Albert-Ludwigs-Universität Freiburg
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| One of our major problems in Charlestown is Japanese Knotweed |
Since the plant can use nitrogen from the air on account of
its symbiotic relationship with bacteria on its roots, and since it also grows
rapidly and produces a lot of biomass, it enriches the naturally nutrient-poor
dune ecosystem with nitrogen, leading to an undesirable fertilization effect.
In addition, it consumes more water than native species. The ecologists Prof.
Dr. Christiane Werner, Christine Hellmann, and Dr. Jens Oldeland have developed
a new approach published in the journal PLOS ONE for
identifying the areas in which the acacia interacts with native species.
The team determined that the invasive species has a negative effect on the development of some native species, while it has no effect on others and even causes several species to grow better.
The interactions between plants and their living and non-living
environment has a decisive impact on the structure and function of ecosystems.
To determine the strength and the spatial zone of influence of such
interactions, the research team uses stable isotopes -- heavy, non-radioactive
forms of elements.
The frequency with which these isotopes occur in materials
in comparison to the much more common light isotopes can vary over space. The
ratio of stable isotopes can therefore provide information on where and how a
material originated.
So-called "isoscapes," a portmanteau of
"isotope" and "landscape," represent in map form how
isotopes are distributed in a landscape.
The team used isoscapes based on the
leaf material of native species to show where the proportion of nitrogen fixed
by the acacia is high and where the invasive species influences the growth of
other species -- whether positively due to additional nitrogen or negatively
due to competition for water.
A dwarf shrub from the Ericaceae family, for
example, exhibits greatly increased nitrogen concentrations and more efficient
photosynthesis in a large radius surrounding acacia, while a stone pine uses
only very small amounts of the additional nitrogen.
A dwarf shrub from the
Fabacean family, on the other hand, which can also use fixed atmospheric
nitrogen, is not influenced by the acacia at all.
The results indicate that the interaction between the acacia and
native plants is species-specific. In addition, the influence varies depending
on the amount of nitrogen or water available to the species.
In order to use
this information to make a map integrating these various aspects, the
scientists conducted a cluster analysis.
This statistical method finds
subgroups in the sampled area that exhibit a similar combination of the
measured values, allowing them to be interpreted as zones of influence.
The
goal of these analyses is to better describe, elucidate, and understand the
complex relationships and dynamics governing natural ecosystems.
Christiane Werner is a professor of ecosystem physiology at the
University of Freiburg's Faculty of Environment and Natural Resources. Her
doctoral student Christine Hellmann conducts her research at the University of
Bielefeld, and Jens Oldeland is a research assistant at the University of
Hamburg.