affect the pathway and magnitude of N loss and the degree of N limitation in these systems. Fire, the major pathway of N loss in ungrazed tallgrass prairie, should result in reduced net N mineralization and N availability. We expected plants to compensate for increased N limitation by increasing their allocation to roots, as manifested in increased soil respiration and C cycling belowground. In contrast, grazing conserves N in the ecosystem by redistributing the N once contained in grass to labile forms in urine and durig. Thus, we predicted that grazing should increase N cycling rates and N availability to plants. Consequently, grazed plants should be less N limited and should allocate less C to roots and more to shoots. This, in turn, should decrease belowground C cycling, manifested as reduced soil CO2 flux. We explored the roles of grazing and burning on root growth in experimental watersheds at Konza Prairie, Kansas, USA. To assess effects of fire on root productivity, we installed root ingrowth cores in two watersheds without grazers that differ in fire frequency: annually vs. infrequently burned (four years since the last fire). To assess effects of grazing, we installed root ingrowth cores in an annually burned watershed grazed by bison and in fenced controls (exclosures). Within bison "grazing lawns," root ingrowth cores were installed in lightly and heavily grazed patches. Concurrently, we measured in situ rates of net N mineralization and soil respiration as indices of soil N and C cycling. Annual burning resulted in a 25% increase in root growth compared to the unburned watershed (four years since last fire), as plants compensated for N limitation by increasing allocation to roots. Grazing had the opposite effect: it decreased root growth, especially in heavily grazed patches (∼30% less than in fenced controls). Grazing by ungulates increased N cycling and availability. Therefore, grazed plants, instead of being N limited, experienced C limitation as shoots regrew and plants allocated less C to roots. Interestingly, root ingrowth on the long-term unburned watershed was as low as in lightly grazed patches in the grazed watershed. Thus, seemingly disparate treatments such as infrequent burning (characterized by accumulation of detritus aboveground) and grazing (periodic biomass removal) both had higher levels of N availability than annually burned prairie in the absence of grazers. Root growth in unburned and grazed watersheds must be limited by resources other than N (e.g., C in grazing lawns or light in infrequently burned prairie). Burning and grazing also altered root tissue chemistry in contrasting ways that further accentuated the root growth differences caused by these treatments. Frequent fires lowered substrate quality of roots (C:N = 60), thus increasing N limitation. In contrast, grazing and infrequent burning improved root tissue quality (C:N = 40), promoting faster cycling of N. These large differences in root growth and tissue chemistry can result in profound ecosystem-level changes. Grazing increased net N mineralization rates from 87% to 617% compared to watersheds without grazers, whereas annual burning decreased it by ∼50% compared to unburned prairie. Although grazing speeded up N cycling, it reduced soil respiration by 50% compared to fenced controls, presumably because of reduced root mass. On the other hand, annual burning increased soil respiration, presumably because of increased root biomass. Ultimately, differences in the quantity and quality of roots provide feedback to affect C and N cycling and help to maintain and even promote the fundamental differences in N cycling between burning and grazing in tallgrass prairie.
Article
The effect of herbivory on grassland whole-plant production is poorly understood. Herbivores can increase grassland aboveground productivity, and laboratory experiments suggest that herbivory should reduce grass root growth. However, few field studies have directly measured the response of grassland root production to herbivores. We examined the effect of native migratory ungulates on grassland primary production by comparing aboveground (NAP) and belowground (NBP) production in grazed vs. ungrazed (fenced) grassland at nine variable sites in Yellowstone National Park. NBP was determined with minirhizotrons to account for root turnover. Grazers stimulated aboveground, belowground, and whole-grassland productivity by 21%, 35%, and 32%, respectively. Root production was stimulated seven times more (217 g/m(2)) than shoot production (30 g/m(2)), indicating that the major effect of herbivores in this system was a positive feedback on root growth. Results refute the prevailing notion that grassland herbivory leads to a reduction in root productivity, and a concomitant decline in soil carbon content, and provide a potential explanation for how organic-rich soil developed in grassland that was grazed throughout millennia.
Article
Traits that enable plants to exploit low-resource environments (eg slow tissue turnover, low transpiration rate, high root:shoot ratio, and high concentrations of plant defences against pathogens and herbivores) are physiological linked to key growth-related traits (low rates of photosynthesis, nutrient uptake, and growth). A genetic change in a switch or underlying trait that turns on this stress resistance syndrome (SRS), which causes it to be expressed over a wider range of environmental circumstances, would effectively convert a high-resource genotype into one that is more stress-tolerant. Because of physiological linkages between growth-related traits and the SRS, any heritable change in a key growth-related trait will pleiotropically affect the SRS. Therefore, heritable changes in these key growth-related traits could be accompanied by evolution of the entire SRS. Evidence for this hypothesis comes from single-gene mutants that differ in many stress-related traits, rapid evolution of metal-tolerant populations that are broadly stress-resistant, and consistent patterns of traits in species along gradient in resource availability. Rapid evolution in response to changing environmental stress may allow many short-lived species to respond to human-induced environmental change and provide opportunities to develop stress-resistance crops. However, the time lag between generations of long-lived species that dominate most natural vegetation may not allow mature individuals of these species to keep pace with rapid global change. -from Authors
Article
A number of parallels are shown between economic theory and plant resource usage. An outline of economic theory leads to 5 predictions concerning plant processes. Resource acquisition, storage, growth, resource loss and sexual reproduction are considered in the light of these predictions. Plants adjust phenology and life history patterns to acquire resources when they are cheap, store these internally and utilise them when conditions are favourable for growth. Plants continue to produce leaves (and perhaps roots) only until the marginal revenue from this increased production is equal to the marginal cost. Plants adjust allocation so that their limitation of growth is more nearly equal for all resources. Plants adjust physiologically to changes in resource availability to reduce extreme exchange ratios; the balance of internal reserves within the plant thereby approaches the proportions that are optimal for growth of most plants.-P.J.Jarvis
Article
The distribution of length, diameter, surface area and volume of roots was measured in northern New South Wales, Australia, under temperate pasture that had been previously grazed at low and high stocking rates for 30 years; these root characteristics were compared with those of roots under ungrazed pasture. The ungrazed pasture was dominated by Phalaris (Phalaris Aquatica), whereas annual grasses and dicotyledons were a large component of the pasture at low and high stocking rates. A fine-meshed (0.250 mm) sieve was used for separating the roots from the soil, and the root characteristics were measured using image analysis techniques. With this sieve size, root length densities were many times higher than published data for astures where a larger mesh sieve had been used for sample preparation. The lengths of root per unit of soil volume (root length densities) were high for all stocking rates and averaged 91 cm cm−3 near the soil surface (0 – 5 cm) declining to 4.0 cm cm−3 at the deepest depth measured (65–75 cm). There was a greater proportion of roots near the soil surface at the higher stocking rates. A greater proportion of fine roots occurred at the higher stocking rates, which was probably due to the differences in botanical composition. Reciprocal, power and logarithmic functions best described the distribution of root length density, root surface area density and root volume density, root surface area density and root volume desnity with depth.
Article
Several investigators claim that removal of or damage to the productive, absorptive, or reproductive tissue of plants by herbiores benefits some plant species by increasing their net primary productivity, seed production, or longevity, and that these changes increase plant fitness and result in the evolution of herbivore-plant mutualisms. Although >40 papers have been cited as presenting experimental evidence in support of these benefits and mutualisms, strong evidence is lacking. Increased plant biomass as a result of tissue removal has been found only under growth-chamber conditions and in cultivated crops. Although herbivores may benefit certain plants by reducing competition or removing senescent tissue no convincing evidence supports the theory that herbivory benefits grazed plants.-from Authors
Article
1 In water-limited environments, the availability of water and nutrients to plants depends on environmental conditions, sizes and shapes of their root systems, and root competition. The goal of this study was to predict root system sizes and shapes for different plant growth forms using data on above-ground plant sizes, climate and soil texture. 2 A new data set of > 1300 records of root system sizes for individual plants was collected from the literature for deserts, scrublands, grasslands and savannas with ≤ 1000 mm mean annual precipitation (MAP). Maximum rooting depths, maximum lateral root spreads and their ratios were measured. 3 Root system sizes differed among growth forms and increased with above-ground size: annuals < perennial forbs = grasses < semi-shrubs < shrubs < trees. Stem succulents were as shallowly rooted as annuals but had lateral root spreads similar to shrubs. 4 Absolute rooting depths increased with MAP in all growth forms except shrubs and trees, but were not strongly related to potential evapotranspiration (PET). Except in trees, root systems tended to be shallower and wider in dry and hot climates and deeper and narrower in cold and wet climates. Shrubs were more shallowly rooted under climates with summer than winter precipitation regimes. 5 Relative to above-ground plant sizes, root system sizes decreased with increasing PET for all growth forms, but decreased with increasing MAP only for herbaceous plants. Thus relative rooting depths tended to increase with aridity, although absolute rooting depths decreased with aridity. 6 Using an independent data set of 20 test locations, rooting depths were predicted from MAP using regression models for three broad growth forms. The models suc-ceeded in explaining 62% of the observed variance in median rooting depths. 7 Based on the data analysed here, Walter's two-layer model of soil depth partitioning between woody and herbaceous plants appears to be most appropriate in drier regimes (< 500 mm MAP) and in systems with substantial winter precipitation.
Article
Summary • The interactive effect of grazing and small-scale variation in primary productivity on the diversity of an annual plant community was studied in a semiarid Mediterranean rangeland in Israel over 4 years. The response of the community to protection from sheep grazing by fenced exclosures was compared in four neighbouring topographic sites (south- and north-facing slopes, hilltop and wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. The herbaceous annual vegetation was highly diverse, including 128 species. Average small-scale species richness of annuals ranged between 5 and 16 species within a 20 × 20 cm quadrat, and was strongly affected by year and site. • Above-ground potential productivity at peak season (i.e. in fenced subplots) was typical of semiarid ecosystems (10–200 g m−2), except on wadi shoulders (up to 700 g m−2), where it reached the range of subhumid grassland ecosystems. Grazing increased richness in the high productivity site (i.e. wadi), but did not affect, or reduced, it in the low productivity sites (south- and north-facing slopes, hilltop). Under grazing, species richness was positively and linearly related to potential productivity along the whole range of productivity. Without grazing, this relationship was observed only at low productivity (−2). • The effect of grazing along the productivity gradient on different components of richness was analysed. At low productivity, number of abundant, common and rare species all tended to increase with productivity, both with and without grazing. Rare species increased three times compared with common and abundant species. At high productivity, only rare species continued to increase with productivity under grazing, while in the absence of grazing species number in the different abundance groups was not related to productivity. • In this semiarid Mediterranean rangeland, diversity of the annual plant community is determined by the interaction between grazing and small-scale spatial and temporal variation in primary productivity, operating mainly on the less abundant species in the community.
Article
1. Prosopis velutina Woot. (Velvet Mesquite) at a site with limited groundwater availability derived a greater percentage of water from shallow soil at the onset of the summer rainy season than did trees at a site with greater availability of groundwater. Predawn leaf water potentials (Ypd) were not a strong indicator of shallow water use for this species with roots in multiple soil layers. 2. We experimentally defoliated P. velutina plants to determine if reduced-canopy photosynthesis would alter vertical patterns of root activity. After natural rain events, hydrogen isotope ratios of xylem sap indicated that defoliated P. velutina took up a greater percentage of its water from shallow soils than did undefoliated plants. 3. Irrigation with deuterium-labelled water further demonstrated that undefoliated plants were able to use shallow soil water. Defoliation appeared to affect the ability of trees to use deep-water sources. 4. Reduced carbon assimilation limited water uptake from deep soil layers. These data highlight that there are internal physiological controls on carbon allocation that may limit water uptake from different soil layers. During periods of high vapour pressure deficit or soil drought, when leaf gas exchange and carbon assimilation decline, this may create positive feedbacks where plants are unable to forage for deep water due to carbon limitations.
Article
40 sites, representing different pasture types in Northwest Spain, were sampled in respect of their floristic composition, distribution of above and below-ground biomass and environmental and physical variables. Five plant community types were identified by classification techniques of plant species composition. These communities were then characterized in terms of the percentage of ground covered by herbaceous and shrub vegetation, stones, rocks and gaps as well as their topographic location and characteristics of the shallow soil (pH, organic matter, nitrogen and calcium content). Bio-mass was assessed in terms of above-ground structures, surface crowns and three below-ground layers to a depth of 10 cm. Three types of grazing regime were distinguished: Concentrated Intense Grazing in early spring (CIG), Extended Intense Grazing throughout the spring (EIG), and Non-Intense Grazing (NIG). Grazing regime showed the highest association with plant community type and three broad categories were identified: xeric stressed pastures, which nevertheless received CIG, mesic pastures with EIG, and three kinds of NIG mesic pastures.The xeric communities had the highest proportion of aboveground biomass, as a consequence of their greater proportion of woody perennials. These xeric communities displayed a more gradual reduction in below-ground biomass with depth than mesic pastures, a likely consequence of the low water content in the upper soil layers. The mesic communities had a high concentration of below-ground biomass in the upper layers when they were intensely grazed. However, when grazing was low (i.e. NIG situations), these communities had greater variability in biomass profiles than any of the other pasture types. Possible causes of the patterns in biomass distribution of the intensely grazed pastures are discussed.
Article
Previous work has shown that below-ground biomass is more concentrated in surface soil layers in intensively grazed mesic grasslands than in moderately grazed grasslands. However, since the mesic grasslands previously studied shared similar compositional traits, the question remained whether grasslands with differing species composition, and intensive defoliation, showed similar biomass distribution patterns. Eight grasslands at four sites distributed along an elevational gradient were investigated. The upper and lower zones of a slope were sampled at each site. Four of these grasslands were grazed by livestock and the other four were grazed and mown. Biomass was divided into above-ground, root crown and three root layers.Species composition varied according to management and topography. Annuals and perennial forbs had relatively more above-ground biomass at the upper part of the slopes, while perennial grasses dominated the lower parts. The above-ground biomass and root biomass at 4 — 7 cm depth attained maximum values in the lower, potentially more fertile, parts of the slopes. Crown biomass increased with altitude at the upper part of the slopes. Despite their differences in composition and structure, seven out of the eight stands showed a remarkable concentration of the below-ground biomass near the soil surface, which decreased drastically with soil depth. This pattern is similar to that observed in the intensively grazed mesic communities studied earlier. This similarity was more evident in the more mesic-like grasslands, since it increased from the upper, potentially drier parts of the slopes, to the lower parts, and, when each topographic position was considered separately, from low to high elevation.
Article
The diversity of responses of individual grasses to defoliation created a controversy about 15 years ago, which still needs clarification. We quantitatively assessed the evidence of defoliation effects on individual grass growth, addressing two main questions: 1) what is the average and variability of the effect of defoliation on plant growth? and 2) what are the associated conditions accounting for the diversity of effects? Regarding the first question, the results showed a negative overall effect of defoliation on plant growth and substantial variability in the defoliation responses of different plant components. There was an intermediate negative effect on total production (which included clipped-off biomass), a large negative effect on final live biomass at harvest, and a minimal effect on root biomass. Regarding the second question (conditions accounting for the diversity of effects), defoliation intensity had no effect on the response to defoliation, but both time for recovery from the last defoliation and the period of time between defoliation events significantly decreased the negative effect of defoliation. Nitrogen availability also altered the effect of defoliation, as plants grown at highest nitrogen levels were more negatively affected by clipping than plants with no supplementary addition of nitrogen. These results indicate that the magnitude of defoliation response by an individual plant differs among plant compartments and this response is modulated by other factors, such as time for recovery after defoliation, and nutrient availability. In general, the effect of defoliation on individual plant production was more negative than reported effects of grazing on ecosystem primary production.
Article
Both theoretical arguments and empirical evidence suggests that herbivory in general and mammalian winter herbivory in particular is important in arctic–alpine ecosystems. Although knowledge of the effect of herbivores on specific plants and communities is quite extensive, little is known about the relative impact of large and small vertebrate herbivores and how it might vary among different habitats. To address this key issue, we established exclosures with two different mesh sizes in forest and nearby tundra at three different sites in four contrasting locations in the forest–tundra ecotone in northernmost Sweden and Norway. Plant community composition was recorded annually in three permanent plots within each exclosure and an unfenced control. Local densities of vertebrate herbivores were estimated in spring and autumn from 1998 to 2002.
Article
1. Many ecological models of plant growth assume balanced growth: that biomass is allocated preferentially to leaves or roots to increase capture of the limiting external resource. An alternative explanation is based on nonlinear (allometric) allocation as a function of plant size. The objective of this study was to test between these two alternative explanations. 2. A total of 1150 plants from 22 different herbaceous species were grown in hydroponic sand culture in factorial combinations of high (1100 µmol m−2 s−1) and low (200 µmol m−2 s−1 PAR) irradiance crossed with a full-strength and a 1/6 dilution of Hoagland’s hydroponic solution. Plants were harvested at 15, 20, 25, 30 and 35 days postgermination, and dry mass was determined for leaf and root components. These data were used to test the hypotheses of balanced growth and of allometric allocation. 3. Both irradiance and nutrient supply affected the slope and intercept of the root : shoot allometry, contrary to the allometric hypothesis but in agreement with the hypothesis of balanced growth; decreased nutrient supply increased allocation to roots; and decreased irradiance increased allocation to leaves. 4. Plants allocated relatively more biomass to roots than to leaves as plants grew larger. In order for the balanced-growth hypothesis to be correct, the net rate of nutrient uptake per unit root mass must have been decreasing relative to the net rate of carbon gain per unit leaf mass. 5. We suggest two reasons why this might be the case: (i) older roots decreased their efficiency of nutrient uptake; and (ii) larger root systems more rapidly decreased the available nutrients between flushes of hydroponic solution. 6. These results support the notion of balanced growth that is found in many ecological models of plant growth.
Article
Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient to the data for each biome (Gale and Grigal 1987). is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-d, where d = depth and Y = the proportion of roots from the surface to depth d. High values of correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (=0.913, 0.943, and 0.943, respectively), with 80–90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (=0.952), while shrubs had only 21% in the same depth increment (=0.978). The rooting distribution of all temperate and tropical trees was =0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was =0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.
Article
Kyllinga nervosa Steud., a sedge from the Serengeti short-grass plains, was subjected to a balanced factorial experiment which included unclipped plants and plants clipped weekly to a 5 cm height, nitrogen supplied as either nitrate or ammonium and three nitrogen concentrations. Tillering rates, green leaf nitrogen, and both green leaf weight and biomass investment in green leaf production increased with nitrogen concentration. Low nitrogen conserved investment in crown production and resulted in adjustments for nitrogen acquisition by increasing biomass allocation to root production. Nitrate nutrition stimulated green leaf weight, tillering rate, nitrogen redistribution and both crown and root nitrogen. Ammonium nutrition increased nitrogen uptake, total plant nitrogen accumulation, reproduction, litter weight and nitrogen loss to decomposers. Clipping increased investment in green leaf production at the expense of stem, root, crown and flower production. Compensatory green leaf production in response to clipping occurred only when plants were grown in ammonium. Clipping stimulated uptake rates of both ammonium and nitrate, and therefore total plant nitrogen accumulation. Results suggest a balanced utilization of both nitrate and ammonium may be necessary for optimal growth in this species.
Article
We conducted a grazing experiment from 1992 to 1996 in Inner Mongolia to explore desertification processes of sandy rangeland. The results show that continuous heavy grazing results in a considerable decrease in vegetation cover, height, standing biomass and root biomass, and a significant increase in animal hoof impacts. As a result, small bare spots appeared on the ground and later merged into larger bare areas in the rangeland. Total bare area reached up to 52% and the average depth of wind erosion was 25 cm in the fifth year of the study. We conclude that sandy rangeland with wind-erodible soil is susceptible to desertification. Heavy grazing of such rangeland should be avoided.
Article
Knowledge about the factors determining habitat use is especially interesting for herbivores living under seasonal climates as they have to deal with food shortage during the drought season. In this context, different-aged individuals are expected to respond differently to seasonal variations because nutritional requirements and predation risk can vary with age. We investigated adult and juvenile European rabbit (Oryctolagus cuniculus) habitat use in a Mediterranean ecosystem of central Spain, during spring, summer and winter. Relationships between adult and juvenile rabbit pellet abundances and 11 environmental variables related to food availability and refuge density were analysed by means of multiple regression, and evaluated using information theory to identify the set of models best supported by the data. Density of warren entrances was the more constant predictor of habitat use for juvenile rabbits in all the seasons. Herbaceous vegetation volume had a negative influence and was the strongest predictor for adult rabbit habitat use in spring and winter. In summer, green vegetation cover became the strongest positive habitat use predictor. These results suggest that adults prefer to forage in low volume swards ensuring a wide sensory range for the detection of approaching predators. However, the arrival of summer and its associated food depletion forces them to shift toward more open productive areas where green vegetation persists, but at the expense of higher predation risk. Seasonal variation induces minor changes in juvenile habitat use due to their strong dependence on warrens. Thus, our results show that rabbit habitat use is influenced by animal age and seasonal variations in resources.
Article
Does grazing by large wild mammals, an intense form of aboveground herbivory, influence belowground productivity? The vast majority of literature data concentrate on short-term pot studies and indicate that clipping consistently retards root growth. Field studies are few and contradictory, but tend to indicate that grazing has little effect on grassland belowground production. We sampled root-soil cores at 0-10 and 10-20 cm increments, at 11 locations across the Serengeti ecosystem, on 10 dates over an annual cycle, sampling monthly during the rainy and early dry seasons and every 2 mo during peak dry season. Fenced and unfenced plots were replicated (n = 2 or 3) at each location. Live roots, identified visually by brightness and texture, were sorted, washed, dried, and weighed. In addition, profiles were sampled at 10-cm increments to 50 cm in fenced and unfenced plots in short, mid-height, and tall grasslands, representing a gradient of grazing, during the month of peak root biomass. Exclosures erected 22-25 yr previously were similarly sampled in short and tall grasslands to a 30-cm depth. Root biomass reached a pronounced minimum in mid-wet season (February) and a decided maximum at the beginning of the dry season (June). Net productivity, based on maximum-minimum biomass, ranged from 100 to 600 g.m(-2).yr(-1) to a 20-cm depth, with minima ranging from 40 to 150 g/m(2) and maxima from 230 to 700 g/m(2), according to location. There was no evidence that grazing reduced root productivity over the annual cycle. Vertical biomass profiles at peak standing crop were similar for short, mid-height, and tall grasslands, with root biomass dropping sharply with depth, except for short grasslands on soils that, atypically, lack a hardpan. In those grasslands, shallow root biomass was lower than in other grasslands, but biomass at depth was distinctly greater. For longterm protected grasslands, root biomasses at peak were identical in short grasslands, whether fenced or unfenced, but fenced tall grasslands had a lower root biomass than grazed plots. We conclude that intense herbivory does not inhibit root biomass or belowground productivity of Serengeti grasslands over either the short or the long term.
Article
The depth at which plants are able to grow roots has important implications for the whole ecosystem hydrological balance, as well as for carbon and nutrient cycling. Here we summarize what we know about the maximum rooting depth of species belonging to the major terrestrial biomes. We found 290 observations of maximum rooting depth in the literature which covered 253 woody and herbaceous species. Maximum rooting depth ranged from 0.3 m for some tundra species to 68 m for Boscia albitrunca in the central Kalahari; 194 species had roots at least 2 m deep, 50 species had roots at a depth of 5 m or more, and 22 species had roots as deep as 10 m or more. The average for the globe was 4.6±0.5 m. Maximum rooting depth by biome was 2.0±0.3 m for boreal forest. 2.1±0.2 m for cropland, 9.5±2.4 m for desert, 5.2±0.8 m for sclerophyllous shrubland and forest, 3.9±0.4 m for temperate coniferous forest, 2.9±0.2 m for temperate deciduous forest, 2.6±0.2 m for temperate grassland, 3.7±0.5 m for tropical deciduous forest, 7.3±2.8 m for tropical evergreen forest, 15.0±5.4 m for tropical grassland/savanna, and 0.5±0.1 m for tundra. Grouping all the species across biomes (except croplands) by three basic functional groups: trees, shrubs, and herbaceous plants, the maximum rooting depth was 7.0±1.2 m for trees, 5.1±0.8 m for shrubs, and 2.6±0.1 m for herbaceous plants. These data show that deep root habits are quite common in woody and herbaceous species across most of the terrestrial biomes, far deeper than the traditional view has held up to now. This finding has important implications for a better understanding of ecosystem function and its application in developing ecosystem models.
Article
Large herbivores can influence plant and soil properties in grassland ecosystems, but especially for belowground biota and processes, the mechanisms that explain these effects are not fully understood. Here, we examine the capability of three grazing mechanisms-plant defoliation, dung and urine return, and physical presence of animals (causing trampling and excreta return in patches)-to explain grazing effects in Phleum pratense-Festuca pratensis dairy cow pasture in Finland. Comparison of control plots and plots grazed by cows showed that grazing maintained original plant-community structure, decreased shoot mass and root N and P concentrations, increased shoot N and P concentrations, and had an inconsistent effect on root mass. Among soil fauna, grazing increased the abundance of fungivorous nematodes and Aporrectodea earthworms and decreased the abundance of detritivorous enchytraeids and Lumbricus earthworms. Grazing also increased soil density and pH but did not affect average soil inorganic-N concentration. To reveal the mechanisms behind these effects, we analyzed results from mowed plots and plots that were both mowed and treated with a dung and urine mixture. This comparison revealed that grazing effects on plant attributes were almost entirely explained by defoliation, with only one partly explained by excreta return. Among belowground attributes, however, the mechanisms were more mixed, with effects explained by defoliation, patchy excreta return, and cow trampling. Average soil inorganic-N concentration was not affected by grazing because it was simultaneously decreased by defoliation and increased by cow presence. Presence of cows created great spatial heterogeneity in soil N availability and abundance of fungivorous nematodes. A greenhouse trial revealed a grazing-induced soil feedback on plant growth, which was explained by patchiness in N availability rather than changes in soil biota. Our results show that grazing effects on plant attributes can be satisfactorily predicted using the effects of defoliation, whereas those on soil fauna and soil N availability need understanding of other mechanisms as well. The results indicate that defoliation-induced changes in plant ecophysiology and the great spatial variation in N availability created by grazers are the two key mechanisms through which large herbivores can control grassland ecosystems. Large herbivores can influence plant and soil properties in grassland ecosystems, but especially for belowground biota and processes, the mechanisms that explain these effects are not fully understood. Here, we examine the capability of three grazing mechanisms-plant defoliation, dung and urine return, and physical presence of animals (causing trampling and excreta return in patches)-to explain grazing effects in Phleum pratense-Festuca pratensis dairy cow pasture in Finland. Comparison of control plots and plots grazed by cows showed that grazing maintained original plant-community structure, decreased shoot mass and root N and P concentrations, increased shoot N and P concentrations, and had an inconsistent effect on root mass. Among soil fauna, grazing increased the abundance of fungivorous nematodes and Aporrectodea earthworms and decreased the abundance of detritivorous enchytraeids and Lumbricus earthworms. Grazing also increased soil density and pH but did not affect average soil inorganic-N concentration. To reveal the mechanisms behind these effects, we analyzed results from mowed plots and plots that were both mowed and treated with a dung and urine mixture. This comparison revealed that grazing effects on plant attributes were almost entirely explained by defoliation, with only one partly explained by excreta return. Among belowground attributes, however, the mechanisms were more mixed, with effects explained by defoliation, patchy excreta return, and cow trampling. Average soil inorganic-N concentration was not affected by grazing because it was simultaneously decreased by defoliation and increased by cow presence. Presence of cows created great spatial heterogeneity in soil N availability and abundance of fungivorous nematodes. A greenhouse trial revealed a grazing-induced soil feedback on plant growth, which was explained by patchiness in N availability rather than changes in soil biota. Our results show that grazing effects on plant attributes can be satisfactorily predicted using the effects of defoliation, whereas those on soil fauna and soil N availability need understanding of other mechanisms as well. The results indicate that defoliation-induced changes in plant ecophysiology and the great spatial variation in N availability created by grazers are the two key mechanisms through which large herbivores can control grassland ecosystems. Large herbivores can influence plant and soil properties in grassland ecosystems, but especially for belowground biota and processes, the mechanisms that explain these effects are not fully understood. Here, we examine the capability of three grazing mechanisms-plant defoliation, dung and urine return, and physical presence of animals (causing trampling and excreta return in patches)-to explain grazing effects in Phleum pratense-Festuca pratensis dairy cow pasture in Finland. Comparison of control plots and plots grazed by cows showed that grazing maintained original plant-community structure, decreased shoot mass and root N and P concentrations, increased shoot N and P concentrations, and had an inconsistent effect on root mass. Among soil fauna, grazing increased the abundance of fungivorous nematodes and Aporrectodea earthworms and decreased the abundance of detritivorous enchytraeids and Lumbricus earthworms. Grazing also increased soil density and pH but did not affect average soil inorganic-N concentration. To reveal the mechanisms behind these effects, we analyzed results from mowed plots and plots that were both mowed and treated with a dung and urine mixture. This comparison revealed that grazing effects on plant attributes were almost entirely explained by defoliation, with only one partly explained by excreta return. Among belowground attributes, however, the mechanisms were more mixed, with effects explained by defoliation, patchy excreta return, and cow trampling. Average soil inorganic-N concentration was not affected by grazing because it was simultaneously decreased by defoliation and increased by cow presence. Presence of cows created great spatial heterogeneity in soil N availability and abundance of fungivorous nematodes. A greenhouse trial revealed a grazing-induced soil feedback on plant growth, which was explained by patchiness in N availability rather than changes in soil biota. Our results show that grazing effects on plant attributes can be satisfactorily predicted using the effects of defoliation, whereas those on soil fauna and soil N availability need understanding of other mechanisms as well. The results indicate that defoliation-induced changes in plant ecophysiology and the great spatial variation in N availability created by grazers are the two key mechanisms through which large herbivores can control grassland ecosystems.
Article
A previously developed dynamic model, NICOLET, designed to predict growth and nitrate content of a lettuce crop, is subjected to (virtual) constant environmental conditions. For every combination of shoot and root environment, the cell sap, here assumed to reside in the "vacuole" compartment, equilibrates at a certain nitrate concentration level. This, in turn, defines the composition of the crop in terms of carbon and nitrogen content in each of the three compartments of the model. Growth under constant environmental conditions is defined as "equilibrium" growth (EG). If, in addition, the source strengths of carbon and nitrogen balance each other, as well as the sink strength of the growing crop, the growth is said to be "balanced" (BG). It is shown that the range of BG approximately coincides with the range of "mild" nitrogen stress, where reduction in nitrogen availability results in a mild reduction of relative growth rate (RGR). Beyond a certain low nitrate concentration in the cell sap, the N-stress becomes "severe" and the loss of growth increases considerably. The model is able to mimic the five central observations of many constant-environment growth-chamber experiments, namely (1) the initial exponential growth and later decline of the RGR, (2) the constant chemical composition, (3) the equality of the RGR and the relative nutrient supply rate (RNR), (4) the proportionality between the N : C ratio and the RNR, and (5) the proportionality between the water content and the reduced N content. Guidelines for the optimal combination of the shoot and root environments are suggested.
A global analysis of root distributions for terrestrial biomes The distribution of soil nutrients with depth: global patterns and the imprint of plants
  • R B Jackson
  • J Canadell
  • J R Ehleringer
  • H A Mooney
  • O E Sala
  • E D Schulze
  • E G Jobbágy
  • R B Jackson
Jackson, R.B., Canadell, J., Ehleringer, J.R., Mooney, H.A., Sala, O.E., Schulze, E.D., 1996. A global analysis of root distributions for terrestrial biomes. Oecologia 108, 389e411. Jobbágy, E.G., Jackson, R.B., 2001. The distribution of soil nutrients with depth: global patterns and the imprint of plants. Biogeochemistry 53, 51e77.
Plant Strategies and the Dynamics and Structures of Plant Communities Above-ground and below-ground biomass relations in steppes under different grazing conditions
  • D Tilman
Tilman, D., 1988. Plant Strategies and the Dynamics and Structures of Plant Communities. Princeton University Press, Princeton. Van der Maarel, E., Titlyanova, A.A., 1989. Above-ground and below-ground biomass relations in steppes under different grazing conditions. Oikos 56, 364e370.
Maximum rooting depths of vegetation types at the global scale Plant responses to multiple environmental factors Evolution of suites of traits in response to environmental stress
  • J Canadell
  • R B Jackson
  • J R Ehleringer
  • H A Mooney
  • O E Sala
  • E D Schulze
Canadell, J., Jackson, R.B., Ehleringer, J.R., Mooney, H.A., Sala, O.E., Schulze, E.D., 1996. Maximum rooting depths of vegetation types at the global scale. Oecologia 108, 583e595. Chapin III, F.S., Bloom, A.J., Field, C.B., Waring, R.H., 1987. Plant responses to multiple environmental factors. Bioscience 371, 49e57. Chapin III, F.S., Autumn, K., Pugnaire, F., 1993. Evolution of suites of traits in response to environmental stress. Am. Nat. 142, S78eS92.