Umweltveränderungen im Bereich der Baumgrenze

Umweltforschung ist in alpinen Ökosystemen besonders zentral, da sich Klimaveränderungen in diesen Regionen gemäss Prognosen stärker auswirken als in den meisten anderen Gebieten der Welt. Forschende verschiedenster Institutionen aus dem In- und Ausland, darunter auch der WSL, führen an der natürlichen Baumgrenze ein Freiluftexperiment mit CO₂-Begasung sowie Erwärmung des Bodens durch. Das Experiment befindet sich in der Nähe von Davos inmitten der Langzeitversuchsfläche Stillberg (2200 m ü. M), wo 1975 beinahe 100'000 Bäume gepflanzt wurden. 

Larix decidua		Pinus mugo spp. uncinata

Bereits seit neun Jahren wird dort die natürliche Vegetation, bestehend aus den zwei Baumarten Larix decidua (Lärche, sommergrün) und Pinus mugo spp. uncinata (Bergföhre, immergrün) sowie dem durch Zwergsträucher dominierten Unterwuchs, einer höheren CO2-Konzentration (+200 ppm) ausgesetzt. Seit 2007 wird ausserdem die Bodentemperatur experimentell erhöht (+4 K). Zusammen ergibt das eine CO2-Konzentration und Erwärmung, die realistisch sind für diese Region im Jahre 2050.

Im Rahmen dieses Umweltveränderungs-Projektes widmen sich die Forschenden einer Vielzahl von ökologischen und biochemischen Fragestellungen (weitere Informationen). Die hier beschriebene Doktorarbeit des SLF beschäftigt sich mit den spezifischen Reaktionen der Pflanzen auf die experimentellen Behandlungen: Grösse, Phänologie (Zeitpunkt verschiedener Schlüsselereignisse wie Blütenproduktion oder Öffnung der Blattknospen), Reaktion auf extreme Klimabedingungen sowie Zusammensetzung der Pflanzengemeinschaften. Die wichtigsten Resultate werden im Folgenden vorgestellt:

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.

Mean annual growth of shoots on randomly selected mid-canopy branches, +/- 1 standard error. * indicates significance at p < 0.05. a) Ambient CO22 (solid line) treatment groups in (dotted line) and elevated COLarix decidua b) CO2 (dotted lines = ambient, solid lines = elevated) and soil warming (black = ambient, red = warmed) treatment groups in Pinus uncinata.

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