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)



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A move across the world

Wordcloud_MinFreq12_Sept2015 Wordcloud_MinFreq25_Sept2015

After 14 years in New Zealand, we are now in the Netherlands and I am about to start my lecturer position at Leiden University. I recently created a word cloud* of abstracts of my published papers to date. The results (above) nicely depict my work at Landcare Research. While I look forward to keeping ties with some of my research and fabulous colleagues in New Zealand and elsewhere, I can’t wait to see what my new position at the Institute of Environmental Sciences (CML Leiden University) will bring!

* Word clouds are great ways to visually analyse text. Words that feature in your text more often are given greater prominence in the ‘cloud’. The number of words that make up your cloud depends on the threshold you place (effectively a minimum number of times that a word has to appear in the text). You can create these word clouds this really easy in some web applications (like worldle.net), but it’s also easy to do in R, and a lot more customisable.

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Fossil ferns

Pikopiko Fossil Forest near Tuatapere in Southland may not be as well-known as the Curio Bay petrified forest but it has one of the richest known Cenozoic* fern floras globally. Walking in between the in situ fossil tree stumps (probably related to Araucaria) on the shores of the Wairau River, you get a sense of the spacing of the trees. Underneath your feet, the fossil litter layers provide insights into this ancient forest community. The diversity and abundance of ferns implies that ferns dominated the evergreen, tall forest understorey just as they do in modern New Zealand rainforests. Eight fern macrofossil groups (parataxa) and 20 very tiny spore types have been identified from the fossil forest. The ancient flora encompasses at least 40% of modern New Zealand fern families, which highlights the long history for some fern genera in the region. The abundance of ferns, the presence of fungi on many leaves and the presence of palms is evidence for warm humid conditions in Late Eocene New Zealand.


Pikopiko fossil forest

Colleagues of mine recently re-examined fossil fern material I used for my MSc thesis together with more fern macrofossils from these Eocene strata (c. 35 million years ago), the first records for New Zealand. The results are published here.

Homes AM, Cieraad E, Lee DE, Lindqvist JK, Raine JI, Kennedy EM, Conran JG 2015. A diverse fern flora including macrofossils with in situ spores from the Late Eocene of southern New Zealand. Review of Paleobotany and Palynology 220: 16-28. doi:10.1016/j.revpalbo.2015.04.007

The discovery of three types of fossil fern fronds bearing sporangia with in situ spores enabled us to roughly identify the ancient ferns and show for the first time that some relate to a present day fern genus (including Blechnum, and Thelypteris subgenus Cyclosorus), some to an extinct group of uncertain affinity and some to a widely known fossil spore form taxon. Five additional fern groups, including another probable Blechnum, could be distinguished on the basis of sterile foliage.

The fern flora recovered from Pikopiko Fossil Forest is significant in being the first record of fern macrofossils from Eocene strata (56 to 33.9 million years ago) in New Zealand, and they provide a wider understanding of the natural history of New Zealand during this time.

*The Cenozoic period is from 65 million years ago to the present day.

Photo credit: GNS

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Invasive plants can help establish native species in harsh places

New Zealand has as many native as invasive plant species. A number of woody invasive species have been rapidly increasing their distribution in recent decades. In many places they are chemically or mechanically controlled often with the implicit, rarely achieved, aim of advancing the restoration of native woody vegetation. However, there may be another option: leave these exotic shrubs – they may be ‘nurse-crops’ that aid the recovery of native vegetation. This seems to be particularly effective in moist environments and where nitrogen-fixing species are involved, such as gorse (Ulex europeaus). In the drier parts of New Zealand, broom (Cytisus scoparius, another nitrogen-fixer) is a real problem, and here this species was thought to be fairly useless as a nurse-crop, until recently (see this post).

In an article that is now available online, we describe a field experiment that tested how five different management treatments of broom cover affected the germination, survival and growth of native tree and shrub species.

Burrows L, Cieraad E, Head N 2015Scotch broom facilitates indigenous tree and shrub germination and establishment in dryland New Zealand. New Zealand Journal of Ecology 39(1) In Press.

We imitated different management techniques that are applied to broom in the dryland zone: in some plots the broom shrubs were sprayed with weedkiller, elsewhere we had bull-dozers drive over the shrubs and leave the debris, or tractors rake all the shrubs (including their roots) and take the debris away, in other plots the shrubs were mulched and the mulch was left, and in the last type of plot, we just didn’t touch the broom shrubs and just left them standing. In all plots, we sowed seeds and planted seedlings of six native tree and shrub species. There was no evidence of unassisted regeneration of native shrubs from plants nearby during our experiment.

We sowed thousands of seeds, and very few seeds germinated and even fewer germinants survived until the end of the experiment (3.5 years after sowing), but some species had much higher germination than others (particularly the hard seeded Kowhai, Sophora microphylla, germinated well). Plots where the broom cover was mulched, crushed or root-raked had very low seed germination and high mortality of planted seedlings, which was apparently due to the soil disturbance and harsh conditions of the open sites that were created. Under the living broom canopy germination and survival rates were significantly higher. This indicates that the positive (facilitative) effects of the living canopy, such as the provision of shade and a moister cooler environment in the dry summer, outweighed any negative effects (probably particularly the increased competition for moisture by the living broom shrub and the native seedlings).

This study suggests that at this dry site, compared with the chemical and mechanical treatments of this woody weed, retaining a live broom canopy was most beneficial for the germination and establishment of planted native woody seedlings. Importantly, it was also the cheapest management option by far, and it may thus be an important strategy to advance the succession of indigenous woody species in these dryland weed communities.

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).