Forests make heatwaves ‘initially warmer’


During heatwaves forests reduce their evaporation, causing the atmosphere to warm up even more, say researchers.

During extremely long periods of heat, however, this reduction enables the forests to continue their evaporation for longer, so the net effect is ultimately one of cooling in relation to the surroundings, explained a team of scientists led by Ryan Teuling from Wageningen University in the Netherlands.

Writing in the journal Nature Geoscience, Dr Teuling worked on the investigation in collaboration with climate researchers from a number of European countries.

The study was prompted by recent heatwaves in Europe, which had raised interest in questions about the influence of land use on temperatures and climate.

Up to now, scientists had assumed that a lack of precipitation during heatwaves automatically led to a reduction in evaporation.

That reduction was thought to be less for forests, because trees, with their deeper root systems, have more water available to them. Examination of the precise role of land use, however, has been largely neglected up to now.

The study found large differences in evaporation strategies during heat waves. Grasslands evaporate more at higher temperatures and stop only when no more water is available.

Forests, in contrast, respond to higher temperatures by evaporating less, which leaves more water at their disposal.

During brief heatwaves, therefore, the greatest warming is found above forests, but during prolonged heat waves the increased evaporation of grasslands ends up causing a shortage of water.

This can lead to exceptionally high temperatures, such as those measured in France in the summer of 2003.

This mechanism might also offer an explanation for the unusually high temperatures near Moscow this summer, the researchers suggest.

In these types of extreme situations, forests in fact have a cooling effect on the climate.

The research was done on the basis of observations made above forests and grasslands in Europe by an extensive network of flux towers. For areas without towers, satellite data were used.

Source: Wageningen University press release

Date: 06/09/2010

Report: Trees in the urban realm


Although the findings from the UK’s Tree and Design Action Group were published in 2008, it is worth reprising here.

The group, comprising of professionals and organisations, looked at the threat facing urban trees, while highlighting the benefits they bring to local communities.

It says that trees enrich the urban landscape by “improving health and well-being for people and the environment”.

It goes on to say that the report also highlights that urban trees mitigate temperature extremes, reduce pollution and increase real estate values.

“In terms of climate change,” the group suggests, “trees have been identified as being a key element of any urban climate change adaptation strategy.

“Trees are uniquely placed to be widely integrated into the urban fabric, providing a shading and cooling mechanism.

“Without this cooling mechanism, cities of the future – London in particular – are likely to be very inhospitable places.”

However, the group says that while there is awareness about the role trees can play in making cities habitable in the future, current design and planning systems make it very difficult to plan for the future.

“The services and infrastructure needed in cities to achieve high densities living generally militates against the presence of trees.

“Climate change will add to these pressures and create a landscape devoid of large trees unless practical steps are taken by a range of professional bodies working in partnership.”

It builds on the London Assembly Environment Committee’s 2007 report “Chainsaw Massacre“, which highlighted the loss of street trees in London. It found that more large tree species were being cut down rather than being replaced.

The Trees in Towns II report, commissioned by the Department for Communities and Local Government, echoed this findings for the rest of England.

Source: Trees and Design Action Group (TDAG)

Date: December 2008

Leaves ‘keep their cool’


This is a bit of an old one, but this story featured on the Nature website by Heidi Ledford about how leaves regulate their temperature in order to maximise their metabolism is worth a mention on these pages:

Whether growing in the heat of Puerto Rico or in the icy chill of northern Canada, tree leaves are able to buffer against the outside temperature, new research has found.

A survey of 39 North American tree species over an area spanning 50° of latitude has shown that plants protect one of their most important functions – photosynthesis – by maintaining average leaf temperatures at around 21 °C, regardless of the weather.

Source: Nature.com

Date: 11/06/2008

Follow

Get every new post delivered to your Inbox.

%d bloggers like this: