Forest Inventory and Analysis (FIA)Annual Inventory Answers theQuestion: What Is Happening toPinyon-Juniper Woodlands?

John D. Shaw, Brytten E. Steed, and Larry T. DeBlander

Widespread mortality in the pinyon-juniper forest type is associated with several years of drought inthe southwestern United States. A complex of drought, insects, and disease is responsible for pinyonmortality rates approaching 100% in some areas, while other areas have experienced little or nomortality. Implementation of the Forest Inventory and Analysis (FIA) annual inventory in several statescoincided with the onset of elevated mortality rates. Adjunct inventories provided supplemental data ondamaging agents. Preliminary analysis reveals the status and trends of mortality in pinyon-juniperwoodlands.

Keywords: forest inventory, FIA, pinyon pines, pinyon-juniper woodlands, drought, mortality, Ipsconfusus, bark beetles, southwestern United States

T he Forest Inventory and Analysis(FIA) is a national USDA ForestService program that conducts for-

est inventory across all land ownerships inthe United States. FIA inventories are basedon a systematic sample, establishing perma-nent sample plots on the ground at an inten-sity of approximately one plot per 6,000 for-ested acres. In the past, FIA conductedperiodic inventories in which all plots in agiven state were measured at once. In west-ern states it commonly required several yearsto measure all the plots in a state. Theplanned periodic revisitation cycle in thewestern United States was 10 years, but ac-tual cycle lengths sometimes approached 20years. In response to user demand for moretimely information, FIA began to test andimplement an annual inventory system in1996 (Gillespie 1999).

The annual inventory system uses the

same systematic sample grid as was used forperiodic inventories, but the plots are evenlydivided into annual panels that are visited ina continuous cycle. In western states, one-tenth of the plots in a state is sampled peryear, producing a 10-year inventory cycle.Plots are distributed throughout the state ineach panel. Consequently, annual panels aretheoretically free from geographic bias. Thissystem provides a continuous flow of data,which allows for more flexible analysis op-tions and provides opportunities to monitorforest change in ways that were not possibleusing periodic inventory data.

Pinyon-juniper woodland is the mostcommon forest type in the American South-west, covering over 36 million acres in 10states and extending into Mexico. The typeis defined by the presence of one or morepinyon species—usually common andsingleleaf pinyon (Pinus edulis Engelm. and

Pinus monophylla Torr.&Frem.)—and oneor more juniper species (Juniperus spp.);pure stands of pinyon usually are not con-sidered a separate type. Pinyon nuts were astaple food for Native Americans of theSouthwest for thousands of years, and theharvest of this valuable resource continuestoday (Lanner 1981). There also are manytraditional and modern uses for pinyon andjuniper wood. The extensive range and vol-ume of the resource has generated interest inintensive use, such as an energy-producingbiomass crop. However, pinyon-juniperwoodland is seen as a weedy invader of pro-ductive grasslands by some.

The Interior West FIA (IW-FIA) pro-gram operates in Arizona, Colorado, Idaho,Montana, Nevada, New Mexico, Utah, andWyoming, which includes most of the rangeof the pinyon-juniper type in the UnitedStates. Annual inventory was implementedin Utah in 2000 and in most of the otherIW-FIA states since then. About the timethat annual inventory was started in the IW,forest managers and researchers began to no-tice an increase in the incidence of insectsand disease in several forest types, includingpinyon-juniper. Much of the increase hasbeen attributed to the drought that spreadacross the Southwest beginning in the late1990s (Figure 1).

As of Jan. 2005, the drought was ongo-ing and significant drought-related mortal-ity had occurred in the pinyon-juniper type

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(Figure 2). Widespread and locally severemortality has inspired efforts to quantify theeffects of drought, insects, and disease overthe past 5 years. Some of these efforts havebeen local in nature and lack the geographicand temporal ranges covered by the FIA pro-gram. Other efforts provided wider coveragebut were focused on areas where mortality

was known to occur. Because the FIA sam-ple is unbiased with respect to plot locationand covers a wide area and extended timeperiod, it provides a unique view of pinyon-juniper woodlands. The systematic samplealso reduces the likelihood of producing er-roneous conclusions that may come fromsurveys conducted only in known areas of

mortality. Therefore, the current mortalityepisode has provided an opportunity to testthe usefulness of the FIA annual inventorysystem for quantifying rapid change in pin-yon-juniper woodlands over a large geo-graphic area (Shaw in press).

Because of the high degree of interest inthe amount of mortality and causal agents,we established an adjunct inventory on asubset of FIA inventory plots to documentdamaging agents in detail. Here, we describethe extent and severity of mortality in thepinyon-juniper forest type and highlightsome of the contributing mortality agents.

Drought and Effects on ForestsThe current Southwest drought began

about 1998 (McPhee et al. 2004), but theexact time of onset varies by location andinterpretation of climatic data. Although lo-cally severe, the current drought appearscomparable in magnitude to droughts in theearly 1900s, in the 1950s, and during manyother dry periods that have been docu-mented using tree-ring–based reconstruc-tions of the past 800 years (Cole et al. 2004,McPhee et al. 2004; Figure 1). In recentmonths, it appears that some areas affectedby drought since the late 1990s are experi-encing a degree of relief (Society of Ameri-can Foresters 2004).

Anecdotal reports of drought-relatedeffects on Southwest forests began in 2000,but a dramatic increase in tree mortality oc-curred in 2002 (Anhold and McMillin2003). There were local reports of up to100% mortality of the pinyon componentin pinyon-juniper woodland. A rapid expan-sion of high-mortality areas was recordedduring aerial surveys between the fall of2002 and the fall of 2003 (Anhold and Mc-Millin 2003). Although locally severe, littleof the observed mortality has been attrib-uted to drought alone. The cause of mortal-ity may be best described as a complex ofdrought, insects, and disease.

Mortality Status and TrendsData from IW-FIA periodic and annual

inventories offer some insight into the pro-gression of drought-related mortality acrossthe Southwest. Periodic inventories fromArizona, New Mexico, and Utah quantifiedpredrought conditions of pinyon-juniperforests and more recent annual inventorydata from Arizona, Colorado, Nevada, andUtah captured the current drought-relatedmortality episode. Although junipers andother species have suffered mortality in some

Figure 1. Climate record from 1895 to 2004 for five southwestern states as recorded in thePalmer Drought Severity Index (PDSI). Data points represent the mean annual PDSI forNational Oceanic and Atmospheric Administration (NOAA) Climate Divisions within eachstate (National Climatic Data Center 1994).

Figure 2. Landscape-scale pinyon mortality in pinyon-juniper woodland in southeasternUtah. (Photo by B. Steed.)

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areas, FIA plot data suggest that, to date,they are largely unaffected in the pinyon-juniper type. For the sake of simplicity, thefigures and trends presented here representonly the pinyon component of the pinyon-juniper type.

The IW-FIA program defines a mortal-ity tree as one determined to have diedwithin 5 years of the plot measurement date.Therefore, a distinction between long-standing dead trees and recently killed treesis made in the field. In the case of periodicinventories, this distinction was necessaryfor the purpose of estimating annual mortal-ity because data were limited to initial plotmeasurements. In reports based on periodicdata, observed mortality is averaged over the5-year period and reported as annual mor-tality in the year before inventory. There-fore, using this method it is assumed that5-year average mortality is representative ofthe reported year. Periodic inventory datasuggest that annual mortality is relativelylow for pinyon and juniper species. Based onrecent periodic inventory data from Arizona(O’Brien 2002), New Mexico (O’Brien2003) and Utah (O’Brien 1999), annualmortality, on a volume basis, was estimatedat 0.08–0.23% for common pinyon, 0.14%for singleleaf pinyon, 0.01% for oneseed ju-niper (Juniperus monosperma (Engelm.)Sarg.), 0.01–0.08% for Rocky Mountainjuniper (Juniperus scopulorum Sarg.), and0.01–0.07% for Utah juniper (Juniperus os-teosperma (Torr.) Little). In contrast, e.g.,estimates of annual mortality for ponderosapine (Pinus ponderosa Laws.) ranged from0.21 to 0.48% in the same inventories. Be-cause these estimates were computed fromperiodic inventories conducted in relativelynormal years (i.e., not excessively droughty),they may be representative of approximate“background” mortality rates. These esti-mates also include mortality due to fire, sobackground mortality rates excluding fireare actually somewhat lower.

Basing mortality estimates on a 5-yearaverage can be advantageous when it is de-sirable to smooth year-to-year variation andrepresent a “typical” year. However, themethod also may mask trends that are ofinterest or produce biased estimates. For ex-ample, consider a periodic inventory thatimmediately followed a severe fire season. Ifmortality data were attributed only to theprevious year, the mortality estimate wouldbe relatively accurate for that year but woulddrastically overestimate mortality for a typi-cal year. On the other hand, the 5-year aver-

age would be closer to the true backgroundmortality rate but still overestimates typicalmortality because of the inclusion of a severefire year. In addition, it is impossible todiscern trends based on a single periodicinventory.

In the case of insect and disease out-breaks, short-term temporal trends are ofsignificant interest. Annual data provide fin-er-scale temporal resolution that may revealshort-term trends. Preliminary analysis sug-gests that the progression of drought-related

Figure 3. County-level estimates of pinyon mortality 2000–2004. Mortality values representthe percentage of mortality of all pinyon trees 5.0 in. DBH and greater (live and mortality),on a basal area basis and on plots classified as the pinyon-juniper woodland type andexcludes mortality attributable to fire and logging. Annual inventory was implemented inUtah in 2000, Arizona in 2001, and Colorado in 2002. Data for Nevada are from a pilotinventory conducted in 2004. Annual inventory still has not been implemented in NewMexico. The range of pinyon-juniper woodlands, based on the ranges of common, single-leaf, border, Mexican, and Parry (Pinus quadrifolia Parl.) pinyons, is shown for reference.Ranges modified using FIA data from digitized maps (USGS 2005).

282 Journal of Forestry • September 2005

mortality has been captured by annual datacollected in southwestern states over the past5 years.

Annual inventory data show thatdrought-related mortality has occurredwidely across the Southwest. Figure 3 showsmortality estimates at the county scale,which is a common FIA reporting unit(USDA Forest Service 2005). In the earlystages of drought (2000–2001), nearly allthe mortality occurring at the county scalewas located in one or two plots in thecounty. As the event progressed, mortalitywas recorded on many more plots, but con-siderable variation in the amount of mortal-ity still occurred among plots within coun-ties.

When all plots are combined at state orregional scales, it is evident that the annualinventory has captured the rapid increase indrought-related mortality across the South-west (Figure 4). There are several aspects ofFigure 4 worth noting. The first is that theannual mortality estimates (as well as thecounty-level estimates in Figure 3) are actu-ally 5-year cumulative mortality values.Given the previous explanation of how an-nual mortality is calculated, it would appearthat the data overestimate mortality in agiven year. However, because data from ear-lier panels show far less mortality than laterpanels, it is possible to infer that the mortal-ity detected in later years, or at least most ofit, occurred just before the time it was ob-

served. Thus, the actual “new” mortality fora given year could be estimated by subtract-ing the cumulative mortality from the previ-ous year. Compilation methods that canproduce such estimates are under investiga-tion.

The similarities and differences in mor-tality rates among states also may be infor-mative. The fact that the trends shown byeach of the statewide curves are similar from2000 to 2003 lends to confidence in the re-sults. However, the differences among statesrevealed by the 2004 data raise a number ofquestions, perhaps the most important be-ing: why? Our preliminary attempts at mod-eling mortality risk in the pinyon-junipertype suggest that mortality may be predictedby a combination of edaphic factors andstand structural traits. This may allow pre-diction of trends in New Mexico, e.g., whereannual inventory still has not been startedbut recent periodic inventory data are avail-able.

Finally, trends revealed by annual dataagree with anecdotal accounts of the pro-gression of mortality. Reports from the fieldindicated that new insect and disease-causedmortality declined during 2004 and mortal-ity, while still increasing, was doing so at adecreased rate. Inventory data from Utahand Colorado appear to confirm field obser-vations, and data from Arizona suggest aslight decrease in the rate of increase. Noinference of trend is possible for Nevada,

which is represented by only a single panel ofannual data. However, the Nevada statewideestimate of 2.9% mortality probably is fiveto 10 times the expected background mor-tality, based on pinyon mortality rates inother states (O’Brien 1999, O’Brien 2002,O’Brien 2003).

Mortality AgentsAlthough mortality has been attributed

to drought alone in some cases, the death ofa tree is commonly attributable to multiplefactors. In conjunction with stress caused bydrought, a number of insects and diseasescan affect the health of pinyons. Theseagents may work singly, killing the tree di-rectly, or in concert, with each agent causingcumulative damage or stress that eventuallyovercomes the tree. Agents of particular im-portance include pinyon ips (Ips confusus(LeConte)), twig beetles (Pityophthorus spp.and Pityogenes spp.), pitch moths (familiesPyralidae (especially Dioryctria spp.) andSesiidae), black stain root disease (Lep-tographium wageneri (Kendrick) Wingfield),and pinyon dwarf mistletoe (Arceuthobiumdivaricatum Engelm.).

The pinyon ips (Figure 5, A and B) isthe most important insect mortality agent,causing the majority of the pinyon mortalityin the Intermountain West (Rogers 1993).However, ips beetles tend to be only moder-ately aggressive, attacking and killing treesstressed by other agents (Hagle et al. 2003).Outbreaks of this native bark beetle areknown to occur during periods of drought(Furniss and Carolin 1977), but also may beassociated with root disease, heavy dwarfmistletoe infection, previous defoliation,dense stand conditions, or poor soil condi-tions (McCambridge 1974, Hessburg et al.1995, Negron and Wilson 2003, Skelly andChristopherson 2003). When beetle popu-lations build, outbreaks may continue for ayear or two even if stressful conditions, suchas drought, are removed. Pinyon ips canhave two to four generations per season,which has resulted in rapid progression ofmortality in some areas.

Other species of bark beetle may attackthe smaller branches of pinyon pines. These“twig beetles” are largely from the generaPityophthorus and Pityogenes and are consid-ered secondary beetles that make use of treesthat are already stressed (Rogers 1993, Cainet al. 1995, Skelly and Christopherson2003). Some beetles from these genera mayattack larger branches and trunks (Cain et al.1995, Skelly and Christopherson 2003). Al-

Figure 4. Statewide mortality trends in Arizona, Colorado, Nevada, and Utah. Pinyonmortality is defined as in Figure 3. Nevada is represented only by 2004 pilot inventorydata. Line labeled as “all” represents the combination of all states in which annual datawere collected in a given year.

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though they usually have little effect on treehealth, these beetles can kill small trees orbranches of larger trees during periods ofdrought (Cain et al. 1995) and have beenassociated with occasionally heavy, localizedmortality. Most twig beetle species producetwo to four generations a year depending onweather and location.

Large masses or streamers of pitch (Fig-ure 5C) are indicative of pitch moth activity.The taxonomy and biology of many of thesespecies appear to be poorly understood(Swift 2004), resulting in difficulty defining

the organism(s) responsible for pitch mass-causing activity on pinyons. Potential agentsinclude pyralid moth larva (Pyralidae), in-cluding several Dioryctria species, and clear-wing moth larvae (Sesiidae) (Cain et al.1995, Skelly and Christopherson 2003,Swift 2004). Injuries inflicted by these spe-cies weaken the host tree, making it suscep-tible to attacks by other insects (Skelly andChristopherson 2003). If sufficient numbersof larvae feed on a tree, the tree may bekilled.

Recent research indicates that the black

stain root disease variant L. wageneri var.wagneri is specific to pinyon species. Blackstain fungi colonize the water-conductingtissues of the roots and lower stem, prevent-ing movement of water to the foliage. Black-colored bands in the sapwood of roots, theroot collar, or lower bole of dying trees (orno more than 6 months dead) indicate infec-tion (Figure 5D). Root-feeding beetles andweevils may spread the fungi as they move tofeed and breed. However, the most impor-tant method of infection is likely throughroot contact with adjacent infected trees(Skelly and Christopherson 2003). Infectedtrees may fade over time but often are killedearly by other agents such as pinyon ips(Hessburg et al. 1995, Skelly and Christo-pherson 2003). The disease does not appearto be widespread (Skelly and Christopher-son 2003), although it has been identified inthe Four Corners area of southwestern Col-orado and southeastern Utah and is impli-cated in the death of many trees in that area(Dr. Bill Jacobi, Colorado State University,personal communication, 2005).

The pinyon dwarf mistletoe (Figure5E) is a parasitic plant that robs its host treeof water and nutrients. It frequently infectsboth common and singleleaf pinyons andcan appear on other pinyon species withinthe mistletoe’s distribution (Hawksworthand Wiens 1996, Mathiasen et al. 2002).Infection reduces tree growth and vigor, pre-disposing it to attack by other agents. Severalstudies (e.g., Negron and Wilson [2003])suggest that dwarf mistletoe-infected treesare more susceptible to, and perhaps pre-ferred by, pinyon ips. However, dwarf mis-tletoe alone can kill trees, especially seedlingsand saplings. The length of time it takes tokill a tree depends on the level of infectionand the age and health of the tree (Hawks-worth and Wiens 1996, Mathiasen et al.2002).

Several other insects and diseases canhave strong visual effects on pinyon trees butgenerally are not important mortalityagents. Some areas thought to be mortalitylocations during windshield or aerial surveyswere, in fact, defoliated by nonlethal agentsand found to have greened up during subse-quent surveys. However, some of these or-ganisms may predispose trees to attack byother agents. Some of the insects and dis-eases that can be found on pinyons still maynot be identified. However, the onset ofdrought-related mortality has generated newinterest in the ecology of the pinyon-juniper

Figure 5. Agents associated with mortality of pinyons in drought-affected areas: (A) pinyonips beetle, (B) entrance hole and frass from pinyon ips, (C) pitch flow caused by pitch moth,(D) black stain root disease, and (E) pinyon dwarf mistletoe. (Photos by: panel A, T. Eager;panel B, University of Arizona; panels C and E, B. Steed; and panel D, W. R. Jacobi.)

284 Journal of Forestry • September 2005

woodlands and the taxonomy and generalbiology of many associated organisms.

The Value of Annual InventoryThe annualized FIA inventory (Gillespie

1999) was implemented with an associatedcomplement of assumptions and unansweredquestions. The onset of widespread drought-related mortality in the Southwest is providingan opportunity to test some of the assumptionsand answer some important questions (Shawin press). In addition, some characteristics ofthis mortality episode have led us to consideralternative methods of compilation and analy-sis of FIA data. For example, the geographicdistribution of mortality suggests that geo-graphic delineations not commonly used inFIA reporting, such as ecoregional units or dis-crete population segments, may better captureregional variation and reduce variance in sta-tistical analyses.

Based on early results, the potential forusing annual panels as independent samplesand time series data appears promising. TheIW-FIA annual inventory system appearsable to detect trend and magnitude of short-term change during a widespread, patchyevent such as drought-related mortality. Italso appears that relatively low levels ofchange can be detected, at least in caseswhere the variable of interest (in this case,background mortality) is typically at low lev-els and relatively constant over time andspace. Status and trends probably can be es-timated with confidence at larger scales. Itmay be possible to draw some conclusions atmedium geographic scales, such as thecounty or national forest, but this ability islargely dependent on the distribution of theforest type of interest within the geographicarea. For example, in less common foresttypes such as limber pine (Pinus flexilisJames) or bristlecone pine (Pinus aristataEngelm.) it may be difficult to detect trendsbecause of the relatively small numbers ofFIA plots that occur in those types.

The FIA annual inventory system isproviding the intended benefits of shorterreporting cycles and fine-resolution tempo-ral trends in the states where it has been im-plemented. It is also providing a valuablerecord of change during an ecosystemwidephenomenon. With full implementation(i.e., all states under annual inventory) andthe addition of new data every year, the re-sulting information will become increas-ingly valuable with time and will continue to

be an important resource for managers andresearchers.

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John D. Shaw (jdshaw@fs.fed.us) is analyst,USDA Forest Service, Rocky Mountain Re-search Station, 507 25th Street, Ogden, UT84401. Brytten E. Steed (bsteed@fs.fed.us) isforest entomologist, USDA Forest Health Pro-tection, Ogden Field Office, 4746 S 1900 E,Ogden, UT 84401. Larry T. DeBlander(ldeblander@fs.fed.us) is analyst, USDA For-est Service, Rocky Mountain Research Station,507 25th Street, Ogden, UT 84401.

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