Vilatersana R., Sanz M., Galian A., Castells E. (2016) The invasion of Senecio pterophorus across continents: multiple, independent introductions, admixture and hybridization. Biological Invasions. 18: 2045-2065.EnlaceDoi: 10.1007/s10530-016-1150-1
Senecio pterophorus (Compositae) is a perennial shrub native to eastern South Africa that was introduced into the Western Cape in South Africa and Australia approximately 100 years ago and into Europe (Italy and Spain) more than 25–30 years ago. In this study, the aims were to unravel the putative sources of the introduced populations and identify the changes in genetic diversity after invasion using molecular markers and phylogeographic and population genetic analyses. We sampled the entire area of distribution for S. pterophorus extensively. Based on the results, three lineages were established along a latitudinal and climatic gradient in the native range (south, central, central/north) with high levels of admixture. Multiple, independent introductions occurred in the four invaded ranges. The central/northern lineage (humid climate) was the primary source for all of the invaded regions (with drier climates), although a secondary role was revealed for the southern lineage in the Western Cape and the central/northern lineage in Australia and Spain. The genetic diversity was slightly lower in the Spanish and Australian populations than that in the native populations. A variety of demographic and genetic processes affected the amount and structure of genetic diversity in the invaded areas, including multiple introductions and admixture (Western Cape, Australia and Spain) as well as pre-invasive hybridization (Italy). The patterns of dispersion support a hypothesis of rapid evolution of S. pterophorus after invasion in response to novel climatic conditions. © 2016, Springer International Publishing Switzerland.
Colomer-Ventura F., Martínez-Vilalta J., Zuccarini P., Escolà A., Armengot L., Castells E. (2015) Contemporary evolution of an invasive plant is associated with climate but not with herbivory. Functional Ecology. 29: 1475-1485.EnlaceDoi: 10.1111/1365-2435.12463
Divergence in plant traits and trait plasticity after invasion has been proposed as mechanisms favouring invasion success. Current hypotheses predict a rapid evolution in response to changes in the abiotic conditions in the area of introduction or to differences in the herbivore consumption pressure caused by a decrease in the enemies associated with the area of origin [e.g. evolution of increased competitive ability (EICA) hypothesis]. The importance of these factors in determining plant geographical divergence has not been yet simultaneously evaluated. Senecio pterophorus (Asteraceae) is a perennial shrub native to eastern South Africa and a recent invader in western South Africa (since ~100 years ago), Australia (>70-100 years) and Europe (>30 years). These areas differ in their summer drought stress [measured as the ratio of summer precipitation to potential evapotranspiration (P/PET)] and their interactions with herbivores. We performed a common garden experiment with S. pterophorus sampled throughout its entire known distributional area to determine (i) whether native and non-native populations diverge in their traits, as well as the plasticity of these traits in response to water availability and (ii) whether climate and herbivory play a role in the genetic differentiation across regions. Plants from the non-native regions were smaller and had a lower reproductive output than plants from the indigenous area. No geographical differences in phenotypic plasticity were found in response to water availability. Herbivory was not related to the plant geographical divergence. In contrast, our results are consistent with the role of climate as a driver for postinvasive evolution, as suggested by adaptation of plants to a drought cline in the native range, the analogous change in plant traits in independently invaded regions and the convergence of vegetative traits between non-native plants and native plants under similar drought conditions. Native and non-native populations of S. pterophorus differed in plant traits, but not in trait plasticity, in response to their local climatic conditions. Our results are contrary to the role of herbivory as a selective factor after invasion and highlight the importance of climate driving rapid evolution of exotic plants. © 2015 The Authors.
Castells E., Peñuelas J., Valentine D.W. (2005) Effects of plant leachates from four boreal understorey species on soil N mineralization, and white spruce (Picea glauca) germination and seedling growth. Annals of Botany. 95: 1247-1252.EnlaceDoi: 10.1093/aob/mci139
• Background and Aims: Natural regeneration of white spruce (Picea glauca) after disturbance has been reported to be very poor. Here a study was made to determine whether C compounds released from understorey species growing together with white spruce could be involved in this regeneration failure, either by (1) changing soil nutrient dynamics, (2) inhibiting germination, and/or (3) delaying seedling growth. • Methods: Foliage leachates were obtained from two shrubs (Ledum palustre and Empetrum hermaphroditum) and one bryophyte (Sphagnum sp.) with high phenolic compound concentrations that have been reported to depress growth of conifers in boreal forests, and, as a comparison, one bryophyte (Hylocomium splendens) with negligible phenolic compounds. Mineral soil from a white spruce forest was amended with plant leachates to examine the effect of each species on net N mineralization. Additionally, white spruce seeds and seedlings were watered with plant leachates to determine their effects on germination and growth. • Key Results: Leachates from the shrubs L. palustre and E. hermaphroditum contained high phenolic compound concentrations and dissolved organic carbon (DOC), while no detectable levels of C compounds were released from the bryophytes Sphagnum sp. or H. splendens. A decrease in net N mineralization was determined in soils amended with L. palustre or E. hermaphroditum leachates, and this effect was inversely proportional to the phenolic concentrations, DOC and leachate C/N ratio. The total percentage of white spruce germination and the growth of white spruce seedlings were similar among treatments. • Conclusions: These results suggest that the shrubs L. palustre and E. hermaphroditum could negatively affect the performance of white spruce due to a decrease in soil N availability, but not by direct effects on plant physiology. © The Author 2005. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
Castells E., Peñuelas J., Valentine D.W. (2004) Are phenolic compounds released from the Mediterranean shrub Cistus albidus responsible for changes in N cycling in siliceous and calcareous soils?. New Phytologist. 162: 187-195.EnlaceDoi: 10.1111/j.1469-8137.2004.01021.x
We studied the effects of Cistus albidus leaf leachates on nitrogen-cycling processes in two siliceous soils (granite and schist) and one calcareous soil. We compared those effects with gross N-transformation rates in soils sampled underneath Cistus. Soils amended with leachates and soils sampled under Cistus had higher NH4+ immobilization and lower nitrification compared with control soils. Gross N mineralization increased under Cistus but decreased in soils amended with leachates. These effects were especially strong in granite soil. To determine whether phenolic compounds were causing those effects, we incubated granite soils with leachate and a leachate fraction containing only nonphenolic compounds. Nonphenolic compounds increased NH4+ immobilization and decreased gross nitrification, while decreases in gross N mineralization were estimated to be caused by phenolic compounds. Our results show that although phenolic compounds leached from green foliage changed gross N mineralization, their effects on net N rates were eclipsed by the changes produced by polar nonphenolic compounds such as carbohydrates. Plant nonphenolic compounds may drive N cycling under Cistus. © New Phytologist (2004).
Castells E., Peñuelas J. (2003) Is there a feedback between N availability in siliceous and calcareous soils and Cistus albidus leaf chemical composition?. Oecologia. 136: 183-192.EnlaceDoi: 10.1007/s00442-003-1258-8
The effects of the Mediterranean shrub Cistus albidus on N cycling were studied in two siliceous (granitic-derived and schistic-derived) and one calcareous soil differentiated by their texture and acidity. We aimed to find out whether soils under C. albidus were affected by the release of C compounds from the canopy, and whether phenolic compound production in C. albidus changed depending on the soil N availability. Calcareous soils, with higher clay content and polyvalent cations, had a higher organic matter content but lower net N mineralization rates than siliceous soils, and C. albidus growing therein were characterized by lower foliar N and phenolic compound concentrations. Under C. albidus, all types of soils had higher phenolic compound concentrations and polyphenol oxidase activity. C. albidus presence and leachate addition decreased net N mineralization and increased soil respiration in siliceous soils, and these changes were related to a higher soil C/N ratio under the canopy. In calcareous soils, however, no significant effects of plant presence on N cycling were found. In the studied plant-soil system it is not likely that higher phenolic compound concentrations were selected during evolution to enhance nutrient conservation in soil because (1) higher phenolic compound concentrations were not associated with lower soil fertilities, (2) C compounds released from C. albidus accelerated N cycling by increasing N immobilization and no evidence was found for decreased gross N mineralization, and (3) soil organic N content was more related to soil chemical and physical properties than to the effects of the C. albidus canopy.
Castells E., Peñuelas J., Valentine D.W. (2003) Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest. Plant and Soil. 251: 155-166.EnlaceDoi: 10.1023/A:1022923114577
The effects of the understory shrub Ledum palustre on soil N cycling were studied in a hardwood forest of Interior Alaska. This species releases high concentrations of phenolic compounds from green leaves and decomposing litter by rainfall. Organic and mineral soils sampled underneath L. palustre and at nearby non-Ledum sites were amended with L. palustre litter leachates and incubated at controlled conditions. We aimed to know (i) whether L. palustre presence and litter leachate addition changed net N cycling rates in organic and mineral soils, and (ii) what N cycling processes, including gross N mineralization, N immobilization and gross N nitrification, were affected in association with L. palustre. Our results indicate that N transformation rates in the surface organic horizon were not affected by L. palustre presence or leachate addition. However, mineral soils underneath L. palustre as well as soils amended with leachates had significantly higher C/N ratios and microbial respiration rates, and lower net N mineralization and N-to-C mineralization compared to no Ledum and no leachates soils. No nitrification was detected. Plant presence and leachate addition also tended to increase both gross N mineralization and immobilization. These results suggest that soluble C compounds present in L. palustre increased N immobilization in mineral soils when soil biota used them as a C source. Increases in gross N mineralization may have been caused by an enhanced microbial biomass due to C addition. Since both plant presence and leachate addition decreased soil C/N ratio and had similar effects on N transformation rates, our results suggest that litter leachates could be partially responsible for plant presence effects. The lower N availability under L. palustre canopy could exert negative interactions on the establishment and growth of other plant species.
Castells E., Roumet C., Peñuelas J., Roy J. (2002) Intraspecific variability of phenolic concentrations and their responses to elevated CO2 in two mediterranean perennial grasses. Environmental and Experimental Botany. 47: 205-216.EnlaceDoi: 10.1016/S0098-8472(01)00123-X
Intraspecific variability of total phenolic compound concentrations and their responses to elevated CO2 were studied in two wild Mediterranean perennial grasses, Dactylis glomerata and Bromus erectus. Ten and nine genotypes of each species were grown in climate-controlled naturally-lit glasshouses under plant intergenotypic and interspecific competition for water, light and nutrients. Carbon source-sink balance hypotheses of resource allocation were also tested. Elevated CO2 induced changes in dry shoot biomass (DM), leaf total non-structural carbohydrate concentrations [TNC] and leaf nitrogen concentrations [N] found in a previous study (New Phytol. 143 (1999) 73) were related to changes in phenolic compound concentrations. Phenolic compound concentrations increased to 15.2% DM in D. glomerata and 86.9% DM in B. erectus under elevated CO2. These changes were more pronounced when expressed on a structural dry mass basis (DMst). Increases in DMst and [TNCst] and decreases in [Nst] were also found according to current resource allocation hypotheses. However, there were no proportional changes between phenolic responses to elevated CO2 and DMst, [TNCst] and [Nst] responses. Phenolic concentrations were highly determined by genetics in both species, but all studied genotypes responded in a similar way to elevated CO2. Considering the present experimental conditions with plants growing in intraspecific and interspecific competition, the absence of CO2 × genotype interaction would lead to little changes of fitness in terms of antiherbivore chemical defence, and, therefore, to low evolutionary consequences in CBSC under the increasing atmospheric CO2 concentrations of the next decades. © 2002 Elsevier Science B.V. All rights reserved.
Peñuelas J., Castells E., Joffre R., Tognetti R. (2002) Carbon-based secondary and structural compounds in Mediterranean shrubs growing near a natural CO2 spring. Global Change Biology. 8: 281-288.EnlaceDoi: 10.1046/j.1365-2486.2002.00466.x
We studied carbon-based secondary and structural compounds (CBSSCs) in Myrtus communis, Erica arborea, and Juniperus communis co-occuring in a natural CO2 spring site and in a nearby control site in a Mediterranean environment. Leaf concentrations of phenolics and CBSSCs, such as lignin, cellulose, and hemicellulose, total nonstructural carbohydrates (TNCs), and lipids were measured monthly (phenolics) and every two months (the other compounds) throughout a year. There was a slight seasonal trend towards maximum concentrations of most of these CBSSCs during autumn-winter and minimum values during the spring season, particularly in Myrtus communis. For most of the CBSSCs and species, there were no consistent or significant patterns in response to the elevated [CO2] (c. 700 μmol mol-1) of the spring site. These results were not due to a dilution effect by increased structural or nonstructural carbon. Therefore, in contrast to many experimental studies of CO2 enrichment, mainly conducted for short periods, there were no greater concentrations of phenolics, and, as in many of these studies, there were neither greater concentrations of the other CBSSCs. These results do not agree with the predictions of the carbon source-sink hypotheses. Possible causes of this disagreement are discussed. These causes include the complex heterogeneous environmental conditions and the variability of resource availabilities in the field, photosynthetic down-regulation, and/or the homeostatic and evolutionary nature of organisms. These results suggest evolutionary adaptive responses to changes in CO2. They also suggest caution in attributing increased CBSSC concentrations to elevated [CO2] at long-term scale in natural conditions, and therefore in their implications for plant-herbivore interactions and for decomposition.
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