Biotic constraints on the invasion of diffuse knapweed (Centaurea diffusa) in North American grasslands

Knapweeds (Centaurea spp.) are among the most invasive of non-indigenous plant species that have colonized western North America over the last century. We conducted a 4-year experiment in a reconstructed grassland to test hypotheses related to the ability of grasslands to resist the invasion of diffuse knapweed (C. diffusa). We experimentally invaded C. diffusa and three native species into areas where we manipulated soil nitrogen (N) and phosphorus (P) availability and removed extant grasses to reduce competition. We evaluated the growth response of these species to these resources and competitive manipulations. Of the native species that were experimentally added, only one species, Ratibida pinnata (prairie coneflower), established in any numbers. Establishment values in intact vegetation were low for both species, but establishment by C. diffusa (0.02%) clearly outperformed that of R. pinnata (0.001%). Under reduced grass competition, establishment was enhanced, but the values for C. diffusa (0.68%) were not statistically different from those of R. pinnata (0.57%). Neither species performed better under higher soil nutrients in the presence of competing grasses. In plots with both species, biomass of the two planted species was positively correlated, but the biomass of both species was negatively correlated with non-added weedy species. Subsequent harvests of C. diffusa indicated that establishment was enhanced in treatments with higher soil nutrients but that the biomass of these plants could only be enhanced when plant competition was also reduced. These results indicate that C. diffusa can establish in intact grasslands at rates higher than natives, but opportunism rather than competitive ability best describes the invasiveness of C. diffusa. Thus, the mechanisms contributing to the establishment of this knapweed species are different from factors identified as contributing to the dominance of this invader.     

Competitive impacts and responses of an invasive weed: dependencies on nitrogen and phosphorus availability

Changes in competitive interactions under conditions of enhanced resource availability could explain the invasion success of some problematic plant species. For one invader of North American grasslands, Centaurea diffusa (diffuse knapweed), we test three hypotheses: (1) under ambient (high resource) conditions, C. diffusa is better able to tolerate competition from the resident community (competitive response), (2) under ambient conditions, C. diffusa strong impacts the competitive environment (competitive effect), and (3) reductions in nitrogen and/or phosphorus availability diminish these advantages. In support of our first hypothesis, C. diffusa was the most tolerant to neighbor competition of the four focal species under current resource conditions. In opposition to our second hypothesis, however, neighborhoods that contained C. diffusa and those where C. diffusa had been selectively removed did not differ in their impact on the performance of target transplant individuals or on resource conditions. Reduction in resource availability influenced competitive tolerance but not competitive impact, in partial support of our last hypothesis. Reduction in soil nitrogen (via sucrose carbon addition) enhanced the degree of neighbor competition experienced by all species but did not change their relative rankings; C. diffusa remained the best competitor under low nitrogen conditions. Reduction of soil phosphorus (via gypsum addition) weakened the ability of C. diffusa to tolerate neighbor competition proportionately more than the other focal species. Consequently, under low phosphorus conditions, C. diffusa lost its competitive advantage and tolerated neighbor competition similarly to the other focal species. We conclude that C. diffusa invasion may be double-edged: C. diffusa is less limited by nitrogen than the other focal species and is better able to utilize phosphorus to its competitive advantage.     

Responses to fertility and disturbance in a low-diversity grassland

We examined variation in species composition in a low-diversity, anthropogenic grassland in response to 11 years of nitrogen (N) manipulation and disturbance. The species-poor grassland (2–3 species/0.5 m²) represents a wide spread vegetation type (>10 million ha in North America) dominated by the introduced perennial grasses Bromus inermis and Agropyron cristatum. Four levels of N and three of soil disturbance were applied in all combinations to plots (5 × 15 m, N = 120) in a completely randomized design each year. Seeds or transplants of 47 species were added to ensure that dispersal was not a barrier to changes in species composition. After 11 years of treatment, all but the most disturbed plots continued to be dominated by B. inermis. The cover of the second-most abundant species, A. cristatum, decreased with disturbance but did not vary significantly with N. Despite the lack of changes in the identity of the dominant species, our environmental manipulations strongly influenced ecosystem characteristics. Added N increased soil available N, and decreased the cover of bare ground and light availability. Soil disturbance decreased aboveground biomass, and increased the cover of bare ground and light availability. Sawdust application, designed to decrease N availability, significantly reduced community biomass, and increased light availability and the cover of bare ground, but did not alter nutrient availability or species composition. The results highlight the difficulty of restoring diversity in species-poor, anthropogenic communities dominated by introduced species, and thus the importance of conserving remnants of diverse natural grasslands.     

The effect of environmental factors on floating fresh marsh end-of-season biomass

We used both stepwise and quantile regression to determine the sources of environmental variation that explained the observed inter-annual variation in end-of-season freshwater floating marsh aboveground biomass over an 18-year period. The vegetation at our study site had high species diversity with an average of 20 species recorded from 10 0.25 m⁻² plots. However, Panicum hemitomon was clearly the dominant contributing 74% of the total biomass. Only three other species (Solidago sempervirens, Vigna luteola, and Thelypteris palustris) were so common that they were sampled in all years. We expected that the most important factors controlling interannual variation in aboveground biomass are temperature and nitrogen availability. We also expected that nitrogen availability to the plants is affected by water movement through and under the mat driven by precipitation (lower N), evaporation (transportation of higher N waters to roots), and local runoff (higher N). Stepwise regression analysis indicated that P. hemitomon average biomass was negatively related to average water level and positively related to maximum water level and had a curvilinear response with TKN. Using quantile regression the best fit for P. hemitomon maximum-biomass with two parameters was obtained using hot days (positive relationship) and maximum water level (negative relationship). Both analytical methods showed maximum water level (negative relationship) and cold front passage (positive relationship) to be the environmental parameters that best explained interannual variation in S. sempervirens biomass. V. luteola biomass was positively related to temperature. Stepwise regression added chloride concentration as an additional positive parameter explaining V. luteola biomass, while quantile regression identified nitrogen as an important positive parameter. Both analytical methods identified pH, TKN, and water level as environmental parameters that were negatively correlated with T. palustris biomass. The overall negative effect of water level on all species was unexpected in this floating mat system. We initially assumed that higher water levels were due to higher runoff which should have a positive effect on biomass. However, higher water levels may also be related to a higher retention time in this fresh-water tidal system, which decreases water exchange and nutrient replenishment.     

氮素和水分添加对贝加尔针茅草原植物多样性及生物量的影响

为了解草原植物群落物种多样性和植物地上生物量对氮沉降增加和降水变化的响应,在内蒙古贝加尔针茅(Stipa baicalensis)草原,分别设置对照(N0)、1.5 g/m2(N15)、3.0 g/m2(N30)、5.0 g/m2(N50)、10.0 g/m2(N100)、15.0 g/m2(N150)、20.0g/m2(N200)和30.0 g/m2(N300)(不包括大气沉降的氮量)8个氮素(NH4NO3)添加梯度和模拟夏季增加降水100mm的水分添加交互试验,研究氮素和水分添加对草原群落植物物种多样性和几种常见植物地上生物量的影响。结果表明:(1)氮素和水分的添加降低了草原群落植物物种多样性,且氮素和水分有显著的互作效应。在水分添加的条件下,随着施氮水平的增加,群落植物物种多样性减小;在无水分添加的条件下,随着施氮水平的增加,群落植物物种多样性呈先增加后减小的"单峰"变化趋势。(2)不同植物对氮素和水分添加的响应不同,随着施氮水平的增加,羊草地上生物量显著增加;贝加尔针茅、羽茅、糙隐子草、寸草苔和冷蒿先增加后减少,呈单峰曲线;星毛委陵菜、牧马豆、扁蓄豆和线叶菊地上生物量则逐渐减少。而且氮素和水分对贝加尔针茅、羽茅、扁蓄豆地上生物量有显著的交互作用。     

Effects of the invasive plant Mikania micrantha H.B.K. on soil nitrogen availability through allelopathy in South China

Allelopathy has been regarded as a mechanism for successful exotic plant invasion. However, it is not clear if and what effects of allelopathic substances may exert on soil nutrient. The exotic plant Mikania micrantha H.B.K. (M. micrantha) has invaded many forests in south China, and recent studies have suggested it has allelopathic potential for other plants and soil microbial community. Thus, we hypothesized that M. micrantha could influence soil nutrients and N transformation through allelopathy. We measured total C and N, NO₃ ⁻, NH₄ ⁺ and pH of the soil beneath M. micrantha and the adjacent open soil, and then measured the above soil properties after treating soil with 3 concentrations of aqueous extracts of M. micrantha (T₁: 0.005 g ml⁻¹; T₂: 0.025 g ml⁻¹; T₃: 0.100 g ml⁻¹). In addition, a bioassay was conducted to determine the allelopathic potential of the soil beneath M. micrantha. The results showed that M. micrantha significantly affected soil nutrients and N transformation. Soil beneath M. micrantha had inhibitory effects on seed germination and seedling growth of test plant, and had significantly higher C, N, ammonia, net nitrification rate than those of open soil. The plant extracts decreased soil pH, and T₁ decreased it the most, and it increased soil C and N, and T₁ represented the greatest increase in both C and N. The extracts also increased both NO₃ ⁻ and NH₄ ⁺ in soil, whereas no significant difference existed among the 3 extract treatments. Compared to the water control, the soil net mineralization rate was higher under T₁, while lower under T₂ and T₃. However, the extracts increased the soil nitrification rates under all the treatments (T₁, T₂ and T₃). Our results suggest that the water soluble allelochemicals of M. micrantha improve soil nutrient availability, through which the invasive plant M. micrantha may successfully invade and establish in new habitats.     

Growth and physiological responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to elevated temperature and to nitrogen fertilization

Picea asperata is a dominant species in the subalpine coniferous forests distributed in eastern edges of Tibetan Plateau and upper reaches of the Yangtze River. The paper mainly identified the short-term influences of experimental warming, nitrogen fertilization, and their combination on growth and physiological performances of Picea asperata seedlings. These seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0; 25 g m⁻² a⁻¹ N) for 6 months. We used a free air temperature increase of overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The temperature increment induced an obvious enhancement in biomass accumulation and the maximum net photosynthetic rate, and decreased AOS and MDA level under ambient nitrogen conditions. Whereas, negative effects of experimental warming on growth and physiology was observed under nitrogen fertilization condition. On the other hand, nitrogen fertilization significantly improved plant growth in unwarmed plots, by stimulating total biomass, maximum net photosynthetic rate (A _max), antioxidant compounds, as well as reducing the content of AOS and MDA. However, in warmed plots, nitrogen addition clearly decreased A _max, antioxidant compounds, and induced higher accumulation of AOS and MDA. Obviously, the beneficial effects of sole nitrogen on growth and physiology of Picea asperata seedlings could not be magnified by artificial warming.     

Interactive effects of N and P on growth but not on resource allocation of Canna indica in wetland microcosms

The interactive effects of three levels of N (mM) (low 0.36, medium 2.1 and high 6.4) and two levels of P (mM) (low 0.10 and high 0.48) on growth and resource allocation of Canna indica Linn. were studied in wetland microcosms. After 91 days of plant growth, there was a significant interactive effect of N and P on plant growth, but not on resource allocation (except for allocation of N to leaves and allocation of P to the stems). The plant growth positively responded to the relatively higher nutrient availability (taller plants with more stems, leaves and flowers), but the growth performance was not significantly different between the medium N-low P and high N-low P treatments. At high P, the total biomass in the high N was about 51% higher than that in the medium N and about 348% higher than that in the low N. The growth performance was related to the physiological responses. The photochemical efficiency (F_v/F_m) increased from 0.843 to 0.855 with an increase in N additions. The photosynthetic rate increased from 13 to 16 μmol m⁻² s⁻¹ in the low P levels and from 14 to 20 μmol m⁻² s⁻¹ in the high P levels with an increase in N applications, but significant difference was only between the low and medium N levels, regardless of the P levels. The tissue concentrations of N increased with an increase in N applications and decreased with an increase in P additions, whereas reverse was true for tissue concentrations of P. The highest concentrations of N and P in leaves were 30.8 g N kg⁻¹ in the high N-low P treatment and 4.9 g P kg⁻¹ in the low N-high P treatment. The percent biomass allocation to aboveground tissues in the high N was nearly twice that in the low N treatments. The N allocation to aboveground tissues was slightly larger in high N than in low N treatments, whereas the P allocation to aboveground tissues increased with an increase in the N addition. Although some patterns of biomass allocation were similar to those of nutrient allocation, they did not totally reflect the nutrient allocation. These results imply that in order to enhance the treatment performance, appropriately high nutrient availability of N and P are required to stimulate the growth of C. indica in constructed wetlands.     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N₂-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.     

Effects of carbon concentration and carbon to nitrogen ratio on the growth and sporulation of several biocontrol fungi

Effects of carbon concentration and carbon to nitrogen (C:N) ratio on six biocontrol fungal strains are reported in this paper. All fungal strains had extensive growth on the media supplemented with 6–12 g l⁻¹ carbon and C:N ratios from 10:1 to 80:1, and differed in nutrient requirements for sporulation. Except for the two strains of Paecilomyces lilacinus, all selected fungi attained the highest spore yields at a C:N ratio of 160:1 when the carbon concentration was 12 g l⁻¹ for Metarhizium anisopliae SQZ-1-21, 6 g l⁻¹ for M. anisopliae RS-4-1 and Trichoderma viride TV-1, and 8 g l⁻¹ for Lecanicillium lecanii CA-1-G. The optimal conditions for P. lilacinus sporulation were 8 g l⁻¹ carbon with a C:N ratio of 10:1 for M-14 and 12 g l⁻¹ carbon with a C:N ratio of 20:1 for IPC-P, respectively. The results indicated that the influence of carbon concentration and C:N ratio on fungal growth and sporulation is strain dependent; therefore, consideration for the complexity of nutrient requirements is essential for improving yields of fungal biocontrol agents.     

Responses of the functional traits in Cleistogenes squarrosa to nitrogen addition and drought

植物功能性状被广泛地用于研究植物对环境变化的响应。糙隐子草(Cleistogenes squarrosa)是内蒙古草原重要的C4物种,其功能性状是如何对水氮环境的变化做出响应的,还不十分清楚。该文采用盆栽实验的方法,进行氮添加(0,10.5,35.0和56.0 g·m–2·a–1)和降水(自然降水和70%平均月降水量)处理,研究糙隐子草整株性状、叶形态性状和叶生理性状对氮添加和干旱的响应。结果表明,氮添加显著影响了糙隐子草的整株性状,氮、水处理及它们的交互作用显著影响了糙隐子草的叶形态性状和叶生理性状。各功能性状对氮添加的响应格局在自然降水和干旱处理下是不同的。根深、茎生物量和茎叶比在干旱条件下低和中氮添加处理中较高,而在自然降水下无明显变化;比叶面积在干旱条件下随氮添加量的增加而增加,而在自然降水下无增加趋势;自然降水下,高氮添加显著刺激了光合速率和蒸腾速率,增加了水分利用效率,而在干旱条件下氮添加对它们没有显著影响;叶片单位面积的氮含量在自然降水下随氮添加量的增加有增加趋势,而在干旱条件下显著降低。在自然降水下,氮添加主要影响糙隐子草的叶形态和生理性状,而在干旱条件下,氮添加主要影响糙隐子草的整株性状和形态性状。总之,糙隐子草的功能性状对氮添加表现出明显的响应,响应格局在不同的水分条件下不同,反映了其对氮水环境变化的弹性适应。     

Effects of European Earthworm Invasion on Soil Characteristics in Northern Hardwood Forests of Minnesota, USA

European earthworms are colonizing worm-free hardwood forests across North America. Leading edges of earthworm invasion in forests of northern Minnesota provide a rare opportunity to document changes in soil characteristics as earthworm invasions are occurring. Across leading edges of earthworm invasion in four northern hardwood stands, increasing total earthworm biomass was associated with rapid disappearance of the O horizon. Concurrently, the thickness, bulk density and total soil organic matter content of the A horizon increased, and it’s percent organic matter and fine root density decreased. Different earthworm species assemblages influenced the magnitude and type of change in these soil parameters. Soil N and P availability were lower in plots with high earthworm biomass compared to plots with low worm biomass. Decreases in soil nitrogen availability associated with high earthworm biomass were reflected in decreased foliar nitrogen content for Carex pensylvanica, Acer saccharum and Asarum canadense but increased foliar N for Athyrium felix-femina. Overall, high earthworm biomass resulted in increased foliar carbon to nitrogen ratios. The effects of earthworm species assemblages on forest soil properties are related to their feeding and burrowing habits in addition to effects related to total biomass. The potential for large ecosystem consequences following exotic earthworm invasion has only recently been recognized by forest ecologists. In the face of rapid change and multiple pressures on native forest ecosystems, the impacts of earthworm invasion on forest soil structure and function must be considered.     

Recruitment and growth in Aconitum septentrionale and Actaea spicata in relation to microbial soil communities manipulated by additions of glucose and nutrients

Sowing experiments were used to study seedling recruitment, growth and biomass allocation patterns in the perennial forest herbs Aconitum septentrionale and Actaea spicata in relation to the microbial soil community. Glucose and nutrients were added every second week over a 3-year period to manipulate soil microbial activity and nutrient availability. The glucose was added (400 g glucose m⁻² yr⁻¹) to reduce the nutrient availability to the plants by increasing soil microbial demands. A full nutrient solution was used to increase the nutrient availability. The experiments were performed in a deciduous forest and in an open field in South East Norway, and our study is based on a consecutive sampling of whole plants with intact root systems to be able to estimate growth and allocation patterns. Both species recruited best in the forest while their growth in the open field was ca. 100 times larger than in the forest. Shoot:root ratios were surprisingly similar in the forest and the open field sites and were only marginally affected by the glucose and nutrient treatments. However, the shoot:root ratios were characterised by highly significant seasonal variations. This was the case for both species and indicates that the shoot:root ratios were under strong ontogenetic control. Recruitment was negatively affected by glucose additions, in particular in the open field. Growth was significantly and negatively affected by glucose additions in the forest. Nutrient additions gave, as expected, a significant increase in growth. The failure of seedling recruitment and inferior growth following glucose additions support the assumption that the soil microbial community is an important determinant of plant recruitment and growth.     

Nitrogen and water availability interact to affect leaf stoichiometry in a semi-arid grassland

The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m⁻² year⁻¹) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.     

Aboveground and Belowground Impacts Following Removal of the Invasive Species Baby's Breath (Gypsophila paniculata) on Lake Michigan Sand Dunes

The removal of invasive species is often one of the first steps in restoring degraded habitats. However, studies evaluating effectiveness of invasive species removal are often limited in spatial and temporal scale, and lack evaluation of both aboveground and belowground effects on diversity and key processes. In this study, we present results of a large 3‐year removal effort of the invasive species, Gypsophila paniculata, on sand dunes in northwest Michigan (USA). We measured G. paniculata abundance, plant species richness, plant community diversity, non‐native plant cover, abundance of Cirsium pitcheri (a federally threatened species endemic to this habitat), sand movement, arbuscular mycorrhizal spore abundance, and soil nutrients in fifteen 1000 m² plots yearly from 2007 to 2010 in order to evaluate the effectiveness of manual removal of this species on dune restoration. Gypsophila paniculata cover was greatly reduced by management, but was not entirely eliminated from the area. Removal of G. paniculata shifted plant community composition to more closely resemble target reference plant communities but had no effect on total plant diversity, C. pitcheri abundance, or other non‐native plant cover. Soil properties were generally unaffected by G. paniculata invasion or removal. The outlook is good for this restoration, as other non‐native species do not appear to be staging a “secondary” invasion of this habitat. However, the successional nature of sand dunes means that they are already highly invasible, stressing the need for regular monitoring to ensure that restoration progresses.     

Plant and Soil N Response of Southern Californian Semi-arid Shrublands After 1 Year of Experimental N Deposition

Large inputs of atmospheric N from dry deposition accumulate on vegetation and soil surfaces of southern Californian chaparral and coastal sage scrub (CSS) ecosystems during the late-summer and early-fall and become available as a pulse following winter rainfall; however, the fate of this dry season atmospheric N addition is unknown. To assess the potential for dry season atmospheric N inputs to be incorporated into soil and/or vegetation N pools, an in situ N addition experiment was initiated in a post-fire chaparral and a mature CSS stand where 10 × 10 m plots were exposed to either ambient N deposition (control) or ambient +50 kg N ha⁻¹ (added N) added as NH₄NO₃ during a single application in October 2003. After 1 year of N addition, plots exposed to added N had significantly higher accumulation of extractable inorganic N (NH₄−N + NO₃−N) on ion exchange resins deployed in the 0–10 cm mineral soil layer and higher soil extractable N in the subsurface (30–40 cm) mineral soil than plots exposed to ambient N. Chaparral and CSS shrubs exposed to added N also exhibited a significant increase in tissue N concentration and a decline in the tissue C:N ratio, and added N significantly altered the shrub tissue δ ¹⁵N natural abundance. Leaching of inorganic N to 1 m below the soil surface was on average 2–3 times higher in the added N plots, but large within treatment variability cause these differences to be statistically insignificant. Although a large fraction of the added N could not be accounted for in the shrub and soil N pools investigated, these observations suggest that dry season N inputs can significantly and rapidly alter N availability and shrub tissue chemistry in Mediterranean-type chaparral and CSS shrublands of southern California.     

Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado

Abundances and interactions among biological control insects and their effects on target invasive plants were monitored within the flower heads and roots of diffuse knapweed, Centaurea diffusa, and in spotted knapweed, Centaurea stoebe, along the Colorado Front Range. Flower weevils, (Larinus species) and root-feeders (Cyphocleonus achates and Sphenoptera jugoslavica) were released on knapweed that already supported biological control gall flies (Urophora species). At a single monitoring site, seed production by C. diffusa declined from 4400 seeds m⁻² in 1997 to zero seeds m⁻² on the monitoring sites in 2006, while the flowering stem density of C. diffusa declined from a peak of almost 30 stems m⁻² in 2000 to zero stems m⁻² in 2006. The average abundance of Urophora and Larinus in flower heads fluctuated independently during the 2001–2006 interval, while the relative abundance of C. achates and S. jugoslavica in roots exhibited a weak inverse relationship that appeared driven by climate effects. The relative abundance of insects on a population of C. stoebe was monitored for five years as Larinus species and C. achates became established on spotted knapweed that already supported Urophora species. Spotted knapweed seed production on our monitoring site declined from 4600 seeds m⁻² in 2003 to zero seeds m⁻² in 2006. Unlike C. diffusa, substantial numbers of rosettes of C. stoebe remained present. Larinus consumed almost all Urophora encountered in C. diffusa, and consumed about 40% of the Urophora in co-infested flower heads of C. stoebe (ca. 10–15% of the total Urophora population). No negative correlations between the relative densities of flower head and root-feeding insects were observed. The effects of these insects on target plants have produced results consistent with the ‘cumulative stress hypothesis’ for biological control of Centaurea species.     

Drought,warming and soil fertilization effects on leaf volatile terpene concentrations in <Emphasis Type="Italic">Pinus halepensis</Emphasis> and <Emphasis Type="Italic">Quercus ilex</Emphasis>

The changes in foliar concentrations of volatile terpenes in response to water stress, fertilization and temperature were analyzed in Pinus halepensis and Quercus ilex. The most abundant terpenes found in both species were α-pinene and Δ³-carene. β-Pinene and myrcene were also abundant in both species. P. halepensis concentrations were much greater than those of Q. ilex in agreement with the lack of storage in the latter species (15205.60 ± 1140.04 vs. 0.54 ± 0.08 μg g⁻¹ [d.m.]). The drought treatment (reduction to 1/3 of full watering) significantly increased the total terpene concentrations in both species (54% in P. halepensis and 119% in Q. ilex). The fertilization treatment (addition of either 250 kg N ha⁻¹ or 250 kg P ha⁻¹ or both) had no significant effects on terpene foliar concentrations. The terpene concentrations increased from 0.25 μg g⁻¹ [d.m.] at 30°C to 0.70 μg g⁻¹ [d.m.] at 40°C in Q. ilex (the non-storing species) and from 2,240 μg g⁻¹ [d.m.] at 30°C to 15,621 μg g⁻¹ [d.m.] at 40°C in P. halepensis (the storing species). Both species presented negative relationship between terpene concentrations and relative water contents (RWC). The results of this study show that higher terpene concentrations can be expected in the warmer and drier conditions predicted for the next decades in the Mediterranean region.     

Tree leaf litter composition and nonnative earthworms influence plant invasion in experimental forest floor mesocosms

Dominant tree species influence community and ecosystem components through the quantity and quality of their litter. Effects of litter may be modified by activity of ecosystem engineers such as earthworms. We examined the interacting effects of forest litter type and earthworm presence on invasibility of plants into forest floor environments using a greenhouse mesocosm experiment. We crossed five litter treatments mimicking historic and predicted changes in dominant tree composition with a treatment of either the absence or presence of nonnative earthworms. We measured mass loss of each litter type and growth of a model nonnative plant species (Festuca arundinacea, fescue) sown into each mesocosm. Mass loss was greater for litter of tree species characterized by lower C:N ratios. Earthworms enhanced litter mass loss, but only for species with lower C:N, leading to a significant litter × earthworm interaction. Fescue biomass was significantly greater in treatments with litter of low C:N and greater mass loss, suggesting that rapid decomposition of forest litter may be more favorable to understory plant invasions. Earthworms were expected to enhance invasion by increasing mass loss and removing the physical barrier of litter. However, earthworms typically reduced invasion success but not under invasive tree litter where the presence of earthworms facilitated invasion success compared to other litter treatments where earthworms were present. We conclude that past and predicted future shifts in dominant tree species may influence forest understory invasibility. The presence of nonnative earthworms may either suppress of facilitate invasibility depending on the species of dominant overstory tree species and the litter layers they produce.