The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

Increased soil N availability may often facilitate plant invasions. Therefore, lowering N availability might reduce these invasions and favor desired species. Here, we review the potential efficacy of several commonly proposed management approaches for lowering N availability to control invasion, including soil C addition, burning, grazing, topsoil removal, and biomass removal, as well as a less frequently proposed management approach for lowering N availability, establishment of plant species adapted to low N availability. We conclude that many of these approaches may be promising for lowering N availability by stimulating N immobilization, even though most are generally ineffective for removing N from ecosystems (excepting topsoil removal). C addition and topsoil removal are the most reliable approaches for lowering N availability, and often favor desired species over invasive species, but are too expensive or destructive, respectively, for most management applications. Less intensive approaches, such as establishing low-N plant species, burning, grazing and biomass removal, are less expensive than C addition and may lower N availability if they favor plant species that are adapted to low N availability, produce high C:N tissue, and thus stimulate N immobilization. Regardless of the method used, lowering N availability sufficiently to reduce invasion will be difficult, particularly in sites with high atmospheric N deposition or agricultural runoff. Therefore, where feasible, the disturbances that result in high N availability should be limited in order to reduce invasions by nitrophilic weeds.

Summary 1.Invasive plants pose a major threat to native plant communities around the globe. Current methods of controlling invasive vegetation focus on eradication of existing populations, and are often effective only in the short term. Manipulating resource availability to give native species a competitive advantage over invasive species could reduce ecosystem vulnerability to invasion and might more effectively control invasive vegetation. We evaluated this approach for controlling invasions of sedge meadow communities by Phalaris arundinacea , a widespread invasive grass in North American wetlands. 2. To test whether lowering nitrogen (N) availability would allow a wetland sedge, Carex hystericina , to suppress Phalaris competitively, we examined Carex and Phalaris competition under a range of inorganic N concentrations (25-400 mg kg − 1 ) in a glasshouse. We lowered N availability in wetland soil using carbon enrichment and repeated harvests of a cover crop, and then created a N gradient by applying NH 4 -N to the N-depleted soil. 3. In soil without carbon added, competition with Phalaris reduced Carex biomass by 91%, while competition with Carex did not influence Phalaris , as is commonly observed in sedge meadows. Phalaris biomass was five times Carex biomass in mixed stands. Conversely, in soil depleted of available N via carbon enrichment, competition with Carex reduced Phalaris biomass by 82%, while competition with Phalaris reduced Carex biomass by only 32%, indicating that Carex is the superior competitor for N. Carex biomass was six times Phalaris biomass in mixed stands in the carbon-enriched soil. 4. Carbon enrichment lowered soil inorganic N by 10-30 mg kg − 1 . NH 4 -N addition mitigated the negative effects of carbon on Phalaris growth and competitive ability, indicating that carbon enrichment altered competitive outcomes by lowering N availability. Greater Carex N uptake efficiency under N-poor conditions appeared to account for the Carex competitive ability for N. 5. Synthesis and applications . Carex dominance in carbon-enriched soil strongly suggests that lowering soil inorganic N to < 30 mg kg − 1 in restored wetlands would allow establishing sedge meadow communities to suppress Phalaris invasions. Low-N soils might be achieved via carbon enrichment, vegetation harvests and reduced N inputs. Reducing community vulnerability to invasion by manipulating resource availability appears to be a promising approach to invasive species management.

Riparian ecosystems, already greatly altered by water management, land development, and biological invasion, are being further altered by increasing atmospheric CO 2 concentrations ([CO 2 ]) and climate change, particularly in arid and semiarid (dryland) regions. In this literature review, we (1) summarize expected changes in [CO 2 ], climate, hydrology, and water management in dryland western North America, (2) consider likely effects of those changes on riparian ecosystems, and (3) identify critical knowledge gaps. Temperatures in the region are rising and droughts are becoming more frequent and intense. Warmer temperatures in turn are altering river hydrology: advancing the timing of spring snow melt floods, altering flood magnitudes, and reducing summer and base flows. Direct effects of increased [CO 2 ] and climate change on riparian ecosystems may be similar to effects in uplands, including increased heat and water stress, altered phenology and species geographic distributions, and disrupted trophic and symbiotic interactions. Indirect effects due to climate-driven changes in streamflow, however, may exacerbate the direct effects of warming and increase the relative importance of moisture and fluvial disturbance as drivers of riparian ecosystem response to global change. Together, climate change and climate-driven changes in streamflow are likely to reduce abundance of dominant, native, early-successional tree species, favor herbaceous species and both drought-tolerant and late-successional woody species (including many introduced species), reduce habitat quality for many riparian animals, and slow litter decomposition and nutrient cycling. Climate-driven changes in human water demand and associated water management may intensify these effects. On some regulated rivers, however, reservoir releases could be managed to protect riparian ecosystem. Immediate research priorities include determining riparian species' environmental requirements and monitoring riparian ecosystems to allow rapid detection and response to undesirable ecological change.

Summary1 Plant allelopathic compounds may have other roles than interspecific interference. We investigated whether (±)-catechin, a phytotoxin exuded from the roots of the exotic invader Centaurea maculosa (spotted knapweed), is also one of the factors that regulates C. maculosa recruitment. 2 Adding activated carbon, which adsorbs organic compounds, to soil around C. maculosa adults in the field increased seedling density by 78% 25 days after sowing, and by 34% 32 days after sowing, suggesting that soil-borne compounds inhibited or delayed recruitment. 3 Analysis of field soils near mature C. maculosa revealed that they can contain exceptionally high (±)-catechin concentrations (mean = 1.55 mg g −1 dry soil, with 60% of samples containing ≥ 1.0 mg (±)-catechin g −1 ). 4 In laboratory experiments, treatment with ≥ 1.0 mg (±)-catechin mL −1 reduced seedling root elongation by > 50%, indicating that field concentrations are sufficient to inhibit C. maculosa recruitment. Provided that 10% methanol was used to maintain (±)-catechin in solution for > 1 day, treatment with ≥ 1.0 mg mL −1 also reduced C. maculosa germination by > 70%. 5 (±)-Catechin maintained in solution with methanol did not significantly reduce C. maculosa seed survival, suggesting that inhibition of germination was due, at least in part, to delayed germination rather than to seed mortality. 6 Depending on the solubility of (±)-catechin in soil and on the duration of its effects on recruitment, C. maculosa may avoid intraspecific competition or regulate the timing of seedling establishment by reducing seedling growth or postponing germination in response to its own phytotoxin. 7 Chemical regulation of C. maculosa recruitment, as demonstrated here, suggests a dual role of (±)-catechin as an allelochemical and an autoinhibitor. The potential for a single plant root exudate to influence both inter-and intraspecific interactions emphasizes the complex effects that plant secondary metabolites may have on plant population and community structure.

The phytotoxin (+/-)-catechin has been proposed to mediate invasion and autoinhibition by the Eurasian plant Centaurea maculosa (spotted knapweed). The importance of (+/-)-catechin to C. maculosa ecology depends in part on whether sufficient catechin concentrations occur at appropriate times and locations within C. maculosa soil to influence neighboring plants. Previous research on catechin in C. maculosa soils has yielded conflicting results, with some studies finding high soil catechin concentrations and other, more recent studies finding little or no catechin in field soils. Here, we report the most extensive study of soil catechin concentrations to date. We examined soil catechin concentrations in 402 samples from 11 C. maculosa sites in North America sampled in consecutive months over 1 yr, excluding winter months. One site was sampled on seven dates, another was sampled twice, and the remaining nine sites were each sampled once on a range of sampling dates. Methods used were similar to those with which we previously measured high soil catechin concentrations. We detected catechin only in the site that was sampled on seven dates and only on one sampling date in that site (May 16 2006), but in all samples collected on that date. The mean soil catechin concentration on that date was 0.65 +/- 0.45 (SD) mg g(-1), comparable to previously reported high concentrations. There are a number of possible explanations for the infrequency with which we detected soil catechin in this work compared to previous studies. Differences in results could reflect spatial and temporal variation in catechin exudation or degradation, as we examined different sites in a different year from most previous studies. Also, large quantities of catechin were detected in blanks for two sampling periods in the present study, leading us to discard those data. This contamination suggests that previous reports of high catechin concentrations that did not include blanks should be viewed with caution. Our results suggest that pure catechin is only rarely present in C. maculosa bulk soils. Thus, although catechin may play a role in C. maculosa invasion, the infrequency of soil catechin that we determined in this study suggests that we cannot be as certain of its role as previous reports of high soil catechin concentrations suggested.

Burns, chartered this team to recommend an ecological basis for ecosystem management. This report is not intended to provide details on all aspects of ecosystem management; it simply provides information and makes recommendations for an ecological basis for ecosystem management. The report is not a decision document. It does not allocate resources on public lands nor does it make recommendations to that effect.The report of this Study Team may be relied upon as input in processes initiated under the National Environmental Policy Act (NEPA), National Forest Management Act (NFMA), Endangered Species Act (ESA), Administrative Procedures Act (APA), and other applicable laws. The information contained in this report is general in nature, rather than site specific. Implementation of ecosystem management and allocation of resources on Forest Service administered lands is the responsibility of the National Forest System in partnership with Forest Service Research and State and Private Forestry. Implementation is done through Forest and project plans that are subject to the NEPA process of disclosing the effects of proposed actions and affording the opportunity for public comment. The Southwestern Region follows a planning process for projects called Integrated Resource Management (IRM).The opinions expressed by the authors do not necessarily represent the policy or position of the U.S. Department of Agriculture, the Forest Service, The Nature Conservancy, or the Arizona Game and Fish Department.The Study Team acknowledges the valuable input of more than 50 individuals from various agencies, universities, professional organizations, and other groups who provided thoughtful comments of an earlier draft of this document. Some of their comments are included in Appendix 3. (Upper left) Road expansion to accommodate increased traffic in forested areas. Photo from Bev Driver. (Upper right) Cypripedium calceolus, a rare species found on the Santa Fe National Forest, New Mexico. Fine filter analyses help protect uncommon species. Photo by Reggie Fletcher. (Lower) Small openings in a ponderosa pine forest created by hotspots in a low-intensity fire. Photo by Ron Moody.

Summary• The flavonol ( ± )-catechin is an allelochemical produced by the invasive weed Centaurea maculosa (spotted knapweed). The full effects of ( ± )-catechin on plant communities in both the native and the introduced ranges of C. maculosa remain uncertain.• Here, by supplementing plant growth media with ( ± )-catechin, we showed that low ( ± )-catechin concentrations may induce growth and defense responses in neighboring plants. Doses of the allelochemical lower than the minimum inhibitory concentration (MIC) induced growth in Arabidopsis thaliana ; plants treated with 25 µg ml − 1 ( ± )-catechin accumulated more than twice the biomass of untreated control plants. Further, pretreatment of A. thaliana roots with low concentrations of ( ± )-catechin induced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 in A. thaliana leaves.• Low doses of ( ± )-catechin resulted in moderate increases in reactive oxygen species (ROS) in the meristems of treated plants, which may have loosened the cell walls and thus increased growth. Experiments with A. thaliana mutants indicated that ( ± )-catechin induces pathogen resistance by up-regulating defense genes via the salicylic acid (SA)/nonexpressor of pathogenesis related protein 1 (NPR1)-dependent pathway.• Our results suggest that the growth and defense-inducing effects of ( ± )-catechin are concentration dependent, as ( ± )-catechin at higher concentrations is phytotoxic, thus suggesting the potential for hormesis to occur in nature.