Environmental change at the alpine treeline

High elevation ecosystems are important in research on environmental change because shifts in climate associated with anthropogenic greenhouse gas emissions are predicted to be more pronounced in these areas than in most other regions of the world. This project involves a Free Air CO₂ Enrichment (FACE) and soil warming experiment located in a natural treeline environment near Davos, Switzerland (Stillberg, 2200 m a.s.l.). The study site is located within the Stillberg long-term research site (further information in german), where close to 100,000 trees were planted in 1975.

Larix decidua	Pinus mugo spp. uncinata

For the past nine years natural vegetation, including the two tree species Larix decidua (European larch, deciduous) and Pinus mugo spp. uncinata (mountain pine, evergreen) and the dwarf shrub-dominated understorey, have been exposed to elevated atmospheric CO₂ concentrations (+200 ppm). A soil warming treatment (+4 K) was added in 2007, resulting in a combination of CO₂ and warming that is realistic for this region in approximately 2050. A broad range of ecological and biogeochemical research is carried out as part of this environmental change project (further information). This PhD research specifically focuses on the responses of plants to the experimental treatments: growth, phenology (timing of key events, e.g. leaf budburst and flower production), response to extreme climate conditions, and community composition. A few major research topics are outlined below.

Tree growth responses to elevated CO₂ and soil warming

Our treeline CO₂ enrichment and soil warming experiment provides the opportunity to address the fundamental question of whether carbon (source) and/or cold temperature (sink) limits tree growth at high elevations. By measuring shoot and stem growth of the two major treeline species, we can also help predict shifts in plant growth and species composition in treeline regions under environmental change. Over the nine years of CO₂ enrichment, there was a consistent pattern of increased annual shoot growth under elevated CO₂ (relative to pre-treatment growth) in larch, although the effect was only significant in 2002. In contrast, pine did not show shoot growth stimulation under elevated CO₂ in any year compared to the pre-treatment values. Soil warming for three growing seasons did not independently affect shoot growth or change the CO₂ response in larch, whereas pine showed more growth in warmed than unwarmed plots during 2008 regardless of CO₂ treatment.

Section of a tree microcore

These shoot growth results suggest that larch is at least partially limited by carbon at this treeline site, although the lack of a strong CO₂ stimulation over several years suggests that other resources might also limit growth. In contrast, pine shoot growth does not seem to be limited by carbon supply, whereas soil warming might alleviate a temperature limitation on growth and/or create more favourable (drier) soil moisture conditions. The different responses of these two major treeline species to environmental changes suggest that future shifts in biodiversity and structure of the treeline are likely. Microcore samples including the last 12+ tree rings were collected during autumn of 2009, and analysis of stem growth will complement the shoot measurements for a robust indication of treatment effects on tree growth.

Dwarf shrub responses to long-term CO₂ enrichment and three years of warming

Dwarf shrubs cover large areas of arctic and alpine regions and play an important role in ecosystem function. As a result, shifts in dwarf shrub abundance and species composition under environmental change are likely to impact many ecological processes. In our treeline environmental change experiment, we focus on three dominant ericaceous dwarf shrub species: deciduous Vaccinium myrtillus (bilberry, Heidelbeere) and Vaccinium uliginosum (bog bilberry, Rauschbeere), and evergreen Empetrum nigrum spp. hermaphroditum (crowberry, Krähenbeere). We investigate several responses to the elevated CO₂ and warming treatments, including spring and autumn phenology, reproductive ecology (e.g. berry production and seed germination success rate), shoot growth, and plot vegetation composition.

In general, measurements of dwarf shrub productivity and reproductive output during the experiment have yielded species-specific responses. For example, over the nine years of the CO₂ treatment there was a consistent trend of longer new shoot increments under elevated CO₂ in Vaccinium myrtillus (9% average increase over all years) but not in V. uliginosum or in Empetrum hermaphroditum. New shoot growth was also influenced by the warming treatment, again only in V. myrtillus: mean length of new shoot increments was 24% greater in warmed plots over the three years of treatment relative to pre-warming years. The different responses of these three dwarf shrub species in several measurements of productivity could lead to shifts in species abundance and distribution over the longer term.

Vaccinium myrtillus	Vaccinium uliginosum	Empetrum hermaphroditum

Reduced early growing season freezing resistance under elevated atmospheric CO₂

The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high-altitude environments could increase under future atmospheric and climate change. We examined possible changes in plant sensitivity to freezing conditions through an experimental freezing study including ten prominent species present in the experimental plots at the Stillberg site (more information). We found that long-term exposure to elevated CO₂ led to greater freezing sensitivity in five of the ten selected species but did not influence the timing of leaf budburst or leaf expansion, suggesting that physiological changes caused by CO₂ enrichment were responsible for the effect. Soil warming advanced leaf expansion and reduced freezing resistance in the dwarf shrub Vaccinium myrtillus. The results of this study suggest that that leaf tissue damage caused by episodic early season freezing events will increase in frequency for some species in the coming decades. The resulting shifts in relative freezing resistance among co-occurring species could, in turn, alter competitive interactions among species.

Papers

Dawes, M.A., Hagedorn, F., Zumbrunn, T., Handa, I.T., Hättenschwiler, S., Wipf, S. and Rixen, C. 2011. Growth and community responses of alpine dwarf shrubs to in situ CO₂ enrichment and soil warming. New Phytologist (191) 806-818. Abstract

Dawes, M.A., Hättenschwiler, S., Bebi, P., Hagedorn, F., Handa, I.T., Körner, C. and Rixen, C. 2011. Species-specific tree growth responses to nine years of CO₂ enrichment at the alpine treeline. Journal of Ecology (99) 383-394. Abstract

Martin, M.A., Gavazov, K., Hättenschwiler, S., Körner, C. and Rixen, C. 2010. Reduced early growing season freezing resistance in alpine treeline plants under elevated atmospheric CO₂. Global Change Biology (16:3) 1057-1070. Abstract

Hagedorn, F., Martin, M.A., Rixen, C., Rusch, S., Zürcher, A., Siegwolf, R., Wipf, S., Escape, C., Roy, J. and Hättenschwiler, S. 2010. Short-term responses of ecosystem carbon fluxes to experimental soil warming at the Swiss alpine treeline. Biogeochemistry (97:1) 7-19.

Contact

• Melissa Dawes