Month: August 2023

Beat The Heat

This week, the weather has been hot for the upper Midwestern U.S. with temperatures topping out above 100°F.  Combined with high dew points in the mid-70s, the heat index temperature was upwards of 110°F.  Here are some ways that plants and animals keep cool in the extreme heat.

Mojave Desert by LezusRocks, Getty Images
Kit fox at entrance to desert den by Stevelenzphoto, Getty Images

Desert environments, where hot conditions exist daily, are home to numerous animal species that live underground.  Heat from the sun penetrates soils and sand layers for several inches, but at 20″ below the surface of the sand in the Mojave Desert, temperatures are fairly constant around 86°F while the surface may be over 110°F.  Few large mammals are known to use burrows, although kit foxes and humans are two examples.  Basements and lower levels in buildings are often preferred areas on hotter days.

Shaded walk in the woods by Felixmizioznikov, Getty Images

Shade from trees is highly effective in creating cooler areas.  Leaves are lighter in color than most soils and paved areas, and leaves reflect much of the sun’s radiation upwards, away from the shady area underneath.  Denser foliage and rough leaf textures enhance the cooling effect.  Under a shade tree, temperatures may drop up to ten degrees and other surfaces no longer exposed to direct sunlight, including our skin, may be 20 to 40 degrees cooler. 

African elephant by Petr Polak, Getty Images: Black-tailed jackrabbit by Rancho_Runner, Getty Images; Fennec fox by Nattanan726, Getty Images

Vasodilation occurs when blood vessels near the skin’s surface expand so more blood flows next to the skin.  Many animals living in hotter climates, including fennec foxes, black-tailed jackrabbits, and African elephants, have large ears with broad, flat surfaces devoid of hair, fur, or other insulation.  The ear’s large surface area is covered by a thin layer of skin and blood vessels.  During hot weather, heat carried in the blood through the ear is readily lost to the outside environment, providing a cooling effect for the rest of the body.

Large crowd by Shaunl, Getty Images Signature

Evaporative cooling is another method through which heat can be dissipated from a body.  Trees use this method by losing water vapor through their leaves during photosynthesis, cooling the air under the leaves.  Humans employ this method in a process called sweating.  We lose warm water through skin pores directly to the air around us.  Sweating also moistens our skin’s surface, indirectly providing additional cooling as air moves across the skins surface and wicks away additional heat.  An important aspect enhancing the effectiveness of this method is our posture.  Being supported by two legs, rather than four, exposes much less surface area to the direct rays of the sun, and much more surface area to air currents.

Forest path by Inga Nielsen, Getty Images

While sweating is good for cooling, it is removing water from the body, which must be replaced for the process to continue.  Exposure to today’s temperature extremes may result in a loss of three gallons of water or more from an average-sized human.  As you are out enjoying nature during hot days, remember to keep hydrating to stay cool and keep other bodily systems healthy.

Ash Trees

In the past, ash trees were one of the more common street trees in many urban areas of the United States.   However, with the introduction of the Emerald ash borer, Agrilus planipennis, a member of the beetle family native to Asia, our native ash populations have suffered severe losses. 

Emerald ash borer, Agrilus planipennis, by Leah Bauer, USDA FS Northern Research Station, Bugwood.org
Emerald ash borer larva by Oregon Department of Agriculture, Mar 2023
Emerald ash borer damage by John Hritz, Aug 2006

Ash trees have been a popular “street tree” species for many urban areas.  It is estimated that by the late 20th century, 20% of all parkway trees in Chicago were ash species.  Green ash, Fraxinus pennsylvanica, likes moist soils, but grows well in most conditions.  White ash, Fraxinus americana, is very similar to green ash, but prefers well-drained sites.  Both have furrowed bark appearing as tight rectangular blocks on young trees, but deepening into longer furrows and ridges resembling a diamond-shaped pattern in mature trees.  Black ash, Fraxinus nigra, often found on wet sites, is covered with irregular, knobby bumps.  All the barks are grayish-brown.

Green ash, Fraxinus pennsylvanica, flowers by DonArnold, Aug 2023
Ash tree samara by Herve Breton, Getty Images

In spring, ash trees grow small green or purple flowers with no petals.  Most are wind pollinated, although several bee species collect the pollen and contribute to some seed development.  Flowers appear on both male and female trees, but only female trees develop seeds.  Seeds are called samaras, or “helicopter seeds,” due to the distinctive whirling pattern they exhibit as they fall off the tree in a lazy spiral to the ground.  The seed is housed in a fully rounded shell at one end of the single wing of each samara.

Green ash, pinnately compound leaves by Robert H. Mohlenbrock. 1989, USDA NRCS Wetland Science Institute (WSI), Lincoln

Its pinnately compound leaves are another distinctive feature of the ash family. A compound leaf has a leaf blade divided into leaflets that are attached individually to the middle vein, each by its own stalk.  The term pinnately refers to the arrangement of leaves, opposite one another along the middle vein.  A terminal leaf, grown at the tip of the vein means there are always an odd number of leaflets, from 5 to 9 leaflets in ash species.  Buds and branches also grow in this pattern, which can be observed on rose plants, as well as hickory and walnut trees.

White ash trees in autumn color by Cathy McCray, Nov 2016

White ash is the main source of wood used in commercial applications.  It is of medium weight and springy, able to absorb a shock and bounce back.  This makes it valuable for bats, tennis rackets, bows and arrow shafts, along with a wide variety of other uses including tool handles, boats, and furniture.  The seeds provide a valuable food source for several bird species including quail, duck, and turkey.

Green ash, Fraxinus pennsylvanica, bark by Kerry Woods, Apr 2022
White ash, Fraxinus americana, bark by Virens, Sep 2009
Black ash, Fraxinus nigra , bark by Robert H. Mohlenbrock, 1995, USDA NRCS Wetland Science Institute (WSI), Chester

Although many, many ash trees have been lost to attacks from Emerald ash borers over the past couple decades, a small number of green ash have proven more resistant.  They provide great shade for parks, yards, and large areas.  In fall, watch as white ash trees turn beautiful shades of gold, deep purple, and maroon.  Be sure to make note of them on your walks and check back in a few months to view their spectacular fall displays.