An Ecologically Integrated
Approach to Management of
Dwarf Mistletoe (Arceuthobium) in Southwestern Forests

Southwest Forest Alliance
May 5, 1995

by
Michael M. Pollock, Ph.D.
Kieran Suckling
Southwest Center for Biological Diversity

An Ecologically Integrated Approach to Managing Dwarf Mistletoe (Arceuthobium) in Southwestern Forests

Abstract

Dwarf mistletoes (Arceuthobium species) are a group of vascular plants that parasitize conifers. These plants are integral part of forested ecosystems, and have existed as part of the coniferous forests of North America since the Miocene. Dwarf mistletoe is important to the ecology of these systems. The fruit, foliage and pollen of dwarf mistletoe are a food source for numerous bird, mammalian and insect species. Dwarf mistletoe alters the growth patterns of infected trees, creating structural complexity within forests in the form of witches brooms and snags, both which are used by numerous wildlife species for nesting, roosting and cover. These mistletoes are considered serious pests (by silviculturalists) throughout much of Western North America because they infect, and can reduce the growth rates of commercially important conifers. In the Southwestern United States, ponderosa pine is the primary commercially important species infected by dwarf mistletoe. Land use activities in Southwest forests during the past 125 years have encouraged the spread of dwarf mistletoes. Many of the silvicultural challenges created by these parasites are exacerbated by ecologically insensitive land management policies such as fire suppression, livestock grazing, and inappropriate silvicultural techniques. In general, dwarf mistletoe only becomes a problem when land managers attempt to create highly productive forests or tree farms to grow timber far in excess of historical production rates. The damaging effects of mistletoe can best be minimized, and their ecological benefits maximized, by recreating forest stands with age, size and density distributions similar to the original, presettlement forests.

Introduction

Dwarf mistletoes (Arceuthobium species) are an integral component of most coniferous forests in western North America. Dwarf mistletoe range from central Canada and southeast Alaska to southern Mexico (Fig. 1). The western United States and northwestern Mexico are the center of diversity for this genus, containing 24 of the known 32 species in the world 31. Arceuthobium utilize a wide variety of conifers as their hosts, although most species prefer pines 31. Two important Arceuthobium species quite prevalent in the Southwest are A. vaginatum and A. douglasii, which infect ponderosa pine (Pinus ponderosa) and Douglas fir (Pseudotsuga menziesii), respectively. This paper examines the ecology of these and other parasite-host complexes, but is primarily focused on applying this information to the management of Southwest ponderosa pine forests.

Arceuthobium are well known to most silviculturalists in the Southwest because they diminish the cash value of Douglas fir and ponderosa pine, the regions most commercially important conifers60. Accordingly, concerted efforts have been made to rid forests of these plants on federal lands. However, such a strategy is not wise (or feasible). Fossil records clearly show that Arceuthobium species have been associated with North American coniferous forests since the Miocene (12-26 million years BP) 39. Over this vast expanse of time, many species have adapted to and are perhaps even dependent on mistletoes. Numerous species of birds, mammals and arthropods have been observed feeding on the fruits and foliage of mistletoe, while other birds and mammals utilize the witches brooms created by the mistletoe as nesting platforms or roosting sites (Appendices 1 and 2). Witches brooms are abnormal branches found on trees infected by Arceuthobium. These brooms contain dense patterns of interconnecting twigs and branches, forming a unique structural element in the forest that has many ecological benefits.

Although dwarf mistletoe and witches brooms are integral components of Southwest forests, the primary focus of forest managers in the National Forests has been to eliminate them. Because of the agencys overwhelming focus on timber production, most dwarf mistletoe research sponsored by the Forest Service has been focused on control methods, with little attention being paid to their ecological importance (e.g. see symposia proceedings by Scharpf and Parmeter 1978, Hawksworth and Scharpf 1984). Dwarf mistletoe control projects have traditionally been pursued by the Forest Service in order to maximize timber production. As a result, attention has been focused on management techniques to produce more merchantable trees at the expense of managing for ecosystem health. There is an urgent need for the Forest Service to reevaluate its current strategy for managing dwarf mistletoe, and to adopt an integrated ecosystem perspective that manages for forest ecosystem integrity, rather than waging a war (sensu Wicker, 1984) against dwarf mistletoe 74. In this paper we examine three crucial issues related to dwarf mistletoe in Southwest forests in the hopes of bringing a more enlightened approach to the management of these forests: (1) The biology and ecological importance of dwarf mistletoe, (2) how fire suppression, livestock grazing and silvicultural practices have altered the interaction of dwarf mistletoe with its environment, and (3) integrated, ecosystem-based strategies for managing dwarf mistletoe.

Figure 1. Distribution of Arceuthobium in North America (from Hawkinsworth & Wiens 1972)

The Importance of Dwarf Mistletoe to the Ecology of Southwestern Forests

Dwarf mistletoe interacts with other plants and animals in Southwest forests in several ways: (1) their fruit and foliage provide a food source to animals (2) the brooms they help form adds short-term structural complexity to the forest, and (3) by hastening the death of trees, they help to create snags, which are an important source of long-term structural complexity and wildlife habitat. Additionally, by influencing the amount of dead wood in a forest, they indirectly affect fire intensities, and thus the successional dynamics of forests. This latter topic will be covered under Fire and Dwarf Mistletoe.

Consumers of Dwarf Mistletoe
There are a number of animals that have been observed eating the fruit and foliage of dwarf mistletoe (Appendix 1). Such species include elk, deer, squirrel, grouse, pygmy nuthatches and mountain chickadees, as well as a number of arthropods ^{7-11, 16, 23, 40, 71-74}. The relative importance of dwarf mistletoe as a food source for such species is not known, but given the prevalence of mistletoe, it is likely that certain species have come to rely upon it as a food source during certain times of the year. It is known that the foliage of dwarf mistletoe provides a source of food in the winter for grazers, when fresh foliage is scarce. Witches brooms create ideal platforms for trapping snow, making them susceptible to breakage. These breakages provide mistletoe foliage to animals such as elk and deer ⁹. Nutritional analyses of dwarf mistletoe indicate that the foliage and berries have high nutritional value for deer, and likely for other species as well ⁷³. Dwarf mistletoe also helps to provide food for other species early in the spring. Branches of conifers such as Douglas fir, that are infected with dwarf mistletoe, break bud (bud out) earlier in the spring than do uninfected twigs, providing a food source for certain insect groups ⁷⁰, which in turn provide food for insectivores.

Witches Brooms as Wildlife Habitat
The habitat created by witches brooms is important to a number of birds and mammals. Forests infected with mistletoe have been strongly correlated with the biodiversity of birds in Ponderosa pine forests ⁸, in part because of the brooms, which provide ideal platforms for nesting and cover. It is well documented that birds of prey such as accipiters and owls use witches brooms as nesting sites ^{20, 40, 53, 45, 46, 12, 11}. These birds seem to prefer the large brooms found up high in older trees. The formation of such a broom requires infection of a large tree, because young trees that are infected tend to die before they reach a large size. These large brooms provide visual protection from predators both above and below. It is quite difficult to determine whether a broom contains a nest when viewed from the ground (as many researchers have discovered). A well-disguised nest in a large broom can also be difficult to detect from the air, providing protection against predators such as the great horned owl ⁴⁶.

Available evidence suggests that the size of the broom needed for nesting is a function of bird size ^{40, 53, 45, 46, 12, 11}. Therefore, it is likely that current logging practices, which preferentially remove large trees, tend to have a disproportionately larger impact on big birds of prey. This habitat need may partially explains why two birds of the Southwest which use brooms as nesting sites, the Mexican spotted owl and the Northern goshawk are listed as endangered and as sensitive, respectively, by the U.S. Fish and Wildlife Service and the U.S. Forest Service. Several recent studies have demonstrated that the spotted owl shows a marked preference for witches brooms as nesting sites ^{68, 24, 65}. Of all the nests examined in these studies, 153 out of 359, or 43%, were associated with witches brooms ^{68, 24, 65}. This is particularly remarkable in light of the fact that large brooms of sufficient size for nesting are rare in Southwest forests. It is quite possible that given an abundance of brooms to choose from, an even higher percentage of spotted owls would nest in these structures.

The list of animals known to utilize brooms as nesting platforms is extensive, but not exhaustive. There have been no thorough studies to determine which Southwestern birds and mammals use brooms. They may well be crucial habitat for other endangered species. Other birds known to use brooms as nesting sites include the Coopers hawk, great gray owl, long-eared owl and others (see inset and Appendix 2 ).

It is well known that species associated with ponderosa pine forests are declining in numbers and that bird diversity is decreasing. Bird surveys conducted yearly since 1968 in logged ponderosa pine forests in New Mexico indicate that 75% of all bird species are declining ⁴⁴. Overall, about 25% of all species associated with ponderosa pine forests are declining ^{21, 32}. How much of this is attributable to the loss of witches brooms is uncertain, but the growing list of birds that use witches brooms as nesting sites that are also experiencing population declines, suggests that the loss of this important habitat feature may at least in part be contributing to the loss of biodiversity in Southwest forests.

Dwarf Mistletoe and Snag Formation

One of the most important ecological functions of dwarf mistletoe is to increase the rate of snag formation in Southwestern forests. Mistletoe has always existed as a stand replacing mechanism that has helped to create ecologically important large snags, while simultaneously opening up the canopy to allow for the establishment of young trees. Mistletoe infections eventually result in death for many trees, and the average life span of infected trees is considerably shorter than uninfected trees ²⁹. In general, infected large trees live longer than infected small trees, although precise relationships between tree size or age and post-infection longevity are not known. Snags are an important resource for many species of cavity nesting birds and mammals ^{63, 52, 48}. The number of snags and cavity nesting birds in Southwest forests are correlated to the degree of mistletoe infection 8. Much of the recent decline in cavity nesting birds in southwest forests has been attributed to the loss of large snags. Bird species diversity and numbers in the Southwest have been positively correlated with the density of mature live ponderosa pines and snags ³. Snags are used by 85% of North American birds ³⁷, at least 49 species of mammals, and many reptiles, amphibians and invertebrates ¹⁹. Thirty percent of all North American birds nest in snags ⁴⁷ and forty bird species nest in ponderosa snags ⁶⁴. Secondary cavity nesters alone make up 33% of breeding bird species, and 40% of total breeding bird pairs in ponderosa pine forests ⁴. Eighty two percent of secondary cavity nesters breed exclusively in dead and dying trees ⁴, while between 60 and 94% of overwintering ponderosa pine-associated birds require snag roosts ⁶⁹. In addition to nesting and roosting sites, snags and broken-tops are used as drumming posts, song perches, hawking platforms and foraging.

Large snags are preferred by primary and secondary cavity nesters. Seventy five percent of cavity nests on the Coconino National Forest are in trees > 24 inches dbh 17, while the mean dbh for trees containing cavity nests on the Apache-Sitgreaves National Forest is 23 inches ⁶³. The availability of suitable nesting cavities is the primary limiting factor in secondary cavity nester populations sizes ^{2, 28, 13, 51, 76, 5}. Studies demonstrate that where unlimited nesting and roost sites are available, other factors such as availability of food, do not affect population sizes ⁴¹. Mature forests are the most favorable to cavity nesters because of their abundance of large dead and dying trees. The removal of these trees in managed forests dramatically decreases the number and diversity of secondary cavity nesters. The loss of natural bird diversity in managed forests has been well documented, and the general decline is largely accounted for in the disappearance of cavity nesting species ²⁷.

Because large trees infected with dwarf mistletoe have been viewed as infection centers for nearby smaller trees, the Forest Service has promoted a policy of removing these large trees ⁵⁵. Such a management strategy removes the next generation of large snags, leading to the long-term loss of prime rearing habitat for cavity nesting birds.

Fire and Dwarf Mistletoe
Fire was historically an integral component of forest development in the Southwest, and helped to control the spread of dwarf mistletoe by: (1) keeping overall tree densities low, (2) selectively destroying infected trees and stands, and (3) pruning infected limbs from live trees. Additionally, smoke from fires may have inhibited dwarf mistletoe seed germination.
Today, the number of trees per acre on some southwestern pine forests are one to two orders of magnitude higher than during pre-European times 15. The reasons for this increase are discussed below under the Land Use Practices section. High stand densities contribute to the spread of dwarf mistletoe by placing more trees within the range of dwarf mistletoe seed dispersal from infected trees.

Estimates of pre-European tree densities range from 9.3 to over 150 trees per acre in the ponderosa pine type, depending upon growing conditions and method of measurement ^{14, 15, 38}. Forests tended to be more open at lower elevations, on southern exposures, and on poor soils. Forest were denser in wetter, cooler areas and on richer soils. While a complete picture of pre-European forests has yet to be established, it is clear that in many areas, todays ponderosa pine forest is thicker than in the past. As a result, mistletoe may spread from tree to tree more easily than in the past.

Additional evidence suggests that some ponderosa pine forests had a clumped distribution, with small clusters of trees imbedded in a grassland matrix ¹⁵. The areas of grass between tree clusters likely formed barriers to the dispersal of mistletoe seed, helping to limit infections to isolated patches of trees. It must be emphasized that the density of presettlement southwest forests was quite variable and that some of the more productive forests were dense enough to facilitate the spread of mistletoe by direct dispersal. We simply use some estimates of presettlement tree densities to show that historically, there were many forests where the rate of spread of mistletoe infections was likely quite low, and infections were not as widespread as they are today.

There is also evidence to suggest that infected trees are weeded out by fires ¹. Because witches brooms contain highly resinous, flammable material, dwarf mistletoe influences the susceptibility of trees to fire. Often, brittle brooms break off of trees, forming a pile of inflammable material at the base of the tree. These broom piles can support a fire of sufficient intensity to kill a tree. In heavily infected stands, there may be enough broom piles and dead trees that the entire stand will conflagrate, killing all trees in a wide area. Such an event, provided it kills all infected trees, serves to sterilize the area of dwarf mistletoe, keeping it free from this parasite for decades ¹. If some infected overstory trees survive however, they can serve as infection centers, raining mistletoe down upon the emerging understory of young trees regenerating after the burn ⁵⁶. The historical record suggests that such large, stand destroying fires were rare in ponderosa pine forests ¹⁴, and it is likely that most fires stimulated by the presence of brooms and other dead wood resulting from mistletoe infections at most killed only single or small clusters of trees.

It also appears that prior to extensive fire suppression, the heat from frequent ground fires was sufficient to prune back mistletoe infected branches in the lower crowns of ponderosa pines ^{57, 58}. This served to limit the extent of mistletoe to the upper reaches of the forest canopy ^{57, 58}. This pruning approach to dwarf mistletoe control has been successfully employed by modern day foresters to increase the longevity of infected stands ⁶².

There is also evidence to suggest that the smoke from fires was deadly to mistletoe seeds. In an experimental study, researchers found that exposure of mistletoe seeds (A. vaginatum, A. americanum and A. cyanocarpum) to wood smoke resulted in a germination rate of less than 5% (as compared to a normal germination rate of 27%) when the seeds were exposed to smoke for an hour ⁷⁸. In general, exposure to smoke inhibited germination after 10 minutes, and virtually no seeds germinated after long exposure times (90 minutes). Although the importance of smoke to inhibiting dwarf mistletoe seed germination under natural conditions is not known, this study suggests that it could be important, especially in light of the documentation by early European explorers that ponderosa pine forests were often quite smoky during the summer months, when ground fires were burning ¹⁴.

Figure 2. The probability of being infected by mistltoe seed decreases exponentially as a function of the distance from the seed source (see text for sources).

The Biology of Dwarf Mistletoe

The Life Cycle of Dwarf Mistletoe and Short Distance Seed Dispersal
Arceuthobium species have a unique and interesting life cycle that is well adapted to surviving in coniferous forests. The life cycle is illustrated in Figure 2, and has been well described by Hawksworth elsewhere ³¹. The summary presented here is a synthesis of that information. The adult plant produces berries which contain a single seed. As the fruit matures, fluid pressure builds up inside, until the berry finally breaks off the stem. At this moment, the released pressure ejects the seed from the fruit at tremendous velocities, sometimes exceeding 60 miles per hour. Dwarf mistletoe can eject their seeds up to 100 feet under optimal wind conditions, but in general most seeds travel no more than 10-15 horizontal feet from their source, and a tree 40 feet from an infected tree is considered safe from infection. The average, long-term rate of spread for dwarf mistletoe by such short-distance seed dispersal is about 1-2 feet per year, in managed stands. The seed contains a viscous coat, which allows it to stick to any object with which it comes in contact, such as the needles of a conifer. The seeds remain stuck to the conifer needles until they absorb moisture (e.g. during a rainstorm), whereupon they slide down to the base of the needle, adjacent to a twig. Young, needle-bearing twigs are most susceptible to infection. Once in this position, they begin to grow a hypocotyl, or small root, and bore into the outer layers of the twig. At this stage, swelling occurs in the infected area on the tree. About two years after the infection, aerial shoots develop, and two years after that, the female plant develops mature fruit. Dwarf mistletoe are dioecious (literally, of two houses) meaning that the male and female flowers are found on separate plants. Although individual shoots of the plant die after four or five years, the vegetative part of the mistletoe continues to grow, and old colonies are recognizable by the dense and misshapen cluster of twigs and branches known as witches brooms ³⁰.

Long distance seed dispersal
Although the seeds of mistletoe are spread to nearby trees by direct contact upon ejection, animals also serve as dispersal agents, and help to explain why mistletoe plants have been found far from the nearest potential seed source ⁴⁹. Birds and small mammals feed on and adjacent to mistletoe as ripe seeds are being ejected, and some of the seeds attach to their bodies. As the animals move to other trees, the seeds may detach. Birds such as mountain chickadees and pygmy nuthatches, which forage on the tips of branches and in the foliage, where seed germination is most likely to be successful, are thought to be particularly important agents of dispersal ³³. Birds apparently do not transfer viable dwarf mistletoe seeds through their feces. Most such seeds passing through the digestive systems of birds will not germinate ^{77, 33}. The success of seeds dispersed by birds is quite low. One study estimated that in a 150 ha plot of ponderosa pine, there was only one successful long distance infection every four years ³³. A number of birds and mammals, including Stellars jays, red crossbills, Cassins finches, red squirrels, flying squirrels and pine martens have been observed carrying mistletoe seeds on their fur or feathers. A complete list of dwarf mistletoe seed vectors is given in Appendix 3.

Pollination
Wind and insects are both important pollination mechanisms for Arceuthobium. There are no specialized pollinators of dwarf mistletoe. Instead they rely on general pollinators and wind. Hundreds of insect species have been identified as carriers of dwarf mistletoe pollen, but only a few dozen have been identified as being important pollinators ^{26, 50}. These pollinators include dipterans (e.g. flies), hymenopterans (e.g. ants) and coleopterans (beetles). The importance of dwarf mistletoe flower parts (e.g. the pollen and nectaries) as a food source for insects is not well studied. For those seeking more information, several excellent reviews on the pollination biology of dwarf mistletoe have been published ^{66, 25, 67}.

Figure 3. Generalized six year life cycle of dwarf mistletoe. Modified from Hawksworth and Wiens (1972).

Tree host selection
Whether some trees are more susceptible than others to dwarf mistletoe infestation has long been a subject of interest to silviculturalists. The dwarf mistletoe of the Southwest are fairly host specific, that is, one type of mistletoe only has one primary host. However, there are secondary hosts. For instance, A. americanum will sometimes infect ponderosa pine even though its primary host is lodgepole pine. Likewise, spruce and true firs will occasionally host A. douglasii even though Douglas fir is the primary host. A. vaginatum, which primarily infects ponderosa pine, occasionally infects lodgepole pine ³¹.

Large areas of apparently susceptible primary hosts are immune to mistletoe infection. For instance, ponderosa pines in the Black Hills area of south Dakota are uninfected by A. vaginatum, while A. douglasii is generally absent from the Douglas fir forests of the West Coast. Why these areas are immune to mistletoe infections is unknown, but it is thought to be related to climate ⁶¹.

Climate is also thought to influence the distribution of dwarf mistletoe on a more local scale. Studies suggest that dwarf mistletoe infestations in ponderosa pine forests are most severe on the driest sites, where tree growth rates are slowest , and least severe in wetter habitats ^{18, 43}. Also, A. americanum is absent form the higher ranges of lodgepole pine, apparently because of low temperatures, while A. vaginatum is absent from lower elevation ponderosa pine forests, presumably because of high summer temperatures ⁷⁵. Topography also affects the distribution of dwarf mistletoe, as they are most abundant on ridges, upper slopes and moderate slopes, and least abundant on steep slopes and valley floors ⁴³. There is also evidence to suggest that dwarf mistletoe preferentially infect trees on slopes with west and southwest exposure, and prefer stands with moderate site indices and low basal areas ⁴³. Collectively, these studies suggest that dwarf mistletoe preferentially infect stressed or slow growing trees.

The Effects of Land-Use Practices on the Ecology of Dwarf Mistletoe

Fire suppression, livestock grazing and logging are the primary land use practices that have altered the ecology of mistletoe in the Southwest. Because these land use practices have so radically altered the overall condition of Southwest forests, the changed ecology of dwarf mistletoe is largely reflective of overall changes in Southwest forest conditions. Fire suppression and livestock grazing have both altered the effect of dwarf mistletoe by increasing its infection rate, primarily by increasing stand densities and thus facilitating the direct tree-to-tree transfer of mistletoe. Livestock grazing increases stand densities by removing grasses and exposing bare mineral soil, which is ideal for pine seedling germination . Additionally, the hooves of livestock press pine seeds into the soil, further facilitating germination. Livestock also trample seedlings, but overall, they promote seedling establishment.

Fire suppression has eliminated the fires that regularly swept through forest understories ¹⁴. Historically, these fires were of sufficient frequency to kill most pine seedlings, helping to keep recruitment rates and stand densities low. Though fire suppression is commonly identified as the primary cause of high stand densities, scientific studies have determined that intensive livestock grazing preceeded active fire suppression policies by several decades. As a result of this grazing, fire occurance was greatly reduced long before active fire suppression occured. Other comparative studies also strongly suggests that livestock overgrazing, not fire suppression, is the primary factor that has allowed stands to become overstocked ^{59, 42, 79}. Studies of paired sites where fire has been suppressed in both sites, but where grazing has occurred in only one site, show that the ungrazed sites have little in the way of conifer regeneration though fire has been suppressed for many decades (e.g. 125 years). Grasses in these systems are known to contain allelochemicals (toxins) that can kill ponderosa pine seedlings ^{35, 54}, further suggesting that competition with grasses is the primary reason why historically, ponderosa pine recruitment rates were low. Such findings strongly suggest that land use practices which destroy the native grass understory may be the primary cause for the rapid increase in pine seedling recruitment. Fire suppression may have played a secondary role in increasing seedling recruitment, as historically, fires only eliminated those seedlings that managed to grow where competition with grasses was minimal.

Past and current logging practices have also contributed to the spread of dwarf mistletoe in Southwest forests. Disturbing soils during timber harvesting, encouraging afforestation at tree densities well above historic levels, and selective harvesting have all contributed to a decline in forest health and encouraged the spread of mistletoe. When soils are disturbed and ground cover is removed during logging operations, stand densities increase because pine seedlings readily germinate and grow on bare mineral soil. Such disturbances are particularly prevalent when heavy machinery is used extensively during timber operations. In general, silviculturalists do not consider such disturbances a problem because undisturbed forests of the Southwest are considered to be stocked at numbers far below their productive capabilities. Thus any activity that artificially increases stand densities, whether it be fire suppression, livestock grazing, or logging activities, is considered beneficial.

Selective tree harvesting that leaves infected overstory trees standing, also contributes to the spread of mistletoe. These large trees rain seeds down upon the new stand that is emerging below, thus infecting an entire new generation of trees, and ensuring that their lives are short. Recognition of this overstory-to-understory transferal of mistletoe infections have lead foresters to conclude that removal of such large trees is the best way to prevent dwarf mistletoe from spreading. However, such thinning practices fail to consider that under normal conditions, there would be minimal tree regeneration (and thus minimal infections) near infected trees, because of fires and competition from grasses. It is more accurate to consider the unnaturally high numbers of regeneration trees as the problem contributing to the spread of mistletoe. Removing the large trees creates a forest that is quite unlike anything that historically existed. This thinning technique, where the largest, most ecologically important trees are removed from the forest, is known as sanitation salvage, and is being practiced by the Forest Service as way to improve ecosystem health. However, the idea that ecosystem health can be restored by simplifying forestsremoving the most important large structural elements in a forest and replacing them with a stand of even aged seedlingsis counter intuitive and ignores basic ecological principles. It is a basic tenet of ecological theory that structurally complex habitats are more diverse than simplified habitats ³⁴, and habitats that are biologically diverse are generally healthy.

If the purpose of removing infected trees is to improve ecosystem health by preventing the spread of mistletoe, then the infected trees can simply be killed and left standing, as mistletoe will not survive on dead trees. Or better yet, simply creating a small donut shaped no tree zone around infected overstory trees, to serve as a barrier to mistletoe seed dispersal, should solve the problem without further impoverishing the forest.

Logging has not only increased the spread of mistletoe in small trees, but has also removed the ecologically important brooms created by dwarf mistletoe infections in large trees. Unfortunately, current forest practices encourage the removal of these structures. Even when the tree are not cut, thinning and pruning operations designed to save stands (so they can later be harvested) typically remove infected trees and limbs from infected trees. These tree and limbs often contain witches brooms, and many of the trees, because they are infected, will soon be snags. Thus, these well intentioned clean up efforts to improve the health of trees, actually reduce the structural elements of a forest that most greatly contribute to overall ecosystem health and biodiversity.

Integrated Management Strategies for Controlling Dwarf Mistletoe

Although dwarf mistletoe is frequently referred to as a problem, that needs to be controlled, it is more correctly viewed as a component of Southwest forests that is increasing in abundance because of land use changes that have occurred in the past 125 years. The spread of mistletoe is symptomatic of the decline in the health of Southwest forests that has occurred because of these changes. Specifically, livestock grazing, fire suppression and certain logging practices have all contributed to the decline in forest health and to the spread of dwarf mistletoe. An integrated management strategy that restores some of the fundamental components and processes that historically existed in these systems would largely eliminate the mistletoe problem. We refer to this strategy as integrated because the components (outlined below) are interrelated. All components need to be incorporated into an overall management plan for any one of them to work correctly. Such an integrated strategy would include the following fundamental components:

1. No cutting of large diameter trees and snags. There is no justifiable ecological or forest health reason for removing large trees or snags. Mistletoe infections are a normal ecological process whereby large trees are converted into snags. The live infected trees today are the snags of tomorrow. Unfortunately, large trees and snags are relatively rare components of Southwest forests because of past and current timber harvesting activities. There really are very few truly large trees left. The smaller ranges of trees today classified by the Forest Service as large (17 in > dbh < 24 in) ³⁶ were not even considered to be merchantable less than 90 years ago ³. However, the largest existing trees do provide the best current habitat, and will provide a future source of large snags. Therefore, they should not be removed. If a tree is infected with mistletoe and it is determined that the tree should be killed because it poses a high risk of infection to other ecologically important trees, then it should be killed and left standing, to serve as snag habitat.
2. Thin understory trees to create stand structure and densities that approximate presettlement conditions. The overstocked young forests of today facilitate the spread of dwarf mistletoe and in some instances, present a fire hazard to large, ecologically important trees. Any thinning operation, however must leave the largest trees in a stand intact. The current Forest Service practice of removing large overstory trees during thinning operations is an ecologically destructive practice that degrades ecosystem health. Understory thinning will slow tree-to-tree spread of dwarf mistletoe or limit it to isolated clusters of trees. All downed small diameter trees and slash should be removed to reduce the forest floor fuel load and minimize fire hazards. Current and historic logging practices which leave large volumes of slash in the forest increase fire threats and beetle infestations.
3. Reestablish regular ground fires in order to minimize seedling survival and to prevent the accumulation of fuel. Although crown fires did occur on occasion, the historical record suggests that they were rare in ponderosa pine forests. Hence, crown fires should not be encouraged. Before fires are reintroduced, forest understories need to be thinned and accumulations of forest floor fuels must be removed to prevent intense fires from killing the few remaining large trees. Although fires, if controlled properly, will improve forest health, it is essential that large trees be protected or the ecological benefits that occur because of burning may be outweighed by the loss of the big trees that take centuries to replace. Given the rarity of large trees, it is extremely important to remove fuel near their bases to prevent their untimely demise. The frequency of ground fires should approximate presettlement fire frequencies.
4. Reduce livestock densities to a level that will allow a relatively continuous ground cover of herbs and grasses to develop where light, soil and moisture conditions would normally support such vegetation. Once forests are thinned and opened up, they will simply return to their pre-thinning densities if livestock remain to prevent the reestablishment of ground cover. In areas where the ground cover has been eliminated, livestock should be completely removed until the vegetation has recovered. Future introductions of livestock in such areas should occur on a limited basis and under careful monitoring. Should the ground cover begin to degrade, livestock should be removed again until vegetative recovery is complete. In general, livestock should only be grazed in forests where there is a relatively complete ground cover, and in these areas, only at densities and frequencies low enough to prevent increases in the extent of exposed mineral soil. Experimental evidence from other arid systems (e.g. eastern Oregon) suggests that removing livestock from degraded areas will improve foraging opportunities ²². This suggests that removing livestock from damaged Southwest forests may, in the long run, improve ecosystem health and provide more livestock forage.

Summary

The spread of dwarf mistletoe throughout Southwest forests is linked to the overall decline in forest health that has resulted from 125 years of excessive grazing, logging and fire suppression. Most problems associated with dwarf mistletoe can be solved by managing these forests for ecosystem integrity rather than timber protection or livestock forage. Much of the spread of dwarf mistletoe is directly attributable to artificial attempts at increasing timber yields above historic levels. Fire suppression, livestock grazing and logging have all contributed to the spread of dwarf mistletoe. Although there are legitimate concerns about the spread of dwarf mistletoe, management plans must recognize the importance of these plants to forest ecology. Dwarf mistletoe is an integral component of Southwest forests, providing food, shelter and nesting sites for wildlife, and in general increasing biodiversity. Land managers need to focus on maximizing the ecological benefits of dwarf mistletoe, while researchers need to learn more about the ecology of these species. Current attempts to improve ecosystem health by removing mistletoe-infected overstory trees does more harm than good, and the practice should be discontinued. An integrated ecosystem management strategy that restores the natural processes and stand conditions that historically occurred in these forest is the best approach for controlling dwarf mistletoe while simultaneously restoring ecosystem health.

References Cited

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Appendix 1. Organisms that are known to eat dwarf mistletoe

Appendix 2. Organisms known to use witches' brooms as nest sites or cover

Appendix 3. Organisms that are known vectors of dwarf mistletoe seed

END

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