Climate change forces new tree choices

Increasingly, climate is changing the rules of the game for the greenery around us. The effects of extreme weather on our tree stock are becoming more and more visible. If it is up to Johan van den Berk, co-owner of Van den Berk Nurseries, we will have to open up more to non-native species to meet these challenges. "We don't know exactly what the climate will be like in 80 years, so we will have to bet on several horses," he said.

For example, some species, such as beech, are clearly struggling with the increasing drought. "We've had a lot of trouble with that over the past five years. Now everyone is waking up again, because it has never been this wet since last October. Other trees can't stand that again, such as maple species. On the other hand, you also have trees that thrive in this climate." These dynamics force us to make conscious choices in planting trees. 

This means going to a diverse mix of native and non-native species that can withstand both drought and wet periods. This message is not falling well with everyone. "There is a current that thinks we should limit ourselves to using only native species. In my view, you can't stick to that. Besides, how exotic are these species really? Until before the ice ages, many species such as Magnolia, Gingko, Metasequoia and Sequoiadendron were common in the Netherlands. So in that respect, I also have a bit of a problem with the term climate tree."

To find out which tree species will be suitable in the future, you have to look at regions with the climate we will have in 80 years, Van den Berk believes. "Then you look at central America, for example, both on the east and west coast, or China, Japan and the Caucasus. Then I think of Quercus texana 'New Madrid,' an oak from Texas, or Tilia mongolica 'Buda' (linden) from Mongolia. These trees can withstand extreme drought and heat. Another American oak is the Quercus imbricaria, which is resistant to moisture in addition to heat. From the Caucasus comes the Zelkova carpinifolia 'Verschaffeltii', which is used to great heights and can withstand different climatic conditions. We will have to make use of the strength of these trees. These have proven to be real survivors." 

But there are more species that Van den Berk thinks are typical future trees. "Think of the Zelkova serrata 'Urban Ruby' or 'Village Green,' the Japanese elm, or the Pauwlonia tomentosa or Pauwlonia elongata. These are typically fast-growing trees in that they have a different metabolism. Because of this C4 metabolism, carbon is bound even much faster. In five years, these trees can grow 10 meters tall, which allows them to hold a lot of CO2 very quickly. We can also gain a world in that I think." Establishing large amounts of the Paulownia cultivars in forestry can capture a lot of CO2 from the air in a fairly short period of time. "It's unfortunate that this species is still sometimes maligned, because it's really non-native."

A good tree policy is not just about planting future-proof trees. "In the city, you have to pay attention to all the integral aspects of trees, such as heat radiation from rocks, or reflection from the sun. Not all trees can withstand that. In addition, you have to know what your goal is. Do you want a tree that grows big quickly? The field maple and the common oak are both champions of biodiversity, but if you want a tree in the city with lots of shade, that grows quickly, these are not the most suitable trees. Then you're better off choosing a sycamore or a Chinese swamp cypress. Not the most bio-diverse, but very suitable for heat in the city. So you always have to make a trade-off, do you want more shade in a city, or more biodiversity?"

That shifting to other species is more than necessary is also evident from scientific research. Led by Mark Ravensloot of Wageningen University, project CSI Trees is researching different climate scenarios. "Ravensloot is investigating what the climate will look like in 2055 and in 2085. He has developed four potential scenarios that could occur. This shows that 70% of the current tree stock will be inadequate within now and 30 years. That`s pretty intense."

Associated with climate change are tree diseases and pests we are increasingly facing, such as the oak processionary caterpillar. "This did not actually occur in the Netherlands 100 years ago. Due to climate change, it has moved into the Netherlands. The pine processionary caterpillar is fortunately not in the Netherlands yet, but it is advancing. Or think of the typesetter in pines and spruces, which has been able to develop easily in recent years due to drought." However, these pests are also the result of our monoculture, Van den Berk emphasizes. "My advice is to plant mixed, which gives insects less chance to spread. But also consider trees that are resistant to diseases or pests such as Sequoia sempervirens 'Winter Blue' or Eucommia ulmoides, the gummy tree."

Van den Berk definitely sees that shift to other tree species among tree growers. "Of course, we too take this into account when purchasing, but we also want to keep our supply as bio-diverse as possible. Moreover, for about five years we have been switching to 100% organically produced trees with certification under the SKAL standard. For a climate-proof future, these aspects also play an important role. Not for nothing are we a partner of NL Greenlabel, an organization that, like us, strives for a sustainable society and continues to put urgent themes like this high on the agenda." It is precisely by seeking partnerships that knowledge and resources can be pooled and the impact of efforts increased, Van den Berk believes. "If we want to make this change properly, we will have to learn to start thinking out of the box. But that's really a long-term conversion process."