The secret killer in your garden

A garden in Devonshire, England, where Agatha Christie worked in a hospital pharmacy during World War I, before becoming the doyenne of mystery writers, is dedicated to growing all of the poisonous plants employed in her mysteries.

Dame Agatha wrote 80 whodunits, and somebody was poisoned in most of them. Her fictive criminals' favorite poison: cyanide. But they also used arsenic, strychnine, digitalis, morphine and taxine -- all derived from plants -- with deadly effect. Visitors to the garden are accosted by a large sign that warns "Do Not Touch." And each plant is labeled with one to five skulls, indicating its level of toxicity.

Here's a mystery closer to home: Somebody is poisoning the moose of Anchorage. It's probably you. And many of your relatives, friends, and neighbors. Because the entire city is a garden laced with poisonous plants.

I haven't noticed any skulls painted on signs in Anchorage, but some of the most poisonous plants in the Devonshire garden are in the Prunus family. Prunus species are popular ornamental trees in Anchorage, often going by the common names of chokecherry, bird cherry, and May Day tree.

Chokecherries are aptly named. Relatives of the cherry tree, they sport attractive blackish drupes, which have a high acid content, making them sour. The acidic fruits can choke or gag a person, but seldom cause death. Unless you are a moose or other ruminant.

This winter, at least three moose calves have died in Anchorage after browsing on chokecherry twigs. Dozens of moose are reported dead in Anchorage every winter, and it is possible that many more moose have succumbed to poisonous plants. It is difficult to inspect every moose found dead in the city, much less perform a detailed necropsy.

For all its imposing bulk and rugged good looks, a moose is a delicate creature. Its huge, four-chambered stomach, well adapted to digesting a winter diet of woody twigs, is particularly sensitive to physical and chemical agitations. Swallowing a few mouthfuls of chokecherry twigs, leaves or seeds can kill a moose in one to two hours. Calves are probably more vulnerable than adult moose because they are smaller.

The deadly ingredient in chokecherry foliage is cyanide gas: hydrogen cyanide or HCN. The cyanide is locked in plant cells, isolated from the enzymes that create the gas. However, wilting, freezing, crushing, and chewing (does this sound like what might happen to a plant eaten by a moose in winter?) releases the gas. So does digestion by the enzymes in a moose's rumen, the first of four chambers comprising its highly evolved stomach. A lethal dose of HCN causes rapid labored breathing, frothing at the mouth, dilated pupils, ataxia, muscle tremors, and convulsions. The moose usually dies within a few minutes of developing these symptoms. The cyanide stops cellular respiration, resulting in respiratory arrest. The moose suffocates.

The almond-like seed inside a peach pit is also capable of producing hydrogen cyanide. Egyptians and Romans used peach seeds to execute people. Hydrogen cyanide, aka Zyklon B, was employed in the Nazi gas chambers. This is seriously toxic stuff.

Fortunately, it's unlikely that you or I will be poisoned by HCN unless it is forced into our alimentary or respiratory systems by an agent of the Ptolemies, Caesars, or Nazis. Human stomachs are highly acidic, so HCN is released much more slowly than in the mildly acidic or alkaline contents of a rumen. And our stomachs don't have the high water content or microfloral enzymes of a moose rumen. Thank goodness, because in my curious youth I cracked a few peach pits and ate the seeds. They tasted and smelled like bitter almonds.

Hydrogen cyanide is also known as prussic acid, a compound preferred by many English murderers, if one is to believe the likes of Agatha Christie and other detective writers. A small dose inhaled by a bound captive or inadvertently self-administered in a nasal spray kills the victim in minutes, leaving no tell-tale clues. Except the smell of bitter almonds. Cagey killers leave a window open to disperse the smell.

Any rancher or farmer worth his or her salt knows chokecherries are hazardous to the health of cattle and sheep. However, it took an astute observation by Dr. Kimberlee Beckmen, the Alaska Department of Fish and Game veterinarian in Fairbanks, to connect the death of a moose calf with the ubiquitous ornamental chokecherry trees of Anchorage. In 2006, Dr. Beckmen asked a local veterinary pathologist, Dr. Kathy Burek, to necropsy a moose calf and determine cause of death. Noting the absence of any obvious clues, Hercule Poirot might have sniffed the moose's breath, hoping to detect the odor of bitter almonds. The moose had been dead too long for that, but Dr. Burek found chokecherries in the rumen. Beckmen, the Miss Marple of moose, confirmed the presence of cyanide. Yes, there was an open window at the crime scene; however, in this case, the dying moose had staggered through it.

Jessy Coltrane, the acting Anchorage area biologist with the department of fish and game, opened the rumen of the last calf killed by chokecherries. She experienced a blast of cyanide gas "strong enough to sting our eyes."

Chokecherry trees are not native to Alaska. We brought them here. Now tens of thousands of these trees adorn yards, parks, and roadsides in every part of the city. Three species are most common in Anchorage: Amur chokecherry, Canada red chokecherry, and May Day tree (or European bird cherry). May Day trees are highly invasive. They have escaped cultivation in Anchorage and are beginning to replace native trees, especially along waterways. A 2010 report on several municipal greenbelts by the Alaska Natural Heritage Program described dense thickets of bird cherry trees, in some areas replacing willows, which are a preferred forage for moose. Some riparian areas in Anchorage are already dominated by May Day trees in both the canopy and understory. Invasive plant specialists call this an infestation.

Chokecherry trees are also among the most common ornamental trees and shrubs in city parks and road rights-of-way. Preliminary surveys by the municipal forester, Scott Stringer, have found chokecherries comprising approximately 25 percent of the individual plants. Chokecherries are equally ubiquitous in yards throughout the city.

Moose do not seem to be fond of chokecherries, which is one reason why they are popular landscape plants in Alaska. In fact, Fish and Game biologists have long recommended planting chokecherries when a homeowner or landscape company wants to avoid browsing by moose or attract songbirds. But by mid-winter, when every moose in Anchorage is losing weight every day because woody browse is scarce, some moose will sample chokecherry twigs and fruits, turning their stomachs into gas chambers.

As bad as all this sounds, chokecherries aren't the most toxic trees in Anchorage. Introduced European and Japanese yews are so toxic to many ruminants, like moose, that the most common diagnostic is "animal found dead." European yew is considered the most dangerous poisonous plant in Britain and is a relatively common cause of poisoning of livestock. Japanese yew is similarly toxic and a much more common ornamental plant in the United States because it is hardier in cold climates. One of many poisonous compounds in yews is taxine, an alkaloid that causes cardiac and respiratory collapse. Taxine is found in all parts of the yew, except the bright red fruits. Symptoms of taxine poisoning are difficulty breathing, trembling, weakness, heart problems, stomach upset and, frequently, sudden death with no other signs. A horse or cow, and presumably a moose, can drop dead within 10 or 15 minutes of consuming a single mouthful of yew. Unlike chokecherries, which are most toxic to ruminants, yews are highly toxic to most mammals, including dogs and people. You don't want Fido to fetch a stick obtained from a yew.

Taxine is found in greatest concentration in the plant in winter. Cause of death is often indicated by observing yew leaves in the mouth or rumen, or by a slight "piney" odor from exposed rumen contents. Deer, at least those inhabiting areas where yews are native, are reputedly able to consume yews with no ill effects. Similarly, chokecherries are eaten by many local birds, which are unaffected by the toxin because they digest only the fruit.

Hundreds of moose are forced into the Anchorage metropolitan area each winter by accumulating snow in the surrounding mountains. As Anchorage has grown, thousands of acres of moose winter habitat have been converted to buildings, parking lots, and lawns. The deliberate cultivation of chokecherries and yews, and the growing infestation of May Day trees in local parks and greenbelts, is liable to make surviving winter even more dicey for local moose.

Some gardeners may embrace a plant that fights back. Moose are notorious for breaking limbs and even killing some ornamental trees. Especially expensive ones. Those who like having moose as neighbors can support the eradication of feral chokecherry trees in local parks, greenbelts, and even roadsides. You might resist planting chokecherry and yew trees in your yard or even remove those already there. Pruning limbs to eliminate twigs and foliage within 10 or 12 feet of the ground might also reduce the chance that you'll find a dead moose calf in your yard some winter.

This column was updated to correct the name of municipal forester Scott Stringer.

Rick Sinnott is a former Alaska Department of Fish and Game wildlife biologist.