Home and Garden
Have you noticed that birch trees are getting a bad rap this week? Residents are calling for their removal from the streets of Christchurch because birch pollen causes allergies.
And, frankly, fair enough if you're one those unfortunate people who suffer from Oral Allergy Syndrome, as reported in the Nelson Mail way back in 2011 (still searchable on the Stuff website).
While plants such as privet and pine are usually the targets of allergy angst, their pollen isn't too bad compared to that of some other plants. But birch pollen is. According to the very informative Auckland Allergy Clinic website (www.allergy clinic.co.nz), birch pollen is a "very potent allergen".
It's also one of the most prevalent allergens in this country. Birches are one of our most common urban trees and, because they are wind-fertilised, produce prolific amounts of pollen when they flower in spring.
While the pollen may give some people a permanently runny nose and itchy, streaming eyes, for others it also causes the much more insidious Oral Allergy Syndrome. This occurs when some people have what is termed a cross-reaction to foods that have a similar molecular makeup to birch pollen. In response to the similar molecules, their bodies produce a similar reaction, but not just when birch trees are in flower.
This cross-reaction can occur at any time of the year, when someone with the condition eats one of the foods that causes it.
Foods known to cause cross-reactions to birch pollen include apples, carrots, cherries, peaches, plums, fennel, walnuts, wheat and potatoes - simply by touching (peeling) them, but only when they're raw. Once cooked, all these foods are OK, as heating destroys the allergens.
The reaction isn't pleasant. It usually causes oral irritation, including swelling and itching of the tongue, throat and roof of the mouth.
The tricky thing, according to those I know with the condition, is that often it goes undiagnosed for years, because people usually don't make the connection between the various allergenic foods and their reaction to birch pollen.
The bad news is that Oral Allergy Syndrome is also caused by grass pollens (another widely prevalent source in this country) and cross-reactions to melons, tomatoes, watermelons, oranges and wheat. Ditto pine pollen and pine nuts, hazel pollen, hazelnuts and filbert nuts.
It's a minefield of allergies and cross-reactions out there, but the constant baddie documented in research findings on cross-allergy reactions is the ubiquitous birch tree.
It is, of course, an exotic species introduced from its Northern Hemisphere home - and, quite rightly, is highly regarded for its elegant, aesthetic appearance and apparent urban-friendly habits.
Birch trees have small leaves that produce lovely bunched foliage and summer shade, then drop early in autumn. They also have attractive white bark and upright trunks, and that most sought-after of qualities for the impatient home gardener - fast growth.
Despite their propensity to get rot and dieback, and the tendency of their small seeds to block swimming pool filters, birches were regarded in the 1970s and ‘80s as the height of good garden fashion and avant-garde suburban style here. They rated well up there on any self-respecting landscape designer's top 10 list of desireable home garden trees.
But gardens, like anything else, are subject to the flippancy of fashion, so silver birches have long gone out of style and, in keeping with the disturbingly xenophobic-like attitudes now held by some eco-police types, are also vilified as unwanted exotics.
While the debate continues over what species - native or exotic - should be grown in streets and cities here, new evidence from Australia shows just how essential trees are in the struggle against searing urban temperatures and heat-related deaths.
Last week's temperatures of over 40 degrees C in Melbourne provided an opportunity to study the effects of shade and temperature moderation from urban trees.
Inner-city areas of Melbourne not only bake in daytime temperatures but become a heat sink, with temperatures rising day after day and staying hot at night because of the thermal inertia inherent in the built environment.
Such conditions result in inner-city temperatures several degrees higher than those of greener, treed, outlying suburban areas.
Furthermore, localised inner-city temperatures under and around individual trees are also highly variable.
Thermal imaging during last week's heatwave showed the impact of Mebourne's trees in reducing heat. There was a whopping 42.4C difference below the canopy of a plane tree (38.3C) compared to nearby surfaces (80.7C) in readings taken in mid-afternoon, when the temperature was 32.4C.
Melbourne council staff says parks, gardens and trees are the key to reducing urban temperatures during such heatwaves, and they want to plant more trees through an urban forestry strategy.
They plan to double the size of the city's canopy over the next 20 years, increasing it from 22 per cent now to 40 per cent by 2040.
And good on them. Not only will it help to cool Melbourne in the battle against the inner-city heat island effect, but a greener urban environment will also help people's wellbeing, as has been well documented worldwide.
And while gums trees may be native to Australia, just as totara and titoki are native here, all are evergreen - and, as anyone who has lived in either Melbourne or Christchurch knows, the warmth and joy of the sun is what you most want during a southern winter. This is why deciduous - and thus largely exotic - trees are most likely the first choice for urban and street tree planting.
Just as towns and cities are highly modified environments, with buildings and paving, exotic trees are part of the modified vernacular landscape. As such they can be like giant, natural air conditioners, providing cool summer shade and allowing warm winter sun.
Lots of suitable species like pin oaks, small-leaved planes and ginkgoes fit the bill. They have to cope with compacted street soils, be prunable and meet engineering design parameters, and have to like the exhaust fumes in the fast lane of the road reserve.
But maybe, for the allergy-challenged, it's best not to plant birches.
For more information on allergies and birch tree Oral Allergy Syndrome, refer to www.allergyclinic.co.nz/guides and www.allergy.org.nz, and hop on their Facebook page to share information.
To read more about Melbourne's heat issues, thermal imaging and trees, go to media.theage.com.au/news/national-news/melbourne-an-urban-heat-island-5082373.html.
To read more about plans to green the city, go to smh.com.au/environment/energy-smart/melbourne-carbonreduction-plan-best-in-the-world-20130905-2t66c.html#ixzz2qi52Apqz.
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