Physiological responses to short-term water and light stress in native and invasive plant species in southern California

Heterogeneous Impact of Water Warming on Exotic and Native Submerged and Emergent Plants in Outdoor Mesocosms

Some aquatic plants present high biomass production with serious consequences on ecosystem functioning. Such mass development can be favored by environmental factors. Temperature increases are expected to modify individual species responses that could shape future communities. We explored the impact of rising water temperature on the growth, phenology, and metabolism of six macrophytes belonging to two biogeographic origins (exotic, native) and two growth forms (submerged, emergent). From June to October, they were exposed to ambient temperatures and a 3 °C warming in outdoor mesocosms. Percent cover and canopy height were favored by warmer water for the exotic emergent Ludwigia hexapetala. Warming did not modify total final biomass for any of the species but led to a decrease in total soluble sugars for all, possibly indicating changes in carbon allocation. Three emergent species presented lower flavonol and anthocyanin contents under increased temperatures, suggesting lower investment in defense mechanisms and mitigation of the stress generated by autumn temperatures. Finally, the 3 °C warming extended and shortened flowering period for L. hexapetala and Myosotis scorpioides, respectively. The changes generated by increased temperature in outdoor conditions were heterogeneous and varied depending on species but not on species biogeographic origin or growth form. Results suggest that climate warming could favor the invasiveness of L. hexapetala and impact the structure and composition of aquatic plants communities.

Eco-physiological performance may contribute to differential success of two forms of an invasive vine, Dolichandra unguis-cati, in Australia

Invasive plant species are hypothesized as being more efficient at resource acquisition and use, resulting in faster growth than co-occurring non-invasive plant species. Nonetheless, some findings suggest that trait differences between invasive and non-invasive species are context dependent. In this study, two forms of an invasive vine species,Dolichandraunguis-cati, were used to test the context-dependent hypothesis.Dolichandraunguis-catiis a weed of national significance in Australia with two different forms: the ‘long pod’ (LP) and ‘short pod’ (SP). The two forms have different levels of distribution on the eastern Seaboard of the continent, with the SP form occurring extensively in both States of Queensland and New South Wales while the LP form is found only in isolated sites in South-East Queensland. This study examines whether differences in eco-physiological performance could be responsible for differential success of the two forms. A partially factorial experiment was set up in controlled conditions where potted plants of both forms were grown under two levels of light, water and nutrient resources (high and low) for 15 months. We measured several traits that are known to correlate with plant performance and resource use efficiency (RUE). The SP form exhibited higher values of carbon assimilation, RUE, number of subterranean tubers and leaf nitrogen than the LP form. However, the LP form produced greater biomass than the SP form, with the difference driven mainly by high resource conditions. The LP form displayed significantly higher phenotypic integration (number of traits significantly correlated) than the SP form in response to all treatments while the SP form exhibited higher phenotypic integration than the LP form in response to high resource conditions only. The SP form displayed traits that are well suited for successful colonization, possibly explaining its increased success in Australia, while the LP form possessed traits of opportunistic plants. Overall, we find that the two forms of the weedy vine deploy different carbon economies in response to resource conditions, which is evidence of the context-dependent trait hypothesis.

Rapid root responses of seedlings exposed to a postdrought water pulse

PREMISE OF THE STUDY

Mediterranean-type climate ecosystems experience significant variability in precipitation within and across years and may be characterized by periods of extreme drought followed by a brief, high-intensity precipitation pulse. Rapid root growth could be a key factor in effective utilization of precipitation pulses, leading to higher rates of seedling establishment. Changes in root growth rate are rarely studied, however, and patterns in seedling root traits are not well explored. We investigated the influence of an extreme postdrought precipitation event on seedlings that occur in southern California coastal sage scrub.

METHODS

We measured root elongation rate, root tip appearance rate, new leaf appearance rate, and canopy growth rate on 18 mediterranean species from three growth forms.

KEY RESULTS

Root elongation rate responded more strongly to the precipitation pulse than did root tip appearance rate and either metric of aboveground growth. The majority of species exhibited a significant change in root growth rate within 1 week of the pulse. Responses varied in rapidity and magnitude across species, however, and were not generally predictable based on growth form.

CONCLUSIONS

While the majority of species exhibited shifts in belowground growth following the pulse, the direction and magnitude of these morphological responses were highly variable within growth form. Understanding the implications of these different response strategies for plant fitness is a crucial next step to forecasting community dynamics within ecosystems characterized by resource pulses.

Plant functional traits of dominant native and invasive species in mediterranean-climate ecosystems

The idea that dominant invasive plant species outperform neighboring native species through higher rates of carbon assimilation and growth is supported by several analyses of global data sets. However, theory suggests that native and invasive species occurring in low-resource environments will be functionally similar, as environmental factors restrict the range of observed physiological and morphological trait values. We measured resource-use traits in native and invasive plant species across eight diverse vegetation communities distributed throughout the five mediterranean-climate regions, which are drought prone and increasingly threatened by human activities, including the introduction of exotic species. Traits differed strongly across the five regions. In regions with functional differences between native and invasive species groups, invasive species displayed traits consistent with high resource acquisition; however, these patterns were largely attributable to differences in life form. We found that species invading mediterranean-climate regions were more likely to be annual than perennial: three of the five regions were dominated by native woody species and invasive annuals. These results suggest that trait differences between native and invasive species are context dependent and will vary across vegetation communities. Native and invasive species within annual and perennial groups had similar patterns of carbon assimilation and resource use, which contradicts the widespread idea that invasive species optimize resource acquisition rather than resource conservation. .

Eco-Physiological Responses of Dominant Species to Watering in a Natural Grassland Community on the Semi-Arid Loess Plateau of China

Altered precipitation regimes significantly affect ecosystem structure and function in arid and semi-arid regions. In order to investigate effects of precipitation changes on natural grassland community in the semi-arid Loess Plateau, the current research examined eco-physiological characteristics of two co-dominant species (i.e., Bothriochloa ischaemum and Lespedeza davurica) and community composition following two watering instances (i.e., precipitation pulses, July and August, 2011, respectively) in a natural grassland community. Results showed that the photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration rapidly increased on the first to third day following watering in both species, and both months. Under watering treatments, the maximum net photosynthetic rates appeared on the second to third day after watering, which increased 30-80% in B. ischaemum and 40-50% in L. davurica compared with non-watering treatments, respectively. Leaf water use efficiency kept stable or initially decreased in both species under watering treatments. Watering in July produced more promoting effects on grass photosynthesis than in August, particularly in B. ischaemum. Community above-ground biomass at the end of the growing season increased after watering, although no significant changes in species diversity were observed. Our results indicated that timing and magnitude of watering could significantly affect plant eco-physiological processes, and there were species-specific responses in B. ischaemum and L. davurica. Pulsed watering increased community productivity, while did not significantly alter community composition after one growing season. The outcomes of this study highlight eco-physiological traits in dominant species may playing important roles in reshaping community composition under altered precipitation regimes.

Intra-annual rainfall regime shifts competitive interactions between coastal sage scrub and invasive grasses

Changes in rainfall distribution, generally predicted by many climate models, can affect resource dynamics and ecosystem function. While little studied, intra-annual rainfall distribution may have particularly strong effects on competitive interactions. Here, we test whether increased rainfall event size and decreased frequency within a growing season can influence competitive dynamics related to the invasion of exotic annual grasses in California coastal sage scrub (CSS). We hypothesized that larger rainfall events and decreased frequency will increase the competitive ability of native CSS species: a deeper root system will permit greater water use during dry periods between pulses and enhance their resource depletion effect on more shallow-rooted grasses. We planted grass and CSS seedlings in an additive competition design under three rainfall treatments: frequent small events, infrequent large events, and infrequent small events. The first two treatments had the same total rainfall but different frequency, while the second and third treatments had the same frequency but different total rainfall. Rainfall treatment altered the competitive interactions between CSS and grasses. In the first year, the competitive effect of annual grasses on shrub seedlings was strongest under the frequent small rainfall regime where they reduced deep soil moisture and light. In year two, the established shrubs began to exert strong competitive effects on grasses, and these effects were strongest under the infrequent small rainfall regime (low total rain) where they reduced shallow soil moisture and decreased grass stomatal conductance. Results suggest that reductions in both rainfall frequency and total rainfall may be important to competitive interactions, and can alter plant community composition and invasion when species have different rooting depths and different responses to soil moisture.

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.

Ecophysiology of invasive plants: osmotic adjustment and antioxidants

Current research into plant invasiveness often attempts to predict the effect of invasions under future climate change, but most studies only focus on ecological aspects. Understanding ecophysiological responses by characterizing physiological markers such as osmotic adjustment or antioxidant protection indicators will help us to project future invasiveness patterns. In this opinion article, we highlight how the information from physiological measurements can be incorporated into effective management strategies. Furthermore, we propose how combining research strategies of physiologists and ecologists could speed up our understanding of the advantageous mechanisms adopted by invasive species. We suggest that a combined approach would also be of considerable benefit for the development of effective governmental biodiversity conservation policies.

The physiology of invasive plants in low-resource environments

While invasive plant species primarily occur in disturbed, high-resource environments, many species have invaded ecosystems characterized by low nutrient, water, and light availability. Species adapted to low-resource systems often display traits associated with resource conservation, such as slow growth, high tissue longevity, and resource-use efficiency. This contrasts with our general understanding of invasive species physiology derived primarily from studies in high-resource environments. These studies suggest that invasive species succeed through high resource acquisition. This review examines physiological and morphological traits of native and invasive species in low-resource environments. Existing data support the idea that species invading low-resource environments possess traits associated with resource acquisition, resource conservation or both. Disturbance and climate change are affecting resource availability in many ecosystems, and understanding physiological differences between native and invasive species may suggest ways to restore invaded ecosystems.

A test for pre-adapted phenotypic plasticity in the invasive tree Acer negundo L

Phenotypic plasticity is a key mechanism associated with the spread of exotic plants and previous studies have found that invasive species are generally more plastic than co-occurring species. Comparatively, the evolution of phenotypic plasticity in plant invasion has received less attention, and in particular, the genetic basis of plasticity is largely unexamined. Native from North America, Acer negundo L. is aggressively impacting the riparian forests of southern and eastern Europe thanks to higher plasticity relative to co-occurring native species. We therefore tested here whether invasive populations have evolved increased plasticity since introduction. The performance of 1152 seedlings from 8 native and 8 invasive populations was compared in response to nutrient availability. Irrespective of nutrients, invasive populations had higher growth and greater allocation to above-ground biomass relative to their native conspecifics. More importantly, invasive genotypes did not show increased plasticity in any of the 20 traits examined. This result suggests that the high magnitude of plasticity to nutrient variation of invasive seedlings might be pre-adapted in the native range. Invasiveness of A. negundo could be explained by higher mean values of traits due to genetic differentiation rather than by evolution of increased plasticity.

Alien plant species with a wider global distribution are better able to capitalize on increased resource availability

• A high ability of alien plant species to capitalize on increases in resource availability has been suggested as an explanation for being globally successful. Here, we tested this hypothesis meta-analytically using existing data from experiments manipulating plant resources (light, water and nutrients). • From these studies we extracted the response to resource increase of biomass, as an indicator of plant performance, and the responses of two traits related to resource capture: root : shoot ratio and specific leaf area (SLA). For 211 species recorded in the Global Compendium of Weeds, we assessed the relationship between effect sizes from such studies and the number of global regions where a species was established. • We found that globally widespread species exhibited greater biomass responses to increases in resources overall, compared to less widespread species. Root : shoot ratio and SLA responses to increased resource availability were not related to species global distribution. • In general, globally widespread alien plant species were better able to capitalize on increased availability of resources, through achieving increased growth and biomass accumulation, while greater plasticity of key resource-capture traits per se did not appear to be related to greater success.