The influence of habitat conditions on the performance of two invasive, annuals — Impatiens glandulifera and Bidens frondosa

The investigations of habitat conditions on the variability of selected population features in the two invasive, annuals with different life-history traits were conducted in the years 2008–2010, in the Polish part of the Carpathian Mountains in communities characterized by the gradual decrease of light availability. The individuals of Impatiens glandulifera were surveyed along roadsides, in willow thickets, as well as inside and along the edges of the riparian forest, whereas the individuals of Bidens frondosa were observed in riverside gravels characterized by a different species composition. Each year, the number and density of individuals (stems) occurring in the particular sites were examined, as well as the height and the fruit production in 30 randomly chosen stems were surveyed. Moreover, the number of seeds per fruit, the diaspore dimensions and the seedling recruitment in laboratory conditions were examined during each season. As the values of height of individuals, number of fruits per stem, number of seeds per fruit, as well as the seedling abundance in some groups were not consistent with the normal distribution and the variances were not homogeneous, the non-parametric Kruskal-Wallis test was used in statistical analyses.Much greater number and density of individuals of Impatiens glandulifera were found in riparian forest, than in willow thickets and along the roadsides. In all sites the number and density of individuals increased steadily in consecutive seasons. The great number and density of Bidens frondosa individuals observed in shady and partly shady sites during the first year of studies raised in the second year and subsequently dramatically decreased in the third season. The lowest number and density of individuals noted in the first season in unshaded site raised substantially in subsequent years. The height of stems, as well as seed and fruit production of both taxa diminished with a decrease of height of neighboring plants. The seed dimensions presented the spatial and temporal variability, whereas the number of seedlings among consecutive years and in successive sites did not differ.The considerable height of the individuals of Impatiens glandulifera, high production of large fruits and seeds in open and dry roadside areas can contribute to more effective ballistic dissemination, while substantial seedling recruitment enables the colonization of new, perhaps more advantageous sites. On the other hand, lower individual height, as well as fruit and seed production and considerable seedling emergence allow the population to last and to gradually extend the area in forest communities. The considerable abundance of the high-statured Bidens frondosa individuals, substantial production of large capitula and achenes contribute to long persistence of populations in open and sun-lit sites. The gradually decrease in the height of the stems, achene number and size observed in partly-shaded and shaded places might allow to long-distance dispersal of seeds by animals, while substantial seedling recruitment might contribute to establishment in new areas.

Altered phenology and temperature sensitivity of invasive annual grasses and forbs changes autotrophic and heterotrophic respiration rates in a semi-arid shrub community

Many invasions, like the wide-spread establishment of annual grasses and forbs in semi-arid shrublands, are associated with climate change. In order to predict ecosystem carbon (C) storage it is critical that we understand how invasion affects soil respiration (Rt). Because plants and microbes have different seasonal dynamics, determining the relative contribution of autotrophic (Ra) and heterotrophic (Rh) respiration provides critical insight into soil C processes. Using automated soil respiration measurements and root exclusion cores we evaluated the moisture and temperature sensitivity of Rt and Rh and calculated the contribution of Ra in native shrub and invaded areas. Invasion increased cumulative Rt by 40% from 695 (±51) g C m−2 under shrubs to 1050 g C m−2 (±44) in invaded areas. Cumulative Rh did not change but invasion altered the seasonal pattern of Rh. Throughout the season Rt and Rh responded positively to temperature increases when soils were wet and negatively when soils were dry. Invasion increased temperature sensitivity of Rt and Rh in wet soils and decreased temperature sensitivity in dry soils. The altered temperature sensitivity of invasives was attributed largely to differences in phenology. Early phenology of invasive grasses caused rapid Ra increases early in the season; late phenology of invasive forbs resulted in the surprising maintenance of diurnal Ra and Rh signals despite high temperatures and low soil moisture. Invasion extended the respiration season of the system. Ability of the invasive community to withstand high temperatures and drought could confer greater resilience if temperature and precipitation patterns in the region change. The high contribution of Ra by invasive annuals means ecosystem C storage will depend heavily on seasonal rainfall dynamics and productivity of invasive annuals. In semi-arid ecosystems even small scale changes in plant community composition alter Rt, Ra and Rh and should be considered when attempting to predict Rt.