Ellen Cieraad's Research

Quantative plant ecology & physiology

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Mountain ecosystems are vulnerable to effects of climate change

Some of the biggest effects of climate warming are being observed in the polar regions; but the climate in mountainous regions are also rapidly changing. For example, the rate at which climate change is happening in the European Alps, is more than double that of the average across the Northern Hemisphere.


Treeline at Craigieburn, New Zealand

Plants are moving pole-ward and uphill trying to keep up with the climate they thrive in. Clear examples of this are treelines that are advancing uphill (but that doesn’t happen everywhere). We know that plants adapted to cold climates are being driven out of their natural ranges – and in the mountainous areas they might run out of space! However, beyond observing changes in the distributions of species, we still have a very poor understanding of the processes that accompany the climatic changes occurring in mountainous areas.

Sweden-based Dr. Jordan Mayor and David Wardle (SLU in Uppsala), organised and set up a study with an international research team, including myself, to investigate whether, at a global scale, we see universal shifts in ecosystem properties across elevational gradients. Our paper was published in Nature this week.

Mayor J, et al. Elevation alters ecosystem properties across temperate treelines globally – Nature, 25 Jan 2017 nature.com/articles/doi:10.1038/nature21027


Treelines in the different study regions (Photos J Mayor – Mayor et al. 2017 Extended data Fig 1)

The long-term and broad-scale changes that are instigated by climate change are hard to study using experiments. So instead, we used elevation as a surrogate for climate warming. This is possible because, as a consequence of warming, in 80 years from now, any particular elevation is expected to experience the temperature that is currently found 300 meters lower. Studying the properties of vegetation and soil along elevational transects near the treelines in seven regions (including the European Alps, Hokkaido Japan, Rocky Mountains USA-Canada, Patagonia, New Zealand, and Australia) allowed us to predict the effects of warming across temperate mountain regions world wide.


Treelines in the different study regions (Photos J Mayor – Mayor et al 2017 Extended data Fig 1)


We found remarkably consistent patterns across these extremely varied mountain regions. Decreasing elevation (increasing temperature) consistently increased the availability of soil nitrogen for plant growth – so we can expect that warming will consistently improve plant nitrogen nutrition. However, plant phosphorus availability was not controlled by elevation (and thus temperature) in the same way. This resulted in a pattern where the balance of nitrogen-to-phosphorus in plant leaves was very similar across the seven regions at higher elevations, but diverged greatly across the regions at lower elevation. This means the nitrogen-to-phosphorus ratio is constrained by low temperatures but at higher temperatures, regional factors and differences between regions become more important.

We also found that with increasing temperature, the patterns in plant nutrition were paralleled by changes in the amount and quality of organic matter in the soil and the microbial community. Our study allowed us to untangle the effects of vegetation type (forest below treeline, and alpine above it) on these patterns, and we found that the changes were at least partly independent of any effect of the vegetation. This means that effects of warming on ecosystem properties will occur irrespective of whether treeline shifts up-slope.

Our results not only suggest that warming could affect the way that plants grow, but also that these changes are linked to effects of warming on soils, especially the cycling of key nutrients that sustain the growth of plants. It provides evidence that expected temperature changes over the next 80 years have the potential to greatly disrupt the functional properties of mountain ecosystems and result in increased disequilibrium in the above- and below-ground ecosystem components, and the links between them.

The changes in mountain ecosystem processes identified in this study may have important implications for which plants grow in mountain ecosystems (affecting biodiversity), and the potential upward shift of treelines. Such shifts are expected have an effect on the local climate itself, and may indeed speed up the warming process, as forests reflect less and retain more heat than lower, less green vegetation.

We used elevational gradients to predict what will happen in mountainous ecosystems as the climate warms – this is a powerful approach to understanding the processes that are occurring at an increasing pace in these areas. However such changes are also likely to occur in lower lying areas. Much remains unknown about how human-driven climate change will affect the Earth in the long-term and over larger spatial scales.


This article in the news: 

Vermont university  How climate change threatens mountaintops (and clean water)
Also reported in EurekAlert – global source Science News Environmental News Network 

Manchester university Study reveals that climate change could dramatically alter fragile mountain habitats. Related content also reported by Phys.org , Reddit.com , New Zealand Ministry of Foreign Affairs , EurekAlert

AlphaGalileo Rise in temperature impacts mountain ecosystem

Leidsch Dagblad (in Dutch) Klimaatverandering verandert boomgrens newspaper clip

De kennis van nu (in Dutch) Wie over 80 jaar nog wil skiën heeft een probleem

Leiden university (in Dutch) Temperatuurstijging tast ecosysteem bergen aan

My news desk (in Swedish) Varmare klimat kan få stor inverkan på bergsekosystem jorden runt

Umea university (in Swedish) Ett varmare klimat kan påverka alpina ekosystem över hela världen

Science at APA (in German) Klimawandel bringt weltweit massive Änderungen für Bergpflanzen

Nature Asia (in Japanese)



Southern Hemisphere treelines are not so warm after all

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Treelines are fairly abrupt vegetation boundaries that have kept researchers intrigued for decades. A lot of work has gone into answering questions like “Why do trees stop growing at certain elevations?” The consensus is that the temperature during the growing season is crucial in determining the location of many treelines around the world. Once you get too high up the mountain, there is simply not enough warmth for trees to grow.

Treeline at Craigieburn

Treeline at Craigieburn, New Zealand

Why do the treeline elevations differ between regions? In the late 1800’s, explorers already noticed that treelines in the Northern Hemisphere formed at much higher elevations than in the Southern Hemisphere. For example, treelines in many parts of the European Alps occur at well over 2000m elevation, but at similar latitudes in the Southern Hemisphere, you’ll find the highest trees at 1500m or lower. The very few temperature records that were available for different treelines suggested that it was also warmer at the treelines in the Southern Hemisphere. To explain this, people proposed that the southern treelines are formed by species from the local flora that either didn’t have the right genes or didn’t have the time to evolve into cold-hardy alpine trees. In a recently published study, we wanted to test whether the temperature during the growing season differed between treelines in the Northern and Southern Hemisphere.

Cieraad E, McGlone MS, Huntley B 2014. Southern Hemisphere temperate tree lines are not climatically depressed.
Journal of Biogeography. doi: 10.1111/jbi.12308

We measured soil and air temperatures at six New Zealand treeline sites for more than 2 years, and compared this with data collected by others at treelines around the world. This study provides, for the first time, a comprehensive analysis of temperatures experienced at multiple treeline sites in New Zealand, and compares this with data collected at treeline sites all over the world. Whilst they are found at lower elevations, New Zealand treelines form at temperatures similar to those at Northern Hemisphere temperate treelines. So, contrary to long-held beliefs, our data shows that New Zealand treelines are not anomalously warm compared to treelines elsewhere. Other researchers have recently shown that the same is true in Patagonia and Chile. Together these results show that the many tree species that form the treeline in the Southern Hemisphere are not wimps, but that they can grow up to a similar temperature limit as their counterparts up north; it just happens that they encounter that temperature at a lower elevation on the mountains. So, the difference in treeline elevation between the Hemispheres can simply be explained by the fact that, at similar latitudes, summers in the Southern Hemisphere are cooler than those in the Northern Hemisphere because of to the oceanic influence on the relatively small landmasses, compared with the more intense heating of the large northern landmasses.

The study has implications for modelling of vegetation communities (temperature correlations of the altitudinal limits of forest types), invasion ecology (projection for invading naturalised tree species above indigenous treelines) and the understanding of evolution of tree species in New Zealand and the Southern Hemisphere.

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Two types of treelines in New Zealand and the climate they experience

The upper elevational limit of forests (or treeline) comes in two main forms: gradual and abrupt treelines. Most natural treelines around the world are a gradual transition from forest to alpine vegetation, where the height of trees gradually declines with elevation until there are only shrubs and grasses left.  These are sometimes also called diffuse treelines. These differing growth forms are an adaptation to the stresses imposed by increasing elevation, including low temperature, wind and snow accumulation. In other parts of the world (in some places in the Southern Hemisphere and in the tropics), there are also abrupt treelines, where tall forests abruptly give way to alpine vegetation.

Abrupt treeline on the St Arnaud Range

In the eastern ranges, like here on the St Arnaud Range, mountain beech forests form an abrupt treeline (photo: Landcare Research)

In New Zealand, we have both types of vegetation transitions (also called ecotones): abrupt treelines formed by mountain beech dominate in the eastern rain-shadow districts, and in the western, wetter regions, gradual ecotones are often formed by a diverse set of species. Even the early explorers recognised that, at similar latitudes, abrupt treelines form at higher elevations than gradual treelines. But nobody has investigated if there is also a difference in the temperature conditions experienced at the contrasting treeline ecotones.

In a newly published study, we measured soil and air temperatures across four gradual and two abrupt treelines ecotones in New Zealand for 2 years, and compared the climatic conditions between the treeline forms.

Cieraad E, McGlone MS 2014. Thermal environment of New Zealand’s gradual and abrupt treeline ecotones. New Zealand Journal of Ecology 38(1):12-25.

We found that air and soil temperatures mirror the change in tree stature in the ecotone. With increasing elevation through the gradual treeline ecotone, temperature decreased gradually. This has been shown elsewhere in the world to: short vegetation is generally warmer than tall vegetation on sunny days with little wind, but in areas with high winds, high humidity and/ or where cloud cover reduces solar radiation, the temperature difference between tall and short vegetation is not so clear. At the abrupt treeline–grassland interface of the abrupt treelines, temperature changed also abruptly: the soil beneath the mountain beech forest was 1° to 2.5°C cooler than soils of sites without trees less than 10 m away. The forest canopy creates its own microclimate by shading the soil beneath, and prevents the soil from warming up.

Gradual treeline on the west coast

At Camp Creek, on the West Coast of the South Island, the tall forest very gradually gives way to alpine vegetation

Trees at the gradual treelines experience similar summer temperatures as those at the abrupt treelines in the east. But temperatures in the shoulder seasons and during winter differed. At the gradual treeline sites, soil hardly ever froze and air temperature did not fall below −6°C. At the abrupt treeline sites freezing soils and snow were much more common. Compared to temperatures experienced at treeline sites in the Northern Hemisphere (for example the European Alps and North America), it was still not that cold – the coldest air frost recorded was −9°C. Trees growing at the New Zealand treeline can easily withstand those sorts of frosts (see a previous study).