The Gunnera trick

Lofty peaks and a very high rate of succession grace the cover of New Phytologist 223:2, courtesy of Alberto Benavent-González. Below he explains the story behind his research.

We are looking at the very front of the Pia Glacier, located at the southern side of the Darwin Range in Tierra del Fuego (Chile). This glacier, as many others in the region, is retreating rapidly and exposing barren substrates which are quickly colonised by several organisms. This process, known as primary colonisation or primary succession, is usually initiated by autotrophic and heterotrophic microorganisms followed by the appearance of the first plants (mostly mosses) and lichens.

Pia Glacier front, with Gunnera Magellanica in the initial stages of colonisation. Courtesy of Alberto Benavent-González.

In the picture, this initial colonisation includes the vascular plant Gunnera magellanica, a plant that is special in that it belongs to the only genus of phanerogams that establishes an endosymbiotic relationship with cyanobacteria (genus Nostoc). Thus, the picture shows the extensive population of Gunnera magellanica colonising the Pia Glacier moraines after just c. four years of exposure.

The picture was taken during our second field trip to the region in 2015 (in the austral summer season). We wanted to compare the initial stages of plant colonisation and development with the older parts of the glacier foreland where a mature forest of Nothofagus spp. was already in place after only 34 years. We used in-situ gas exchange measurements in combination with in-situ N-fixation determinations and nutrient / isotope discrimination contents of samples to explore the ecophysiology of dominant plant species.

Nothofagus betuloides and N. antarctica individuals at mid-successional stages (c. 10 year of exposure) in front of the Pia Glacier. Courtesy of Alberto Benavent-González.

Primary succession at the Pia Glacier is the fastest reported so far in glacier forelands. Only three decades are required after initial soil exposure to achieve a forest, with some trees reaching 10 metres in height. This fast succession, together with leaf nutrient analyses of leaves of several shrubs and trees along the chronosequence in front of the retreating glacier, suggested no limitation by nutrients.

This did not match with the generally accepted assumptions around newly exposed glacier forelands, usually thought to be nutrient poor. In fact, a similar situation also exists at the Pia Glacier where soils initially have undetectable N content, and external N-inputs to the system (from glacial melt and atmospheric deposition), are also at global low levels. Our aim was to gather information about what was happening in this special place to have promoted such a rapid vegetation succession, and our suspicions pointed to the native herb G. magellanica as a tentative candidate to provide large amounts of nitrogen to the habitat via symbiotic N-fixation.

Our studies confirmed that G. magellanica has very high rates of N-fixation and, very unusually, occurred – and could therefore contribute N – at all stages of the succession, from initial bare ground to forest understory. However, it was clear that biological N-fixation was not the full story.

Forest successional stage with dense cover by Gunnera magellanica dominating the herbaceous layer. Courtesy of Albert Benavent-González.

Nitrogen stable isotope values in plant leaves along the chronosequence suggested a major role for symbiotic mycorrhizae. The mycorrhizae provide a major and rapid channel for N and other nutrients to the host plants, which has a major effect because nutrient release via mineralisation is usually slow in cold regions. In fact, the mycorrhizal involvement is so powerful that soil microbial N mineralisation seems to be absent at Pia Glacier, although this requires further research for confirmation.

In any case, we now know that the rapid succession at Pia Glacier foreland is based on two major symbioses by the plants, the first with a cyanobacterium in G. magellanica and then with mycorrhizal fungi in all plants present.

Glacier run-off bordering the moraine with dense vegetation colonisation. Courtesy of Alberto Benavent-González.

Alberto Benavent-González

Read the paper: Benavent‐González, A., Raggio, J., Villagra, J., Blanquer, J. M., Pintado, A., Rozzi, R., Green, T. A. and Sancho, L. G. (2019) High nitrogen contribution by Gunnera magellanica and nitrogen transfer by mycorrhizas drive an extraordinarily fast primary succession in Sub‐Antarctic Chile. New Phytologist. doi: 10.1111/nph.15838

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