Ellen Cieraad's Research

Quantative plant ecology & physiology

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Moving below-ground?

Most of my work has focussed on the above-ground parts of plants and ecosystems, but over the last few years, I have tutored PhD students in statistics and the use of the programming language R, and this has led to me being involved into some of their projects focussing on below-ground processes.

One of these projects, led by Anna Zakharova (who graduates later this month, congratulations!), looks at understanding the response of soil carbon (C) to disturbance and land management practices. Because soils are the largest pool of carbon (C) in terrestrial ecosystems (globally containing more than two-thirds of total ecosystem C), any changes in the soil storage can have dramatic consequences for the global storage/release of carbon and soil productivity (think global food crops!), not to mention the subsequent global warming due to increased COin the atmosphere. But not all soil C is equal when it is predisposed to conditions where it may be lost. The portion called labile C (approximately 5% of the soil organic matter) is particularly vulnerable to being lost, because it is poorly protected by the soil particles, and microbes present in the soil will easily consume it when the soil structure is disturbed.

Until recently, it was really hard to measure how much labile C gets lost when you disturb a soil, and what determines the extent of the loss, but in a paper that was published online today, we tested whether a new method may do exactly that.

Zakharova A, Beare MH, Cieraad E, Curtin D, Turnbull MH, Millard P 2014. Factors controlling labile soil organic matter vulnerability to loss following disturbance as assessed by measurement of soil-respired δ13CO2. European Journal of Soil Science. doi: 10.1111/ejss.12209

As it turns out, the isotopic analysis of soil respired CO2 proved a powerful technique to improve our understanding of soil properties controlling potential loss of labile C after soil disturbance.

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).