Tree of heaven or hell? On the ecology of Ailanthus altissima (Mill.) Swingle
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Date
2019Type
- Doctoral Thesis
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Abstract
Forests provide a multitude of ecosystem services for society. Their provisioning
is increasingly challenged by rapid changes in climate and socio-economy,
and the spread of non-native tree species. To ensure the continuous provision
of ecosystem services, sound scientific knowledge on the potential of tree species
to cope with and adapt to these changes is required.
In this thesis, I focus on the ecology of the tree species Ailanthus
altissima (Mill.) Swingle native to China, which was introduced to many continents,
including Europe. Since the 1950s, the tree has spread in lowland forests
in southern Switzerland that used to be dominated by Castanea sativa Mill.
Chapter 1: Recent reports of regeneration of A. altissima in relatively
undisturbed forests in southern Switzerland seemed to contradict the characterisation
of the species as shade intolerant. Therefore, we studied the shade
tolerance of juvenile A. altissima in six sites at various colonisation stages.
Results indicate that A. altissima is more shade tolerant than early-successional
Larix decidua Mill. and Pinus sylvestris L.. This was more pronounced
for vegetative regeneration (i.e., root sprouts) that may profit from resources
of canopy trees. Therefore, we conclude that A. altissima displays a low to
intermediate shade tolerance in early regeneration.
Chapter 2: The establishment of A. altissima has primarily been studied
in managed forests or after a severe natural disturbance. Hence, we analysed
the factors determining the presence and abundance of generative regeneration
of A. altissima in forests that lack a severe, large-scale disturbance. It was
found that high rock cover, low litter cover, and high light availability are the
most important factors that determine A. altissima occurrence, whereas its
abundance was positively influenced by high light availability, low litter cover,
and high browsing on competing regeneration. The results of this study suggest
that A. altissima will be able to further expand its distribution in lowland forests in southern Switzerland currently dominated by C. sativa.
Chapter 3: Short-term growth reactions of trees to weather conditions can reveal growth optima and limits, and thus help to predict growth responses to climate change. To this end, we studied hourly stem growth of mature A. altissima and co-occurring C. sativa in two sites in southern Switzerland, comprising four years of continuous dendrometer measurements. The weather conditions in which the trees achieved highest hourly growth and conditions in which largest proportions of seasonal growth occurred were determined. A. altissima achieved highest growth rates and increased amounts of growth in warmer and drier weather conditions compared to C. sativa, especially in the site with lower interspecific competition. Hence, we conclude that A. altissima has a greater potential than C. sativa to cope with predicted future climate conditions.
Chapter 4: Species that are able to establish and spread outside their native range are mainly analysed in the introduced range only. However, studies in the native range may increase the understanding of their success in colonising new environments. Hence, we analysed and compared radial growth patterns of A. altissima in both its native and non-native range in eastern China and southern Switzerland, respectively. Growth rates were similar despite lower amounts of growing season precipitation in China. Furthermore, negative correlations between annual radial growth and average seasonal temperature at the warmest site suggest that the growth of A. altissima starts to be limited by average growing season temperatures > 21 °C. These results confirm the findings of Chapter 3 as well as the high drought tolerance of A. altissima reported in other studies.
Overall, I expect that the distribution of A. altissima will increase in southern Switzerland due to a rising propagule pressure and its tolerance of the warm and dry conditions expected under a changing climate. Furthermore, higher frequencies of disturbance events (e.g., fire or windthrow) and the expected further increase in mortality rates of C. sativa due to pathogens and drought may lead to greater dispersal opportunities for A. altissima. An increase in the distribution and density of A. altissima could, however, lead to a higher pathogen load, thus limiting the further spread of the tree.
Non-native species are frequently viewed in a negative light (e.g., invasive or aggressive) hindering an objective assessment of their impacts. Thus, I advocate an ecosystem functions and services centred assessment, which is open to both positive and negative effects. Moreover, it is important to consider the socio-ecological context in such assessments, which may result in spatially diverse outcomes and management approaches. For example, a non-native tree species may be promoted for industrial wood production but suppressed in nature conservation areas.
This thesis extends our knowledge of the ecological characteristics of A. altissima at different life stages. Thus, it adds to the fundamental understanding that is required to anticipate future scenarios of forest composition. This may then be used to predict the provisioning of ecosystem functions and services. Such predictions are a prerequisite for informed management decisions and practices. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000364287Publication status
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Contributors
Examiner: Bugmann, Harald![cc](/themes/Mirage2//images/orcid_icon.png)
Examiner: Wunder, Jan
Examiner: Conedera, Marco
Examiner: Kowarik, Ingo
Publisher
ETH ZurichSubject
Ailanthus altissima (tree of heaven); Castanea sativa (sweet chestnut); Non-native tree; Invasive tree; Shade tolerance; Colonisation; Gap Light Index (GLI); Browsing; litter cover; Propagule presure; Point dendrometer; Maximum growth; Radial growth; Tree-ring; Climate growth relationship; Vessel lumen area; Wood anatomyOrganisational unit
03535 - Bugmann, Harald / Bugmann, Harald
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