Patterning ecological restoration after weeds

The United Nations Decade on Ecosystem Restoration aims to prevent, halt, and reverse the degradation of ecosystems on every continent. Disturbances stemming from anthropogenic or natural causes make plant community restoration challenging. The introduction of fast-growing weeds that generate high biomass and produce copious seed is most threatening to plant communities. A paradigm shift in ecosystem restoration is needed that emphasizes traits and affected ecological processes similar to weeds. The repeated introduction of seed from native plants with weedy characteristics follows the propagule pressure and evolution of invasiveness hypotheses. In targeting areas with heavy weed populations, native plants could establish and more successfully develop into functioning plant communities

Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates

Comparing biosphere–atmosphere carbon exchange across monsoon (warm-season rainfall) and Mediterranean (cool-season rainfall) regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of the different precipitation seasonality. Considerable crossover behavior occurred between the two rainfall regimes. As expected in these usually water-limited ecosystems, precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature and/or light also played an important role in regulating GEP and NEE at all sites. If significant rainfall occurred in the winter at the Arizona sites, their behavior was characteristically Mediterranean whereby the carbon flux responses were delayed till springtime. Likewise, the Spanish Mediterranean sites showed immediate pulse-like responses to rainfall events in non-winter periods. The observed site differences were likely due to differences in vegetation, soils, and climatology. Together, these results support a more unified conceptual model for which processes governing carbon cycling in semiarid ecosystems need not differ between warm-season and cool-season rainfall regimes.This paper is the result of a fellowship funded by the OECD Co-operative Research Programme: Biological Resource Management for Sustainable Agricultural Systems to R.L. Scott. This paper has been supported in part by the Andalusian regional government project GEOCARBO and GLOCHARID (P08-RNM-3721), European Union Funds (ERDF and ESF), the Spanish flux-tower network CARBORED-ES (Science Ministry project CGL2010-22193-C04-02), and the European Commission collaborative project GHG Europe (FP7/2007-2013; grant agreement 244122)

Agricultural Research Service Weed Science Research: Past, Present, and Future

The U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) has been a leader in weed science research covering topics ranging from the development and use of integrated weed management (IWM) tactics to basic mechanistic studies, including biotic resistance of desirable plant communities and herbicide resistance. ARS weed scientists have worked in agricultural and natural ecosystems, including agronomic and horticultural crops, pastures, forests, wild lands, aquatic habitats, wetlands, and riparian areas. Through strong partnerships with academia, state agencies, private industry, and numerous federal programs, ARS weed scientists have made contributions to discoveries in the newest fields of robotics and genetics, as well as the traditional and fundamental subjects of weed-crop competition and physiology and integration of weed control tactics and practices. Weed science at ARS is often overshadowed by other research topics; thus, few are aware of the long history of ARS weed science and its important contributions. This review is the result of a symposium held at the Weed Science Society of America\u27s 62nd Annual Meeting in 2022 that included 10 separate presentations in a virtual Weed Science Webinar Series. The overarching themes of management tactics (IWM, biological control, and automation), basic mechanisms (competition, invasive plant genetics, and herbicide resistance), and ecosystem impacts (invasive plant spread, climate change, conservation, and restoration) represent core ARS weed science research that is dynamic and efficacious and has been a significant component of the agency\u27s national and international efforts. This review highlights current studies and future directions that exemplify the science and collaborative relationships both within and outside ARS. Given the constraints of weeds and invasive plants on all aspects of food, feed, and fiber systems, there is an acknowledged need to face new challenges, including agriculture and natural resources sustainability, economic resilience and reliability, and societal health and well-being

Interactions among Plant Species and Microorganisms in Salt Marsh Sediments

The interactions among Spartina patens and sediment microbial populations and the interactions among Phragmites australis and sediment microbial populations were studied at monotypic sites in Piermont Marsh, a salt marsh of the Hudson River north of New York, N.Y., at key times during the growing season. Arbuscular mycorrhizal fungi (AMF) effectively colonized S. patens but not P. australis, and there were seasonal increases and decreases that coincided with plant growth and senescence (17 and 6% of the S. patens root length were colonized, respectively). In sediment samples from the Spartina site, the microbial community and specific bacterial populations were at least twice as large in terms of number and biomass as the microbial community and specific bacterial populations in sediment samples from the Phragmites site, and peak values occurred during reproduction. Members of the domain Bacteria, especially members of the α-, γ-, and δ-subdivisions of the Proteobacteria, were the most abundant organisms at both sites throughout the growing season. The populations were generally more dynamic in samples from the Spartina site than in samples from the Phragmites site. No differences between the two sites and no differences during the growing season were observed when restriction fragment length polymorphism analyses of nifH amplicons were performed in an attempt to detect shifts in the diversity of nitrogen-fixing bacteria. Differences were observed only in the patterns generated by PCR or reverse transcription-PCR for samples from the Spartina site, suggesting that there were differences in the overall and active populations of nitrogen-fixing bacteria. Regression analyses indicated that there was a positive interaction between members of the δ-subdivision of the Proteobacteria and root biomass but not between members of the δ-subdivision of the Proteobacteria and macroorganic matter at both sites. In samples from the Spartina site, there were indications that there were bacterium-fungus interactions since populations of members of the α-subdivision of the Proteobacteria were negatively associated with AMF colonization and populations of members of the γ-subdivision of the Proteobacteria were positively associated with AMF colonization

Effects of surface and sub-surface soil horizons on the seasonal performance of Larrea tridentata (creosotebush)

1. In warm arid and semiarid environments, the accumulation of clay minerals produces increasingly well developed soil horizons with the passage of time. Differences in the strength of development of two prominent soil horizons, silt- and clay-rich surface vesicular (Av), and clay-enriched subsurface argillic (Bt), may strongly influence the amount and seasonal continuity of plant-available water and the physiological activity of long-lived desert shrubs. Three sites were selected on an alluvial piedmont (bajada) in the Mojave Desert that varied in surface and subsurface horizon development. The first site, a deep deposit of stabilized dune sand, entirely lacked soil horizon development. The second site had a well developed surface stone pavement and underlying Av horizon, but lacked an argillic horizon in the sandy subsoil. The third had a well developed surface pavement and Av horizon, and a deeper, well developed clay-rich argillic horizon. Seasonal water potential and gas-exchange responses of the evergreen desert shrub Larrea tridentata[DC.] Cov., and volumetric soil water content (θ), were measured monthly in 1996 on these three soils in order to test the hypothesis that desert pavements, Av and subsurface Bt horizons differentially affect the effectiveness and utilization of seasonal precipitation. 2. Predawn and midday water potentials (ψpd and ψmid), net photosynthetic rates (Anet), and stomatal conductances (gs) in L. tridentata were highest in the deep, sandy dune soils lacking horizons that could restrict surface and subsurface infiltration. Plants growing in these soils also showed no physiological response to summer precipitation events. Following a single large precipitation event during the growing season (3·8 cm), the water potentials, Anet and gs in L. tridentata were similar in the first (sand dune) and second (pavement and Av horizon) sites. However, plant performance on these soil surfaces showed marked seasonal declines, and did not respond to a small pulse of summer rainfall. Plants growing at the third site (older soils with strongly developed pavement, Av and Bt horizons) had very low gas-exchange rates and water potentials. However, following convectional summer thunderstorms L. tridentata showed improved water relations and gas exchange in these soils. 3. Midday water potentials were frequently anomalously higher than predawn water potentials, up to +6 MPa late in the growing season, especially on soil surfaces with well developed soil horizons. This anomaly was due to seasonal decreases in ψpd accompanied by invariant midday ψ. In general, ψpd was correlated with θ across the depths measured. 4. Correlations between ψmid with θ at 35 cm increased dramatically with increasing Bt horizon development, suggesting that seasonal ψmid may have been due to the vertical translocation of water (\u27hydraulic lift\u27). 5. Our results show that subsurface and surficial soil horizon development differentially affects the seasonal availability of water for desert plants. During wetter parts of the season, subsurface horizons limit the degree of water availability, while surface soil characteristics have the greatest influence on the effectiveness of summer precipitation. These findings suggest that the effectiveness of climatic precipitation and attendant plant utilization of water resources in warm desert systems may depend on the physical soil condition

Reproductive allocation and seed production in Bromus madritensis ssp. rubens at elevated atmospheric CO2

1. Two trends are consistent across the response of plant species to growth at elevated CO2: decreased leaf nitrogen content and increased photosynthetic gas exchange. While both of these are very important to the understanding of plant and ecosystem responses to climate change, little research has evaluated the consequences of these patterns on reproductive allocation and seed production. 2. For this reason, Bromus madritensis ssp. rubens was grown in ambient (360 μmol mol–1), × 1·5 ambient (550 μmol mol–1) and elevated (700 μmol mol–1) CO2 environments to compare the relationship between allocation to growth and reproduction as a function of CO2 growth environment. 3. There were no differences in final total biomass or reproductive mass between CO2 growth environments. There were significant decreases in reproductive mass per unit total mass and per unit vegetative mass, but not per unit leaf surface area (LSA), as growth CO2 environment increased from 360 to 700 μmol mol–1 CO2. Despite these decreases, the number of seeds produced per unit LSA in elevated CO2 significantly increased as compared to ambient CO2. These results may be owing to a shift in allocation to greater investment in vegetative growth as compared to reproduction under elevated levels of atmospheric CO2. 4. Prior to reproduction, there were no significant differences between CO2 treatments in carbon uptake by leaves. In contrast, plants grown in elevated CO2 did not show a decline in photosynthetic rate during seed filling, suggesting that nitrogen may not have been re-translocated from leaves to seeds as apparently occurred in ambient plants. 5. Patterns measured here may partially explain the parental effect of CO2 environment exhibited in Bromus. Seeds produced from elevated parental CO2 growth conditions lead to seedlings that produce smaller leaves that are delayed in development and smaller roots as compared to structures produced by seeds from ambient-grown parents. 6. Because the success of Bromus is partially owing to its ability to produce large numbers of viable seeds, these changes in reproductive allocation and subsequent seedling performance with respect to growth in an elevated CO2 environment may have impacts on community composition in the Mojave Desert

Effects of elevated CO2 and temperature stress on ecosystem processes

In this chapter, we examine the effects of temperature stress and elevated CO2 on ecosystem processes, including primary production, nutrient cycling, and landscape water and energy balance. Although we examine the interplay between changing CO2 and rising temperature on ecosystem processes, an important focus of this book is on the effects of environmental stress in concert with elevated CO2 on biological systems. Environmental stress has been defined in many ways, and often varies depending on whether it is applied to a cultivated production system or to an ecosystem. In its broadest sense, environmental stress is any environmental factor that lowers primary production below its optimum (Osmond et al., 1987). In this context, any nonoptimum temperature can be considered stressful. However, a more narrow view of environmental stress would require that an environmental condition (e.g., freezing/chilling temperatures, heat stress) cause some type of damage to the plant in question. Severe damage to be seen to have clear impacts on ecosystem processes. However, there are very few elevated-CO2 studies that have examined this important type of temperature response in plants. Also of interest from an ecosystem perspective is recurring environmental stress, which can result in either physiological hardening in plants (which may be irreversible within a given growing season) or genetic responses of populations over time, such that the ecosystem equilibrates to nonoptimum condition through a loss of productivity and/or a shift to more stress-tolerant genotypes and functional types (Chapin, 1991)

Cool-season whole-plant gas exchange of exotic and native semiarid bunchgrasses

The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (θ25cm) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55 mm in 2009) and El Niño-enhanced accumulations (154 mm in 2010) in a SE Arizona savanna. Across both years, h25cm was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Niño, indicating greater CO2 assimilation. Ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUEe = NEE/ET), while bush muhly WUEe remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUEp = GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUEe may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates

Effects of elevated CO2 (FACE) on the functional ecology of the drought-deciduous Mojave Desert shrub, Lycium andersonii

Elevated CO2 may improve the productivity of cool-season active (‘drought-deciduous’) shrubs in the deserts of southwestern North America by reducing early-season phenological constraints imposed by low leaf area when photosynthetic capacity is high and later-season physiological limitations from declining photosynthesis and midday water potentials. Altered productivity under elevated CO2 would depend on the specific responses of short-shoots that only provide early-season leaf area display, and long-shoots which determine annual growth increment in these plants. We measured plant water relations, photosynthetic gas exchange, and growth in short- and long-shoots of the drought-deciduous shrub, Lycium andersonii, under Free Air CO2 Enrichment (FACE) in the field in an intact Mojave Desert ecosystem. We were specifically interested in the differential effects CO2 enrichment would have on short-shoots and actively growing long-shoots during canopy development. Net photosynthesis (Anet) was similar in elevated compared with ambient CO2, but stomatal conductance (gs) was reduced by 27% in both shoot types. L. andersonii growing in elevated CO2 had larger leaves on short-shoots, and more leaves per shoot length on long-shoots. Enhanced leaf growth did not counter lower gs, and midday plant water potential was similar between treatments. In both short- and long-shoots, down-regulation of light-saturated photosynthetic electron transport rate (Jmax) occurred under elevated CO2. However, the balance between rubisco efficiency (estimated by the maximum carboxylation rate of rubisco, Vcmax), and electron transport capacity (Vcmax/Jmax) remained constant in short-shoots, but increased in elevated CO2 grown long-shoots. Apparent quantum requirement was similar, while light-saturated photosynthetic rates (Amax) decreased by approximately 30% under elevated CO2 in both shoot-types. These results suggest that elevated CO2 lowered investment to photosynthetic electron transport capacity and whole-plant water use, even when leaf growth was stimulated. Such canopy dynamics are likely to enhance the ability of this drought-deciduous species to better cope with the highly variable inter- and intra-annual climate regimes characteristic of North American deserts

Low temperature tolerance and cold acclimation for seedlings of three Mojave Desert Yucca species exposed to elevated CO2

Leaf tolerance to low temperatures, as determined by vital stain uptake and chlorophyll a fluorescence, was compared for seedlings of three Yucca species native to the south-western United States: Yucca brevifolia, which is distributed throughout the Mojave Desert;Yucca schidigera, which occurs in both coastal and desert California; and Yucca whipplei, which is primarily coastal but occurs in portions of the Mojave Desert. Seedlings maintained at day/night glasshouse air temperatures of 40/25°C or 20/5°C, and under ambient (360 μmol mol−1) or elevated (700 μmol mol−1) levels of CO2were compared to test the hypothesis that cold acclimation and freezing tolerance are enhanced by exposure to elevated CO2. Plants maintained at elevated CO2 had greater low-temperature tolerance compared to controls, yet a larger shift in survival was attributable to the downward shift in day/night temperatures. Low-temperature tolerance was similar to extreme minimum air temperatures for the collection sites averaged over the period 1961 to 1990. For seedlings exposed to elevated CO2, low-temperature tolerance was −11·9°C for Yucca brevifolia, −9·6°C for Y. schidigera, and −13·5°C for Y. whipplei. Elevated CO2 caused excitation energy transfer in Photosystem II (measured as FV/FM) to be maintained at lower temperatures for Yucca brevifolia and Y. whipplei. ΦPSII at low temperatures was increased due to elevated CO2for Y. brevifolia only. The results suggest that survival during episodic sub-zero temperature events will be enhanced for seedlings of these three yucca species in a future elevated CO2 environment