Belowground Competition Research Articles

The above-ground competition between common bean (Phaseolus vulgaris L.) and barnyardgrass (Echinochloa crus-galli L.) affected by nitrogen application

Competition for shared above-ground (e.g. light) and below-ground (e.g. water and nutrients) resources among neighbouring plants is one of the main processes that affect cropping systems. This study aimed to evaluate the effects of weed shading on growth and yield production of bean plants when grown in weedy condition without below-ground competition. The experiments were organized as a split plot design with three replications. The main plots consisted of the nitrogen application rates including of 0, 70, 140 and 280 kg N ha−1 and the sub plots consisted of barnyardgrass densities including 0 (D0), 2 (D2) and 20 (D20) plants m−2. The N application rates were 0 (N0), 25 (N25), 50 (N50), 100% (N100) and 200% (N200) of the recommended N rate on the basis of seed yield goal. Biomass of common bean decreased by 30% for N0, 41% for N50, 35% for N100 and 38% for N200, as weed pressure increased from D0 to D20.. Mean reduction in 100-seed weight, seed number and seed yield caused by high weed pressure was 11, 20 and 41% compared with weed free, respectively. The weed biomass and N uptake increased by 66 and 159% as the nitrogen dose increased from N0 to N200. The highest biomass and N uptake was obtained from N200 treatment. We conclude that agronomically optimum N rates for seed yield was obtained at 140 kg N ha−1 within the range of N fertilization rates used across all weed pressure levels.

Advanced Aboveground Spatial Analysis as Proxy for the Competitive Environment Affecting Sapling Development.

Tree saplings are exposed to a competitive growth environment in which resources are limited and the ability to adapt determines general vitality and specific growth performance. In this study we analyzed the aboveground spatial neighborhood of oak [Quercus petraea (Matt.) Liebl.] and beech (Fagus sylvatica L.) saplings growing in Germany, by using hemispherical photography and terrestrial laser scanning as proxy for the competitive pressure saplings were exposed to. The hemispherical images were used to analyze the light availability and the three-dimensional (3D) point clouds from the laser scanning were used to assess the space and forest structure around the saplings. The aim was to increase the precision with which the biomass allocation, growth, and morphology of the saplings could be predicted by including more detailed information of their environment. The predictive strength of the models was especially increased through direct neighborhood variables (e.g., relative space filling), next to the light availability being the most important predictor variable. The biomass allocation patterns within the more light demanding oak were strongly driven by the space availability around the saplings. Diameter and height growth variables of both species reacted significantly to changes in light availability, and partly also to the neighborhood variables. The leaf morphology [as leaf-area ratio (LAR)] was also driven by light availability and decreased with increasing light availability. However, the branch morphology (as mean branch weight) could not be explained for oak and the model outcome for beech was hard to interpret. The results could show that individuals of the same species perform differently under constant light conditions but differing neighborhoods. Assessing the neighborhood of trees with highly precise measurement devices, like terrestrial laser scanners, proved to be useful. However, the primary response to a dense neighborhood seemed to be coping with a reduction of the lateral light availability aboveground, rather than responding to an increase of competition belowground. The results suggest continuing efforts to increase the precision with which plant environments can be described through innovative and efficient methods, like terrestrial laser scanning.

Young shade trees improve soil quality in intensively managed coffee systems recently converted to agroforestry in Yunnan Province, China

The trend of soil degradation in intensive open coffee systems is well-documented. This study highlights the impact of young shade trees on soil quality only 4 years after their intercropping with coffee. 18 young shade trees belonging to three tree species (Cinnamomum camphora, Bishofia javanica and Jacaranda mimosifolia) were selected in an intensive coffee system in Southern Yunnan. Soil samples (0–20 cm) were tested for chemical composition, soil communities and soil enzyme activities under their canopies and in open areas, both in coffee rows and inter-rows, once during the rainy and once during the dry season. Additionally, root systems were characterized using trenches. Soil water profiles and litterfall were monitored along the production cycle. Coffee yield was recorded for two consecutive years. We detected a positive impact of all shade tree species on soil chemical, biological and biochemical components, especially during the dry season. This positive impact included higher soil organic matter (+10%) and more abundant soil microbial communities (+64%) under shaded coffee than under open coffee. Furthermore, shaded coffee trees yielded as much as open coffee trees, except under C. camphora, probably due to high below-ground competition. These results demonstrate that carefully selected shade trees can rapidly contribute to preserving and/or restoring soil quality in intensive coffee systems, while maintaining high coffee yield.

Calla lily intercropping in rubber tree plantations changes the nutrient content, microbial abundance, and enzyme activity of both rhizosphere and non-rhizosphere soil and calla lily growth

Rubber is a globally important crop species and is widely grown in Hainan, China; the development of efficient agricultural practices is essential when attempting to maximize productivity. The effects of interspecific competition for nutrients and calla lily (Alocasia macrorrhizos L.) intercropping on calla lily growth, development, and nutrient uptake were investigated in rubber tree (Hevea brasiliensis Müll. Arg.) plantations. The nutrient abundances, enzyme activity, and number of microbes present in rhizosphere and non-rhizosphere soil were determined. Barriers were inserted between the crops to separate their root systems and nullify belowground competition to facilitate the analysis of intercrop competition. We found that in the rubber tree/calla lily intercropping system, nutrient content and soil urease activities in rhizosphere and non-rhizosphere soil significantly decreased, and nitrogen (N) and potassium (K) taken up by calla lilies, and the growth and biomasses of calla lilies significantly decreased. In contrast, soil sucrase, acid phosphatase, and catalase activities and the quantity of fungi in rhizosphere and non-rhizosphere soil significantly increased, and phosphorus (P) uptake, root length, and the ratio of root to shoot dry weights in calla lilies also significantly increased. Intercropping significantly decreased the abundances of both bacteria and actinomycetes and decreased total microbial abundance in rhizosphere soil, but significantly increased their abundances in non-rhizosphere soil. The nutrient content (excluding organic matter content and pH) in rhizosphere soil was lower than in non-rhizosphere soil. These findings indicate that nutrient competition between rubber trees and calla lilies was significant in this intercropping system, with rubber trees holding a competitive advantage. Intercropping significantly inhibited calla lily growth by significantly decreasing soil nutrient content. The abundances of soil microbes and changes in root architectures correspond to the responses of calla lilies to nutrient competition, allowing the adjustment of the plants’ nutritional needs during growth and development.

Root trait-mediated belowground competition and community composition of a temperate steppe under nitrogen enrichment

Shifts of plant community composition with enhanced atmospheric nitrogen (N) deposition in grasslands have occurred globally. Despite extensive studies on the effects of enhanced N deposition on plant species composition of grassland community, few studies have focused on belowground ecological processes and duration of N addition. In situ long-term (14-year: 2004–2017) and short-term (4-year: 2014–2017) N-addition experiments in the same sites were conducted in a temperate steppe of northern China. We investigated the effects of N-addition on plant community composition and root traits by comparing results of short-term and long-term N addition. Nitrogen-evoked changes in plant community composition were primarily dependent on N-addition duration and functional groups. Short-term N addition favored growth of grasses, and depressed growth of forbs, while long-term N addition favored growth of sedges, and concurrently depressed growth of grasses and forbs, ultimately leading to a time-dependent shift of plant community composition from co-dominance by grasses and forbs to dominance by grasses, then to dominance by sedges. Similar to the observed aboveground biomass, short-term N addition led to an increase in the relative belowground biomass (the proportion of belowground biomass of individual plant to community belowground biomass) of the grass Stipa krylovii and a decrease in the relative belowground biomass of the forb Artemisia frigida, respectively. In contrast, long-term N addition enhanced the relative belowground biomass of the sedge Carex korshinskyi, and reduced the relative belowground biomass of the grass S. krylovii and the forb A. frigida. Moreover, traits of absorptive fine roots (first-two orders) in the three species (grass: S. krylovii; forb: A. frigida; sedge: C. korshinskyi) that are representatives of three functional groups differed in their responses to short-term and long-term N addition, thus rendering them differentially competitive for acquisition of belowground resource. The changes in root traits of the three species in responses to N addition of varying duration were associated with their relative aboveground biomass. Root traits play important roles in mediating the competition for belowground resource and changes in plant community composition of temperate grasslands under N enrichment. Our findings provide novel insights into N-deposition-induced changes in steppe community composition by linking root traits of functional groups and duration of N addition.

Fine-root traits are linked to species dynamics in a successional plant community.

Despite the importance of fine roots for the acquisition of soil resources such as nitrogen and water, the study of linkages between traits and both population and community dynamics remains focused on aboveground traits. We address this gap by investigating associations between belowground traits and metrics of species dynamics. Our analysis included 85 species from a long-term data set on the transition from old field to forest in eastern North America (the Buell-Small Succession Study) and the new Fine-Root Ecology Database. Given the prominent roles of life form (woody vs. non-woody) and species origin (native vs. exotic) in defining functional relationships, we also assessed whether traits or their relationships with species dynamics differed for these groups. Species that reached their peak abundance early in succession had fine-root traits corresponding to resource acquisitive strategies (i.e., they were thinner, less dense, and had higher nitrogen concentrations) while species that peaked progressively later had increasingly conservative strategies. In addition to having more acquisitive root traits than native species, exotics diverged from the above successional trend, having consistently thinner fine roots regardless of the community context. Species with more acquisitive fine-root morphologies typically had faster rates of abundance increase and achieved their maximal rates in fewer years. Decreasing soil nutrient availability and increasing belowground competition may become increasingly strong filters in successional communities, acting on root traits to promote a transition from acquisitive to conservative foraging. However, disturbances that increase light and soil resource availability at local scales may allow acquisitive species, especially invasive exotics, to continue colonizing late into the community transition to forest.

Herbaceous Encroachment from Mountain Birch Forests to Alpine Tundra Plant Communities Through Above- and Belowground Competition

Alpine plant communities are highly sensitive to global warming. One of the consequences of the warming is encroachment by herbaceous plants from forests at low elevations into alpine ecosystems. In the Changbai Mountains, narrowleaf small reed (Deyeuxia angustifolia (Kom.) Y. L. Chang) from mountain birch forests encroached upward into alpine tundra, gradually replacing native tundra shrubs such as Rhododendron (Rhododendron aureum Georgi). How encroaching plants affect native plant communities is not fully understood. In this study, we analyzed above- and belowground biomass of alpine plant communities at five encroachment levels to investigate how biomass allocation changed at species and community scales. Our research showed that native plants are forced to change their morphology to cope with competition, at both above- and belowground levels, from encroaching plants. We found that (1) R. aureum increased the shoot height and leaf area in order to compete with D. angustifolia; (2) above- and belowground biomass of D. angustifolia increased while above- and belowground biomass of R. aureum decreased with increasing levels of encroachment; and (3) D. angustifolia encroachment reduced the total biomass of alpine tundra. Encroachment by herbaceous plants has a long-term negative impact on the ability of tundra plants to sequester carbon in the alpine tundra of the Changbai Mountains.

How are nitrogen availability, fine-root mass, and nitrogen uptake related empirically? Implications for models and theory.

Understanding the effects of global change in terrestrial communities requires an understanding of how limiting resources interact with plant traits to affect productivity. Here, we focus on nitrogen and ask whether plant community nitrogen uptake rate is determined (a) by nitrogen availability alone or (b) by the product of nitrogen availability and fine-root mass. Surprisingly, this is not empirically resolved. We performed controlled microcosm experiments and reanalyzed published pot experiments and field data to determine the relationship between community-level nitrogen uptake rate, nitrogen availability, and fine-root mass for 46 unique combinations of species, nitrogen levels, and growing conditions. We found that plant community nitrogen uptake rate was unaffected by fine-root mass in 63% of cases and saturated with fine-root mass in 29% of cases (92% in total). In contrast, plant community nitrogen uptake rate was clearly affected by nitrogen availability. The results support the idea that although plants may over-proliferate fine roots for individual-level competition, it comes without an increase in community-level nitrogen uptake. The results have implications for the mechanisms included in coupled carbon-nitrogen terrestrial biosphere models (CN-TBMs) and are consistent with CN-TBMs that operate above the individual scale and omit fine-root mass in equations of nitrogen uptake rate but inconsistent with the majority of CN-TBMs, which operate above the individual scale and include fine-root mass in equations of nitrogen uptake rate. For the much smaller number of CN-TBMs that explicitly model individual-based belowground competition for nitrogen, the results suggest that the relative (not absolute) fine-root mass of competing individuals should be included in the equations that determine individual-level nitrogen uptake rates. By providing empirical data to support the assumptions used in CN-TBMs, we put their global climate change predictions on firmer ground.

Effects of water level and competition pattern on ecological stoichiometry characteristics of a typical wetland plant <i>Polygonum hydropiper</i> in Lake Dongting

以洞庭湖典型湿地植物辣蓼为目标植物，短尖苔草为邻近植物，通过控制实验研究不同水位（30、0和-30 cm）和竞争（无竞争、全部竞争、地上竞争和地下竞争）模式下目标植物生长及生态化学计量特征的变化.结果表明：水位处理显著影响不同竞争模式下的生物量积累，辣蓼生物量随水位增加显著降低；-30 cm水位无竞争模式下生物量最大，为10.84±1.52 g.在30 cm和0 cm水位梯度下，不同竞争模式下的生物量间无显著差异.但-30 cm水位下，地下竞争模式下的辣蓼生物量积累较全竞争模式和地上竞争模式下显著增多，说明非胁迫条件下，辣蓼和苔草的竞争以地上竞争为主.水位处理对辣蓼叶片、茎和根的氮、磷含量影响显著，30 cm水位下，叶片氮、磷含量显著高于其他水位下的含量.在-30 cm水位下，叶片C：N和C：P显著高于其他水位下的比值，分别为48.08±3.85、590.3±43.4.相比于对照处理（无竞争），竞争作用下的辣蓼总氮含量降低，而C：N值增加，N：P值降低，这可能是因为竞争作用导致辣蓼对氮的吸收减少所致.;Taking Polygonum hydropiper as the target plant and Carex brevicuspis as the adjacent plant, the changes of growth and ecological stoichiometry characteristics of P. hydropiper under different water levels(30 cm, 0 cm and -30 cm)and competition patterns (no competition, full competition, above-ground competition and below-ground competition) were studied through outdoor control experiments. The results showed that:the biomass accumulation of P. hydropiper was significantly affected by water level under different competition patterns, and which decreased significantly with the increase of water level. The maximum biomass was 10.84±1.52 g in the no competition pattern with the -30 cm water level. Under 30 cm and 0 cm water levels, the biomass accumulation of P. hydropiper had insignificant difference under different competition patterns. However, at -30 cm water level, the biomass accumulation of P. hydropiper under the below-ground competition pattern was significantly higher than that under the full competition pattern and the above-ground competition pattern, which indicated that the competition between P. hydropiper and C. brevicuspis was dominated by the above-ground competition under the non-stress condition. The nitrogen and phosphorus contents in leaves, stems and roots of P. hydropiper were significantly affected by water level. The nitrogen and phosphorus contents of leaves were significantly higher at 30 cm water level than those at other water levels. At -30 cm water level, the ratio of C:N and C:P was significantly higher than those at other water levels, which was 48.08±3.85 and 590.3±43.4, respectively. Compared with the control treatment, competition decreased the total nitrogen content of P. hydropiper, while the C:N ratio increased and the N:P ratio decreased, which may be due to that the competition reduced the nitrogen absorption of P. hydropiper.

Restoration of burned and post-fire logged Austrocedrus chilensis stands in Patagonia: effects of competition and environmental conditions on seedling survival and growth

In Andean Patagonia, Argentina, severe wildfires produce high mortality in Austrocedrus chilensis forests. Owing to its high timber quality, A. chilensis trees are generally salvage logged right after wildfires. Post-logged areas result in open, denuded stands with low herbaceous or shrub cover, which precludes natural A. chilensis regeneration. In severely burned A. chilensis stands, we determined how different site conditions (salvage logging, SL, and without logging, WL), and the combination of different methods of control of above- and belowground competition and micro-environmental factors (incident radiation, soil temperature and soil moisture) affected survival, growth and hydric status of planted A seedlings. Two growing seasons after plantation, seedling survival was lower than 10% at SL sites, whereas it was near 90% at WL sites. Four seasons after establishment, and regardless of competition treatments, no seedlings survived at SL sites, whereas over 75% survived at WL sites. Radiation attenuation by canopy of WL sites benefitted A. chilensis seedling survival, and no additional control of early successional herbaceous species should be necessary to attain high seedling survival 4 years after restoration. Selective logging, by leaving ~50% of burned snags, and active restoration practices, may help reconcile economic needs of society and ecological requirements of A. chilensis for recovering former structure and functions.

Testing mechanisms underlying responses of plant functional traits to flooding duration gradient in a lakeshore meadow

Flooding duration is a major factor determining wetland plant growth and community composition. While there have been extensive studies on how direct effects of flooding duration on plant functional traits, very few have tested the indirect effects on these traits, including those arising from competition. We hypothesized that there would be a shift in importance of competition ability and flooding tolerance to functional traits along the flooding duration gradient. We investigated plant functional traits along a flooding duration gradient for two-dominant wetland plant species (Carex cinerascens and Phalaris arundinacea) in a lakeshore meadow of Poyang Lake, China. We found that these two species tended to have different strategies to adapt to flooding stress. P. arundinacea tended to adopt escape strategy with high growth rate (e.g. greater height, specific leaf area, and leaf N and P content) at longer flooding duration, while C. cinerascens tended to adopt more quiescence strategy, with only 5 of 13 of its traits showing a significant response. With shorter flooding duration, both species tended to have lower leaf N and P content, indicating greater nutrient limitation. Meanwhile, relative abundance of C. cinerascens with relatively higher belowground nutrient competition ability (i.e. greater root/shoot biomass ratio) decreased with flooding duration. These results suggest that belowground competition intensifies at shorter flooding duration, because of increased nutrient limitation with increased productivity. Therefore, our study indicates that both flooding stress and nutrient competition could affect plant functional traits along a flooding duration gradient.

Does the root to shoot ratio show a hormetic response to stress? An ecological and environmental perspective

Root/shoot (R/S) ratio is an important index for assessing plant health, and has received increased attention in the last decades as a sensitive indicator of plant stress induced by chemical or physical agents. The R/S ratio has been discussed in the context of ecological theory and its potential importance in ecological succession, where species follow different strategies for above-ground growth for light or below-ground competition for water and nutrients. We present evidence showing the R/S ratio follows a biphasic dose–response relationship under stress, typical of hormesis. The R/S ratio in response to stress has been widely compared among species and ecological succession classes. It is constrained by a variety of factors such as ontogeny. Furthermore, the current literature lacks dose–response studies incorporating the full dose–response continuum, hence limiting scientific understanding and possible valuable application. The data presented provide an important perspective for new-generation studies that can advance current ecological understanding and improve carbon storage estimates by R/S ratio considerations. Hormetic response of the R/S ratio can have an important role in forestry for producing seedlings with desired characteristics to achieve maximum health/productivity and resilience under plantation conditions.

Variation of Fine Roots Distribution in Apple (Malus pumila M.)–Crop Intercropping Systems on the Loess Plateau of China

In arid and semi-arid areas, interspecific below-ground competition is prominent in agroforestry systems. To provide theoretical and technical guidance for the scientific management of apple–crop intercropping systems, a field study was conducted in the Loess Plateau of China to examine the variation of fine roots distribution in apple–crop intercropping systems. The fine roots of apple trees and crops (soybean (Glycine max (L.) Merr) or peanuts (Arachis hypogaea Linn.)) were sampled to 100 cm depth at ten distances from the tree row using the stratified excavation method. The results showed that the vertical distribution of fine roots between intercropped apple trees and intercropped crops were skewed and overlapped. Apple–crop intercropping inhibited the fine roots of apple trees in the 0–60 cm soil depth, but promoted their growth in the 60–100 cm soil depth. However, apple–crop intercropping inhibited the fine roots of intercropped crops in the 0–100 cm soil depth. For the fine roots of each component of the apple–crop intercropping systems, variation in the vertical distribution was much greater than variation in the horizontal distribution. Compared with monocropped systems, apple–crop intercropping caused the fine roots of intercropped apple trees to move to deeper soil, and those of intercropped crops to move to shallower soil. Additionally, apple–crop intercropping slightly inhibited the horizontal extension of the fine-root horizontal barycentre (FRHB) of intercropped apple trees and caused the FRHB of intercropped crops to be slightly biased towards the north of the apple tree row. Variation of the fine roots distribution of each component of the apple–soybean intercropping system was greater than that of the apple–peanut intercropping system. Thus, the interspecific below-ground competition of the apple–peanut intercropping system was weaker than that of the apple–soybean intercropping system. Intense competition occurred in the apple–peanut intercropping system and the apple–soybean intercropping system was in sections whose distance ranged from 0.5–1.3 and 0.5–1.7 m from the tree row, respectively. The interspecific below-ground competition was fiercer on the south side of the apple tree row than on the north side.

Responses of planted Populus tremuloides seedlings to grass competition during early establishment

Root systems of aspen seedlings display limited architectural plasticity in response to below-ground competition, but seedlings compensate for restricted rooting space and reduced root system size, by optimizing water uptake. Below-ground competition with grasses often plays a critical role during tree seedling establishment, but many underlying mechanisms are not well understood. We used a controlled field experiment to study how trembling aspen (Populus tremuloides Michx.) seedlings compete with smooth brome grass (Bromus inermis L.) for space during the first 3 years of seedling establishment and how it affected aspen seedling development. Our study showed that competition with grasses had a limited impact on architectural plasticity of aspen seedlings. Seedlings faced with competition from smooth brome were overall smaller and most architectural parameters, with the exception of shoot height, appeared simply scaled down proportionally. Shoot height changed less than other parameters, because aspen competing with grass allocated relatively more carbon to shoots than roots and adopted a slender shoot morphology to quickly overtop the competition. Aspen growing with grass competition had significantly smaller root systems. Both lateral extent and maximum rooting depth were reduced by ~ 50%. In response to the restricted rooting space, roots of aspen seedlings faced with grass competition had a lower specific root length. Root carbohydrate reserves were not affected by competition; however, aspen roots growing with grass competition had higher soluble sugar concentrations which may be associated with the observed three times higher water uptake efficiency per unit root biomass. Our findings suggest that aspen seedlings have limited capacity for architectural plasticity in response to root competition, but are at least temporarily able to compensate for reduced root system size and rooting space, by optimizing water uptake efficiency.

Soil properties and climate mediate the effects of biotic interactions on the performance of a woody range expander

AbstractExpansion of trees into grasslands and old fields is a complex process that leads to a decline of biodiversity and reduces rangeland availability. Climate and land‐use change contribute to accelerated rates of range expansion. However, the role of biotic interactions in promoting or hindering woody range expanders is still unclear. We investigated the combined effects of abiotic (soil properties and climate) and biotic (simulated grazing and intra‐ and interspecific plant–plant interactions) factors on a woody range‐expander, Juniperus virginiana, which has been spreading into grasslands and old‐field habitats in North America. We hypothesized that interspecific competition would negatively affect growth and survival of J. virginiana due to belowground competition with herbaceous species; grazing would favor J. virginiana via competitive release, and intraspecific interactions would be beneficial to tree seedlings during early life stages by means of facilitation. We also predicted that a thicker winter snowpack would have a positive impact on tree growth by providing protection from frost damage. In a multisite field experiment, we exposed J. virginiana seedlings to intra‐ and interspecific interactions, as well as simulated grazing of surrounding herbaceous species. These treatments were repeated at three different sites that vary in soil properties and that are situated along a precipitation gradient. Additionally, we conducted a snow‐manipulation experiment at one of the sites. We conducted our monitoring for two consecutive growing seasons characterized by very different rainfall conditions, the second growing season receiving between 6% and 14% more rainfall than the first. Under lower rainfall availability, interspecific interactions between tree seedlings and herbaceous species negatively affected seedling growth rates. However, this effect was detectable only during the drier year and at the site characterized by more favorable soil properties. During winter, we found that deeper snow cover was associated with decreased growth rate of plants, probably due to repeated freeze‐thaw cycles. Our results indicate that the role of biotic interactions had an effect only under harsh climatic conditions and that abiotic factors may affect range expansion directly and indirectly via biotic interactions.

Is Competition for Soil Resources by Carex pensylvanica Restricting Tree Seedling Growth in a Temperate Northern Hardwood Forest?

In response to ecological disturbances, sedge species like Carex pensylvanica form dense monocultures on the forest floor. These “sedge mats” have been shown to severely inhibit plant growth, and limit understory species diversity. In recent years, concern has grown that they may also be restricting the regeneration of economically valuable tree species like sugar maple through belowground competition. To determine if and how Carex pensylvanica may be impacting tree seedling growth through belowground competition I located and exclosed 44 tree seedlings in areas of representatively dense sedge at two forest sites at the Queen’s University Biological Station. I removed sedge from half the plots, and measured soil resource availability and seedling growth response at all plots throughout the growing season. I predicted that sedge would negatively impact the growth of tree seedlings by decreasing the availability of soil resources. I found that the presence of sedge did not affect seedling growth over one growing season, but that it did impact soil resource availability by increasing the availability of surface soil moisture and decreasing the availability of soil nitrate, changes which may have implications for seedling growth beyond the single growing season studied. My results were also site specific, indicating that location is important when managing sedge impact on tree regeneration. Understanding the impact of sedge on tree seedling regeneration is important for predicting changes in the trajectory of forest communities and for informing the management of economically valuable species like sugar maple.

Temporal changes of fine root overyielding and foraging strategies in planted monoculture and mixed forests

BackgroundMixed forests are believed to enhance ecosystem functioning and sustainability due to complementary resource use, environmental benefits and improved soil properties. The facilitation between different species may induce overyielding. Meanwhile, the species-specific fine root foraging strategies and tradeoffs would determine the structure and dynamics of plant communities. Here the aim was to investigate the admixing effects of fine-root biomass, vertical distribution and morphology in Pinus massoniana–Cinnamomum camphora mixed plantations and corresponding monocultures at 10-, 24- and 45-year old stands.ResultsThe fine root biomass in the Pinus–Cinnamomum mixed forest exerted a certain degree of overyielding effect. These positive admixing effects, however, did not enhance with forest stand development. The overall relative yield total ranged from 1.83 and 1.51 to 1.33 in 10-, 24- and 45-year-old stand, respectively. The overyielding was mainly attributed to the over-performance of late successional species, Cinnamomum, in mixed stands. The vertical fine root biomass distribution model showed fine roots of pioneer species, Pinus, shifted to the superficial layer when mixed with Cinnamomum. Furthermore, the specific root length (SRL) of Pinus was significantly higher in Pinus–Cinnamomum mixed stands than that in monocultures, and the magnitude of differences increased over time. However, the vertical fine-root distribution and SRL for Cinnamomum did not show significant differences between monoculture and mixtures.ConclusionsOur results indicated that the magnitude of fine root overyielding in mixed forests showed a high degree of consistency with the total amount of fine root biomass itself, suggesting the overyielding effects in mixed forests were correlated with the degree of belowground interaction and competition degree involved. The late successional species, Cinnamomum, invested more carbon to belowground by increasing the fine root biomass in mixtures. While the pioneer species, Pinus, adapted to the presence of the species Cinnamomum by modification of vertical distribution and root morphological plasticity in the mixtures. These species-specific fine root foraging strategies might imply the differences of forest growth strategies of co-occurring species and contribute to the success and failure of particular species during the succession over time.

Light energy partitioning, photosynthetic efficiency and biomass allocation in invasive Prunus serotina and native Quercus petraea in relation to light environment, competition and allelopathy

This study addressed whether competition under different light environments was reflected by changes in leaf absorbed light energy partitioning, photosynthetic efficiency, relative growth rate and biomass allocation in invasive and native competitors. Additionally, a potential allelopathic effect of mulching with invasive Prunus serotina leaves on native Quercus petraea growth and photosynthesis was tested. The effect of light environment on leaf absorbed light energy partitioning and photosynthetic characteristics was more pronounced than the effects of interspecific competition and allelopathy. The quantum yield of PSII of invasive P. serotina increased in the presence of a competitor, indicating a higher plasticity in energy partitioning for the invasive over the native Q. petraea, giving it a competitive advantage. The most striking difference between the two study species was the higher crown-level net CO2 assimilation rates (Acrown) of P. serotina compared with Q. petraea. At the juvenile life stage, higher relative growth rate and higher biomass allocation to foliage allowed P. serotina to absorb and use light energy for photosynthesis more efficiently than Q. petraea. Species-specific strategies of growth, biomass allocation, light energy partitioning and photosynthetic efficiency varied with the light environment and gave an advantage to the invader over its native competitor in competition for light. However, higher biomass allocation to roots in Q. petraea allows for greater belowground competition for water and nutrients as compared to P. serotina. This niche differentiation may compensate for the lower aboveground competitiveness of the native species and explain its ability to co-occur with the invasive competitor in natural forest settings.

Pitcher plant facilitates prey capture in a sympatric congener

Carnivorous plants avoid below-ground competition for nitrogen by utilizing an alternative nitrogen resource—invertebrate prey, but it remains unclear if sympatric carnivorous plants compete for prey resources. The aim of this study was to investigate if exploitative prey-resource competition occurs between the two sympatric pitcher plant species, Nepenthes rafflesiana and N. gracilis in Singapore. We first investigated if prey-resource partitioning occurs between these two species, and then investigated niche shift in N. gracilis by examining its pitcher contents along an in situ gradient of N. rafflesiana interspecific competition. Our results showed clear evidence of resource partitioning between the two species, but contrary to the expectation of competition, proximity to N. rafflesiana pitchers correlated with higher total prey numbers in N. gracilis pitchers. Our multivariate model of prey assemblages further suggested that N. rafflesiana facilitates N. gracilis prey capture, especially in several ant taxa that are trapped by both species. Concurrently, we found strong evidence for intraspecific competition between N. gracilis pitchers, suggesting that prey resources are exhaustible by pitcher-predation. Our results show that resource partitioning can be associated with facilitative interactions, instead of competition as is usually assumed. Facilitation is more typically expected between phylogenetically distant species, but divergences in resource acquisition strategies can permit facilitation between congeners.

Strong competitive effects of African savanna C4 grasses on tree seedlings do not support rooting differentiation

Abstract:Rooting differentiation between established trees and grasses has been well documented in savannas, but it remains unclear to what extent tree-grass rooting differences affect competition between newly established seedlings and grasses. To examine this question, a greenhouse experiment was conducted at the University of Missouri, USA. Twenty 3-mo-old seedlings each of two African savanna tree species (Acacia nigrescens and Colophospermum mopane) were grown for 8 mo with two crossed factors: grass competition and irrigation depth. Strong negative effects of grass competition on final seedling biomass and leaf photosynthetic and stomatal conductance occurred in both tree species, but no effects of irrigation depth were detected. There was a clear tree species by grass competition interaction, suggesting interspecific variation in competitive response. The results emphasize the importance of below-ground competition with grasses for physiological and morphological responses of tree seedlings, while minimizing the importance of tree-grass rooting depth differences as a factor in modulating the competitive response of trees to grasses at the seedling stage.