Heat Resistance of Trees

Talking Trees with Lillian Jadd . Welcome to Talking Trees . In today's episode , we explore a comprehensive review of how rising temperatures and heat waves impact city trees . This article delves into the physiological , biochemical and molecular responses of trees under thermal stress , examining critical processes like photosynthesis , respiration and water regulation .

Hey everyone , thanks for joining us for another Deep Dive . You know we're all about exploring the latest research and sharing practical insights , and today we're tackling a topic that's becoming increasingly crucial in our field .

Yeah , that's right .

Understanding how rising temperatures and those increasingly frequent heat waves are impacting our urban forests .

You know , as arborists , we're the ones on the front lines making those decisions about which trees to plant , how to protect them and how to manage these ecosystems in the long term , which ?

trees to plant , how to protect them and how to manage these ecosystems in the long term . It's a huge responsibility , especially when you consider that climate models are predicting even more intense and frequent heat waves in the coming decades .

Right , and one of the key things we need to grasp is the urban heat island effect .

Oh yeah , absolutely . We're all familiar with the concept , of course .

Right , of course , but it's worth emphasizing just how much hotter cities can get compared to those surrounding rural areas .

All that creek and asphalt building density , it all traps and radiates heat , creating these little islands of heat .

Exactly , and that means our urban trees are starting at a disadvantage right . They're facing these higher baseline temperatures than their counterparts out in the forests .

Yeah , it's like they're running a marathon in a sauna .

Ha ha , I like that analogy . It's a good way to picture it . But of course trees have their own unique ways of coping with heat , which we'll delve into later .

Right , but in terms of just the baseline temperature they have to deal with , it's significantly higher in those urban environments .

Absolutely , and that's before we even factor in those heat waves .

So thinking practically , how does this impact our work ? Say , for instance , we're selecting trees for a new park in a dense urban area .

Well , that's where understanding the urban heat island effect becomes crucial . Take that study from Turin Italy . For example , they use remote sensing to actually map out the urban heat island and then they use that data to inform their urban forestry strategies .

So they weren't just planting trees haphazardly , they were being strategic about it .

Exactly , they were maximizing those cooling benefits . And what's interesting is , they found that this approach not only helped mitigate the heat island effect , but it actually reduced the energy demand for cooling buildings in those areas .

Wow . So we're not just planting trees , we're actually shaping microclimates .

Precisely that's the power you have as arborists .

That's amazing . Okay , so we've talked about the urban heat island effect , but what actually

I mean we all see the obvious signs , like leaf scorch Sure , but it's important to remember that heat stress impacts trees at every level .

Oh yeah .

From the cellular function to the overall health of the whole tree .

So it's not just what we see on the surface . There's a lot happening beneath the bark .

You got it . We see those visible symptoms , but the internal damage , the things we don't immediately see , those are critical too .

You're talking about things like impaired photosynthesis , increased respiration and oxidative stress .

Exactly , it's this kind of slow burn that can weaken the tree over time .

And it makes them more susceptible to other problems like pests and diseases .

Absolutely Remember that heat wave in Europe back in 2003? .

Oh yeah , I remember reading about that . It was devastating .

It actually turned the whole region into a net CO2 source .

A CO2 source . That's terrifying , what happened .

Forest productivity was so badly hit that the trees were actually releasing more CO2 than they were absorbing .

Wow , and that's just from one heat wave . It's a stark reminder of what we're up against .

Right , and it wasn't just mature trees that were affected either . There's been research looking at the impacts of spring heat waves on those emerging leaves .

Those new leaves are so vulnerable , it's like they haven't had time to toughen up yet .

Exactly . There's a study by Filewood and Thomas from 2014 . They looked at sugar maples and they found that even a short spring heat wave could cause significant leaf loss , reduce photosynthesis and potentially impact the tree's long-term carbon gain .

So even a short burst of intense heat early in the growing season can have lasting consequences .

Absolutely , and that's something we need to be more aware of as arborists . It's not just about surviving the summer heat anymore . We need to be thinking about the whole year .

Right , it's a whole new ballgame with these changing climate patterns . Exactly those emerging leaves in spring . They're so sensitive , it's a critical growth period and if those new leaves get damaged by heat , well , it can really set the tree back .

Absolutely so . We need to be extra vigilant during those spring months , making sure those young trees have the resources they need to withstand those early heat waves .

Like . We need to give them a good head start .

Exactly A strong foundation to build on .

Like being a tree coach .

Haha , I like that .

We're here to guide them to success .

That's a great way to think about it . We're in this together with the trees .

We are All right . So we've talked about the urban heat island effect and the impacts of heat stress on trees , but now let's dive into some of the strategies we can use to protect our urban forests in these increasingly hot environments .

Sounds good , let's get practical . And you know , it's not just the heat itself we need to think about , it's the combination of stresses .

Oh yeah , for sure , Heat and drought that often go hand in hand .

Exactly , it's like a one-two punch for those trees .

So how does that actually play out physiologically ? I mean inside the tree .

Well , think about Photocy for a minute .

Okay .

Correa and their team did some fascinating work with Eucalyptus globulus .

Okay .

That's a species we see a lot in urban landscapes .

Right , and they found that heat directly damages the photosynthetic machinery within the leaves . Oh wow , so the heat is basically messing with the tree's ability to convert sunlight into energy .

Exactly . And then , on top of that , you add drought into the mix which forces those stomata to close up ? You know , to conserve water .

Makes sense .

But when those stomata close , it limits the uptake of carbon dioxide .

Ah , so it's like a double whammy the heat is damaging the system and the drought is cutting off the supply .

Got it , and that's why we see trees really struggle during those combined heat and drought events .

It's like a recipe for disaster .

It can be , and this is where irrigation becomes so important .

Especially in those urban environments where water can be so limited .

Absolutely . Even trees that we generally consider to be drought tolerant , they might need a little extra help during those extreme conditions .

So we can't just rely on natural rainfall . We have to be proactive .

Right .

We need to be monitoring soil moisture and making sure those trees are getting the water they need . Okay , so we've talked about irrigation , now let's shift gears a bit and talk about assessing heat stress . I mean , we're all experts at identifying those visual signs the leaf scorch , the wilting .

Right , but the problem is those are often late indicators . By the time we see those symptoms , the damage might already be done .

It's like waiting for the check engine light to come on before taking your car in for service .

Exactly , and that's where physiological tests can really come in handy .

They give us a much earlier and more precise picture . Exactly so . You're talking about things like chlorophyll , fluorescence , electrolyte leakage , root growth potential tests .

Exactly .

These aren't exactly everyday tools for most arborists .

Right , but they can be incredibly valuable . Think of it like having so , for example , chlorophyll fluorescence measures , the efficiency of photosynthesis at the leaf level .

Okay .

So it can detect those subtle changes in photosynthetic activity that we might not be able to see with our naked eye .

So we can actually see how well the tree is converting sunlight into energy .

Exactly . Then you've got electrolyte leakage , which measures damage to cell membranes .

Okay .

So basically it's like a stress test for the tree's cells .

If those cell membranes are leaking , it means the tree's in trouble .

Right . And then , finally , we have root growth potential tests , which are particularly useful for young trees . Strong root growth is essential for water and nutrient uptake .

Especially during those times of stress .

Exactly so . Instead of waiting for those visual cues which can be lagging indicators , we can use these physiological tests to get a real-time assessment of how the tree is coping .

It's like preventative medicine for trees .

I love that analogy . It's a great way to think about it .

Okay . So we have all these tools to assess heat stress , but even with all that knowledge , selecting the right trees for these increasingly hot environments , it can still be tricky .

It definitely is . There's no magic bullet , and we can't just assume that a tree from a warmer climate will automatically thrive in a hot urban environment .

Right . There are so many factors at play .

Exactly . I've been hearing a lot about provenance selection lately .

Oh yeah , me too . What's that all about , and how does it relate to heat tolerance ?

So provenance selection recognizes that there's genetic variation within a species .

OK .

So trees from different parts of a species range .

They might have different adaptations even if they look very similar . So a sugar maple from Alabama might have a different level of heat tolerance than a sugar maple from Minnesota , exactly , even though they're both sugar maples . Interesting . So how do we go about finding those trees that are best suited for our particular urban environments ?

Those are great starting points , okay , but we need to be a bit cautious about relying solely on those general climate tolerances .

Because there might be variation within the species .

Exactly . There are some studies that have shown conflicting results , so we need to be careful about making those broad generalizations .

So we need to do our homework and look for research that specifically focuses on the providence of the trees we're considering .

Absolutely . For example , there's a study by Rubikowski and their team from 2012 . They looked at red maples and northern red oaks from different providences and they found significant differences in heat tolerance among the trees from those different regions . Interesting and there's also Weston Boweral's research from 2007 on red maple genotypes from contrasting climates .

So there's real evidence to support this idea of providence selection .

There is , but we definitely need more research on a wider range of species to make those truly informed decisions .

And that's where we , as arborists , can really make a difference .

Absolutely

We're the ones out there seeing these trees in action , seeing how they perform in different conditions .

Exactly so . Let's move on to some practical strategies for protecting trees from heat stress once they're in the ground .

Okay , let's talk solutions . We've covered a lot of the challenges , but I'm sure our listeners are eager to hear about what they can actually do to help their trees thrive .

All right . So let's talk about some of those solutions , some things we can do to help our trees beat the heat . You know , I think everyone listening is eager to get to this part .

Absolutely . You know , we can't just rely on natural rainfall .

Right , especially in urban areas where water can be so scarce .

So even trees in irrigated areas might need some supplemental watering during those really intense heat waves .

Exactly . We need to be monitoring soil moisture and adjusting irrigation schedules as needed .

And it's not just the amount of water , but also the timing .

Right Deeper . Infrequent watering is generally better than frequent shallow watering .

Because it encourages those deeper roots .

Exactly those deep roots are so important for drought tolerance .

Okay , so we've got irrigation . What about nutrient management ? Can we give our trees a little extra boost to help them handle the heat ?

Well , there's definitely research that suggests that certain nutrients can play a role in enhancing heat tolerance .

Okay , like what ?

Well , calcium , for example .

Okay .

Calcium is involved in cell wall stability , which is important for maintaining cell integrity under stress .

So it helps prevent those cell membranes from getting leaky .

Exactly . And then you've got silicon , which has been shown to improve drought tolerance in some plants .

How so .

It enhances water uptake and reduces transpiration .

So it's like a water conservation tool for the tree .

Exactly . And then you have potassium and magnesium .

Right , those are essential nutrients .

They are . They're important for photosynthesis and overall plant health , which can help trees cope with stress in general .

So it sounds like we need to be thinking about a really balanced nutrient program that addresses those specific needs of trees in these heat-stressed environments .

Absolutely , and then there's also a growing interest in the use of biostimulants .

Oh yeah , Biostimulants , they're fascinating .

Right , they're substances that can stimulate

It's almost like we're trying to train trees to be more resilient .

It is . It's a really cool concept , but the research on trees specifically is still fairly limited .

Okay , what do we know so far ?

Well , there are some studies that suggest certain biostimulants can improve drought tolerance and reduce oxidative stress .

Okay , so there's promise .

There is , but we need more research to really understand how they work and how effective they are for different species .

So more field trials , more real-world data .

Exactly , and that's where you know citizen science projects and collaborations between researchers and arborists . I think those are going to be key .

Absolutely . We need to work together on this .

We do , okay . So we've talked about irrigation , we've talked about nutrient management , we've talked about biostimulants . Now there's one more tool we should discuss , and that's chemical enhancement .

Okay , this is where things can get a little bit controversial , right , using chemicals to actually boost a tree's heat tolerance .

It can be a sensitive topic . There's definitely a need for caution and responsible use .

Right .

But research has shown that certain chemicals like triazoles they can induce stress protective compounds in plants .

I know triazoles are found in some fungicides and growth regulators .

How do they actually work to enhance heat tolerance . Well , they essentially trigger the production of antioxidants and other protective molecules within the tree , and that helps to reduce the damage caused by the heat stress .

So it's like giving the tree a temporary shield .

Exactly . It's not a permanent solution , but it can be a really valuable tool in certain situations .

Like protecting newly planted trees during a particular bad heat wave .

Exactly , but it's really important to remember that there are potential drawbacks .

Of course we have to weigh those risks and benefits .

Absolutely , and we need to make sure that we're using these chemicals judiciously and in accordance with all regulations .

It's all about making those informed decisions Based on the best available science and the specific needs of those trees that we're managing .

I think that's a great point to end on . You know we've covered a lot of ground .

We have , from the urban heat island effect to all the different strategies we can use .

Right and it's clear that understanding and addressing this whole issue of heat stress it's absolutely vital for the future of our urban forests .

Absolutely , and it's not just a scientific challenge , it's a call to action for all of us it is . We need to work together share our knowledge and be relentless in our pursuit of solutions . Our urban forests are depending on us . They are Well . Thank you so much for joining us on this deep dive into the world of heat stress trees .

Thank you for joining us on today's episode of Talking Trees . We've journeyed through the complex challenges posed by rising temperatures and heat waves on our urban trees , exploring how thermal stress affects their photosynthesis , respiration and water balance . We've also uncovered the fascinating biochemical and molecular mechanisms from osmolite accumulation to heat