ANTS•TREES A LIFELONG RELATIONSHIP From the first growth spurts of the tiniest seedling to the final days of the mightiest giant, ants are there, shaping the lives of trees.

BY AARON M. ELLISON

WHEN THINKING OF HOW ANTS INTERACT a closer look at the many healthy seed- with trees, a lot of people may think lings, saplings and trees near your of carpenter ants eating trees - home. Anywhere you look, you and the wood in their home. will probably find a worker In fact, in both our forests of one of the many ant and houses, these deni- species associated with zens of hollow trees and the trees in our forests. rotting rafters are merely Follow her back to her the final stage of a lifelong nest, and you'll start to relationship between trees learn about the intertwined and many kinds of ants. Take lives of ants and trees.

38 I WINTER 2014 AM ERICANFORESTS.ORG OF SEEDLINGS AND SOIL In this way, ants are integral to the life to pH -neutral: Ifthe surrounding soil is The trail ofworkers often will lead back of a tree from the very beginning. Ants acidic, ant nests tend to be more basic, to a volcano-like heap of soil. Depending create the best compost there is; anthills and vice-versa. All ofthis means that a on the species, such anthills can range are localized hotspots of nutrients. seedling that germinates from a seed that in size from a tiny pile of sand grains Their digestive cycle helps to create the was lucky enough to land on the sweet, that is less than an inch across to a huge nutrient-rich soil young trees need. As rich soil of an anthill will often get a mound several feet high and many cubic omnivores, ants collect and store large head-stait in the race for the canopy. yards in volume. This is where the amounts of nutrient-rich prey. As they lifelong connection between ants and process this food, their wastes further SCRATCH MY BACK, trees begins. Anthills are the product enrich the soil. Ant nests are also close I'll SCRATCH YOURS of tens to tens of thousands of burrow­ As trees grow, they are set upon by true ing, tunneling worker ants that have bugs (order Hemiptera) that feed on excavated mineral soil while build- sap, such as aphids, among many other ing temperature-controlled earthen herbivorous insects. In some cases, the chambers in which to live, store food, role ants play at this stage in a tree's life protect the queen and rear her brood. can actually cause a population boom In the formerly glaciated parts of for these insects, as many of our most North America- most of Canada and common and abundant ants, including much of the northern reaches of the Fuzzy ants (Lasius species) and species United States - there are no native ofFormica (Latin for "ant," not the plas­ earthworms. In these areas, much of tic countertop) care for the bugs. They the topsoil was created by ants. In fact, stand guard over the bugs, protecting ants create soil up to 10 times faster them from predators and occasionally than earthworms, excavating as much moving them from place to place to tap as 30,000 pounds of soil per acre every new areas ofthe tree, all in exchange for year, creating about 4 inches of new the privilege of dining on their excreta. soil per millennium in the process. Only rarely will a mature tree succumb

Anthills range in size from the tiny pile of the Labor Day ant, Lasius neoniger, to the large mound hill built by the Allegheny mound ant, Formica exsectoides and the mound nest of the European red wood ant, Formica lugubris.

AMERICANFORESTS.ORG WINTER2014 I 39 Plant-ants actively patrol the surface of the tree, aggressively fighting off herbivores such as caterpillars. to this population boom, but seedlings or a variety oftrees produce specialized small saplings may not survive it. structures to feed and/or house ants. In response, some trees have evolved One of the classic examples of a co­ a work-around to the ant-bug mutual­ evolutionary mutualism - two species ism, and feed the ants directly. In turn, evolving in tandem to the benefit of both Two New World black ants, Formica neogagates, haul a caterpillar back to for a reward similarly sweet to that - involves 'ant-plants' and 'plant-ants' their nest. which they would get from the bugs, the in tropical forests. Ant-plants, such as ants protect the trees. Central America's bull-horn acacias and This type of ant-tree mutualism is trees of the genus Cecropia, have hollow their hollow stems, respectively - in common throughout the tropics, where structures - thorns and internodes in which ants form their colonies. The trees also secrete sugar and protein-rich food bodies that make up a significant portion ofthe ants' diet. In return, the plant-ants actively patrol the surface ofthe tree, ag­ gressively fighting off herbivores such as caterpillars, as well as pruning off vines that could otherwise engulf the tree. These tightly-coupled mutualisms between ants and trees are still unknown in the temperate zone, though they do exist between ants and many common forest understory herbs, such as trillium and bloodroot, which provide similar

40 I WINTER 2014 AMERICANFORESTS.ORG food rewards for dispersing their seeds. Nonetheless, trees in temperate zones derive nutrients from the waste products - decaying food and nutrient-rich feces and excretions - ofthe many species of ants that nest and forage under bark and in the canopy. This kind of indirect mu­ tualism - trees provide ants with shelter; ants feed on herbivorous insects; ant wastes nourish the tree - is widespread in most forests. Trees also can benefit from the voracious appetites of ground-dwelling predatory ants - another indirect mutualism between trees and ants. For example, in the eastern U.S., Formica neogagates opportunistically feeds on a wide range of caterpillars, including gypsy moths and eastern tent caterpillars. Other Formica species of Formica obscuripes, preys not only on ants prey on sawflies that mine birch caterpillars, but also on conifer sawfly leaves. Mound-building red wood ants larvae. In general, when predatory ants (Formica rufa-group species) prey on are abundant, trees set more seeds, a wide range of insects. In the western even if those ants also tend aphids and U.S., one of these mound-builders, other bugs.

AMERICANFORESTS.ORG WINTER 2014 I 41 THE TREE RETURNS TO THE SOIL Like many other ants, carpenter beginning. Many logs also nurse a new Bark protects a tree from the elements, ants are omnivorous. They range wide­ cohort of seedlings, which will in turn but over time, fissures open, cracks ly, foraging for food, primarily at night, support a new generation of ants and appear, beetles burrow, water seeps in hundreds of yards from their nest. their allies. and fungus follows. As the tree's life When the colony gets too big for their So before putting ant baits around comes to an end, ants are there, as they original tree house, they form satellite the woodpile, remember that these have been since the beginning. As the colonies in other rotting trees - and little creatures, which in aggregate far aging tree's heartwood softens and rots, sometimes houses - nearby. The sat­ outweigh all of Earth's other verte­ carpenter ants move in, removing the ellite colonies remain connected to the brates - including people - really do rotten wood and opening up space in natal nest where the queen remains; keep the world turning . .... which to nest. Their huge colonies, some workers move food and larvae to and with more than 15,000 workers, can live from the satellites. Aaron M. Ellison is the senior research for decades; carpenter ants overwinter Carpenter ants signal their pres­ fellow in Ecology at Harvard University's by huddling together for warmth, feeding ence as the sawdust they remove from Harvard Forest, and lead author of A on fats stored in their bodies and - when inside the tree builds up at its base. Field Guide to the Ants of New England it gets really cold - filling themselves When the dying tree finally falls, the (Yale University Press, 2012). Learn with glycerol, a chemical like antifreeze decaying bole, with the help of soil­ more about ants and contribute your that stops ice crystals from forming nesting ants, returns its nutrients to own observations and insights at inside their bodies. the soil, bringing the cycle back to the www.NEants.net.

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