High resource capture and use efficiency and prolonged growth season contribute to invasiveness of Eupatorium adenophorum

To explore the traits contributing to invasion success of Eupatorium adenophorum, a noxious invasive perennial forb throughout the subtropics in Asia, Oceania, Africa, and USA, we compared the differences in ecophysiology and phenology between the invader and native E. japonicum under eight treatment combinations of two irradiances and four nitrogen additions in a two-year shadehouse experiment. The invader had significantly higher mass-based light-saturated photosynthetic rate (P _max) than its native congener in all treatments, contributing to higher photosynthetic nitrogen-, phosphorus-, and energy-use efficiencies. The higher P _max of the invader was associated with its higher nitrogen concentrations in the photosynthetic apparatus, which resulted from higher leaf nitrogen allocation to photosynthesis. The invader had higher specific leaf area and stomatal conductance at most of the treatments, also contributing to its higher P _max. The invader was not constrained by the negative correlation between leaf lifespan and specific leaf area or P _max. Leaf lifespan and total leaf area of the invader were greater than those of the native. From November to March the native congener was leafless, whereas the invader maintained a large area of leaves with relatively high P _max. Biomass accumulated in these months accounted for more than 40 % of the total biomass of the invader. Our results indicate that both the ability to capture and utilize resources efficiently and the ability to use resources when they are unavailable to natives contribute to invasion success of E. adenophorum and emphasize the importance of exploring multiple, non-mutually exclusive mechanisms for invasions.     

Photosynthesis, nitrogen allocation and specific leaf area in invasive Eupatorium adenophorum and native Eupatorium japonicum grown at different irradiances

The mechanisms underlying biological invasions are still not well elucidated. In this study, ecophysiological traits of invasive Eupatorium adenophorum and native E.   japonicum were compared at 10 irradiances in field. I hypothesized that the invader may allocate a higher fraction of leaf nitrogen (N) to photosynthesis and have higher light-saturated photosynthetic rate ( P _max) and specific leaf area (SLA) than E.   japonicum . The invader had a significantly higher ability to acclimate to high irradiance than E.   japonicum , while it showed a similar shade-tolerant ability. The invader indeed allocated a higher fraction of leaf N to photosynthesis than E.   japonicum , which, with its high leaf N content ( N _A), resulted in a higher N content in photosynthesis ( N _P), contributing to its higher biochemical capacity for photosynthesis and P _max. However, the invader had a significantly lower SLA than E.   japonicum , contributing to its higher P _max but increasing its area-based leaf construction cost. The abilities to acclimate to a wider range of irradiance and to allocate a higher fraction of leaf N to photosynthesis, and the higher P _max, N _A, N _P and leaf area ratio may contribute to the invasion of the invader. High SLA is not always necessary for invasive species.     

Innate and evolutionarily increased advantages of invasive <Emphasis Type="Italic">Eupatorium adenophorum</Emphasis> over native <Emphasis Type="Italic">E. japonicum</Emphasis> under ambient and doubled atmospheric CO<Subscript>2</Subscript> concentrations

Both innate and evolutionarily increased ecophysiological advantages can contribute to vigorous growth, and eventually to invasiveness of alien plants. Little effort has been made to explore the roles of innate factors of alien plants in invasiveness and the effects of CO₂ enrichment on alien plant invasions. To address these problems, we compared invasive Eupatorium adenophorum, its native conspecific, and a native congener (E. japonicum) under ambient and doubled atmospheric CO₂ concentrations. Native E. adenophorum from Mexico grew slower than invasive E. adenophorum but faster than native E. japonicum under both CO₂ concentrations. The faster growth rate of invasive E. adenophorum was associated with higher photosynthetic capacity and leaf area ratio. For invasive E. adenophorum, the higher photosynthetic capacity was associated with higher nitrogen (N) allocation to photosynthesis, which was related to lower leaf mass per area; the higher leaf area ratio was due to lower leaf mass per area and higher leaf mass fraction. Tradeoff between N allocations to photosynthesis versus defenses was found. CO₂ enrichment significantly increased relative growth rate and biomass accumulation by increasing actual photosynthetic rate for all studied materials. However, the relative increase in growth was not significantly different among them. CO₂ enrichment did not influence N allocation to photosynthesis, but increased N allocation to cell walls. The reduced leaf N content decreased N content in photosynthesis, explaining the down-regulation of photosynthetic capacity under prolonged elevated CO₂ concentration. Our results indicate that both innate and evolutionary advantages in growth and related ecophysiological traits contribute to invasiveness of invasive E. adenophorum, and CO₂ enrichment may not aggravate E. adenophroum’s invasion in the future.     

Similar responses in morphology,growth, biomass allocation,and photosynthesis in invasive <Emphasis Type="Italic">Wedelia trilobata</Emphasis> and native congeners to CO<Subscript>2</Subscript> enrichment

Both global change and biological invasions threaten biodiversity worldwide. However, their interactions and related mechanisms are still not well elucidated. To elucidate potential traits contributing to invasiveness and whether ongoing increase in CO₂ aggravates invasions, noxious invasive Wedelia trilobata and native Wedelia urticifolia and Wedelia chinensis were compared under ambient and doubled atmospheric CO₂ concentrations in terms of growth, biomass allocation, morphology, and physiology. The invader had consistently higher leaf mass fraction (LMF) and specific leaf area than the natives, contributing to a higher leaf area ratio, and therefore to faster growth and invasiveness. The higher LMF of the invader was due to lower root mass fraction and higher fine root percent. On the other hand, the invader allocated a higher fraction of leaf nitrogen (N) to photosynthetic apparatus, which was associated with its higher photosynthetic rate, and resource use efficiency. All these traits collectively contributed to its invasiveness. CO₂ enrichment increased growth of all studied species by increasing actual photosynthesis, although it decreased photosynthetic capacities due to decreased leaf and photosynthetic N contents. Responses of the invasive and native plants to elevated CO₂ were not significantly different, indicating that the ongoing increase in CO₂ may not aggravate biological invasions, inconsistent with the prevailing results in references. Therefore, more comparative studies of related invasive and native plants are needed to elucidate whether CO₂ enrichment facilitates invasions.     

Nitrogen allocation and partitioning in invasive and native Eupatorium species

There is a trade-off between nitrogen (N) allocation to photosynthesis and to defence. Invasive species may reduce N allocation to defence because of the absence of natural enemies. Thus, I hypothesised that invasive species may allocate a higher fraction of total leaf N to photosynthesis and have higher light-saturated photosynthetic rate ( P _max) and photosynthetic N-use efficiency (PNUE) than closely related native species. To test these hypotheses, invasive Eupatorium adenophorum and native E.   chinense and E.   heterophyllum were compared in a limestone shrub. Unlike expectation, the invader did not allocate a higher fraction of leaf N to photosynthesis than the natives. However, it was more efficient in photosynthetic N partitioning than the natives. It partitioned a higher fraction of the photosynthetic N to carboxylation and showed higher use efficiency of the photosynthetic N, while the natives partitioned a higher fraction of the photosynthetic N to light-harvesting components. Total leaf N content was not significantly different among the three studied invasive and native species. For the invader, the higher fraction of leaf N allocated to carboxylation resulted in the higher N content in carboxylation and in both carboxylation and bioenergetics, which led to higher P _max, and therefore to higher PNUE, water-use efficiency, respiration efficiency and apparent quantum yield. These physiological advantages of the invader and its higher leaf area ratio may contribute to its invasiveness.     

A congeneric comparison shows that experimental warming enhances the growth of invasive Eupatorium adenophorum

Rising air temperatures may change the risks of invasive plants; however, little is known about how different warming timings affect the growth and stress-tolerance of invasive plants. We conducted an experiment with an invasive plant Eupatorium adenophorum and a native congener Eupatorium chinense, and contrasted their mortality, plant height, total biomass, and biomass allocation in ambient, day-, night-, and daily-warming treatments. The mortality of plants was significantly higher in E. chinense than E. adenophorum in four temperature regimes. Eupatorium adenophorum grew larger than E. chinense in the ambient climate, and this difference was amplified with warming. On the basis of the net effects of warming, daily-warming exhibited the strongest influence on E. adenophorum, followed by day-warming and night-warming. There was a positive correlation between total biomass and root weight ratio in E. adenophorum, but not in E. chinense. These findings suggest that climate warming may enhance E. adenophorum invasions through increasing its growth and stress-tolerance, and that day-, night- and daily-warming may play different roles in this facilitation.     

Influence of Elevated CO2 Concentration and Nitrogen Source on Photosynthetic Traits in the Invasive Species Eupatorium adenophorum (Asteraceae)

Increases in the concentration of atmospheric CO2 and plant invasion are two important problems that face humans worldwide. In some plants, exposure to a short term elevated concentration of CO2 (SE［CO2］) promotes photosynthesis, but the promotion of elevated ［CO2］ (E ［CO2］) to photosynthesis might disappear after long term treatment (so called “CO2 acclimation”); this might result from the associated inhibition of nitrate assimilation. The present study investigated the physiological effects of short term (8 days) and long term (40 days) exposure to E［CO2］ when these were combined with different forms of inorganic N (full N; nitrate (NO3-) N) in the invasive species Eupatorium adenophorum. Exposure to E［CO2］ increased the biomass of Eadenophorum, regardless of the duration of exposure to E［CO2］ and the type of inorganic N that was supplied. E［CO2］ could promote the photosynthesis of Eadenophorum seedlings fertilised with non depleted Hoagland solutions (full N). For plants fertilised with NH4+ depleted Hoagland solution (NO3- N), LE［CO2］ treatment promoted the photosynthesis of Eadenop horum, but the promotion of photosynthesis by E［CO2］ disappeared under SE［CO2］ conditions. Photosynthetic pigments contents were determined to estimate potential changes in the photosynthetic capacity of Eadenophorum. For plants fertilised with non depleted Hoagland solution, there were no significant differences in chlorophyll among the three ［CO2］ treatments, but the treatment of SE［CO2］ increased the levels of chlorophyll in leaves. The apparent promotion of biomass accumulation and photosynthesis at LE［CO2］ without a decrease in chlorophyll indicates that Eadenophorum might not acclimate to long term exposure to E［CO2］. NH4+ depletion did not affect the capacity of LE［CO2］ to promote the photosynthesis of Eadenophorum. Thus, considering some plants fertilised with NO3- acclimating to LE［CO2］, Eadenophorum might be more competitive in areas where the soils are relatively poor in NH4+ as levels of atmospheric CO2 continue to rise.     

CO2浓度升高和不同氮源对紫茎泽兰生长及光合特性的影响

大气CO2浓度升高和植物入侵是全世界面临的两大重要问题。CO2浓度升高促进植物的光合作用,但在某些植物中,这种促进作用出现在短期高浓度CO2下,而在长期高浓度CO2处理下消失（称为CO2驯化）,被认为源于高浓度CO2对光呼吸和NO-3同化的抑制。通过比较研究不同形式氮源（全氮、硝态氮）和短期（8 days）、长期（40 days）CO2浓度升高处理对入侵植物紫茎泽兰生理特征的影响,结果表明在全氮供应下,短期和长期CO2浓度升高均促进了紫茎泽兰的光合;氨态氮缺失情况下,长期CO2浓度升高促进紫茎泽兰的光合,而短期CO2浓度升高对紫茎泽兰的光合没有促进作用;缺NH＋4下,短期高浓度CO2提高了叶片叶绿素含量,长期CO2升高又使其回复到正常CO2下的较低水平。这些结果表明紫茎泽兰并不会对长期的CO2升高产生驯化,即长期CO2升高会促进紫茎泽兰的光合作用,而且这一促进作用不受土壤中缺NH＋4的影响。鉴于培养介质中缺NH＋4会导致一些植物产生“CO2驯化”,未来CO2浓度升高情况下,在缺NH＋4的土壤中,紫茎泽兰的竞争力可能会更强。     

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

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.     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

AMF colonization and community of a temperate invader and co-occurring natives grown under different CO2 concentrations for 3 years

连续3年不同CO₂浓度下一种温带外来入侵植物和两种共存本地植物丛枝菌根真菌群落及侵染率 大气CO₂浓度升高等全球变化过程不仅能促进外来植物入侵,也能改变土壤丛枝菌根真菌(AMF)的群落结构,但我们并不清楚大气CO₂浓度升高促进外来植物入侵是否与其对外来入侵植物和 本地植物AMF共生的影响有关。为回答这一问题,我们在环境和倍增CO₂浓度下连续3年栽培一年生外来入侵植物瘤突苍耳(Xanthium strumarium)与两种共存的一年生本地近缘植物,比较了AMF侵染率、土 壤养分和土壤AMF群落组成的差异。研究结果表明,大气CO₂浓度升高只在少数情况下提高根系AMF侵染率,并且瘤突苍耳AMF侵染率的提高并不比本地种多。在环境CO₂浓度下,栽培第一年瘤突苍耳的AMF侵染侵染率显著高于两种本地植物;而栽培第二年和第三年与两种本地植物的差异不显著,因为两种本地植物的AMF侵染率随种植时间的增加而增加,而瘤突苍耳AMF侵染率受种植时间的影响较小。物种、CO₂浓度和种植时间对AMF侵染率的影响与它们对土壤养分和AMF群落的影响有关,土壤养分对AMF侵染率的影响可能比AMF群落组成的影响更大,因为后者也受土壤养分的影响。上述结果表明,与本地植物相比,入侵植物能更快地与AMF形成共生关系,有利于其适应和入侵新生境;在探究全球变化如大气CO₂浓度升高等对外来植物入侵的影响时,需要考虑AMF的影响和时间效应。     

Comparisons of plastic responses to irradiance and physiological traits by invasive Eupatorium adenophorum and its native congeners

To explore the traits contributing to invasiveness of Eupatorium adenophorum and to test the relationship between plasticity of these traits and invasiveness, we compared E. adenophorum with its two native congeners at four irradiances (10%, 23%, 40%, and 100%). The invader showed constantly higher performance (relative growth rate and total biomass) across irradiances than its native congeners. Higher light-saturated photosynthetic rate (P(max)), respiration efficiency (RE), and nitrogen (PNUE) and water (WUE, at 40% and 100% irradiances only) use efficiencies contributed directly to the higher performance of the invader. Higher nitrogen allocation to, stomatal conductance, and the higher contents of leaf nitrogen and pigments contributed to the higher performance of the invader indirectly through increasing P(max), RE, PNUE and WUE. The invader had consistently higher plasticity only in carotenoid content than its native congeners in ranges of low (10-40%), high (40-100%) and total (10-100%) irradiances, contributing to invasion success in high irradiance by photo protection. In the range of low irradiances, the invader had higher plasticity in some physiological traits (leaf nitrogen content, nitrogen contents in bioenergetics, carboxylation and in light-harvesting components, and contents of leaf chlorophylls and carotenoids) but not in performance, while in the ranges of high or total irradiances, the invader did not show higher plasticity in any variable (except Car). The results indicated that the relationship between invasiveness and plasticity of a specific trait was complex, and that a universal generalization about the relationship might be too simplistic.     

Simulated nitrogen deposition enhances the performance of an exotic grass relative to native serpentine grassland competitors

Previous research suggests that atmospheric nitrogen (N) deposition may facilitate the invasion and persistence of exotic plant species in serpentine grasslands, but the relative impact of increased N availability on native and exotic competitive dynamics has yet to be clearly elucidated. In this study, we evaluated how increased N deposition affects plant performance and competitive dynamics of five native grasses and forbs (Plantago erecta, Layia gaillardioides, Lasthenia californica, Vulpia microstachys, and Cryptantha flaccida) and the most common invasive grass in Bay Area serpentine grasslands, Lolium multiflorum. Using a growth chamber system, we exposed Lolium in monoculture, and native species grown both in monoculture and in competition with the exotic Lolium, to all four possible combinations of gaseous nitrogen dioxide (NO₂; a dominant atmospheric N pollutant) and soil ammonium nitrate (NH₄NO₃). In monocultures, gaseous NO₂ and soil N addition each increased shoot biomass in Lolium and the natives Layia and Cryptantha. Lolium competitive ability (mean relative yield potential??RYP) increased in response to NO₂ addition plus soil N addition against all native competitors. Lolium and most native species did not show differences in photosynthetic rate and stomatal conductance in response to N addition. Our findings indicate that increasing N deposition and subsequent N accumulation in the soil may confer a competitive advantage to the exotic Lolium over native species by stimulating greater biomass accumulation and N allocation to photosynthetic tissue in the invader.     

Strengthening Invasion Filters to Reassemble Native Plant Communities: Soil Resources and Phenological Overlap

Preventing invasion by exotic species is one of the key goals of restoration, and community assembly theory provides testable predictions about native community attributes that will best resist invasion. For instance, resource availability and biotic interactions may represent “filters” that limit the success of potential invaders. Communities are predicted to resist invasion when they contain native species that are functionally similar to potential invaders; where phenology may be a key functional trait. Nutrient reduction is another common strategy for reducing invasion following native species restoration, because soil nitrogen (N) enrichment often facilitates invasion. Here, we focus on restoring the herbaceous community associated with coastal sage scrub vegetation in Southern California; these communities are often highly invaded, especially by exotic annual grasses that are notoriously challenging for restoration. We created experimental plant communities composed of the same 20 native species, but manipulated functional group abundance (according to growth form, phenology, and N‐fixation capacity) and soil N availability. We fertilized to increase N, and added carbon to reduce N via microbial N immobilization. We found that N reduction decreased exotic cover, and the most successful seed mix for reducing exotic abundance varied depending on the invader functional type. For instance, exotic annual grasses were least abundant when the native community was dominated by early active forbs, which matched the phenology of the exotic annual grasses. Our findings show that nutrient availability and the timing of biotic interactions are key filters that can be manipulated in restoration to prevent invasion and maximize native species recovery.     

Non-native grass invasion alters native plant composition in experimental communities

Invasions of non-native species are considered to have significant impacts on native species, but few studies have quantified the direct effects of invasions on native community structure and composition. Many studies on the effects of invasions fail to distinguish between (1) differential responses of native and non-native species to environmental conditions, and (2) direct impacts of invasions on native communities. In particular, invasions may alter community assembly following disturbance and prevent recolonization of native species. To determine if invasions directly impact native communities, we established 32 experimental plots (27.5 m²) and seeded them with 12 native species. Then, we added seed of a non-native invasive grass (Microstegium vimineum) to half of the plots and compared native plant community responses between control and invaded plots. Invasion reduced native biomass by 46, 64, and 58%, respectively, over three growing seasons. After the second year of the experiment, invaded plots had 43% lower species richness and 38% lower diversity as calculated from the Shannon index. Nonmetric multidimensional scaling ordination showed a significant divergence in composition between invaded and control plots. Further, there was a strong negative relationship between invader and native plant biomass, signifying that native plants are more strongly suppressed in densely invaded areas. Our results show that a non-native invasive plant inhibits native species establishment and growth following disturbance and that native species do not gain competitive dominance after multiple growing seasons. Thus, plant invaders can alter the structure of native plant communities and reduce the success of restoration efforts.     

Immobilizing nitrogen to control plant invasion

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.     

Elevated CO<Subscript>2</Subscript> increases energy-use efficiency of invasive <Emphasis Type="Italic">Wedelia trilobata</Emphasis> over its indigenous congener

Increasing atmospheric CO₂ concentration is regarded as an important factor facilitating plants invasions by stimulating invasive species growth. However, the physiological mechanisms by which invasive plants increase at the expense of existing native plants are poorly understood. Plant growth is always related to energy-use process including energy assimilation and expenditure, and thus examination of energetic properties could provide mechanistic insight into growth responses to increased CO₂. The aims of this study were to examine the effect of rising CO₂ on the growth and energetic properties of alien invasive species (Wedelia trilobata (L.) Hitchc.) and its native congener (Wedelia chinensis (Osbeck.) Merr.) in South China, and to determine if the specific energetic properties of invasive species at elevated CO₂ favoring its growth. Elevated CO₂ stimulated a greater increase in biomass production for invasive W. trilobata (58.9%) than for its indigenous congener (48.1%). Meanwhile, elevated CO₂ altered the energetic properties differently upon species. For invasive W. trilobata, elevated CO₂ significantly increased total energetic gain via photosynthetic activity (A _total), but decreased energetic cost of biomass construction (CC), and thus enhanced photosynthetic energy-use efficiency (PEUE) by 85.3%. In contrast, the indigenous W. chinensis showed a slight increase in PEUE by 43.8%. Additionally, W. trilobata individuals grown in elevated CO₂ increased energy allocation towards stems. Statistic analysis revealed significant associations between growth characteristics (relative growth rate and biomass) and energetic properties (CC and PEUE), suggesting the greater growth stimulation in invasive species could be partly explained by its specific energetic properties in elevated CO₂ concentration. The invasive species showed a greater increase in energy-use efficiency under elevated CO₂, which consequently facilitated its growth. It might be a physiological mechanism promoting success of invasion with ongoing increase in atmospheric CO₂ concentration.     

The roles of biotic resistance and nitrogen deposition in regulating non-native understory plant diversity

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and non-native forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.     

紫茎泽兰的光合作用特征及其生态学意义

本文研究了紫茎泽兰(Eupatorium adenophorum Spreng)光合作用强度的变化规律及其与环境主要生态因子的关系,比较了它与某些农作物叶片净光合速率的差异,得到如下结果: 1.紫茎泽兰是一种阳性偏阴的C_3类草本植物,其光合作用的光饱和点约为40000 lx,光补偿点约为700 lx,且具有80 ppm左右的CO_2补偿点。 2.紫茎泽兰的最大净光合速率能达到23毫克CO_2/平方分米·时左右,叶片净光合速率的日变化规律呈双峰曲线型(主峰在10时左右,次峰在16时左右)。在一年中有较长的时间,它的光合速率保持着较高的水平。 3.生长在一般菜园土上的紫茎泽兰,当土壤含水量降至17％左右时,叶片光合速率接近0:而且,受过干旱处理的紫茎泽兰植株,在恢复供水后的第三天,其光合速率只达到原来的53％。 根据以上结果,结合受紫茎泽兰危害地区干湿季分明的特点,提出干季是防除紫茎泽兰的最佳季节。     

Soil biota reduce allelopathic effects of the invasive Eupatorium adenophorum

Allelopathy has been hypothesized to play a role in exotic plant invasions, and study of this process can improve our understanding of how direct and indirect plant interactions influence plant community organization and ecosystem functioning. However, allelopathic effects can be highly conditional. For example allelopathic effects demonstrated in vivo can be difficult to demonstrate in field soils. Here we tested phytotoxicity of Eupatorium adenophorum (croftonweed), one of the most destructive exotic species in China, to a native plant species Brassica rapa both in sand and in native soil. Our results suggested that natural soils from different invaded habitats alleviated or eliminated the efficacy of potential allelochemicals relative to sand cultures. When that soil is sterilized, the allelopathic effects returned; suggesting that soil biota were responsible for the reduced phytotoxicity in natural soils. Neither of the two allelopathic compounds (9-Oxo-10,11-dehydroageraphorone and 9b-Hydroxyageraphorone) of E. adenophorum could be found in natural soils infested by the invader, and when those compounds were added to the soils as leachates, they showed substantial degradation after 24 hours in natural soils but not in sand. Our findings emphasize that soil biota can reduce the allelopathic effects of invaders on other plants, and therefore can reduce community invasibility. These results also suggest that soil biota may have stronger or weaker effects on allelopathic interactions depending on how allelochemicals are delivered.