A Comparison of Rating Systems for Dwarf Mistletoe-Induced Witches' Brooms in Ponderosa Pine

A Comparison of Rating Systems for Dwarf Mistletoe-Induced Witches’ Brooms in Ponderosa Pine

Thomas J. Parker, Camp Navajo, ATTN: AZIA-GC-DE-E, P.O. Box 16123, Bellemont, AZ 86015, and Robert L. Mathiasen, School of Forestry, Box 15018, Northern Arizona University, Flagstaff, AZ 86011. Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021

ABSTRACT: We compared four methods of assessing dwarf mistletoe infection and estimating volume of dwarf mistletoe-induced witches’ brooms in ponderosa pine (Pinus ponderosa). Witches’ brooms can provide important wildlife habitat and can influence fire behavior. We used Hawksworth’s dwarf mistletoe rating system (DMR), Tinnin’s broom volume rating system (BVR), an estimate of the percentage of live crown occupied by witches’ brooms (PCB), and a new system, total broom volume (TBV). We rated 12,536 trees for southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum) infection and broom volume. Using Hawksworth’s DMR and Tinnin’s BVR systems, we were not able to distinguish trees with different witches’ broom volumes at all sample sizes. At all sample sizes, TBV ratings accurately and precisely identified amount of witches’ broom volume. We recommend that TBV be used to quantify witches’ broom volume in ponderosa pine where wildlife habitat and fire ecology are primary concerns. West. J. Appl. For. 19(1):54–59. Key Words: Arceuthobium vaginatum, ponderosa pine, dwarf mistletoe, witches’ broom, infection rating, broom volume rating.

Many species of dwarf mistletoes (Arceuthobium spp., The six-class dwarf mistletoe rating (DMR) system was Viscaceae) induce dense, profuse masses of branches known originally developed to quantify southwestern dwarf mistletoe as witches’ brooms (Hawksworth and Wiens 1996). Witches’ infection (Arceuthobium vaginatum subsp. cryptopodum) in brooms are valuable to wildlife (Hawksworth and Wiens ponderosa pine and Douglas-fir dwarf mistletoe infection (A. 1996, Mathiasen 1996, Parks et al. 1999, Hedwall 2000, douglasii) in Douglas-fir (Pseudotsuga menziesii) Garnett 2002). At least eight raptor species are known to nest (Hawksworth and Lusher 1956, Hawksworth 1977). However, in witches’ brooms, including federally threatened species in the DMR system, witches’ brooms are not a criterion for such as the Mexican spotted owl (Strix occidentalis lucida) rating. Although dwarf mistletoe infection is commonly (Hawksworth and Wiens 1996). Witches’ brooms may also associated with the development of witches’ brooms, severely increase crown scorching and decrease tree survival in infested infected ponderosa pines frequently do not have witches’ stands that are burned by prescribed fire (Alexander and brooms (Hawksworth 1961). Because the DMR system regards Hawksworth 1975, Harrington and Hawksworth 1990, Conklin infected branches as the criterion for developing its rating, and Armstrong 2001). In order to evaluate broom volume for rather than the number of brooms or broom volume, it can be fire management and assessment of wildlife habitat, a method applied to trees with or without witches’ brooms. The six- of quantifying broom volume is necessary; however, no class DMR system has become the standard for quantifying broom rating system has been developed for ponderosa pine the severity of dwarf mistletoe infection (technically DMR is (Pinus ponderosa) in the Southwest. a seven-class system, since the zero given for uninfected trees is also a class) (Hawksworth 1977). NOTE: Thomas Parker can be reached at (928) 773-3318; Fax: (928) 773- Tinnin (1998) proposed the broom volume rating (BVR) 3271; E-mail: [email protected]. The authors wish to extend their appreciation to Gregg Garnett, Janine McCabe, and system to quickly provide an infection rating that approximated Hendrick Herlyn for their assistance with data collection. Brian DMR in Douglas-fir with large witches’ brooms. Dwarf Howell assisted with the development of the total broom volume mistletoe infection in Douglas-fir typically results in witches’ rating system. Michelle Corse provided comments on the manuscript, and Carol Chambers and Bill Block provided guidance broom formation (Tinnin and Knutson 1980). The BVR on the thesis. This research was supported in part by funds system does not require counting branches. This makes it an provided by the USDA For. Serv. Rocky Mountain Research excellent system to use when trees have dense brooms that Station and the School of Forestry Mission Research Board, Northern Arizona University. Copyright © 2004 by the Society of make branches difficult to see. Because Tinnin (1998) intended American Foresters. the BVR system to result in a rating similar to DMR, crown

54 WJAF 19(1) 2004 thirds with at least one infection but without brooms receive infected, the third was rated as 1. If a crown third a rating of one on a 0–2 scale. As a result, trees without brooms was uninfected, it was rated as 0. The ratings for but with infections in each crown third receive a BVR of three each third were added, for a possible rating of 0–6. on a 0–6 scale. Conversely, because the BVR system gives the If the bole had an infection but no portion of the live same rating to a portion of the crown that is less than half canopy did, the tree was rated as 1. occupied by brooms, up to 48% of the live crown could be occupied by brooms and the tree would still receive a BVR of 2. Broom volume rating (BVR, Tinnin 1998). The three. Broom volume rating, therefore, is not sensitive to small BVR system incorporates both broom volume and broom volumes, but witches’ broom volume may not need to infection. The live crown of each tree was visually encompass half of the live crown of the tree in order to affect divided into thirds. If more than one half the volume fire behavior (Brown 1975, Koonce and Roth 1985, Conklin of a crown third was occupied by brooms, it was and Armstrong 2001) or to provide wildlife habitat (Hedwall rated as 2. If one half or less of the volume of a 2000, Garnett 2002). crown third was occupied by brooms, but it had at A further limitation of both the DMR and BVR systems least one dwarf mistletoe infection, it was rated as is that they only rate the live crown of trees. Live crown 1. If a third was uninfected, it was rated as 0. Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021 ratios vary widely; therefore, a rating of two in the lower Ratings for each third were added together for a portion of the crown may refer to infections or brooms possible rating of 0–6. For BVR and all broom either near the ground or high in the canopy. Brooms ratings, we considered a witches’ broom to be an occurring near the ground are more important to fire abnormal cluster of branches associated with dwarf behavior than brooms above the scorch height because mistletoe infection that was at least 20 cm in all they can become ladder fuels (Koonce and Roth 1985). three dimensions (depth, height, and width). In Conversely, brooms occurring higher in the crown will addition, when viewed directly below the broom usually survive low intensity surface fires and may thus be without binoculars, needles, twigs, and branches of more valuable to wildlife. Therefore, a witches’ broom the broom structure had to occupy at least 70% of rating system should distinguish broom locations. the horizontal plane in order for a structure to be A dwarf mistletoe rating system that effectively uses considered a broom. witches’ brooms has not been developed for ponderosa pine. The objective of this study was to evaluate the ability of three 3. Total Broom Volume (TBV, Parker 2001). The rating systems—DMR, BVR, and a new rating system known entire height of the tree from ground level to treetop as the total broom volume rating system, or TBV—to accurately (not only the height of live crown as in the other two quantify the extent of tree canopy that was occupied by systems) was visually divided into thirds. The witches’ brooms in ponderosa pine in northern Arizona. bottom third of ponderosa pines in our study were frequently not occupied by live crown. In these Methods cases, in order to visualize a crown volume for the bottom third, the existing live crown (in some cases We examined ponderosa pines for infection by southwestern several meters above ground level) was visually dwarf mistletoe in 16 dwarf mistletoe-infested ponderosa pine projected to the ground from the lowest live branches stands (32–73 ha) in northern Arizona. Stands were at least 1 parallel to the bole. In other words, the width of the km distant from one another and ≥99% ponderosa pine by bottom third was the width of the dripline of the basal area. Stands were located on the Coconino and Kaibab tree. If a third of the tree had no brooms, it was rated National Forests west of the San Francisco Peaks. We as 0. If the volume of a tree third was 1–33% established 40 circular plots (radius 11.3 m, 0.04 ha) per stand occupied by brooms, it was rated as 1. If the relative from a random starting point on a 200 m grid. We examined volume was 34–66% occupied by brooms, it was all trees larger than 12.7 cm in diameter at 1.4 m above the rated as 2. If it was ≥67%, it was rated as 3. The sum ground (dbh), and used binoculars to look for dwarf mistletoe of the ratings of the tree thirds had a maximum of shoots in all examined trees. Binoculars were especially 9 in order to more precisely quantify different useful to locate dwarf mistletoe shoots high in the crown and broom volumes. If a single broom occupied more on cloudy days (Geils and Mathiasen 1990). Once a tree’s than one tree third, its contribution to each third degree of dwarf mistletoe infection and witches’ broom volume was rated separately. In contrast to the other broom was determined, we rated it using four methods: rating systems we used, both live and dead brooms 1. Six-class dwarf mistletoe rating system (DMR, were considered when rating trees using the TBV Hawksworth 1977). The live crown of each tree system. was visually divided into equal thirds. The number of branches diverging from the bole in a crown 4. Estimate of the percentage of live crown occupied third was then counted. If more than half of the by brooms to the nearest 10% (PCB). This was a branches in a crown third were infected, the third visual estimate of the percentage of the volume of was rated as 2. If less than half the branches in a the live crown occupied by live brooms. Trees third were infected, and at least one branch was without brooms were rated as 0. If the live crown of

WJAF 19(1) 2004 55 a tree was 1–14% occupied by brooms it was able to identify trees with moderate broom volumes (>25%). assigned PCB = 10. A tree with a live crown from With the total sample of trees, we used a three-way ANOVA 15–24% occupied by brooms was assigned as PCB with factors DMR, BVR, and TBV. For factors with significant = 20, and so forth, up to 100%. effect tests we used Tukey-Kramer HSD multiple comparison tests to test for differences in group means between trees It should be noted that all of the dwarf mistletoe rating without brooms (PCB = 0), trees with 1–14% of live crown systems that we used were subjective. In order to establish occupied by brooms (PCB = 10), and trees with 25–34% of consistency, the senior author was present when all ratings live crown occupied by brooms (PCB = 30). were performed. Repeatability between studies is difficult to attain in all research using rating systems without quantifiable measures (Block et al. 1987). The question of repeatability Results may best be addressed through training provided during In the 16 stands we sampled (stand DMR 0.1–3.7), we rated meetings of professional societies and during forestry a total of 12,536 ponderosa pines. Of these, 4,183 trees were coursework. However, there will always be some interobserver infected by dwarf mistletoe (Table 1). There were few trees variability in applying dwarf mistletoe rating systems. with TBV ratings ≥3 and also few PCB ratings ≥30 (Table 1). Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021 We used PCB to test whether DMR, BVR, and TBV were In contrast, the number of trees rated BVR 1–3 was much able to distinguish trees that did not have witches’ brooms greater than for PCB ratings 10–30 or TBV ratings 1–3. For from trees that had brooms with different volumes. We used infected trees, DMRs were widely distributed from 0–6 (Table PCB because there was no established standard for estimating 1). No trees received TBV ratings >6, despite the fact that the broom volume in ponderosa pine, PCB was easy to understand, TBV rating system had a possible maximum score of 9. and it could be used to identify broom volume with relatively With the subsample of 100 trees, multiple linear fine resolution. However, PCB, as well as all dwarf mistletoe regression of the five independent variables (stand, DMR, ratings, was a subjective estimate. Therefore, on a subsample BVR, TBV, and PCB) on measured percentage of crown of 100 trees we tested the effectiveness of PCB by measuring occupied by witches’ brooms resulted in an r2 = 0.73, (F- broom volume and calculating the percentage of the live test P < 0.001) and indicated that only PCB and TBV were crown occupied by brooms. We randomly chose trees from significantly correlated to the dependent variable (effect plots in all 16 stands that had an average PCB of 5% or higher. test P < 0.01). Percentage of crown volume occupied by We measured the size of brooms in three dimensions (height, brooms (PCB) was positively related to measured depth away from the bole, and width perpendicular to depth). percentage of crown occupied by witches’ brooms (rs= For witches’ brooms close to the ground we used a meter stick 0.97, P < 0.001). Because PCB was highly correlated to to measure broom dimensions. We also used a fiberglass tape measured percentage of crown volume occupied by brooms, measure to measure crown width in two dimensions. For we considered the PCB method an effective measure of brooms too high in the crown to measure by hand, and to broom volume with which to determine whether the other measure tree height and height to live crown, we used a Haglöf three rating systems were able to distinguish trees that had Vertex III hypsometer. For each of the 100 trees, we used no brooms from trees that had different volumes of brooms. witches’ broom measurements to calculate approximate broom 3 Total broom volume (TBV) was also positively related to volumes in m . We further used average crown width, height measured percentage of crown occupied by witches’ of live crown, and the formula for calculating volume of a cone brooms in the subsample of 100 trees (r = 0.91, P < 0.001). to calculate approximate total crown volume. In order to calculate the measured percentage of crown volume occupied Table 1. Number of ponderosa pines by mistletoe by brooms, we divided broom volume by total crown volume rating classes for dwarf mistletoe rating (DMR) 1, and multiplied by 100. broom volume rating (BVR) 2, total broom volume We applied multiple linear regression to the subsample of (TBV) 3, and percent live crown occupied by brooms 100 trees to determine the relationship among five independent (PCB) 4. variables (stand, DMR, BVR, TBV, and PCB), and the Rating class DMR 1 BVR 2 TBV3 PCB4 dependent variable (measured percent of crown volume ...... (No. of trees)...... occupied by brooms). For significant independent variables 0 8353 8353 11,413 11,444 1 958 1185 723 609 we used Spearman’s rank correlation procedure to illustrate 2 590 1171 280 258 the direction and strength of association between individual 3 681 1565 78 111 independent variables and the dependent variable (Neter et al. 4 486 207 30 64 1996). For significant independent variables we also employed 5 874 50 10 21 Tukey-Kramer HSD multiple comparison tests in order to test 6 594 5 2 16 7——04 differences in group means between trees without brooms 8——03 (PCB = 0), trees with 1–14% of live crown occupied by 9——04 brooms (PCB = 10), and trees with 25–34% of live crown 10 — — — 2 occupied by brooms (PCB = 30) (Zar 1999). These three PCB 1 Hawksworth (1977), ratings range from 0–6. 2 ratings were selected for comparisons because we wanted to Tinnin (1998), ratings range from 0–6. 3 TBV ratings range from 0–9. test the ability of rating systems to distinguish trees with small 4 Rating classes should be multiplied by 10 to obtain the PCB broom volumes (<15%) from trees without brooms, and to be rating classes used in the text, ratings range from 0–100.

56 WJAF 19(1) 2004 A Tukey-Kramer HSD multiple comparison test (alpha = brooms (PCB = 0), trees with broom volumes of 1–14% 0.01) indicated that TBVs were significantly different (PCB = 10), or trees with broom volumes of 25–34% (PCB between trees without brooms (PCB = 0), trees with broom = 30) (Figure 1). Because PCB was the rating system most volumes of 1–14% (PCB = 10), and trees with broom highly correlated to measured percentage of crown volume volumes of 25–34% (PCB = 30) (Figure 1). occupied by brooms, we considered the PCB method an We then removed TBV and PCB from the multiple effective measure of broom volume with which to gauge linear regression and ran the regression again to determine the effectiveness of the other three rating systems using separately the effect of DMR, BVR, and stand on the the total sample of infected trees. dependent variable. This multiple linear regression resulted A three-way ANOVA with factors DMR, BVR, and in an r2 = 0.57 (F-test P < 0.001), and indicated that DMR TBV on the entire sample of 4,183 infected trees was and BVR were significantly correlated to the dependent significant (P < 0.001) and effect tests on all three factors variable (effect test P < 0.01) when TBV and PCB were not were significant (P < 0.001). Tukey-Kramer HSD multiple part of the equation. The effect test indicated that the stand comparison tests indicated that group means were different variable was not correlated to the dependent variable (P = between trees without brooms (PCB = 0), trees with 1– 0.27). Broom volume rating was positively related to 14% of crown volume occupied by brooms (PCB = 10), Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021 measured percentage of crown occupied by witches’ and trees with 25–34% of crown volume occupied by brooms (rs= 0.61, P < 0.001). However, a Tukey-Kramer brooms (PCB = 30) for all three factors (DMR, BVR, and HSD multiple comparison test, (alpha = 0.05) indicated TBV) (P < 0.01) (Figure 2). Mean DMRs for trees without that BVRs were not significantly different between trees brooms (PCB = 0), trees with 1–14% of live crown occupied without brooms (PCB = 0) and trees with broom volumes by brooms (PCB = 10), and trees with 25–34% of live of 1–14% (PCB = 10), although there was a significant crown occupied by brooms (PCB = 30) were 3.12, 3.79, difference between trees with broom volumes of 25–34% and 4.40 respectively (Figure 2). Confidence intervals (PCB = 30) and trees with broom volumes of 1–14% (PCB were wider for trees with 25–34% of live crown occupied = 10) (Figure 1). Dwarf mistletoe rating was weakly by brooms (PCB = 30) than for trees with 1–14% of live positively correlated to measured percentage of crown crown occupied by brooms (PCB = 10) and trees without occupied by witches’ brooms in the subsample of 100 trees brooms (PCB = 0) using all three rating systems (Figure (r = 0.15, P = 0.11). However, a Tukey-Kramer HSD 2). Total broom volume rating showed the narrowest multiple comparison test (alpha = 0.05) indicated that confidence intervals for all three extents of broom volume TBVs were not significantly different between trees without (Figure 2).

Figure 1. Subsample, N = 100 trees. Means and 99% confidence Figure 2. Full sample, N = 4,183 trees. Means and 99% confidence intervals for the six-class dwarf mistletoe rating system (DMR, intervals for the six-class dwarf mistletoe rating system (DMR, Hawksworth 1977), the broom volume rating system (BVR, Hawksworth 1977), the broom volume rating system (BVR, Tinnin 1998), and the total broom volume rating system (TBV) in Tinnin 1998), and the total broom volume rating system (TBV) in northern Arizona. PCB 0 = trees without brooms; PCB 10 = trees northern Arizona. PCB 0 = trees without brooms; PCB 10 = trees with 1–14% of live crown occupied by witches’ brooms; PCB 30 with 1–14% of live crown occupied by witches’ brooms; PCB 30 = trees with 25–34% of live crown occupied by witches’ brooms. = trees with 25–34% of live crown occupied by witches’ brooms.

WJAF 19(1) 2004 57 Discussion the mean DMR or BVR of a stand, the more likely that witches’ brooms were present. In contrast, mean TBV varied Statistical analyses were conducted at two different scales by less than 0.01 of a TBV rating point between the subsample in this study: the subsample of 100 trees in which percentage and the full sample: mean TBV of trees without brooms was of crown occupied by brooms was measured, and the full 0.0 and 0.01, respectively between the subsample and full sample of 4,183 infected trees in which PCB was used as the sample; mean subsample and full sample TBV of trees with 1– “gold standard” to evaluate the extent of crown volume 14% of crown volume occupied by brooms was 1.21 and 1.11, occupied by brooms. With the subsample of 100 trees, it was respectively; mean subsample and full sample TBV of trees possible to distinguish trees without brooms from trees with with 25–34% of crown volume occupied by brooms was 2.00 small amounts of broom volume, and trees with small amounts and 2.08, respectively). With either sample size it was easier of broom volume from trees with moderate amounts of broom to distinguish the amount of broom volume using TBV than volume, using only TBV and PCB. However, with the full using DMR or BVR. sample of 4,183 infected trees, DMR, BVR, and TBV were all Despite its name, Tinnin (1998) intended BVR to provide able to distinguish between the three amounts of broom an infection rating, not a broom volume rating. Although BVR volume. This difference in results is partially due to higher was intended to result in ratings similar to DMR for Douglas- Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021 statistical power in the sample of 4,183 infected trees than in fir (Tinnin 1998), it did not provide ratings similar to DMR for the subsample of 100 trees. In the full sample of 4,183 infected ponderosa pine. For instance, only 55 of 4,183 infected trees trees, 99% confidence intervals for mean TBV ratings for (less than 2%) were rated a BVR of 5 or 6, but 1,468 of those trees with 25–34% of live crown occupied by brooms (PCB trees (35%) received a DMR of 5 or 6. This is most likely 30) shrank from ± 0.17 in the subsample of 100 trees to ± 0.05 because dwarf mistletoe infection in ponderosa pine, unlike in the full sample. With the higher statistical power of the Douglas-fir, often does not result in witches’ broom formation larger sample, mean DMR = 3.12 for trees without brooms (Hawksworth 1961). Severely infected crown thirds that do (PCB = 0) was significantly different from mean DMR = 3.79 not have brooms can rate only a maximum BVR of 1, but rate for trees with broom volumes of 1–14% of crown volume. a DMR of 2. The effect is that DMR can attain a value of 6 Mean BVR = 2.01 for trees without brooms (PCB = 0) was where there are many infections but no brooms, whereas BVR significantly different from mean BVR = 2.34 for trees with can only attain a value of 3. Therefore, in tree species in which broom volumes of 1–14% of crown volume. Being able to dwarf mistletoe infection typically does not result in witches’ distinguish between trees with or without brooms is biologically broom formation, DMR should be used as the primary method important. When dealing with samples of thousands of of rating infection. ponderosa pines at landscape scales, (e.g., watersheds, regions) In its current form, BVR can best be used as an evaluation both DMR and BVR can make this distinction statistically. of broom volume in ponderosa pine with very large sample However, in the more usual case of a land manager evaluating sizes, or as a measure of infection in Douglas-fir. Broom the risk of crown fire or the amount of nesting habitat in a volume rating could, however, be quite easily adapted to more single stand with a sample of a few hundred trees, DMR and accurately assess broom volume in all tree species. The BVR cannot statistically make this important biological current BVR rating system stipulates that if a crown third is distinction. Total broom volume is the only rating that we one half or less occupied by brooms but has at least one dwarf evaluated that could achieve this at the smaller sample size. mistletoe infection, it receives a 1. The restriction of relying Mean DMR estimates varied between the subsample of on infections rather than brooms when no brooms are present, 100 trees and the full sample of 4,183 infected trees. Trees results in ratings more similar to DMR in Douglas-fir. More without brooms in the subsample had a mean DMR = 3.87, but accurate broom volume ratings would result by stipulating in the full sample, trees without brooms had a mean DMR = that crown thirds would receive a rating of 1 only if they had 3.12. Trees with 1–14% of crown volume occupied by brooms at least one broom, and brooms occupied one half or less of the had a mean DMR = 4.37 in the subsample, and 3.79 in the full crown third volume. Such a modified BVR would not have sample. Broom volume ratings performed better but also TBVs advantages of identifying brooms close to the ground varied between samples (e.g., trees with 1–14% of crown by dividing the entire height of the crown into thirds, or of volume occupied by brooms had a mean BVR = 2.70 in the providing a rating that ranges from 0 to 9 to more precisely subsample, and 2.34 in the full sample). Variability between evaluate witches’ broom volume. However, such a method samples was partially an artifact of the sampling design of the would be more comparable to DMR and therefore easier for subsample as compared to the sampling design of the full rating crews to use. Such a new broom volume rating system sample. The full sample consisted of all infected trees whether would need to be distinguished by name from its predecessor, or not they had witches’ brooms. Trees of the subsample were which continues to be used in Douglas-fir. We suggest selected from plots that had PCB > 5, and therefore tended to “modified broom volume rating” (M-BVR). be more severely infected than trees from the full sample. Although the DMR system does not use witches’ broom Mean DMR and BVR of trees varied depending on how volume for rating trees, in our study more severely infected severely infested the area was from which the sample was ponderosa pine trees did tend to have greater amounts of taken. Our results indicate that it is difficult to state that a stand witches’ broom formation. Hawksworth (1961) reported has an absolute amount of witches’ broom volume based on its that the proportion of trees with brooms increased markedly mean DMR or BVR. It is only possible to state that the higher as the DMR class increased; however, he also reported that

58 WJAF 19(1) 2004 many severely infected ponderosa pines in northern Arizona reasons: (1) TBV was more highly correlated to measured did not form brooms. Most of the trees in our study (66%) percentage of crown occupied by witches’ brooms than either with severe dwarf mistletoe infection (DMR 5 or 6) did not DMR or BVR; (2) It was possible at both sample sizes to form witches’ brooms. Hawksworth (1961) further distinguish trees with different amounts of broom volume suggested that pines that become infected throughout their with TBV; (3) TBV estimates did not vary based on sample crowns in a relatively short time do not form witches’ size as did DMR and BVR; and (4) TBV provides information brooms. Our research suggests a weak negative relationship on the vertical distribution of brooms within the total height between basal area and witches’ broom volume (rs = -0.43, of the tree. This is useful for predicting fire behavior and P = 0.07). If a century of fire suppression has increased evaluating wildlife habitat. We recommend the use of the tree densities in ponderosa pine forest, witches’ broom TBV rating system to quantify witches’ broom volume in volume in these forests may be lower than it was before ponderosa pine in order to evaluate both dwarf mistletoe fire suppression. The causes of witches’ broom formation effects on fire behavior and to assess wildlife habitat. in ponderosa pine, however, require further research. We recommend that, rather than a raw estimate of percent Literature Cited Downloaded from https://academic.oup.com/wjaf/article/19/1/54/4717678 by guest on 30 September 2021 of crown occupied by witches’ brooms such as PCB, a rating ALEXANDER, M.E., AND F.G. HAWKSWORTH. 1975. Wildland fires and dwarf such as TBV or M-BVR be used to evaluate witches’ broom mistletoes: A literature review of ecology and prescribed burning. volume in ponderosa pine. The PCB method does not allow USDA For. Serv. Gen. Tech. Rep. RM-14. 12 p. BLOCK, W.M., K.A WITH, AND M.L. MORRISON. 1987. On measuring bird for data collection by crown thirds; therefore, the location of habitat: Influence of observer variability and sample size. Condor brooms within the tree is not indicated. Breaking the crown 72:182–189. into thirds also allows the pathologist to consider a smaller BROWN, J.K. 1975. Fire cycles and community dynamics in lodgepole pine forests. P. 430–456 in Proc. of symp. on Management of lodgepole pine area when estimating witches’ broom volume. In addition, the ecosystems, Baumgartner, D.M. (ed.). Washington State Univ. Coop. rating systems have the advantage of assigning ratings to a Ext. Serv., Pullman. range of mistletoe infection (e.g., 1–33% or 1–49% = 1). This CONKLIN, D.A., AND W.A. ARMSTRONG. 2001. Effects of three prescribed fires on dwarf mistletoe infection in southwestern ponderosa pine. USDA makes evaluating broom volume quicker than if a percentage For. Serv. SW Region, For. and For. Health Rep. R3-01-02. 17 p. was selected for each third and averaged to derive PCB. GARNETT, G.N. 2002. Wildlife use of witches’ brooms induced by dwarf Furthermore, forest inventory crews are often familiar with mistletoe in ponderosa pine forests of northern Arizona. M.Sc. thesis, N. Ariz. Univ., Flagstaff. 75 p. the six-class dwarf mistletoe rating system that requires visually GEILS, B.W., AND R.L. MATHIASEN. 1990. Intensification of dwarf mistletoe dividing a tree into thirds and then rating each third separately on southwestern Douglas-fir. For. Sci. 36:955–969. (Hawksworth 1977). HARRINGTON, M.G., AND F.G. HAWKSWORTH. 1990. Interactions of fire and dwarf mistletoe on mortality of southwestern ponderosa pine. P. 234– We sampled over 4,000 dwarf mistletoe-infected ponderosa 240 in Proc. of symp. on Effects of fire management of southwestern pines but did not rate a tree with a TBV > 6. This is partly natural resources, Krammes J.S. (tech. coord.). USDA For. Serv. Gen. because the live crown of ponderosa pine rarely extends to the Tech. Rep. RM-191. HAWKSWORTH, F.G. 1961. Dwarf mistletoe of ponderosa pine in the Southwest. ground. The TBV system divides the total height of the tree USDA For. Serv. Tech. Bull. 1246. 112 p. into thirds, so the bottom third rarely receives a rating of >1. HAWKSWORTH, F.G. 1977. The 6-class dwarf mistletoe rating system. USDA But because mistletoe-infected ponderosa pines frequently For. Serv. Res. Note RM-48. 7 p. HAWKSWORTH, F.G., AND A.A. LUSHER. 1956. Dwarf mistletoe survey and have brooms near the ground, the rating for the bottom crown control on the Mescalero Apache Reservation, New Mexico. J. For. third is an important component of the rating. In addition, in 54:384–390. severely infected trees the top third may often be dead and thus HAWKSWORTH, F.G., AND D. WIENS. 1996. Dwarf mistletoes: Biology, pathology, and systematics. USDA For. Serv. Agric. Handb. 709. receive a rating of one, even when several brooms are present Wash., DC. 410 p. in that third. Trees with a TBV > 6 do, however, occur based HEDWALL, S.J. 2000. Bird and mammal use of dwarf mistletoe-induced on our field observations in northern Arizona. Total broom witches’ brooms in Douglas-fir in the Southwest. M.Sc. thesis, N. Ariz. Univ., Flagstaff. 66 p. volume ratings would more regularly attain the maximum of KOONCE, A.L., AND L.F. ROTH. 1985. The effects of dwarf mistletoe on fuel 9 if TBV were applied to trees species that more typically form in precommercial ponderosa pine stands. P. 66–72 in Weather—The brooms. drive train connecting the solar engine to forest ecosystems. Proc. of 8th conf. on Fire and forest meteorology, Donoghue L.R., and R.E. Martin We considered only live brooms when assigning PCB (eds.). Soc. of Am. For., Bethesda, MD. ratings, but considered both live and dead brooms for TBV MATHIASEN, R.L. 1996. Dwarf mistletoes in forest canopies. Northwest Sci. ratings. As a result, 25 trees with only dead brooms rated a 70:61–70. NETER, J., M.H. KUTNER, C.J. NACHTSHEIM, AND W. WASSERMAN. 1996. TBV of 1 or 2 but received a PCB of 0. A TBV system Applied linear statistical models. Ed. 4. McGraw-Hill, Boston, MA. considering only live brooms could be used if land managers 1408 p. want to assess broom volume in ponderosa pine for wildlife PARKER, T.J. 2001. Bird communities in dwarf mistletoe-infested ponderosa pine forests. M.Sc. thesis, N. Ariz. Univ., Flagstaff. 90 p. rather than for fire management. When dead brooms were PARKS, C.G., E.L. BULL, R.O. TINNIN, J.F. SHEPHERD, AND A.K. BLUMTON. included in the rating, as was done in our study, TBV was a 1999. Wildlife use of dwarf mistletoe brooms in Douglas-fir in northeast good indicator of the amount of broom volume that could Oregon. West. J. Appl. For. 14:100–105. TINNIN, R.O. 1998. An alternative to the 6-class dwarf mistletoe rating influence fire behavior in dwarf mistletoe-infested ponderosa system. West. J. Appl. For. 13:64–65. pine stands. TINNIN, R.O., AND D.M. KNUTSON. 1980. Growth characteristics of the In conclusion, the TBV rating system was the most accurate brooms on Douglas-fir caused by Arceuthobium douglasii. For. Sci. 26(1):149–158. and precise method to quantify witches’ broom volume in ZAR, J.H. 1998. Biostatistical analysis. Ed. 4. Prentice Hall, Upper Saddle dwarf mistletoe-infected ponderosa pines for the following River, NJ. 929 p.

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