Anthony Manea

Competitive interactions between native and invasive exotic plant species are altered under elevated carbon dioxide

We hypothesized that the greater competitive ability of invasive exotic plants relative to native plants would increase under elevated CO2 because they typically have traits that confer the ability for fast growth when resources are not limiting and thus are likely to be more responsive to elevated CO2. A series of competition experiments under ambient and elevated CO2 glasshouse conditions were conducted to determine an index of relative competition intensity for 14 native-invasive exotic species-pairs. Traits including specific leaf area, leaf mass ratio, leaf area ratio, relative growth rate, net assimilation rate and root weight ratio were measured. Competitive rankings within species-pairs were not affected by CO2 concentration: invasive exotic species were more competitive in 9 of the 14 species-pairs and native species were more competitive in the remaining 5 species-pairs, regardless of CO2 concentration. However, there was a significant interaction between plant type and CO2 treatment due to reduced competitive response of native species under elevated compared with ambient CO2 conditions. Native species had significantly lower specific leaf area and leaf area ratio under elevated compared with ambient CO2. We also compared traits of more-competitive with less-competitive species, regardless of plant type, under both CO2 treatments. More-competitive species had smaller leaf weight ratio and leaf area ratio, and larger relative growth rate and net assimilation rate under both ambient and elevated CO2 conditions. These results suggest that growth and allocation traits can be useful predictors of the outcome of competitive interactions under both ambient and elevated CO2 conditions. Under predicted future atmospheric CO2 conditions, competitive rankings among species may not change substantially, but the relative success of invasive exotic species may be increased. Thus, under future atmospheric CO2 conditions, the ecological and economic impact of some invasive exotic plants may be even greater than under current conditions.

Do invasive alien plants benefit more from global environmental change than native plants

Abstract Invasive alien plant species threaten native biodiversity, disrupt ecosystem functions and can cause large economic damage. Plant invasions have been predicted to further increase under ongoing global environmental change. Numerous case studies have compared the performance of invasive and native plant species in response to global environmental change components (i.e. changes in mean levels of precipitation, temperature, atmospheric CO2 concentration or nitrogen deposition). Individually, these studies usually involve low numbers of species and therefore the results cannot be generalized. Therefore, we performed a phylogenetically controlled meta‐analysis to assess whether there is a general pattern of differences in invasive and native plant performance under each component of global environmental change. We compiled a database of studies that reported performance measures for 74 invasive alien plant species and 117 native plant species in response to one of the above‐mentioned global environmental change components. We found that elevated temperature and CO2 enrichment increased the performance of invasive alien plants more strongly than was the case for native plants. Invasive alien plants tended to also have a slightly stronger positive response to increased N deposition and increased precipitation than native plants, but these differences were not significant (N deposition: P = 0.051; increased precipitation: P = 0.679). Invasive alien plants tended to have a slightly stronger negative response to decreased precipitation than native plants, although this difference was also not significant (P = 0.060). So while drought could potentially reduce plant invasion, increases in the four other components of global environmental change considered, particularly global warming and atmospheric CO2 enrichment, may further increase the spread of invasive plants in the future. &NA; We performed a phylogenetically‐controlled meta‐analysis to assess whether there is a general pattern of differences in invasive and native plant performance under each component of global environmental change, using a database of studies that reported performance measures for 74 invasive alien plant species and 117 native plant species. We found that elevated temperature and CO2 enrichment increased performance of invasive alien plants more strongly than was the case for native plants. Invasive alien plants tended to also have a slightly stronger positive response to increased N deposition and increased precipitation than native plants, but these differences were not significant. Invasive alien plants tended to have a slightly stronger negative response to decreased precipitation than native plants, although this difference was also not significant. Figure. No caption available.

Exotic C4 grasses have increased tolerance to glyphosate under elevated carbon dioxide.

The increase in atmospheric CO2 levels can influence the growth of many invasive exotic plant species. However, it is not well-documented, especially for C4 plants, how these growth responses will alter the effectiveness of the worlds most widely used herbicide for weed control, glyphosate. We aimed to address this question by carrying out a series of glasshouse experiments to determine if tolerance to glyphosate is increased in four C4 invasive exotic grasses grown under elevated CO2 in nonlimiting water conditions. In addition, traits including specific leaf area, leaf weight ratio, leaf area ratio, root ∶ shoot ratio, total leaf area, and total biomass were measured in order to assess their contribution to glyphosate response under ambient and elevated CO2 levels. Three of the four mature grass species that were treated with the recommended concentration of glyphosate displayed increased tolerance to glyphosate under elevated CO2. This was due to increased biomass production resulting in a dilution effect on the glyphosate within the plant. From this study, we can conclude that as atmospheric CO2 levels increase, application rates of glyphosate might need to be increased to counteract the growth stimulation of invasive exotic plants. Nomenclature: Glyphosate.

Competitive interactions between established grasses and woody plant seedlings under elevated CO2 levels are mediated by soil water availability

The expansion of woody plants into grasslands has been observed worldwide and is likely to have widespread ecological consequences. One proposal is that woody plant expansion into grasslands is driven in part by the rise in atmospheric CO2 concentrations. We have examined the effect of CO2 concentration on the competitive interactions between established C4 grasses and woody plant seedlings in a model grassland system. Woody plant seedlings were grown in mesocosms together with established C4 grasses in three competition treatments (root competition, shoot competition and root + shoot competition) under ambient and elevated CO2 levels. We found that the growth of the woody plant seedlings was suppressed by competition from grasses, with root and shoot competition having similar competitive effects on growth. In contrast to expectations, woody plant seedling growth was reduced at elevated CO2 levels compared to that at the ambient CO2 level across all competition treatments, with the most plausible explanation being reduced light and soil water availability in the elevated CO2 mesocosms. Reduced light and soil water availability in the elevated CO2 mesocosms was associated with an increased leaf area index of the grasses which offset the reductions in stomatal conductance and increased rainfall interception. The woody plant seedlings also had reduced ‘escapability’ (stem biomass and stem height) under elevated compared to ambient CO2 levels. Our results suggest that the expansion of woody plants into grasslands in the future will likely be context-dependent, with the establishment success of woody plant seedlings being strongly coupled to the CO2 response of competing grasses and to soil water availability.

Leaf Area Index Drives Soil Water Availability and Extreme Drought-Related Mortality under Elevated CO2 in a Temperate Grassland Model System

The magnitude and frequency of climatic extremes, such as drought, are predicted to increase under future climate change conditions. However, little is known about how other factors such as CO2 concentration will modify plant community responses to these extreme climatic events, even though such modifications are highly likely. We asked whether the response of grasslands to repeat extreme drought events is modified by elevated CO2, and if so, what are the underlying mechanisms? We grew grassland mesocosms consisting of 10 co-occurring grass species common to the Cumberland Plain Woodland of western Sydney under ambient and elevated CO2 and subjected them to repeated extreme drought treatments. The 10 species included a mix of C3, C4, native and exotic species. We hypothesized that a reduction in the stomatal conductance of the grasses under elevated CO2 would be offset by increases in the leaf area index thus the retention of soil water and the consequent vulnerability of the grasses to extreme drought would not differ between the CO2 treatments. Our results did not support this hypothesis: soil water content was significantly lower in the mesocosms grown under elevated CO2 and extreme drought-related mortality of the grasses was greater. The C4 and native grasses had significantly higher leaf area index under elevated CO2 levels. This offset the reduction in the stomatal conductance of the exotic grasses as well as increased rainfall interception, resulting in reduced soil water content in the elevated CO2 mesocosms. Our results suggest that projected increases in net primary productivity globally of grasslands in a high CO2 world may be limited by reduced soil water availability in the future.

Reductions in native grass biomass associated with drought facilitates the invasion of an exotic grass into a model grassland system

The invasion success of exotic plant species is often dependent on resource availability. Aspects of climate change such as rising atmospheric CO2 concentration and extreme climatic events will directly and indirectly alter resource availability in ecological communities. Understanding how these climate change-associated changes in resource availability will interact with one another to influence the invasion success of exotic plant species is complex. The aim of the study was to assess the establishment success of an invasive exotic species in response to climate change-associated changes in resource availability (CO2 levels and soil water availability) as a result of extreme drought. We grew grassland mesocosms consisting of four co-occurring native grass species common to the Cumberland Plain Woodland of western Sydney, Australia, under ambient and elevated CO2 levels and subjected them to an extreme drought treatment. We then added seeds of a highly invasive C3 grass, Ehrharta erecta, and assessed its establishment success (biomass production and reproductive output). We found that reduced biomass production of the native grasses in response to the extreme drought treatment enhanced the establishment success of E. erecta by creating resource pulses in light and space. Surprisingly, CO2 level did not affect the establishment success of E. erecta. Our results suggest that the invasion risk of grasslands in the future may be coupled to soil water availability and the subsequent response of resident native vegetation therefore making it strongly context- dependent.

Are fire resprouters more carbon limited than non-resprouters? Effects of elevated CO2 on biomass, storage and allocation of woody species

Resprouting ability is a key functional trait determining plant responses and vegetation dynamics after disturbances such as fire that shape most global biomes. It is likely that rising atmospheric CO2 concentrations will alter resource allocation patterns in plants which in turn will alter resprouting responses. In this study, we asked: (1) do resprouters have greater allocation to storage than non-resprouters?; (2) if so, do resprouters account for this negative carbon balance by having reduced growth?; and (3) do resprouters have a relatively weaker growth response compared to non-resprouters under elevated CO2 levels due to their increased allocation to storage? To address these questions, we grew congeneric species-pairs of shrubs common to south-eastern Australia with contrasting resprouting abilities under ambient and elevated CO2 levels. We found that resprouters in general allocated more resources to storage (root non-structural carbohydrates and biomass) and had less total biomass than non-resprouters. Under elevated CO2 levels both sprouting types increased biomass production, suggesting they were carbon limited. Surprisingly, the resprouters allocated this additional carbon to biomass rather than to storage. This suggests that although elevated CO2 levels may not affect resprouting ability directly in resprouters, it may enhance other aspects of persistence such as escapability and bud protection. Furthermore, non-resprouters may also benefit from the additional carbon by being able to set seed more quickly and increase seed production thus enhancing their recruitment after fire. Thus, the relative benefits of elevated CO2 levels on resprouters versus non-resprouters remain equivocal.

Leaf flammability and fuel load increase under elevated CO2 levels in a model grassland

Fire is a common process that shapes the structure of grasslands globally. Rising atmospheric CO2 concentration may have a profound influence on grassland fire regimes. In this study, we asked (1) does CO2 and soil P availability alter leaf flammability (ignitibility and fire sustainability); (2) are leaf tissue chemistry traits drivers of leaf flammability, and are they modified by CO2 and soil P availability?; (3) does CO2 and soil P availability alter fuel load accumulation in grasslands; and (4) does CO2 and soil P availability alter the resprouting ability of grassland species? We found that leaf flammability increased under elevated CO2 levels owing to decreased leaf moisture content and foliar N, whereas fuel load accumulation increased owing to decreased foliar N (slower decomposition rates) and increased aboveground biomass production. These plant responses to elevated CO2 levels were not modified by soil P availability. The increase in leaf flammability and fuel load accumulation under elevated CO2 levels may alter grassland fire regimes by facilitating fire ignition as well as shorter fire intervals. However, the increased root biomass of grasses under elevated CO2 levels may enhance their resprouting capacity relative to woody plants, resulting in a shift in the vegetation structure of grasslands.