Urban Trees vs. Forest Friends: Who Handles Heatwaves Better?

Hey There, Let’s Talk About Trees!

Okay, so picture this: our planet is getting a bit of a fever, right? Climate change is turning up the heat, and our poor trees are feeling the stress. We were super curious about how trees in different spots handle the pressure, especially when things get really hot. So, we decided to dive deep and compare trees hanging out in a busy urban area versus their cousins chilling in a forest.

We specifically looked at trees in Milton Keynes, UK (that’s our urban jungle) and a deciduous forest nearby in Aspley Heath. Why these two? Well, they’re close enough to get similar weather vibes, but the urban environment throws in extra challenges like concrete heat and less natural space. We wanted to see how they coped between 2018 and 2023, focusing on those crucial summer months when heatwaves really hit.

Our Tree Health Tracker: NDVI

To figure out how healthy the trees were, we used something called NDVI. Think of NDVI as a greenness meter you can see from space. Satellites look at how much red and near-infrared light plants reflect – healthy, leafy plants bounce back a lot of near-infrared and soak up red, giving us a high NDVI score. Low scores mean the plants might be stressed or not doing so great. We grabbed this data from satellites (MODIS for tracking changes over time and Sentinel-2 for detailed maps).

We also pulled in data on climate factors: leaf temperature (which we figured out from air temperature), wind speed, surface pressure, and how much moisture was around. We wanted to see which of these factors really influenced the trees’ NDVI scores.

What’s a Heatwave for a Tree?

Now, the weather folks have their definition of a heatwave, usually based on what makes *us* uncomfortable. But trees? They have different limits! Based on some lab experiments we did on urban trees, we found critical temperature thresholds where tree health starts to dip. The first big red flag was around 38°C, and things got even tougher above 42°C. We used these tree-specific thresholds to identify heatwave events in our study period.

Looking at the satellite data from 2018 to 2023, we spotted several times when leaf temperatures soared past that 38°C mark. And get this – in July 2022, urban leaf temperatures in Milton Keynes actually hit a scorching 42.9°C! That’s well into the danger zone for trees.

Getting Technical (But Keeping It Simple!)

To understand the messy relationships between climate factors and tree health, we needed a smart statistical tool. We used something called Generalised Linear Mixed Models (GLMMs). These models are great because they can handle complex data, like tracking the same trees over time and dealing with things that might vary year-to-year (like seasonal differences). They helped us see the average impact of each climate factor while also accounting for that year-to-year wobble.

One tricky bit in studies like this is when your predictor variables are buddy-buddy – like humidity and transpiration (how plants release water) often go hand-in-hand. This is called multicollinearity, and it can mess up your results. We used a technique called Principal Component Analysis (PCA) to combine related moisture factors into a single “moisture index” so our models could give us clearer answers about each factor’s true impact. We also checked for time lags – does today’s heat affect the tree today, tomorrow, or even later? We found that leaf temperature had an immediate effect on NDVI, but also a smaller, delayed effect the very next day.

What the Data Told Us

Alright, the juicy part! Our models crunched the numbers, and here’s what we learned about how different climate factors influenced tree health (NDVI) in the urban and forest areas:

• Leaf Temperature: Generally, warmer leaf temperatures were linked to healthier NDVI, but there was a big difference between the two locations. Forest trees showed a stronger positive bump in NDVI with rising temperatures (a beta coefficient of 0.45), while urban trees got a smaller boost (beta of 0.32). What does a beta mean here? Since we standardised everything, it means a 3.82°C rise in leaf temp was associated with a 0.45 increase in forest NDVI but only a 0.32 increase in urban NDVI. This suggests urban trees aren’t getting the same benefits from warmth, probably because of that urban heat island effect making things *too* hot or adding other stresses.
• Wind Speed: Wind wasn’t great for trees in either place, but it was *worse* in the city. Higher wind speeds meant lower NDVI. The negative effect was stronger for urban trees (beta -0.21) than forest trees (beta -0.17). Maybe buildings create wind tunnels, or urban trees are just more exposed?
• Surface Pressure: This one was a positive influence! Higher surface pressure (often linked to stable, clear weather) meant better tree health in both spots, slightly more so in the urban area (beta 0.17) than the forest (beta 0.14).
• Moisture Availability: Water, water everywhere (or not!). Moisture was super important for tree health, but again, the forest trees benefited more (beta 0.39) than urban trees (beta 0.30). This makes sense – cities have lots of concrete and pavement, making it harder for water to get to the roots.

We also saw that the forest consistently had higher NDVI values overall compared to the urban area. And remember that super hot summer of 2022? Both areas saw a dip in NDVI then, really showing how sensitive they are to extreme heat and drought.

The Timey-Wimey Bit: Lags

That time-lag analysis was pretty cool. It confirmed that leaf temperature hits tree health immediately (big effect at lag 0, beta 0.64). But there’s also a hangover effect – the temperature from the day before still had a smaller, but noticeable, impact on NDVI the next day (lag +1, beta 0.18). This is especially important for urban trees, suggesting they might take longer to recover after a hot day.

So, What’s the Big Takeaway?

Our study really highlights that urban trees are facing a tougher time than their forest buddies when it comes to climate stress. While moderate warmth might be okay, the combined effects of higher urban temperatures (especially during heatwaves), stronger wind impacts, and limited access to moisture mean they’re more vulnerable.

This isn’t just interesting science; it has real implications for how we manage our green spaces. Urban planners and foresters need different game plans. For cities, we need strategies to help trees cope: maybe planting species that are more heat and wind tolerant, finding ways to get more water to their roots (less concrete, more permeable surfaces!), and designing cities to reduce those wind tunnel effects.

In forests, the focus might be more on maintaining natural resilience – keeping the soil moist, protecting the canopy, and generally letting the ecosystem do its thing while keeping an eye on long-term climate shifts.

Looking Ahead

Of course, this study is just one piece of the puzzle. We used satellite data, which is awesome for seeing the big picture, but it doesn’t tell us about individual tree species or super fine-grained details. Future studies could combine this satellite view with on-the-ground measurements (like checking leaf chlorophyll) and look even more closely at how different species handle the heat and how long those lagged effects really last.

Ultimately, as climate change keeps pushing boundaries, understanding these differences between urban and forest ecosystems is key. It helps us make smarter decisions about where to plant, what to plant, and how to care for our trees so they can keep providing us with all those amazing benefits – shade, clean air, beauty – for years to come.

Source: Springer