Weeds in a changing environment

Abstract

In the course of the past decades, various impacts of climate change have been recognized to pose a challenge to arable production systems. Among others, shifts in the species composition of weed communities, particularly the rising importance of highly competitive neophytic weed species and sleeper weeds, cause yield losses and additional management costs. This thesis aimed to provide insights into the response to changed environmental conditions of a selection of arable weeds. We examined the effects of the projected future climatic conditions in Lower Saxony (northwest Germany) on emergence, morphological traits, habitat suitability and potential geographical distribution of the weed species. We used different methods to address these targets: a mechanistic processed-based experimental approach, as well as a correlative maximum entropy approach of bioclimatic niche modelling (software MaxEnt). Focus was put on three weed species, which may alter their distribution under the predicted conditions of environmental change: Abutilon theophrasti, Datura stramonium and Iva xanthiifolia were selected as examples of a guild of highly competitive, ruderal, stress-tolerant, thermophile, annual C3 weeds, which have a distinct effect on the agricultural productivity, or, in case of I. xanthiifolia, which cause human harm. They are currently expanding their range in Europe, but are still differing in their biogeographic importance for Lower Saxonys’ agro-ecosystems. In a first experimental study, a manipulative approach was used to analyse the effects of elevated temperature (ambient temperature +2.5°C), reduced soil moisture (soil moisture content between -0.1 to -1.5 MPa) versus ambient temperature conditions and soil moisture content of -0.0036 MPa in four soil type (loess, clay, peat and sand), and their interactions on the emergence characteristics of the target weeds. The emergence patterns were significantly influenced by the environmental conditions. A linear-mixed effects model and a survival analysis revealed that drought conditions decreased the number of emerged seedlings and the probability that seedlings will emerge in most environmental settings, especially in combination with warmer temperature and in sand rich soils. In dry conditions, emergence was highest in peat compared to the other soil types. The results showed that for I. xanthiifolia the warmer temperature treatment and the reduced water availability were outside of the optimum conditions for emergence. Within the second experimental study, the stress treatment factors which were applied to the emergence stage were continued during the establishment stage. Higher temperatures and reduced soil water content produced negative effects on the overall growth performance - as measured by total biomass and traits indicating biomass allocation (e.g., leaf area, root length)- for the three weed species. Peat and clay rich soils provided better growing condition than other soil types under warming and drought. I. xanthiifolia did not respond to the tested conditions in a consistent way, but positive growth performance of this species was most strongly related to good moisture supply and colder temperatures. For A. theophrasti and D. stramonium, we observed a tendency of a higher total biomass in clay and peat rich soils than in the other soil types under warm-dry conditions. These two species showed a higher plasticity than I. xanthiifolia in all of the tested environmental treatments. The results from the first and the second experimental studies demonstrated that the three weeds which were assumed to have similar environmental requirements responded in rather idiosyncratic ways the variety of microsite conditions. Under future climate, emergence and growing conditions for plants may differ more distinctly than today between sand and peat rich sites. We further concluded from our results that future climatic conditions may impair the emergence and growth performance of I. xanthiifolia. For A. theophrasti and D. stramonium, we deduced tolerance to a wide range of environmental conditions. These two species may benefit from the predicted future climate. In the ecological niche modelling approach on a small, regional scale, it was demonstrated that there will be moderate to high impacts on the distribution of the target weed species in Lower Saxony as based on a gain and loss of suitable habitat areas. The environmental influences varied with regard to their relevance for the three weeds. The modelled prediction, and the experimental findings did differ in the case of A. theophrasti. The model predicted a decline in potential distribution and habit suitability for this species. This was in disagreement with our experimental findings, were we estimated a high plasticity and a wide range of tolerance of the species. The results of the modelling approach indicated further an increase of D. stramonium and a decrease of I. xanthiifolia in potential distribution and habitat suitability for northwest Germany, which was conform to the findings from our experiments. We demonstrated that even for weed species with similar ecology, under future climatic conditions dissimilar responses concerning morphological traits and distributional shifts have to be expected. In conclusion, this thesis provides evidence that an integrative approach, combining manipulative environmental experiments, and ecological niche based modelling on the current and future potential distribution and habitat suitability data, enabled a better understanding of how the target weeds may react under future environmental conditions, than a separated analysis would have allowed. Such a comprehensive approach may lead to greater precision in predicting the impacts of a changing environment on plant species and ecosystems processes for Lower Saxony.