The climate update – exotics in extreme heat

Posted on August 9, 2021 by lembrechtsjonas

In ‘The Climate Update’ gives Arne Ven, climate change advisor in the Global Change Ecology center of the University of Antwerp, us a recap of the news – good and bad – about climate change. Part 11 is about heat and exotic plants. This blog was prepared in collaboration with guest author Jonas Lembrechts (University of Antwerp) and first appeared in Dutch here.

The earth is getting warmer

When it is summer here, the news about forest fires, heat waves, temperature records … sky-rockets. In the early summer of 2021, large parts of Eastern Europe, Western America and Canada saw extreme heat. ^(1,2,3) These extreme events have repeatedly been linked to climate change. ⁽⁴⁾ According to one study, human-induced climate change caused one-third of heat-related deaths between 1991 and 2018. Older people and those with chronic conditions such as asthma are especially vulnerable. ⁽⁵⁾ And it’s not over: even if we meet the Paris climate goals (limiting warming to 2°C and preferably 1.5°C above pre-industrial levels), we’re going to face ‘bloody hot times’. There are scientific studies that say that, worst case, many millions of people – including some of the world’s poorest – will be routinely exposed to potentially deadly temperatures of up to 56 °C and above by the year 2100. ^(6,7) These extreme events will have enormous consequences, including for nature: some species will go extinct, while others will try to migrate to colder places. On top of that, new species – so called ‘exotics’ – are routinely brought in by humans. As it is getting hotter, especially those from originally much warmer regions could become of concern.

Cities are getting warmer

The increasing heat is not felt equally everywhere. In Flanders, for example, the sandy Campina region is generally warmer than the coast, while cities suffer more from heat than the countryside. Scientists have recently proven that exotic species from warm regions are more common in urban areas than in the countryside. This can be explained by the fact that, due to climate change, cities are warmer (and cool down less at night⁽⁸⁾) than the countryside: the heat island effect. Not only is it often several degrees hotter in cities than in the countryside, it is also often drier there, because pavement and other impermeable surfaces prevent water from seeping into the soil ⁽⁹⁾

*Photos: exotic species often like cracks in concrete (left), paving stones (middle), and transitions between stones and walls (right). © Charly Géron*

A recent study by the University of Antwerp and Liège showed that exotics, originating from warmer climates, also experience heat stress in warm locations in our country during extreme heat waves. This shows that the effects of the heat island effect on plants are not as straightforward as thought: although the exotics probably benefit from the warmer winter temperatures in the city, they also need shade during hot summer days to survive. ⁽¹⁰⁾

Your garden is getting warmer

Temperature differences do not only occur between city and countryside, they can even be felt at street level. Sometimes “extreme weather” can be very localized and depend on the environment, for example with Meditteranean temperatures – and associated plant species – on south-facing walls. These local hotspots can have a whole cascade of effects on humans (e.g. there are many more heat deaths in urban environments than in green environments) and on nature (e.g. exotic species start to crowd out native vegetation in locations with a suitable microclimate). However, as these differences can be so local, it means that one might have a significant influence on the local impact of extreme events, and make your living space more resilient to drought and heat yourself. This was explained by Jonas Lembrechts in an online seminar of the University of Flanders: planting trees – because trees provide shade and cool by evaporating water, not mowing your grass too short, ensuring a higher diversity of plant species in your garden, removing impermeable surfaces to ensure rainwater can infiltrate the soil, … all this can help to reduce the local impact of extreme weather events ⁽¹¹⁾.

Meanwhile, scientists at the University of Antwerp are working with citizens (= citizen science) to investigate how we can better deal with the effects of increasingly extreme summers. ⁽¹²⁾ How do we ensure that our gardens remain a cooling place during a heat wave? And how do we better arm our gardens, as well as our parks, fields, and natural areas, against drought? Which of the above factors (planting trees, growing grass…) work best against heat, drought or even heavy rain? All this is being investigated in the project ‘CurieuzeNeuzen in de Tuin’. For this purpose, almost 5000 citizens have placed a small weather station (affectionally called a ‘lawn dagger’) in their lawn. These instruments transmit microclimate data on soil temperature and soil moisture for the whole of Flanders to inform us about the best measures to protect our gardens against extreme events. With the extreme rain Flanders has seen this summer, such a project has become more acute than ever.