Belowground Competitive Ability Research Articles

Beneath it all: Size, not origin, predicts belowground competitive ability in exotic and native shrubs1,2

Abstract Traits associated with root morphology and nutrient uptake rate may contribute to the competitive ability of invasive species by determining their access to soil nutrients and their ability to extract those resources. Here, we tested the hypotheses that (a) exotic woody shrubs would be superior belowground competitors for nitrogen in heterogeneous soil resulting from key aspects of root architecture and (b) larger plants would be superior belowground competitors. We tested these hypotheses using two native shrubs, Rubus allegheniensis and Viburnum dentatum, and two invasive exotic shrubs, Rubus phoenicolasius and Berberis thunbergii, all four of which can become abundant in plant communities in the eastern United States. We grew replicate plants from each species with interspecific competitors, with intraspecific competitors, and individually in a randomized layout in a greenhouse in two temporal blocks. Each experimental container had a central soil patch amended with 15N-labeled litter. We meas...

Contrasting root behaviour in two grass species: a test of functionality in dynamic heterogeneous conditions

Root systems are highly plastic as they express a range of responses to acquire patchily distributed nutrients. However, the ecological significance of placing roots selectively in nutrient hotspots is still unclear. Here, we investigate under what conditions selective root placement may be a significant functional trait that determines belowground competitive ability. We studied two grasses differing in root foraging behaviour, Festuca rubra and Anthoxanthum odoratum. The plants were grown in stable and more dynamic heterogeneous environments, by switching nutrient patches halfway through the experiment. A. odoratum was a factor of two less selective in placing its roots into nutrient-rich patches than F. rubra. A. odoratum produced overall higher root length densities with higher specific root length than F. rubra and acquired more nutrients. A. odoratum appeared to be the superior competitor, irrespective of the nutrient dynamics. Our results suggest that root behaviour consisting of producing high root length densities at relatively low biomass investments can be a more effective foraging strategy than placing roots selectively in nutrient hotspots. When understanding the functionality of root traits among different species, specific root length may play a key role.

Seasonal Root Biomass and Distribution of Switchgrass and Milk Vetch Intercropping under 2:1 Row Replacement in a Semiarid Region in Northwest China

A root system has the plasticity to adapt to environmental change and species interaction. Root biomass and distribution were studied in three comparable stands in the fifth growth year (2005) in a semiarid region of northwest China: (1) pure switchgrass, (2) pure milk vetch, and (3) mixed switchgrass and milk vetch in 2:1 row replacement. The measurements were carried out three times (April, September, and November) in 2005 during the vegetation period using a soil core method (9 cm). For each species, the vertical distribution of root biomass (RB) was measured in six consecutive layers (0–20, 20–40, 40–60, 60–90, 90–120, and 120–150 cm deep) between rows, between plants, and at the center of the plant, respectively. Results indicated a flexible distribution strategy of switchgrass that tended to increase soil exploitation and space sequestration efficiency in soil layers. Milk vetch followed a more conservative strategy: its roots reached the same soil depth in the pure and mixed stands but showed greater root densities in shallower soil layers in the latter. Under intercropping, RB input and root/shoot ratio in switchgrass were reduced, while milk vetch put more photosynthate into root growth. The shift toward a more superficially distributed system of milk vetch in the mixture with switchgrass together with the high RB and wide vertical and horizontal distribution of switchgrass in the study indicates the greater belowground competitive ability of switchgrass in the mixture.

Soil water availability and plant competition affect the yield of spring wheat

This study was conducted to determine the effect of both inter-cultivar and intra-cultivar competition on the growth of three spring wheat ( Triticum aestivum L.) cultivars released in different periods that had different root:shoot ratios, differed in water extraction from dry soil, and differed in height. Two water regimes were imposed to compare competitive effects under irrigated and rainfed conditions. Our main hypotheses were that the different distribution of biomass between shoot and root in old and new wheat cultivars will alter their competitive ability, that differences in root size will alter their competitive ability through different water uptake patterns, and differences in the gradient for water uptake will alter their competitive ability in different environments by enabling water extraction from drier soil. In monoculture, the recent cultivars had significantly more grain yield and higher water use efficiency for grain (WUE G) than the old cultivar. Under the two water regimes the old cultivar had more root biomass, and extracted water in deeper soil layers, whereas the modern cultivars extracted more water in dry soil layers. The old cultivar benefited from inter-cultivar competition in terms of both grain yield and above-ground biomass accumulation at the expense of the modern cultivars, which showed significantly reduced growth in mixtures compared to in monoculture. Our study suggested that the below-ground competitive ability of cultivars may have a negative relationship with the grain yield and WUE G in monoculture. The yield superiority of modern and recent cultivars was mostly due to increased above-ground biomass, kernel number and WUE G and a smaller proportion of root biomass. Our results demonstrate that inter-plant competition is an important factor affecting spring wheat productivity in contrasting environments, but the extent and intensity of these effects depend on the adaptation of root traits to available soil water. Reducing root growth redundancy and enhancing the ability to deplete more soil water are clearly adaptive features for wheat for water-limited conditions.

Belowground competition in a broad-leaved temperate mixed forest: pattern analysis and experiments in a four-species stand

We investigated fine root biomass and distribution patterns in a species-rich temperate Carpinus–Quercus–Fagus–Tilia forest and searched for experimental evidence of symmetry or asymmetry in belowground competition. We conducted extensive root coring and applied the recently introduced in situ-root growth chamber technique for quantifying fine root growth under experimentally altered intra- and interspecific root neighbourhoods in the intact stand. In 75% of all soil cores, fine roots of more than two tree species were present indicating a broad overlap of the root systems of neighbouring trees. Quercus trees had more than ten times less fine root biomass in relation to aboveground biomass or productivity (stem growth) and a much higher leaf area index/root area index ratio than Carpinus, Fagus and Tilia trees. The root growth chamber experiments indicated a high belowground competitive ability of Fagus in interspecific interactions, but a low one of Quercus. We conclude that (1) interspecific root competition is ubiquitous in this mixed stand, (2) root competition between trees can be clearly asymmetric, and (3) tree species may be ranked according to their belowground competitive ability. Fagus was found to be the most successful species in belowground competition which matches with its superiority in aboveground competition in this forest community.

Research advances in plant competition in agroforestry systems

Plant competition plays an important role in the success and sustainability of agroforestry systems. Minimizing competi- tion and maximizing resource utilization increase crop yield and overall productivity of agroforestry systems. This paper introduces the concept and theory of plant competition, reviews competition for light, water and nutrient among trees and crops, and summarizes allelopathy and competition related plant traits in agroforestry systems. Aboveground competition for light among trees and crops is a major constraint under humid climate conditions, while belowground competition for water predominates in semi-arid tropics and temperate agroforestry systems. In agroforestry systems, plant competitive ability for light is decided by leaf area, canopy structure, et al. Root traits related to belowground competitive ability include occupied soil space, morphological and physical elasticity and spatial and temporal soil partitions. It is suggested that future research on agroforestry systems focus on comparisons among different components in different climatic conditions, competition processes and physiological mechanisms, plant allelopathy and the role and mechanism of mycorrhizae in plant competition, interactions between above- and below-ground competition and the effect of global climate change on plant competition of the agroforestry systems.

Evaluation of switchgrass and sainfoin intercropping under 2:1 row-replacement in semiarid region, northwest China

Field experiments were carried out under natural conditions to compare the aboveground biomass, root growth and distribution, and topsoil nutrition contents of switchgrass ( Panicum virgatum ) and sainfoin (Onobrychis viciaefolia ) grown in sole cropping and 2:1 row-replacement intercropping in semiarid loess region on Loess Plateau of northwest China. The sole and intercropping was compared based on the aboveground biomass, water use efficiency (WUE), soil organic matter (SOM) and total nitrogen (TN), and root biomass and distribution. The aboveground biomass production was measured every year at the end of growth seasons in 2001 - 2005. Root biomass and vertical distribution were studied only in 2005. Topsoil SOM and TN contents were measured discontinuously during the study period. Results showed that intercropping reduced the aboveground biomass production of sainfoin significantly compared with its pure stand. The WUE of sole switchgrass was significantly the highest in each year among the three stands, while the intercropped stand was significantly higher than sole sainfoin on five-year averaged. The aggressivity of sainfoin to swichgrass decreased along with the growth years. Before 2005, the land equivalent ratio (LER) was bigger than 1.0 and actual yield loss (AYL) was positive. Under intercropping, root biomass input and root: shoot ratio in switchgrass reduced, while sainfoin inputs more photosynthate to root growth. Switchgrass had high root biomass and wide distribution vertically and horizontally indicates a higher belowground competitive ability in the mixture. It indicated a flexible distribution strategy of switchgrass tending to increase soil exploitation and space sequestration efficiency in soil layers. SOM and TN increased significantly for the three stands at the end of the fourth growth year, especially for the mixture. Switchgrass and sainfoin intercropped under 2:1 row-replacement can be a short-term rotation tillage choice with respect to soil management in the area. Key words:

Kin recognition in an annual plant

Kin recognition is important in animal social systems. However, though plants often compete with kin, there has been as yet no direct evidence that plants recognize kin in competitive interactions. Here we show in the annual plant Cakile edentula, allocation to roots increased when groups of strangers shared a common pot, but not when groups of siblings shared a pot. Our results demonstrate that plants can discriminate kin in competitive interactions and indicate that the root interactions may provide the cue for kin recognition. Because greater root allocation is argued to increase below-ground competitive ability, the results are consistent with kin selection.

Belowground ecological interactions in mixed-species forest plantations

Belowground competitive and complementary interactions define productivity and sustainability of mixed-species plantations to a large extent, but have received relatively little attention in the scientific literature. Belowground competitive ability is correlated with such traits as root length density, surface area and root plasticity. Competition for light can be asymmetrical with larger plants disproportionately shading smaller ones, but competition for soil resources is more symmetric. Niche separation and reduced competition belowground are commonly observed as the mechanisms that lead to coexistence of species in mixed-species plantations. Chemical interference through allelopathy may play a role in controlling the productivity of some mixed-species plantations. This paper reviews some of the well-known ecological principles by drawing examples from both temperate and tropical mixed-species plantations. We conclude that our ability to design successful mixed-species systems is constrained by limited information on belowground competitive and complementary interactions. Future research priorities that will help us expand our knowledgebase are identified.

Root distribution of Norway spruce in monospecific and mixed stands on different soils

Vertical root distribution was studied in pure Norway spruce ( Picea abies (L.) Karst.) stands and in adjacent mixed stands of spruce and European beech ( Fagus sylvatica L.) with comparable site conditions. The investigation was carried out in two regions of Austria with different bedrock material. One site was located on nutrient rich, poor-aerated stagnic cambisol, the other on nutrient poor, well-aerated podsolic cambisol. A modified trench profile wall technique was used to study the coarse root distribution on altogether 160 m 2 of profile walls. The fine root biomass was estimated using soil core cylinders. Coarse roots were more abundant on nutrient poor than on nutrient rich site, showing the greatest cross-section (CS-)area in the humus and upper mineral soil layer in all of the four investigated stands. No significant difference in total root CS-area was found between pure and mixed stand in both regions. Coarse roots of Norway spruce were markedly stronger limited to the upper soil layers in mixture with beech than in pure spruce stands. This limitation to the upper soil was especially pronounced for the mixed stand on stagnic cambisol. The fine root biomass was greater in spruce stand on nutrient poor, well-aerated soil than on stagnic soil, whereas the mixed stands did not differ. The fine root system of spruce was markedly shallower in stagnic than in podsolic cambisol in pure stands and, compared to the basal area, spruce fine roots were under-represented in the mixed stands on both soil types. Like the coarse roots, fine roots of spruce showed a clear shift towards the upper soil layers in mixture with beech. This shift towards a more superficially distributed root system of spruce in mixture with beech together with the under-represented fine root biomass of spruce in these stands indicates a higher belowground competitive ability of beech compared to spruce on both nutrient rich and nutrient poor sites.

SOIL NUTRIENT HETEROGENEITY ALTERS COMPETITION BETWEEN TWO PERENNIAL GRASS SPECIES

Differences in root foraging behavior between species have been well documented, but the effects of these differences on belowground competitive ability are only beginning to be studied. Here we report the results of a competition experiment in homogeneous and heterogeneous soils between two species that differ in their ability to acquire nutrients from patchy environments. The perennial grasses Festuca rubra and Anthoxanthum odoratum have comparable growth rates, but results of previous studies with isolated plants designated the latter species as the more effective forager, probably due to its higher physiological plasticity (stronger increase in nutrient uptake rates per unit root mass in enriched nutrient patches). We introduced nutrient soil heterogeneity at two spatial scales. In a fine-grained heterogeneous treatment, the nutrient-rich patches were smaller and more concentrated than in a coarse-grained heterogeneous (checkerboard) treatment. Overall, the level of nutrient availability in these hete...

Vertical distribution and radial growth of coarse roots in pure and mixed stands of Fagus sylvatica and Picea abies

The vertical distribution of roots greater than 2 mm diameter was determined from digital images covering 116-m2 profile walls in soil pits in pure stands of European beech (Fagus sylvatica L.) and of Norway spruce (Picea abies (L.) Karst.) and in a mixed stand of both species. Radial root growth was assessed for roots greater than 5 mm diameter by growth ring analysis. Beech roots showed maximum density at the 10- to 20-cm depth, whereas the density of spruce roots decreased exponentially with increasing soil depth. Roots of both species reached the maximum excavation depth (1 m) in their monospecific stands. However, the root system of spruce was shallower in the mixture with beech, where large roots (diameter (d) >20 mm) were limited to the upper 10 cm. Beech roots reached the same rooting depth as in the pure stand but showed higher root densities in deeper soil layers. Neither root diameter nor root growth of any species was correlated with soil depth. Radial root growth of beech exceeded that of spruce significantly in both pure and mixed stands. Radial growth rate of beech roots further increased when mixed with spruce. The enhancement of beech root growth in the mixed stand suggests a higher belowground competitive ability of beech compared with spruce; as a result the spruce root system developed even shallower in the mixed versus in the pure stand.

Plant Competition Underground

Belowground competition occurs when plants decrease the growth, survival, or fecundity of neighbors by reducing available soil resources. Competition belowground can be stronger and involve many more neighbors than aboveground competition. Physiological ecologists and population or community ecologists have traditionally studied belowground competition from different perspectives. Physiologically based studies often measure resource uptake without determining the integrated consequences for plant performance, while population or community level studies examine plant performance but fail to identify the resource intermediary or mechanism. Belowground competitive ability is correlated with such attributes as root density, surface area, and plasticity either in root growth or in the properties of enzymes involved in nutrient uptake. Unlike competition for light, in which larger plants have a disproportionate advantage by shading smaller ones, competition for soil resources is apparently more symmetric. Belowground competition often decreases with increases in nutrient levels, but it is premature to generalize about the relative importance of above- and belowground competition across resource gradients. Although shoot and root competition are often assumed to have additive effects on plant growth, some studies provide evidence to the contrary, and potential interactions between the two forms of competition should be considered in future investigations. Other research recommendations include the simultaneous study of root and shoot gaps, since their closures may not occur simultaneously, and improved estimates of the belowground neighborhood. Only by combining the tools and perspectives from physiological ecology and population and community biology can we fully understand how soil characteristics, neighborhood structure, and global climate change influence or are influenced by plant competition belowground.