Great Basin Naturalist
Volume 59 Number 1 Article 7
1-22-1999
Factors associated with wetwood intensity of Populus fremontii (Fremont cottonwood) in Arizona
T. S. Hofstra Arizona State University, Tempe
J. C. Stromberg Arizona State University, Tempe
J. C. Stutz Arizona State University, Tempe
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Recommended Citation Hofstra, T. S.; Stromberg, J. C.; and Stutz, J. C. (1999) "Factors associated with wetwood intensity of Populus fremontii (Fremont cottonwood) in Arizona," Great Basin Naturalist: Vol. 59 : No. 1 , Article 7. Available at: https://scholarsarchive.byu.edu/gbn/vol59/iss1/7
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FACTORS ASSOCL>\TED WITH WETWOOD INTENSITY OF POPULUS FREMONTII (FREMONT COTTONWOOD) IN ARIZONA
1:S. Hufstra', I.C. Strombergl", and j.C. Stur,'
AIISTRAcr.-Wetwood is a condition of Populus fremont" and other tree species characterir.ed by staining and W'dtcr soaking in the heartwood, bleeding from wounds and stem junctions, and leaf and br-mch diehack. A field sUlvey imli. cated that welwood symptoms were present in populations of POlJulus fremontii at all!7 riparian sites .mrveyed in Ari· zona. However, incidence and severity ofblecding symptom~ varied within Key worlL.·: wetw()Q(~ POpUllL.. [remontii. di"ease severily, riparian lutbital. btlcleria, canopy decline. \i\letwood occurs in the heartwood of Popu.~ involved in the production of external symp Ius jremontii Wats (Fremont cottonwood) and toms associated with internal wetwood symp other tree species and is characterized by dark toms (Rasmussen-Dykes and Jacobi 1995) brown staining and infusion of water (Ward Regardless of causative factors, biotic and and Pong 1980, Murdoch and Campana 1981). abiotic environmental conditions are known to High internal pressures develop in trees with influence the development ofwetwood in tree internal wetwood symptoms resulting in the species. Wetwood symptoms have been shown bleeding or "fluxing" ofliquid from wounds and to increase with host age, but symptoms do stem junctions. ""hen contaminated with bac occur in ve,y young trees (Hartley et aI. 1961, teria, yeasts, and other fungi, the liquid forms a Ward and Pong 1980), In some cases moist or fetid, foaming mass known as slime flux. Profuse swampy sites are associated with high inci bleeding can cause liquid to flow down bark, dences of wetwo IDc(llU"l.roenl of t'1~llt Biology, Arinma St-.,tc l:"ni~ty. Temp" lo7.85287. 2Author to WhOllll''''leiiX'IlUe"ce should he mousse 85 86 GREAT BASIN NATURALIST [Volume 59 symptoms and canopy dieback. Bleeding was (PMC) present was estimated for each tree, tak observed from all woody plant parts including ing into consideration the presence of broken, roots and the entire length of stems. Sources missing, or otherwise damaged limbs and ofbleeding included branch junctions, ends of trunks, branch dieback, and wilting. Maximum broken limbs, insect bore holes, frost ribs, and canopy was defined as the maximum canopy other wounds of undetermined nature. Exami development that would occur under ideal nation of cores from trunks of trees with these growing conditions. external symptoms revealed darkly discolored, Host and Environmental water-soaked wood. The objectives of this in Conditions vestigation were to ascertain the extent ofwet wood symptoms in P. fremontii populations in We measured each P ji'ernontii tree for Arizona and to determine some host and envi trunk diameter at 1.5 m above ground level ronmental factors associated with high inci (dbh), distance (m) from the trunk to the edge dences and severity ofwen:vood symptoms. of the nearest channel, and distance to the nearest adjacent P. frenwntii. Because willows MATERIALS AND METHODS (Salix spp.) were observed also to have wet wood symptoms, we recorded distance to the Wetwood Symptoms nearest willow tree. Elevation (m) above the We quantified severity and incidence of channel thalweg was estimated for each tree. wetwood symptoms and extent of canopy Height of flooding debris was recorded as an dieback for E fremontii at 17 riparian sites in indication of magnitude of the most recent Arizona (Fig. 1). Sites were chosen to encom flood (statewide flooding occurred in winter pass a range of environmental conditions in 1995). Each tree was inspected for the pres which P. fremontii occur. At sites with narrow ence of wounds, mistletoe, tent caterpillars, riparian corridors, we randomly selected 50 P. and bark beetle damage. frernontii trees. At sites with wide flood plains, For each site we visually characterized tree the 50 trees were selected in stratified random species composition as P. frenwntii dominated, fashion by dividing the flood plain into lateral P frernontii-Salix dominated, or "mixed" (veg strips of varying distances from the active etated by a mixture of P frernontii, Salix sp., channel. This insured the sampling of trees in Juniperus sp., Quercus sp., Platanus wrightii, the range ofsize classes present. Fraxinus sp., and/or Alnus sp.). Surface flow We used a 5-point scale (Table 1) to quantifY frequency was characterized as either ephem severity of wetwood symptoms for each tree. eral or perennial, based on literature review, Severity was assessed based on quantity and conversations with site managers, or review of size ofbleeding sources (including branch junc USGS stream gage data. Channel morphology tions, branch stubs, cracks, and other wounds) was described as either multichanneled (i.e., per tree. Bleeding sources were divided into braided) or confined to a well-defined single minor and major sources based on size, flow channel; this variable was designated as chan rate, and presence of bark incrustation. The nel stability. Predominant substrate particle size severity scale had greatest sensitivity at the was visually classified as silt, sand, gravel, or low end of the scale and lost sensitivity at the cobbles. Information on site elevation (as a sur top end in that all trees receiving a rank of 5 rogate for site temperature) and stream gradi did not have equivalent amounts of wetwood ent was obtained from topographical maps. symptoms. We defined incidence in a binary Statistical Analysis fashion based on presence or absence of wet wood symptoms. All trees scoring a rank of 1 '''Tithin sites, Pearson correlation analysis on the severity scale were considered to be was used to examine the relationship of wet free ofwetwood symptoms. Incidence was cal wood severity with tree size, distance to the culated as the percentage of trees with symp stream channel and above the thalweg, and toms relative to the total sample oftrees. These distance to nearest neighbors. Correlation methods may underestimate incidence because analysis also was used to assess the relation not all individuals that show internal wetwood ship between individual severity of wetwood symptoms also express bleeding symptoms bleeding symptoms and dbh, using data pooled (Toole 1968). The percent ofmaximum canopy across sites. 1999J WETWOOD IN POPULUS FREMONTII 87 RESULTS We observed wetwood hleeding symptoms Upper Verde River on E frenwntii at all 17 sampling sites. Inci dence and mean severity ofwetwood bleeding symptoms varied substantially within and among sites, Within sites, incidence and sever HlSsayampa E meral ity varied strongly with tree size, Incidence and mean severity increased from the smallest Hauayampa Pur8Miai size class to the largest size class present (Table 2, Fig. 2), and individual severity of wetwood bleeding symptoms was significantly 200 20 correlated with individual dbh (P < 0.01, r = c:;;; = • 0.55, n 850, data pooled across sites). Dis A tance to channel and elevation above channel for each tree were also significantly correlated with severity at several sites; however, dis tance and elevation relative to channel were also significantly correlated with host dbh at P San Pedro Ephemeral < 0.05. For size class 1, incidence ranged among sites from 14% at Wet Beaver Creek to 80% at Empire Wash, and mean severity ranged from Cienega. Pe nnlal 1.1 at Wet Beaver Creek to 2.6 at the Santa Empire Wash I Cruz Ephemeral site (Table 2). For size class g ""'l a, Ephe ral 2, values for incidence ranged among sites Santa Cw Perennial from 43% to 87%, and mean severity ranged Sooolla Cleek Low Garden CMycn Upper irde anyon from 1.9 to 4.5. Popu/w; fremontii spacing, stand Santa ruz Ephemeral San Pe Perennial composition, substrate texture, and channel stahility were all significantly correlated (at P FIg. L Locations of the 17 study sites found in riparian < 0.05) with incidence or severity of size class areas in Arizona where wetwood incidence and severity 1 or 2 (Fig. 3, Table 3). Stream flow frequency were assessed. and bark beetle incidence were, respectively, negatively and positively correlated with wet wood incidence (at P < 0.10; Table 3). However, To facilitate among-site analysis, we assigned substrate texture and density-related variables trees to 1 of 6 size (dbh) classes (1-32 em, (mean E frenwntii spacing, stand composition) 33-64 em, 65-96 em, 97-128 em, 129-160 em, were the only variables included in the multi 161-192 em). Incidence and mean severity ple regression models (Table 4). Substrate tex were compiled by size class for each site. ture was strongly correlated with several other Relationships between incidence and mean site variables, including channel stability (P < severity values for size classes 1 and 2 (the 0.01, r = 0.73), stream gradient (P < 0.01, r = only size classes present at all 17 sites) and 0.56), stand composition (P < 0.05, r = 0.50), mean site values for host and environmental and site elevation (P < 0.08, r = 0.44). Popu conditions (using a mix of cardinal and ordinal lu.s fremontii spacing was significantly corre variables) were analyzed with Pearson correla lated among sites only with mean E fremontii tion analysis and forward stepwise multiple dbh (P < 0.01, r = 0.65; i.e., correlation be regression analysis (SPSS). Correlation analy tween larger trees and lower density). sis also was used to examine interrelationships Site factors not significantly correlated with of site variables. Average site value for percent incidence and severity included site elevation, maximum canopy (PMC) was correlated with height of flooding, stream gradient, mean dis site averages for incidence and severity of tance to nearest willow, physical wounding wetwood symptoms and other site and host (primarily flood damage), and incidences of variables, mistletoe and tent caterpillars, The incidences 88 GREAT BASIN NATURALIST [Volume 59 TABLE L Severity scale and definitions used to quantify severity of wetwood symptoms have been re bleeding symptoms ofwetwood. ported hy others as well. As P. fremontii in Bleeding symptoms observed Severity class creased in size (and age; Hinchman and Birke No bleeding sources 1 land 1995), severity and incidence ofwetwood symptoms increased. Toole (1968) and Murdoch One minor bleeding source 2 and Campana (1981) similarly found that wet wood intensity increased with tree age in east Two minor bleeding sources, ern cottonwood and elm, although Etheridge or one major bleeding source 3 and Morin (1962) reported wetwood to he Three to four minor bleeding sources, more prevalent in younger balsam fir. Many or two to three major bleeding sources 4 foresters agree that wetwood is associated with older trees (Ward and Pong 1980), hut they More than four minor bleeding sources, also report that Populus is susceptihle to wet or more than three major bleeding sources 5 wood at all ages. As substrate particle size decreased, inciw dence and severity of wetwood symptoms in of mistletoe, tent caterpillar, and bark beetle P. fremontii increased. This relationship may damage were highly variable hetween sites. be a consequence of increased water-holding These organisms were not detected in trees at capacity and decreased aeration that occur at most sampling sites, but they occurred at finer soil textures and is thus consistent with higher incidences (>30%) at a few sites. For observations of high incidences of wetwood example, hark heetle infestation was detected for some tree species at moist or swampy sites at these levels at only 2 sites (Santa Cruz (Ward and Pong 1980). However, wetwood Ephemeral and Perennial). expression increased as surface flow frequency Mean site value for the percentage ofmaxi decreased, as evidenced by negative correla mum canopy (PM C) was significantly corre tions between surface flow frequency and lated (P < 0.01, r = -0.62, n = 17) with mean severity of bleeding for class 1 trees. More in site severity. As severity of bleeding symptoms vestigation is needed regarding wetwood symp increased, so did canopy decline. Natural prun tom expression in Fremont cottonwood in rela ing is common, especially in the lower canopy tion to soil moisture. of dense stands; however, there was no signifi Similar to other studies (Hartley et al. 1961, cant correlation between PMC and stand den Bauch et al. 1975), we found some correlation sity (P > 0.10, r = 0.23). Percent optimal between wounding (i.e., bark heetle damage) canopy decreased significantly with increasing and the occurrence of wetwood symptoms. site elevation, but not with other measured Lack ofsignificant relationships for other types factors. ofwounding may have resulted from the highly DISCUSSION variable nature of wounds between sites, as well as from an inability to detect and accu We found wetwood symptoms to be wide rately quantifY wounding. For example, flood spread in P. fremontii populations at riparian related wounds located near the root crown ecosystems in Arizona. It is likely that values could be buried at some sites under a meter or for wetwood incidence were greater than this more of sediment (Stromherg et al. 1991). investigation reports because incidence calcu Relationship hetween density-related vari lations were based on external symptoms only. ables (mean tree spacing, stand composition) Toole (1968) reported that liquid flowed from and wetwood incidence and severity has re core wounds in only 30% of affected cotton ceived little research attention. High stand wood trees. In addition, Murdoch and Cam densities may affect wetwood incidence and pana (1981) report that elm trees in the west severity by increasing competition for nutri ern U.S. show more variability in internal dis ents and water and affecting growth rates of ease development and significantly less hleed individual cottonwood trees. Density also ing and symptom expression than those in the affects the microenvironment of a stand, lead eastern U.S. ing to changes in light intensity, temperatures, Certain host and site characteristics found and relative humidity, all of which may affect to be associated with higher incidences and wetwood incidence and severity. An intriguing 1999] WETWOOD IN POPULUS FREMONTll 89 TABLE 2. lnddenve and mean severity ofwetwooO symptoms on Populus .tremontii by size (dbh) dass for 17 riparian sites in 4 Arizona watersheds. Incidence (%) Severity Site 1" 2 3 4 5 6 1 2 3 4 5 6 VERDE RIVER BASIN Wet Beaver J4 43 50 1.2 1.9 1.7 Lower Sycamore 26 53 JOO J.4 1.9 3.0 Upper Sycamore 29 69 1.4 1.9 I..ower Verde 64 86 1.9 2.6 Upper Verde 48 94 60 100 1.9 3.3 3.2 5.0 MWDLF. GUll.. RIVER BASIN Hassayampa Perennial 35 96 100 1.5 3.2 3.0 Hassayampa Ephemeral 77 100 2.3 4.0 SANTA CI\UZ RIVEII BASIN Santa Cruz Perennial 35 75 1.5 2.8 Sonoita Creek 30 JOO JOO JOO 1.4 3.5 4.2 5.0 Cienega Perennial 76 JOO 100 2.2 2.8 2.0 Empire Wash 80 80 100 100 2.5 2.8 5.0 5.0 Santa Cruz Ephemeral 76 100 100 100 JOO 2.6 3.7 5.0 5.0 5.0 Cienega Ephemeral 55 100 100 100 100 100 2.J 4.5 4.5 5.0 5.0 5.0 SAN PEDRO RIVER BASIN Upper Garden Canyon 41 83 75 1.5 2.3 2.3 Lower Garden Canyon 33 100 100 1.4 2.7 3.0 San Pedro Perennial 66 100 2.1 3.3 San Pedro Ephemeral 83 100 2.2 2.5 "Si7£ class 1 - 1-32 em dbh, class 2 _ 33--61 em, class 3 _ 65--00 em, class 4 _ 97-128 em, e1R"' 5 - 129-160 em, das., 6 - ]61-192 ern question is whether the very high incidence of Presently, conditions along some rivers in this wetwood at some study sites reflects unnatu study contrast with historical descriptions. For rally high stand densities for E fremontii. example, historical descriptions of the San Irruptions of tree populations-and associated Pedro refer to a mosaic of forest and marsh irruptions of disease or insect outbreaks-can land rather than to corridors of high-density be an indicator ofreduced ecosystem integrity forest stands. Changes in tree density may he (Costanza et a1. 1992). Such irruptions can a response to local extirpation of beaver, an result from human management practices that agent of geomorphic and biological change diverge from evolutionary histOly of a popula that historically served to create a more tion or ecosystem (Covington and Moore 1994), patchy, heterogeneous flood plain community. 100 ----- 5 o Severity e Incidence 100 90 • • • • 80 ~ 80 ------.. ~ *-8 • 0 60 8 70 ------.:~ 0 ~• 60 ~• 0 '~-...... ~-...... ,~ 40 ~ • . 0 0 50 • 1 . 20 40 • J30 o , 0 1 (0-32) 3 (65-96) 5 (129-160) 201~-~~~~~-,--r-,~~---j 2 (33-64) 4 (97-128) 6 (161-192) o 1 2 3 4 5 6 7 8 9 10 11 12 Size class Mean P. fremontii spacing (m) Fig. 2. Mean severity and incidence of wetwood bleed Fig. 3. Mean site incidence of wetwood symptoms in ing symptoms observed for 6 stern diameter (em) si7,e Populus fremontii (values averaged for size class 1 and 2 classes of Populus fremontii for an sites sampled in trees, dbh 1-64 ern) in relation to mean host spacing (i.e., Ari:wna. density). 90 GREAT BASIN NATURALIST [Volume 59 TABLE 3. Correlation coefficients between site variables and wetwood incidence and severity values for 2 size classes ofPopulus fremontii (see Table 2). Incidence Incidence Severity Severity class 1 class 2 class 1 class 2 Mean spacing of P. frem TABU~ 4. Multiple regression coefficients between site variables and wetwood incidence and severity values for 2 size dusses ofPopulus fremontii (see Table 2). Dependent variable Independent variables Incidence) class I Substrate te."ture 0.42 Severity, cla~'s 1 Substrate texture 0.44 Inciden(.:e, class 2 P. fremontii spacing 0.48 P. fremontii spacing + stand composition 0.513 Severity, class 2 Stand composition 0.30 • Given the quantitative relationship expressed CO&TANZA, R.B., B.G. NORroN, AND B. HAKELL, EDITORS. between wetwood symptom bleeding severity 1992. Ecological integrity: protecting earth's life sup~ port systems. Island Press, Washington, DC. 269 pp. and canopy decline, wetwood is ecologically CoVrNGTON, WW, AND M.M. MOORE. 1994. Southwestern significant for P jremontii riparian communi ponderosa forest structure. Journal of Forestry 92: ties. Such a relationship has not been previ 39-47. ously reported for any host species and has ETHERJDCE, O.E.. AND L.A. MORIN. 1962. Wetwood for potential consequences (positive and negative) mation in balsam Hr. Canadian Journal of Botany for iosect and bird species that variously de 40,1335-1345. JiAJm.EY, C., RW DAVIDSON, AND 8.S. CRANDALJ.... 1961. pend on live or dead cottonwood caoopy for Wetwood, bacteria, and increased pH in trees. USDA habitat. Canopy decline may ultimately lead to Forest Service, Forest Products Laboratory Report lIee death. FUrther slndy ofwetwood incidence 2215. and severity in reJatioo to biotic iotegrity of P. HINCHMAN. Y.H., AND K.w. BIRKEUND. 1995. Age predic fremontli communities is warranted. tion based OD stem size for riparian cottonwood stands. Southwestern Naturalist 40:406--409. HORNE, C.W, EDITOR. 1983. Texas plant diseases band ACKNOWLEDGMENTS book. Texas Agricultural Extension Service Publica tion B-ll40. We thank Jana Fry for producing maps and KNUTSON, D.M. 1973. The bacteria in sapwood and heart assisting in the field, and Dr. Duncan Patten wood of trembling aspen (Popul1lS tremuloides). and anonymous individuals for reviewing the Canadian Journal of Botany 51:498---500. MURDOCH, C.W, AND R]. CAMPANA. 198L Bacterial wet manuscript. wood. Pages 31--34 in R.J. Stipes and RJ. Campana, editor, Compendium ofelm diseases. LITERATURE CITED _-,' 1983. Bacterial species associated with wetwood ofelm. Phytopathology n1270-1273. BAUCH, J., W. HOLL, AND D.H. ENDEWARD. 1975. Some RASMUSSEN-DYKES, C., AND WR. JACOBI. 1995. Bacterial aspects of wetwood formation in fir. Holzforschung wetwood. Colorado State University Cooperative 29,198-205. Extensioo Publication 2.910. G..-t"'fER, C.J. 1945. Wetwood of elms. IJlinois NaturaJ His SCKINK, 8., lC. WARD, AND J.G. ZEIKUS.1981. Microbiology tory Survey BuHetin 23:407-448. of wetwood: role of anaerobic bacterial populations 1999] WETWOOD IN POPULUS FREMONTII 91 in living trees. Journal of General Microbiology 'WILCOX, W,W., w.y PONG, AKD J.R. PAR\lETER. 1973. 123,313-322. Effects of mistletoe and other defects on lumber SHIGO, A.L., J. STANKEWICH, AND B.J. COSENZA. 1971. Clo quality in white fir. Wood and Fiber 4:272-277. stridium sp. associated with discolored tissues in liv ZEIKUS, ].G., AND D.L. HE:'-INING. 1975. Methanobacterium ing oaks. Phytopathology 61:122-123. arbophilicum sp. nov. An obligate anaerobe isolated STROMBERG, le., D.T. PATTE~, AND B.D. RICHTER. 1991. from wetwood ofliving trees. Antonie van Leeuwen Flood flows and dynamics ofSOHaran riparian forests. hoek 410543-552. Rivers 2:221-235. ZEIKUS, ].G., AND ].e. '''lARD. 1974. Methane formation in TEIDEMANN, G., J. BAUCH, AND E. BOCK. 1977. Occur living trees: a microbial origin. Science 184:1811-1883. rence and significance of bacteria in living trees of Populus nigra L. European Journal of Forest Pathol Received 6 February 1997 ogy 7,364-367. Accepted 1 April 1998 TOOLE, E.R. 1968. Wetwood in cottonwood. Plant Disease Report 52:822-823. \VARD, J.e., AND \VY. PONG. 1980. Wetwood in trees: a timber resource problem. USDA Forest Service PNW-1l2.