Competition For Soil Resources Research Articles

Mycorrhizal networks and distance from mature trees alter patterns of competition and facilitation in dry Douglas-fir forests

The distribution of dry Douglas-fir forests in western North America is expected to shift northward with climate change and disappear from the grassland interface in the southern interior of British Columbia. This shift may be accentuated by clearcutting, a common harvesting practice that aims to reduce the competitive effects of residual mature trees on new regeneration, but in so doing, ignores their facilitative effects. In this study, we investigated the net effects of competition from and facilitation by mature trees retained on harvested sites on seedling establishment in the dry interface Douglas-fir forests. We demonstrate that access to a mycorrhizal network (MN) and proximity to trees have important influences on seedling performance. On six sites, we established trenched plots around 24 mature Pseudotsuga menziesii var. glauca (Douglas-fir) trees, then planted Douglas-fir seedlings into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, or 250-microm pores, or without mesh) or into impermeable bags (grown in isolation) at four distances (0.5, 1.0, 2.5, or 5.0 m). Seedling survival tended to be greater and water stress lower where seedlings had full access to the MN. Seedling height, shoot biomass, needle biomass, and nutrient uptake peaked at 2.5-5.0 m from mature trees. Seedlings 0.5 m from mature trees had lower CO2 assimilation rates and wood delta(13)C compared to seedlings 5.0 m away. Competition for soil resources was highest near mature trees but facilitation was relatively greater at further distances, resulting in a zone of net benefit for seedlings. These results show that intraspecific tree-seedling interactions are both competitive and facilitative in dry Douglas-fir forests, and that they are spatially dependent. After disturbance, maintaining residual mature trees may be important for their beneficial regeneration zones.

Variations in relative stomatal and biochemical limitations to photosynthesis in a young blackbutt (Eucalyptus pilularis) plantation subjected to different weed control regimes

Foliar gas exchange and carbon (delta(13)C) and oxygen (delta(18)O) isotope ratios were measured in a young blackbutt (Eucalyptus pilularis Sm.) plantation subjected to four weed control treatments defined by the width of the weed-free strip maintained for the first 12 months after planting. Treatments were: 2-m-wide weed-free strip (50% of plot area, 2.0MWC), 1.5-m-wide weed-free strip (37.5% of plot area, 1.5MWC), 1-m-wide weed-free strip (25% of plot area, 1.0MWC) and no weed control (NWC). Our objectives were to determine (1) if decreasing the width of the weed control strip (decreasing herbicide use) affected growth and leaf photosynthesis of the plantation, and (2) the effects of the weed control regimes on variations in relative stomatal and biochemical limitations to photosynthesis. Trees in the 1.0MWC treatment had lower foliar light-saturated photosynthetic rate (A(sat)) than trees in the 2.0MWC treatment. An increase in metabolic limitation was responsible for the decrease in A(sat) in the 1.0MWC trees, which was also partly confirmed by the isotopic data. Compared with trees in the 1.0MWC, 1.5MWC and 2.0MWC treatments, A(sat) of NWC trees was significantly lower, a difference that was attributable mainly to stomatal limitation and to a lesser extent to biochemical limitation. The results support the conclusion that different weed control regimes cause differences in relative stomatal and biochemical limitations to plantation photosynthesis. This report contributes to a growing body of literature on competition for soil resources between trees and weeds. Our results highlight the usefulness of the stable isotopic method in supporting analysis of the response of net photosynthesis to varying intercellular CO(2) concentration for determining the relative stomatal and non-stomatal limitations to photosynthesis under experimental conditions in the field.

Tree and Crop Growth and Soil Resource Availability in Hardwickia binata Roxb Agroforestry Systems in the Indian Desert

Integrating trees in agricultural fields may be beneficial in controlling land degradation and improving productivity in dryland areas. Two-year-old Hardwickia binata Roxb trees were intercropped for 7 years to examine tree-crop interactions and the effect of tree integration and the cropping pattern on land productivity. The tree integrated treatments comprised a Vigna radiata (L.) R. Wilcz crop with H. binata in all years (fixed crop) (FC) and a rotation crop with H. binata in alternate years (RC). Sole crop treatments provided controls for the V. radiata crop (CFC) and rotational crop (CRC) treatments. Pennisetum glaucum (L.) R.Br., V. radiata, Sesamum indicum (L.) and Cymopsis tetragonoloba (L.) Taub were harvested in the RC/CRC plots in 1996, 1997, 1998, 1999, 2000, 2001, and 2003. Tree growth and growth increments did not differ between the FC and RC treatments (p > 0.05). Crop yield (grain + dry matter) was highest (p < 0.01) in the FC treatment in 1996, 1997, 1998, 1999, and 2000, indicating that the presence of trees was beneficial. The observation that yield was highest in the CFC/CRC treatments in 2001 and 2003 indicates a competitive effect of trees for soil resources in these years. Soil water content (SWC) in September was lowest in 2002 and highest in 2003. The effect of cropping pattern on SWC was significant (p < 0.05) only in 2000, 2001, 2002, and 2003, whereas SWC differed (p < 0.01) between 0–25-, 25–50-, and 50–75-cm soil layers. Soil water content to 75-cm soil layer was lower by 0.4 (1996) to 18.9 mm (2003) in tree-integrated treatments than in the control treatments, affecting crop yield. Soil organic matter, NH4-N, NO3-N, and PO4-P in the 0–25-cm soil layer were increased by tree inputs (p < 0.05). H. binata was apparently beneficial in increasing productivity even during droughts, and the benefits may be further improved by root trenching to reduce competition for soil resources in relatively low rainfall years.

Responses of Fraxinus excelsior seedlings to grass-induced above- and below-ground competition

The competitive interactions between woody seedlings and herbaceous vegetation have received increasing interest in recent years. However, little is known about the relative contributions and underlying mechanisms of above- and below-ground competition between species. We used a novel experimental approach to assess the responses of Fraxinus excelsior seedlings to different combina- tions of root and shoot competition imposed by the grass Dactylis glomerata under greenhouse condi- tions. Seedling growth was significantly reduced by competition for soil resources, but neither biomass nor height were significantly affected by shoot competition for light. Competitive response indices based on biomass confirmed that below-ground competition was more important than above-ground competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were almost all significantly affected by root competition; these responses varied depending on the trait exam- ined. In contrast, morphological responses to shoot competition were limited. In the absence of root competition, seedlings showed a significant increase in the biomass allocated to leaves and a greater leaf area ratio in response to shoot competition. Our findings suggest that morphological modifications help to mitigate the negative effects of competition, but the expression of plasticity may be suboptimal due to resource constraints. The present study also highlights the importance of appropriate experimental controls and analysis to avoid confounding effects of experimental design and ontogeny on the interpreta- tion of competitive responses.

Partição da competição por recursos do solo e radiação solar entre cultivares de soja e genótipos concorrentes

As plantas competem por recursos do meio situados abaixo e/ou acima da superfície do solo. A separação física da competição entre plantas possibilita conhecer a importância relativa de cada fração, bem como apontar possíveis diferenças em competitividade entre espécies. Objetivou-se neste trabalho separar os efeitos individuais decorrentes da competição por recursos do solo ou radiação solar, entre soja e plantas concorrentes. Foram realizados seis experimentos em vasos na UFRGS, em Porto Alegre-RS, sendo dois em 2001 e quatro em 2002. Os tratamentos testados resultaram das combinações de dois genótipos concorrentes (cultura e competidor) e quatro condições de competição (ausência de competição, competição por recursos do solo e radiação solar, competição por recursos do solo e competição por radiação solar). Os cultivares de soja IAS 5 e Fepagro RS 10 representaram a cultura, enquanto o nabo forrageiro e o cultivar de soja Fundacep 33 foram os competidores. Determinaram-se variáveis morfofisiológicas em plantas de soja e de nabo forrageiro. O crescimento das plantas de soja foi mais afetado pela competição por recursos do solo, sendo o cultivar RS 10 mais competitivo do que IAS 5. O nabo forrageiro não interferiu no crescimento dos cultivares de soja, porém cresceu mais na presença da cultura.

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.

Fine Root Distribution in Dehesas of Central-Western Spain

A Dehesa is a structurally complex agro-silvo-pastoral system where at least two strata of vegetation, trees and herbaceous plants coexist. We studied the root distribution of trees (Quercus ilex L.) and herbaceous plants, in order to evaluate tree and crops competition and complementarity in Dehesas of Central Western Spain. 72 soil cores of 10 cm diameter (one to two metre deep) were taken out around 13 trees. Seven trees were intercropped with Avena sativa L. and six trees were in a grazed pasture dominated by native grasses. Soil coring was performed at four distances from the tree trunks, from 2.5 (beneath canopy) till 20 m (out of the canopy). Root length density (RLD) of herbaceous plants and trees was measured using the soil core-break method. Additionally, we mapped tree roots in 51 profiles of 7 recently opened road cuts, located between 4 and 26 m of distance from the nearest tree. The depth of the road cuts varied between 2.5 and 5.5 m. Herbaceous plant roots were located mostly in the upper 30 cm, above a clayey, dense soil layer. RLD of herbaceous plants decreased exponentially with depth until 100 cm depth. Holm-oak showed a much lower RLD than herbs (on average, 2.4 versus 23.7 km m−3, respectively, in the first 10 cm of the soil depth). Tree RLD was surprisingly almost uniform with depth and distance to trees. We estimated a 5.2 m maximum depth and a 33 m maximum horizontal extension for tree roots. The huge surface of soil explored by tree roots (even 7 times the projection of the canopy) could allow trees to meet their water needs during the dry Mediterranean summers. The limited vertical overlap of the two root profiles suggests that competition for soil resources between trees and the herbaceous understorey in the Dehesa is probably not as strong as usually assumed.

Short-term Nitrogen Fixation by Legume Seedlings and Resprouts After Fire in Mediterranean Old-fields

Fires may greatly alter the N budget of a plant community. During fire nitrogen is lost to the atmosphere. Although high light availability after fire promotes N2-fixation, the presumably high soil N availability could limit N2-fixation activity. The latter limitation might be particularly acute in legume seedlings compared with resprouts, which have immediate access to belowground stored carbon. We wished to learn whether early post-fire conditions were conducive to N2-fixation in leguminous seedlings and resprouts in two types of grassland and in a shrubland and whether seedlings and resprouts incurred different amounts of N2-fixation after fire. We set 18 experimental fires in early autumn on 6 plots, subsequently labelling 6 subplots (2 × 2 m2) in each community with 15NH4 +-N (99 atom % excess). For 9 post-fire months we measured net N mineralisation in the top 5 cm of soil and we calculated the fraction of legume N derived from the atmosphere (%Ndfa) in seedlings and resprouts. We used two independent estimates of the amounts of N derived from non-atmospheric sources in potentially N2-fixing plants: mean soil pool abundance and the 15N-enrichment of non-legumes. Despite substantial soil net N mineralisation in all burned community types (about 2.6 g Nm−2 during the first nine months after fire), the %Ndfa of various legume species was 52–99%. Legumes from both grasslands showed slightly higher N2-fixation values than shrubland legumes. As grassland legumes grew in more belowground dense communities than shrubland legumes, we suggest that higher competition for soil resources in well established grass-resprouting communities may enhance the rate of N2-fixation after fire. In contrast to our hypothesis, legume seedlings and resprouts from the three plant communities studied, had similar %Ndfa and apparently acquired most of their N from the atmosphere rather than from the soil.

Mechanisms underlying the interaction betweenPinus halepensisand the native late‐successional shrubPistacia lentiscusin a semi‐arid plantation

Pinus halepensishas been extensively planted in semi‐arid areas throughout the world. This has often led to slow‐growth stands that: a) suffer from insect plagues, b) promote nutrient depletion and c) fail to promote the recovery of native vegetation. The introduction of native late‐successional shrubs in these stands could stimulate successional processes, improve soil conditions and enhance their resilience against disturbances. The main objective of this study was to evaluate the mechanisms underlying the interaction betweenPinusand the native late‐successional shrubPistacia lentiscusin a semi‐arid plantation. By using manipulative field and laboratory experiments, we evaluated direct (competition for soil resources and allelopathic effects) and indirect (competition with herbaceous understorey) interactions betweenPinusandPistacia. We found no effect ofPinuslitter and root exudates onPistaciagrowth. In the field,Pistaciaseedlings planted under the canopy ofPinusshowed higher survival than those planted in open areas with sparse vegetation. Girdling ofPinustrees did not affect the performance of plantedPistaciaseedlings, but suppression of the herbaceous understorey significantly enhanced both survival and physiological status ofPistaciaseedlings planted under the canopy ofPinus. The magnitude of the interference by herbaceous understorey was considerably higher than that byPinus. Our results provide evidence that a negative indirect interaction betweenPinusandPistacia, mediated by the herbaceous understorey, is taking place in the afforestation studied, and can help to explain the low rates of colonisation of late‐successional woody shrubs typically observed in semi‐aridPinushalepensisplantations.

Influence of nitrogen on competition between purple nutsedge, maize and soybean

The divided box technique in an additive design was used to study the relative roles of shoot and root competition between purple nutsedge, maize and soybean as influenced by different levels of nitrogen fertilization. Measures of resource complimentarity and the relative aggressivity indicated that during the early stages of growth, shoot competition was important to both crops and purple nutsedge probably due to the rapid establishment rate of purple nutsedge. Root competition for soil resources was more severe than competition for aerial resources in retarding the growth of maize and soybean. Nitrogen fertilization significantly increased maize and nutsedge height, leaf area and biomass. However, soybean height, leaf area and biomass were not significantly influenced by nitrogen. Competitive balance indices of either mixture were not significantly influenced by nitrogen fertilization. The effects of full (shoot + root) competition were more dramatic than the effects of either shoot or root competition operating independently. Purple nutsedge was the aggressor in resource acquisition.

Loblolly Pine Stand Early Development Under Reserve-Tree Silvicultural Systems in East Texas

Abstract Even-aged reproduction methods retaining reserve trees that persist through the development of the succeeding rotation have been poorly studied in loblolly pine (Pinus taeda L.), yet have potential for satisfying a variety of landowner objectives. Two research sites were established in the East Texas Pineywoods region to examine the development of the regeneration component under five residual basal area densities (0, 10, 20, and 40 ft2/ac with natural regeneration on Site I; 0 and 30 ft2/ac with both natural regeneration and planting on Site II). After 7 yr on Site I and 5 yr on Site II, all reserve-tree methods allowed the development of a sufficient number of trees for producing a new generation beneath the residual overstory. However, suppression of the understory pines was evident, and varied in direct proportion to the basal area of the residual overstory. Volume growth of the residual overstory trees was low on an areal basis due to the death of several large residuals. A trenching experiment showed that the largest effect of the residual overstory on understory development was due to light attenuation rather than competition for soil resources. Advantages offered by reserve-tree methods for loblolly pine have to be assessed in view of the suppressed development of the regeneration. South. J. Appl. For. 24(1):11-16.

Competition in tree row agroforestry systems. 1. Distribution and dynamics of fine root length and biomass

Complementarity in the distribution of tree and crop root systems is important to minimise competition for resources whilst maximising resource use in agroforestry systems. A field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya to compare the distribution and dynamics of root length and biomass of a 3-year-old Grevillea robusta A. Cunn. ex R. Br. (grevillea) tree row and a 3-year-old Senna spectabilis DC. (senna) hedgerow grown with Zea mays L. (maize). Tree roots were sampled to a 300 cm depth and 525 cm distance from the tree rows, both before and after maize cropping. Maize roots were sampled at two distances from the tree rows (75–150 cm and 450–525 cm) to a maximum depth of 180 cm, at three developmental stages. The mean root length density (Lrv) of the trees in the upper 15 cm was 0.55 cm cm−3 for grevillea and 1.44 cm cm−3 for senna, at the start of the cropping season. The Lrv of senna decreased at every depth during the cropping season, whereas the Lrv of grevillea only decreased in the crop rooting zone. The fine root length of the trees decreased by about 35% for grevillea and 65% for senna, because of maize competition, manual weeding, seasonal senescence or pruning regime (senna). At anthesis, the Lrv of maize in the upper 15 cm was between 0.8 and 1.5 cm cm−3. Maize root length decreased with greater proximity to the tree rows, potentially reducing its ability to compete for soil resources. However, the specific root length (m g−1) of maize was about twice that of the trees, so may have had a competitive uptake advantage even when tree root length was greater. Differences in maize fine root length and biomass suggest that competition for soil resources and hence fine root length may have been more important for maize grown with senna than grevillea.

The relative contributions of sexual reproduction and clonal propagation in Opuntia rastrera from two habitats in the Chihuahuan Desert

1 The clonal cactus Opuntia rastrera shows predominantly sexual reproduction in grasslands (GH) and clonal propagation in nopaleras (NH). We assessed the effects of light, herbivory, water availability and the habitat an offspring came from on the survival and growth of sexual or clonal offspring (i.e. seedlings and cladodes), through 3‐ and 4‐year common garden and short‐term greenhouse experiments. 2 Shading by nurse plants increased seedling survival in the field by an order of magnitude, and a small additional advantage due to predator protection by grasses was observed. Strong herbivory transforms a facultative nurse–protégé relationship for seedlings into an obligatory one. 3 In the greenhouse seedlings grew better under shade, but in the field the production of the first cladode was delayed in seedlings in the more shaded GH. Competition for soil resources may be more intense under a dense grass tussock than under a open shrub, thus affecting the nurse–protégé relationship. Seedling survival under nurse plants was similar in GH and NH, but higher plant cover suggests that a larger number of seedlings will establish in GH in the long term. 4 Cladode survival was higher in NH. Cladodes were more successful than seedlings at establishing in intercanopy areas, possibly due to physiological differences as well as their ability to survive partial predation. Cladode survival in intercanopy areas may explain the enhanced clonal propagation in the more open NH scrubland, together with their susceptibility to the flooding which affects GH. 5 The high seedling and cladode survival in the greenhouse experiments contrasted with that observed in the field, indicating that survival is determined by the interaction between herbivores, plants and abiotic conditions rather than the physiological aptitude of the plants.

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.

Tillering Response of Barley to Shifts in Light Quality Caused by Neighboring Plants

Tillering is important in establishing the yields potential of spring barley (Hordeum vulgare L.). Two field experiments were conducted for 3 yr to test if early season shifts in the red/far‐red ration (R/FR) modulate barley tillering patterns. Experiments were designed to determine (i) the extent to which shifts in light quality associated with the presence of neigboring plants modulate barley tillering patterns in the absence of shading and competition for soil resources, (ii) barley responses to sudden changes in light quality during the tillering phase, (iii) the influence of plant spacing on response to shifts in light quality, and (iv) the impact of the response to shifts in light quality on productivity. Plans within treatment rows were grown 18 cm from densely sown border rows of barley. Barriers set into the soil assured negligible competition between border and treatment rows for soil resources during the early growing season. The borders functioned to reflect light, especially far‐red (FR), which reduced R/FR within the treatment rows without shading them. Tiller production was reduced in bordered plants relative to unbordered controls. The removal of borders at 20 d after emergence (DAE) caused an increase in treatment row tiller production, whereas the imposition of boreders at 20 DAE halted tillering. Shoot survival was increased by the presence of borders. Border effects on final spike number, biomass production, and grain yield per plant were inconsistent during the experiments. We conclude theat early season shifts in light quality associated with FR refelection from neighboring plants contribute to modulating barley tillering under field conditions.

Mechanisms of Primary Succession Following Deglaciation at Glacier Bay, Alaska

In primary succession following deglaciation at Glacier Bay, Alaska, we tested the hypothesis that the major effect of initial nitrogen—fixing colonizers is to facilitate establishment of late—successional dominants and that other possible causes of successional change (e.g., life history factors governing seed rain and competitive interactions among species) need not be invoked. Environment changed dramatically through the first 200 yr of succession. Soil organic matter increased 10—fold in the upper mineral soil with corresponding increases in soil moisture, total nitrogen (N), and capacity to support plant growth and declines in bulk density, pH, and total phosphorus (P). Plant growth in pioneer soils tended to be simultaneously limited by both N and P, as well as by unknown factors (perhaps lack of mycorrhizae), whereas only P limited growth in older soils. Light availability to seedlings declined through succession. Early—successional species (Epilobium latifolium, Dryas drummondii) had smaller seeds, younger age at first reproduction, shorter life—span, and shorter height at maturity than did mid—successional (alder, Alnus sinuata) and late—successional species (sitka spruce, Picea sitchensis). Seed rain of alder and spruce was negligible in the pioneer stage, increased prior to the stage in which a species was dominant, and was greatest in the stage in which a species dominated. Vegetation in each successional stage inhibited germination and initial establishment of sown alder and spruce seeds (except a tendency of the "black—crust") algal/microbial community in the pioneer stage to enhance survivorship). Removal of the surface litter layer generally enhanced germination and survivorship, particularly of alder. Comparisons of germination in the greenhouse and the field indicated that climatic or indirect vegetation effects (e.g., differential seed predation) and allelopathy also reduced germination and establishment in vegetated communities. Naturally occurring spruce seedlings grew most rapidly in the Dryas and alder stages and most slowly in the spruce stage. Similarly, growth of spruce seedlings transplanted into each successional stage was facilitated by the Dryas (nonsignificantly) and alder stages but inhibited by the spruce stage, relative to earlier successional stages. Facilitation of growth of natural and transplanted spruce seedlings by Dryas and alder stages was associated with higher N and P uptake and tissue nutrient concentrations, whereas nutrient uptake and concentration in spruce seedlings declined in the spruce stage. By contrast, transplanted alder seedlings grew rapidly and accumulated most nutrients in the pioneer stage and were strongly inhibited by subsequent stages. The facilitative effect of Dryas and alder comes primarily from inputs of organic matter and associated N. Addition of alder litter stimulated nutrient uptake and growth of transplanted spruce seedlings in the pioneer and Dryas stages, whereas shading had no effect on growth of spruce seedlings. Root trenching and planting of spruce near isolated alders indicated that, although the net effect of alder is facilitative, alder also inhibits growth of spruce seedlings through competition for soil resources. Strong root competition also occurs in the spruce stage. Alder competitively inhibits Dryas, primarily by shading but also through the physical and allelopathic effects of its litter. In general, both at Glacier Bay and elsewhere, life history traits determine the pattern of succession. Changes in competitive balance accompanying successional changes in environment provide the mechanism for changes in species dominance. Initial site conditions (and facilitation, where present) influence the rate of change and final state of community composition and productivity. We conclude that no single factor or mechanism fully accounts for primary succession at Glacier Bay.