Great Basin Naturalist
Volume 57 Number 3 Article 2
7-31-1997
Growth and survivorship of Fremont cottonwood, Gooding willow, and salt cedar seedlings after large floods in central Arizona
J. C. Stromberg Arizona State University, Tempe
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Recommended Citation Stromberg, J. C. (1997) "Growth and survivorship of Fremont cottonwood, Gooding willow, and salt cedar seedlings after large floods in central Arizona," Great Basin Naturalist: Vol. 57 : No. 3 , Article 2. Available at: https://scholarsarchive.byu.edu/gbn/vol57/iss3/2
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 57(3), © 1997, pp, 198-208
GROWTH AND SURVIVORSHIP OF FREMONT COTIONWOOD, GOODDING WILLOW, AND SALT CEDAR SEEDLINGS AFTER LARGE FLOODS IN CENTRAL ARIZONA
J.e. Stromberg!
ABSTRACT.-During winter 1993, Arizona experienced regional river flooding. Floodwaters at the Hassayampa River eroded floodplains and created a 50-m-wide scour zone available for colonization by pioneer plant species. The slow rate and long duration ofthe floodwater recession allowed establishment of spring-germinating native trees (mainly Fre mont cottonwood [Populus fremontii] and Goodding willow [Salix gooddingii] as well as summer-germinating species including the introduced salt cedar (Tamarix chinensw and related species). Goodding willow and Fremont cottonwood seedlings showed zonation in the floodplain, while salt cedar was equally abundant in zones with saturated and dry sur face soils. Floodplain elevation (and soil moisture) influenced shoot growth rate to different degrees among the 3 species. For example, Goodding willow seedlings were significantly taller in areas with saturated soils than dry surface soils; Fre mont cottonwoods were taller in the dry surface soil areas; and salt cedar were equally short in both soil moisture zones. Other factors that differentially influenced abundance or growth rates included competition with herbaceous species (Melilotus spp., an introduced plant, locally preempted salt cedar establishment) and herbivory (selective browsing by livestock at 1 river site reduced the natural height advantage of the native tree species). I draw on the results of this descriptive field study to suggest ways in which stream flows and floodplain land use can be managed to restore ecologi cal conditions that favor native tree species over the introduced and widespread salt cedar.
Key words: riparian habitats, floods, Populus fremontii, Salix gooddingii. Tamarix chinensis.
Fremont cottonwood (Populus frerrwntii) and by Fremont cottonwood, Goodding willow, and Goodding willow (Salix gooddingii) are pioneer herbaceous vegetation (Stromberg et a1. 1993). species that establish episodically after winter! Larger floods of longer duration, in contrast, spring flood flows along rivers of the desert have been observed to facilitate establishment Southwest (Stromberg et a1. I991, Everitt 1995). of various species of Tamarix (Stevens and Winter/spring floods scour competing vegeta Waring 1985, Ohmart et al. 1988, Griffin et a1. tion, deposit and rework alluvial sediments, and 1989). provide supplemental moisture during the short During winter 1993, Arizona experienced period in spring during which the seeds dis regional river flooding. Although instantaneous perse and germinate. Salt cedar (Tamarix chi peaks were on par with other recent large nensis and related species), an invasive riparian floods, the 1993 event ranked as one of the shrubby tree native to Eurasia, also is adapted most severe in state history when collectively to establish after flood disturbance but estab considering magnitude, duration, and volume lishes more opportunistically tban Fremont (House 1995). The 1993 floods caused mucb cottonwood and Goodding willow (Brock 1994). geomorphic and vegetational change, includ Salt cedar initiates seed dispersal later in the ing channel widening (Huckleberry 1994) and season but disperses its seeds over a longer mass wasting of floodplains supporting velvet period of time (Warren and Turner 1975). Thus, mesquite (Prosopis velutina) woodland and depending in part on timing and duration, Fremont cottonwood-Goodding willow forest. floods may either preclude or enhance salt The floods also created extensive habitat for cedar establishment. For example, salt cedar riverine marshland and young stands ofcotton seedlings were scarce after a small spring flood wood and willow (Stromberg et a1. 1997). I in tbe Hassayampa River in 1991 because the undertook this study with the objective of de narrow band of germination space created by termining how abundance, distribution, growth, the rapidly receding floodwaters was colonized and survivorship of seedlings and vegetative
lCcnler fill" Environmental Studi~, Arizona State University, Thmpe AZ 85287-3211.
198 1997] NATIVE TREE SURVIVORSHIP AFTER FLOODING 199 sprouts of Fremont cottonwood, Goodding wil TABLE 1. Density (stemsJha ofstems >2 em) of native and low, salt cedar. and various shrubs were influ exotic pioneer species in forest stands at 3 central Arizona study rivers. enced by the 1993 floods, and by hydrologic events in subsequent years. Goodding Fremont and Bonpland Sait cottonwood willows cedar STUDY SITES Hassayampa River 287 412 45 Study sites are located on perennial reaches Santa Maria River 312 416 195 of3 central Arizona Sonoran rivers, One site is Date Creek 45 60 5 in the Arizona Nature Conservancy's Hassa yampa River Preserve in northwest Maricopa County (elevation ca 600 m). The Hassayampa River has a mean annual flow rate of0.5 m3s-1 lishment of plot markers (rebar) that were at the Morristown gage (USGS #9516500), removed or buried during floods. Floodplain located about 4 km downstream of the pre topography and changes in plot surface eleva serve. Surface sediments are predominantly tion due to sediment deposition and scour were sand. Surface water has an electrical conduc determined based on repeat cross-sectional tivity of 600-700 uS/em. The area was grazed surveys of the Hassayampa River transects by cattle prior to 1987, but the main land use at (Stromberg et al. 1997). Plots (1 x 1 m) were the preserve is now ecotourism. The Santa sampled for density and height ofwoody seed Maria River study site is in the Alamo Lake lings and vegetative stem sprouts 3-5 times Wildlife Management Area (elevation 365 m), per year during 1993, 1994, and 1995. Shoot in Mohave and La paz counties, and is grazed height was measured for the tallest individual by feral burro and trespass cattle. Mean annual 3 1 per species per plot. Plants were assumed dead flow rate of the Santa Maria is 2 m s- at the ifabsent from plots, although live root or stem Bagdad gage (USGS #9424900), located about fragments may have been dispersed from plots 5 km upstream of the study reach. The Date in floodwaters. The nonparametric Kruskal Creek site is on Arizona State land, leased by Wallis test was used to test for significant dif Date Creek Ranch (elevation 880 m). Date ferences in survivorship of the 1995 flood be Creek is a tributary to the Santa Maria River tween 1993 seedling cohorts ofFremont cotton and is not gaged. The Date Creek floodplain is wood, Goodding willow, and salt cedar. Spear grazed by cattle from November to March. man rank correlation analysis was used to deter Floodplains ofall 3 rivers are vegetated pri mine whether seedling survivorship ofthe 1995 marily by Fremont cottonwood-Goodding wil flood (using pooled values for all 3 tree species) low forests, velvet mesquite woodlands, burro varied with distance from the 1993 channel brush (Hymerwcka morwgyra) scrublands, and edge. SPSS for Windows was used for all sta seep willow (Bacclwris salicifolia) stands (Brown 1982). Other woody species include Arizona tistical analyses. ash (FraxinWi velutina), Bonpland willow (Salix At all 3 rivers, 50 randomly located 1 x 1-m bonplandiana), coyote willow (Salix exigua), plots were sampled in June and October of arrow weed (Tessaria sericea), and screwbean 1994 within 1 or 2 soil moisture zones in the mesquite (Prosopis pubescens). Salt cedar had scoured flood channel. Fifty plots were in a lower density than Fremont cottonwood or zone with saturated surface soils or shallow willows at study sites at all 3 rivers (Table 1), standing water «3 em), and 50 were in a zone based on sampling of saplings and trees (stems with slightly higher surface elevation (10-30 >2 em at breast height) in 1994 with the em higher) and dry or damp surface soils (as of point-quarter method (20 points per river). June 1994). At Date Creek the saturated soil zone was not extensive, and plots were sam METHODS pled only in the dry surface zone. Woody seed lings in the plots were sampled for density, At the Hassayampa River, data were collected and for height and browse status (browsed or at a total of 100 permanent plots distributed unbrowsed) ofthe tallest individual per species. along 8 transects established at the preserve in Plants were classified as browsed if any of the 1987. Plots were located at known distances shoots had been eaten. If there were <20 along the transect line, allowing for reestab- individuals of Fremont cottonwood, Goodding 200 GREAT BASIN NATURALIST [Volume 57 willow, or salt cedar in the 50 plots, stem 250,------~------___, height and browse status were measured in ---~._--_._--l---._. additional random plots to increase sample I. 200t--+cc-- size to 20 individuals per species per zone. -¥J 150 _._.-.--'-.---~----.-- ._-- Herbaceous cover, by species, was visually ~ estimated in the plots using gridded plot ~ 100·-- frames to reduce sampling error. >- 50 --- One-way analysis ofvariance (ANOVA) with :g post-hoc Sheffe's multiple comparison test was ~ 0'1---' '-"'--- --_..- _._-_.._._-, -'--- :; used to detect statistical difference in stem density (n = 50 plots) and stem height (n = 20 individuals per species) between Fremont cot Q N DJ FM AMJJASQ N DJ FMAMJJAS QNDJ FMA MJJ tonwood, Goodding willow, and salt cedar with 1993 1994 1995 in zones of each river. Independent sample Fig. 1. Mean daily flows in the Hassayampa River at the Student's t tests were used to compare density Morristown gage during water years 1993, 1994, and (n = 50 plots) and stem height (n = 20) ofeach 1995. species between the saturated and dry surface zones of the Hassayampa and Santa Maria rivers. Additionally, plots within the dry surface zone at the Hassayampa River were subse elevation of the scour zone relative to 1993 quently divided into those with >50% and (Stromberg et aJ. 1997). The Santa Maria also <50% cover of sweet clover (Meli/otus albus had large flood peaks. Instantaneous dis and M. officinalis), and densities of salt cedar charges in the Santa Maria peaked in 1993 at and Fremont cottonwood were compared be 347 m3s-1 on 8 January, 440 m3s-1 on 9 Febru tween the 2 groups with Student's t tests. ary, and 356 m3s-1 on 20 February. 1993 Seedling Cohort RESULTS TEMPORAL AND SPATIAL DrsTRIBuTION.-As Surface Flow the Hassayampa River floodwaters receded in The 1993 flood in the Rassayampa River had 1993, woody plants germinated in exposed moist soils in the following sequence: Fremont a 25-yr recurrence interval, with instantaneous cottonwood (March-April), Goodding willow flow rates peaking at 745 m3s-1 on 8 January (April-May), salt cedar (May-September), 1993. This was followed by flood peaks of 218 arrow weed (July-September), and seep wil m3s-1 on 17 January, 328 m3s-1 on 9 February, low (July-September; Fig. 2). Trace amounts and 439 m3s-1 on 20 February. Surface flow «0.01 stem/m2) of Bonpland willow were was above average during spring and summer recorded from Date Creek (data not shown), of 1993 and was present through August at the but this species was not present at the Has Morristown gage, located in a frequently dry sayampa River site. reach of the Rassayampa River (Fig. 1). Flood Woody seedlings showed zonation in the waters in 1993 had an estimated depth of 3.0 floodplains ofthe Rassayampa and Santa Maria m on low floodplain terraces and a velocity of rivers (Table 2). Fremont cottonwood was sig 2.1 m s-1 Floodwaters enlarged the channel nificantly more abundant in dry surface zones, from about 3 to 50 m wide and caused a net while Goodding willow, coyote willow, and lowering of tbe floodplain surface. Throughout arrow weed were significantly more abundant 1994, instantaneous flow rates never exceeded in zones with saturated soils, Salt cedar and 2 m3s-1. DUling 1995, instantaneous flow rates seep willow were equally abundant in both peaked at 566 m3s-1 (15- to 20-yr recurrence zones. Within the dry surface zone at the Has interval) in February. Flows remained high in sayampa River, however, salt cedar seedlings spring and summer of 1995 but diminished varied significantly (t test, P < 0.05) in density more rapidly than in 1993. Surface flow was depending on cover ofyellow and white sweet present at the Morristown gage through June clover (Fig. 3). Salt cedar seedlings were nearly 1995. The 1995 floods deposited sediment on absent in plots with high sweet clover cover large portions of tbe floodplain, raising the but averaged 15 stems/m2 in plots with little 1997] NATIVE TREE SURVIVORSHIP AFTER FLOODING 201
140 At the end of the 1993 growing season • ----POFR (November), Fremont cottonwood seedlings -+- 2 120 1993 cohorts 1995 cohorts had a mean density of20 stems/m in the Has SAGO sayampa River flood channel as a whole, fol -+- 100 TACH lowed by salt cedar (13 stems) and Goodding -&- willow (9 stems). Values had declined to about ~ BASA 4 stems/m2 for each species by October 1994. - 80 -+- ~ lESE The 1995 floods caused high mortality of these remaining 1993 seedling cohorts. In Hassa yampa River study plots, Fremont cottonwood and Goodding willow seedlings had 96% 40 mortality, and salt cedar had 100% mortality. Between-species diHcrences in survivorship 20 were not statistically significant at P < 0.05. However, there was a significant correlation o ~ (r = 0.50; P < 0.05; n = 28) between woody o 6 12 18 24 30 36 seedling survivorship and distance of the plot Months past Jan. 1, 1993 from the edge of the channel as of 1993. Fre mont cottonwoods and Goodding willows that Fig. 2. Density of woody seedlings established after floods in 1993 and 1995 in the Hassayampa River (POFR did survive were mainly in narrow bands along == Populus frellwntii, SAGO = Salix guoddingii, TACH = the edge of the scour zone, with some surviv Tamarix chinensis, BASA = Baccharis salicifolia, TESE = ing despite deposition ofup to 1 m ofsediment. 1essaria sericea). GRowrn RATES AND BROWSE RATES.-On average in the Hassayampa River floodplain, seedlings ofnative tree species (Goodding wil low and Fremont cottonwood) were taller than or no sweet clover. Fremont cottonwood seed salt cedar and native shrub species (seep willow lings were equally dense in plots with low and and arrow weed; Fig. 5). Among tree species, high values for sweet clover cover. Yellm:v and Goodding willows were significantly taller than white sweet clover are introduced biennials ~remont cottonwood and salt cedar dUling their and had high cover in 1993 and 1994 in the 1st and 2nd growing seasons in the saturated dry surface zone of the Hassayampa River soil zones at the Hassayampa and Santa Maria Preserve. rivers (Fig. 6). In dry surface zones at river sites, ABUNDAl\'CE AND SURVIVORSHIP.-At the the 3 species did not differ significantly in Hassayampa River, Fremont cottonwood was height after 1 growing season. After 2 growing the most abundant species in the dry soil zone seasons native tree species were significantly after 1 growing season, and Goodding willow taller than salt cedar in the dry surface zone at was most abundant in the saturated soil zone the Hassayampa and Santa Maria rivers but (Fig. 4). After 2 growing seasons, Fremont cot not at Date Creek. Date Creek was the only tonwood and salt cedar seedlings had declined study site with high rates of browse by live to approximately equal densities in the dry soil stock, with 89% of the Fremont cottonwood zone, as had Goodding willow and salt cedar seedlings and .5% of the salt cedar seedlings in the saturated soil zone. At the Santa Maria classified as browsed as ofJune 1994. In most River, Fremont cottonwood and Goodding wil cases browsing resulted in loss of the terminal low seedlings showed trends of being more shoot. Browse rates were <5% per species at abundant in the dry surface and saturated soil the other 2 rivers. zones, respectively. Salt cedar seedlings, how Fremont cottonwoods were significantly taller ever, were significantly more ahundant than in the dry surface zone than in the saturated either native tree species in both zones at the soil zones at both the Hassayampa and Santa Santa Maria River after a single growing season; Maria rivers, after 1 and 2 growing seasons and densities did not differ significantly between (Table .3). In contrast, Goodding willows tended species after 2 growing seasons. Seedling den to be taller in the saturated soil zone, but dif sities did not differ significantly between species ferences behveen zones were significant only at at Date Creek. the Hassayampa River during the 1st growing TABl.E 2. Stem density (no./m2) of 1993 seedling cohorts of Populus fremontii (POFR), Salix gooddingii (SACO), Tamari:r chinensi$ (TACH), BQ{;cham sal.ia/olia (BASA), Tessaria ~ seri<:ea. (TESE), and Salix e2:igua (SAEX) at the Hassayampa River Preserve (HR), Santa Maria River (SM), and Date Creek (DC), by moisture rone. Values shown are means ± standard l'O errors. ~t" indicates values less than 0.1. June 1994 October 1994 POFR SAGO TACH BASA TESE SAEX POfR SAGO TACH BASA TESE SA EX Dry surface: HR 38.3 ± 5.9" 3.2 ± 0.6 11.4± 4.4 1.2 ± 0.1 0.0 ± 0.0 0.0 + 0.0 10.2 ±4,4* 0.4± 0.2 7.8 ± 1.7 0.4 + 0.1 0.0 ± 0.0 0.0 ± 0.0 Saturated soils: HR 1.2±0.3 38.1 ±5.7* 10.8± 7.5 0.9 ± 0.4 O.3±O.1* 0.0+ 0.0 1.1 +0.4 18.5 ± 5.7* 9.6±2.6 0.3 + 0.1 0.3±0.1* 0.0 ± 0.0
Dry surface: 8M 5.9 ± 0.6* 0.3 ± 0.2 34.3 ±5.9 1.3±0.4 0.1 +0.1 0.0 + 0.0 2.5 ±O.7* O.O± 0.0 5.U 1.4 0.3 + 0.1 O.O±O.O O.O±O.O Saturated soils: SM 0.1±0.1 3.7±O.5* 27.6±4.7 1.3 ± 0.7 5.2±O.7* 0.3 ±O.O'" 0.1 ± 0.1 2.6 ± 1.2'" 7.4 ± 2.4 0.1 ± 0.1 L9±O.6* 0.1 ±O.I
Dry surface: DC 2.3 ± 0.6 0.2 + 0.1 0.9 ± 0.3 0.6 ± 0.2 0.0 ± 0.0 t 1.0 ± 0.4 0.0 ± 0.0 0.8 ± 0.3 0.4 + 0.1 O.O± 0.0 I
*Sigl:llReant difference between moisrore ~ones at P < 0.05 o f;i ~ ~ -Z z TABLE 3. Stem height for 1993 seedling cohorts of Populus fremontii (POFR), Salix gooddingii (SAGO), and Taman.1: chinensis (TACH) at the Hassayampa River Preserve (HE), Santa Maria River (SM), and Date Creek (DC), by moisture 2 zone, after 1 and 2 growing seasons. Values shown are means + standard errors. ~ One growing seasona Two growing seasonsb ~ POFR SAGO TACH POFR SAGO TACH Dry surface: HR 18±9'" 77+8 52±5 128 ± 8* 125 ±II 53±7 Saturated soils: HR 45 ±6 103 ±9* 53+7 7l±6 149 ± II 45+5
Dry surface: 8M 60±6* 60±8 40±6 I'll ± 8· 74+6 Saturated soils: SM 35±5 SO±S 41 +5 67±S 116 ± 12 71± II
Dry surface: DC 46±6 60±9 47+6 55±5 45+4
·Me....ured inJune 1994 l>Meamred in October 1994 - ·Signiflcant difference \>etweenJ1'lOis:ture:lXlO(l,.1 P < 0,01 ~ ::- 3 "...,"" 1997] NATIVE TREE SURVIVORSHIP AFTER FLOODING 203
50 lings were an order of magnitude more abun I High cover dant than vegetative sprouts.
~ Low cover 40 = '"E DISCUSSlON AND -0 MANAGEMENT IMPLICATIONS -c ~ 30 Salt cedar was subdominant to Fremont cot c tonwood and Goodding willow in mature for "0'" • est stands at the 3 rivers included in this study. C> 20 .-c To maintain or relegate salt cedar to a sub "0 dominant role, stream flows and floodplain (I)'" '" 10 lands need to be managed to produce ecologi cal conditions that favor native species. This study identified several factors that influence 0+----'-- establishment rates ofsalt cedar vs. native tree • POFR TACH speCies. FLOOD TIMING, MAGNITUDE, DURATION, AND Fig. 3. Stem density of 1993 seedling cohorts of Popul.us STAGE DECLINE RATE.-The large magnitude fremontii (POFR) and Tamarix chiMnsis (TACH) in plots and long duration of the 1993 and 1995 winter with <50% cover of Melilotus species (n = 32; mean cover of 76 + 7) and >50% (n = 18; mean cover or 5 + 1), floods created extensive fluvial surfaces avail as of June 1994. Values shown are means and standard able for colonization by woody pioneer plant error bars. species that do not tolerate competitive herba ceous cover. The long, slow decline in the liver stage (flow rates did not decline to base levels season. Salt cedar height did not differ signifi until several months after the flood peaks) cantly between zones. allowed germination of woody species with a variety of temporal regeneration niches. Exten 1995 Seedling Cohort sive scour prevented herbaceous species from Seedlings of all 3 tree species again germi rapidly colonizing the moist sediments that nated in 1995 in the moist, scoured zone ex were exposed throughout the summer, and thus posed by the slowly receding floodwaters. In germination sites were available for spring the Rassayampa River floodplain, densities of germinating native trees (Fremont cottonwood the 1995 seedling cohorts ranged among the 3 and Goodding willow) as well as for salt cedar, species from 1 to 4 stems/m2 as ofOctober 1995, a species with biseasonal seed dispersal peaks compared to 9--20 stems/m2 for the 1993 cohorts in June-July and August-September (Rorton as of November 1993. Dry surface soils were 1957, 1977, Warren and Turner 1975). relatively more abundant and saturated soils Stream flows on regulated rivers are being less abundant in 1995 than 1993, partly due to naturalized in several ways to facilitate estab floodplain aggradation in 1995. Fremont cotton lishment of Fremont cottonwood and other wood seedlings were the most abundant ofthe native species (Stanford et aI. 1996, Poff et aI. 3 tree species in 1995 and outnumbered salt 1997). The same could be done to reduce estab cedar by a ratio ofabout 2:1 as ofOctober 1995. lishment of salt cedar. Scouring flows should Seedlings ofall 3 species tended to have shorter be released during early spring in potential re stems in 1995 than in 1993, but the difference cruitment years for cottonwoods and willows. between years was significant at P < 0.05 only The flood flows need to be of sufficiently high for Goodding willow (Fig. 7). magnitude and duration to rework sediments Densities of vegetative sprouts were greater and create hare surfaces lying within about 1 m in 1995 than during 1993. There were 0.25, of the "hase-flow" alluvial groundwater table 0.36, and 0.02 stems/m2 for Fremont cotton (Stromberg et aI. 1991, 1993). Flows should wood, Goodding willow, and salt cedar, respec peak prior to Fremont cottonwood and Good tively, as of October 1995, compared to <0.01 ding willow spring seed dispersal periods (from stems per species in November 1993. Many of about February to April, depending on eleva the 1995 sprouts originated from flood-pros tion). River stage should decline during the trated 1993 seedlings. In 1995 and 1993, seed- short (month long) periods of cottonwood and 204 GREAT BASIN NATURALIST [Volume 57
Dry Soil Surface Dry Soil Surface 50 " ~POFR 40 " ~SAGO ~ N E BII!!mI TACH -0 30 c ~ ;i?;' U) c 20 Q) 0 10
0 HR SM DC HR SM DC Age=1yr Age=2yr
Saturated Soils Saturated Soils 50 " 40 ~ N E 0 -c 30 ~ 1 ~ U) c 20 Q) 0 10
0 " HR SM DC HR SM DC Age=1yr Age=2yr
Fig. 4. Stem density of 1993 cohorts of Populus fremontii (POFR), Salix gooddingii (SAGO), and Tamal'ix chinensis (TACH), in dry surface and saturated soil zones, after 1 and 2 growing seasons (HR == Hassayampa River, 8M == Santa Maria River, DC= Date Creek). Values shown are means and standard error bars. Asterisk indicates that a species had signifi.cantly different density from the other species at P < 0.05.
"" ~ Fig. 5. Shoot heights of woody seedlings established '993 ,1994 POFR after the 1993 flood (POFR = Populus fremontii, SAGO "" = Salix gooddingii, TACH == Tamam chinensis, BASA == '00 -SAGO Baccharis salicifolia, TESE = Tessaria sericea). ./ -TACH ~ ./ • BASA ./ / -TESE / :::1/ LO //, ~ , I 9 12 1~ 18 N Months past Jan. 1, 1993 " 1997] AnVE TREE SURVIVORSHIP AFTER FLOOD! 'G 205
Dry Soil Surface Dry Soil Surface 200 ~ POFR 175 f®@! SAGO ~ * E 150 Saturated Soils Saturated Soils 200 175 * ~ 150 ~5 .s::; 125 * .2'- ., 100 .s::; .,E 75 C/J- 50 25 0 HR SM DC HR SM DC Age=1yr Age=2yr Fig. 6. Stem height of 1993 cohorts of Populus fi'emontii (POFR), Salix gooddingii (SAGO), and Tmn willow seed viability to expose moist germina nating native pioneer species such as seep wil tion ,ite, (Mahoney and Rood 1993, Braatne low; however, if initial flood scour is not too et al. 1997). The rate of stage decline (and of great, this species will rapidly regenercl.te asex alluvial groundwater decline) during early seed ually (Stromberg et aI. 1991). ling growth ,hould not exceed rates of root Floods that occur after seed germination also growth of Fremont cottonwood and Goodding may iniluence relative abundance of salt cedar willow seedling,. These rates are about 1-3 cm vs. cottonwood, and willow,. In this study the per day and vary with soil texture (Mahoney 1993 seedling cohorts of Fremont cottonwood, and Rood 1992, Segelquist et a!. 1993). To Goodding willow, and ,alt cedar all had high reduce salt cedar establishment, river stage mortality from the 1995 flood, and survivor should stabilize prior to the onset of salt cedar ship did not differ significantly between the 3 germination in late spring. This also could species. However; some studies suggest that salt reduce germination rates for summer-germi- cedar may be Jess tolemnt ofthe scouring effect' 206 GREAT BASIN NATURALIST [Volume 57 100 -,----~------_.- tial responses to soilinoisture and soil aeration. c.=J 1993 cohort Goodding willow sboot height was greatest in W2%l f995 cohort • conditions of saturated soils or standing water, whereas Fremont cottonwood shoot height was greatest in areas with dry surface soils but shal .~ low water tables. Salt cedar were equally short 5 60 -~ in both zones. Comparison between years sug .~ gests that salt cedar is as tall as the native tree ~ E species under conditions of reduced water ~ 40 U)- availability. Yearlings of all 3 tree species were somewhat shorter in 1995 than in 1993 (most likely due to lower river stage and deeper water 20 tables); however, the difference between years was greater-for native trees. Other studies dem o+---L- onstrate that salt cedar is more drought toler POFR SAGO TACH ant than native cottonwoods and willows and Species thus may have a competitive advantage on drier floodplains with deeper groundwater levels Fig. 7. Stem height of 199:3 Imel 1995 cohorts of Populus (Stevens 1987, Busch and Smith 1995). There frernontii (POFH.), Salix gooddingii (SAGO), and Tamarix is a need for additional controlled studies of chinensis (TACH) after 1 growing season at the Has seedling growth (Siseoe 1993, Shafroth et al. sayampa .Hiver Preserve. Values shown are means and standarrl error bars. Asterisk indicates signficunt differ 1995) to quantify soil moisture ranges over Clll:C (P < 0.05) between cohorts. which native species have a competitive edge. Once such ranges are described, activities that reduce flmdplain water availability (such as sur offloods, but more tolerant of prolonged inun face water diversion and groundwater pump dation, than cottonwood and willow trees (War ing) could be managed to produce soil mois ren and Turner 1975, Irvine and West 1979, ture levels and groundwater depths that favor Everitt 1980, Stevens and Waring 1985). Salt growth and survivorship of native species dur cedar seedlings may have low survivorship of ing establishment periods. floods because oftheir distribution in the flood GRAZING.-Although this study was not ex plain. Because most salt cedar seeds germinate plicitly designed to determine effects of live later than }-<'"'remont cottonwood and Goodding stock on riparian tree seedlings, there were willow seeds, salt cedar seedlings sometimes differences iIi cattle browsing between sites are more abundant on sites close to the chan which resulted in significant differences in nel where risks of mortality from subsequent seedling heights. At the 2 (legally) ungrazed floods are high (Stromberg et ai, 1991). Also, ri.v.er sites, Fremont cottonwood ha~ a large there may be physiological differences between height advantage over salt cedar in certain areas species in tolerance for flood scour or burial of the floodplain (i.e., dry surface soil zone). At by sediments. For example, low rates of shoot Date Creek, which is grazed/browsed by cattle growth may increase the probability of com from November to March, selective browsing plete burial of salt cedar seedlings. Existence on Fremont cottonwood seedlings caused them of differential survivorship thresholds for I'lC to lose their height advantage over salt cedar. tors such as shear stress and sediment deposi Between-species differences in seedling shoot tion should be experimentally tested. If there height ultimately may determine the composi are differences, release of occasional large, tion of the stand dominant, given that shorter scouring floods in alluvial, perennial rivers in plants may have greater mortality due to light the Southwest may be a management strategy limitation, All 3 species, inclnding salt cedar, to increase the mortality ofsalt cedar seedlings appear to be shade intolerant. Additionally, re and saplings relative to that of native pioneer duction in shoot height may reduce a seed tree species. ling's ability to survive deposition of sediment SOIL MOISTUHE.-Differences in tree seed during flood events. To favor native tree species ling shoot height between soil moisture zones in on cattle-grazed rivers, recruitment zOlles this study were most likely a result ofdifferen- should be ·protected year-round from livestock 1997] NATIVE TREE SURVIVORSHIP AFTER FLOODING 207 during at least 2 growing seasons to allow systems of the southwestern U.S. Ecological Mono seedlings to grow above browse heigbt. graphs 65:347-370. EVERITT; B.L. 1980. Ecology of saltcedar-a plea for re HERBACEOUS PLANT COMPETITION.-1n local search. Environmental Geology 3:77-84. ized areas ofthe Hassayampa River floodplain, , ___.. 1995. Hydrologic factors in regeneration of Fre the exotic herbaceous species yellow and white mont cottonwood along the Fremont River, Utah. ' sweet clover preempted establishment of salt Natural and anthropogenic influences in fluvial geo cedar by germinating in spring and rapidly morphology. Geophysical Monographs 89:197-208.' GRIFFIN, G.R, D.M. SMITH, S.R. MORTON, G.E, ALLEN, covering the ground surface prior to salt cedar AND K.A. MASTERS. 1989. Status and implications of germination. Fremont cottonwood seeds, in the invasion of tamarisk (Tamarix aphylla) on the contrast, germinated prior to sweet clover. Finke River, 1\-orthern TerritOlY, Australia'. Journal of Fremont cottonwood seedlings were o'ver Environmental Management 29:297-315. topped by sweet clover during early summer, HORTON, J.S. 1957. Inflorescence development in Tamarix pentandra Pallas (Tamaricaceae).. Southwestern Nat but they were exposed to full sunlight after uralist 2:135~139. the midsummer death of sweet clover and had . 1977. The development and perpetuation of the high survivorship. This suggests a possible --pe-'rmanent tamarisk type in the phreatophyte zone management strategy for salt cedar, at least of the Southwest. USDA Forest Service General Technical Report RM-43:124-127. within small areas. If native annual species HOUSE, p.K. 1995. Hydroclimatological and paleohydro were identified that germinated after (or simul logical context of extreme winter flooding in Ari~ taneously \vith) native trees but prior to salt zona, 1993. Arizona Geological Survey Open-File cedar, it migbt be feasible to disperse seeds of Report 95-12:1-25. the annual berb into the floodplain at an appro HUCKLEBERRY, G. 1994. Contrasting channel response to floods on the middle Gila River, Arizona. Geology priate time after large floods. However, care 22,1083-1086. should be taken to insure that such an effort IRVINE, J.R, AND N.E. \VEST. 1979. Riparian tree species would reduce abundance only ofsalt cedar and distrib~Ltion and succession along the Lower Esca not of "desirable" summer-germinating species lante River, Utah. Southwestern Naturalist 24: 331-346. such as seep willow. MAHONEY, l:\1., AND S.B. ROOD. 1992. Response of a hy brid poplar to water table decline in different sub ACKNOWLEDGME?'JTS strates. Forest Ecology and Management 54:141-156. ---'---c' 1993. A model for assessing the effects of altered We thank the Arizona Nalure Conservancy, river flows on the recruitment of riparian cotton woods. Pages 228-232 in B. Tellman, H.l Cortner, Date Creek Ranch, and Arizona Game and M.G. ·Wallace, L.P. DeBano, and RH. Hamre, tech Fish Department for allowing access to study nical coordinators, Riparian management: common sites, and anonymous reviewers for their help threads and shared interests. USDA Forest Service ful comments. Joelle Don de'Ville and Leigh General Technical Report RM-226. OHMART, RD., B.W ANDERSO)l, AND WC. HUNTER. 1988. , Hedrick assisted with field sampling. The ecology of the lower Colorado River from Davis Dam to the Mexico-United States international LITERATURE CITED boundary: a community profile. United States Fish and Wildlife Service Biological Report 85(7.19): BRAATNE, lR, S.B. ROOD, AND P.E. HEILl' 1996. A general protocol for restoration of regulated STROMBERG, le., D.T. PATIEN, AND B.D. RICHTER. 1991. rivers. Regulated Rivers: Research and Management Flood flows and dynamics of Sonoran riparian forests. 12,391-413. Rivers 2:221-235. STEVENS, L.E. 1987. The status of ecological research on STROMBERG, le., B.D. RICHTER, D.T. 'PATIEN, AND L.G. tamarisk (Tamaricaceae: Tamarix ramosissima) in Ari WOLDEN. 1993. Response of a Sonoran riparian for zona. Pages 99-107 in M.R. Kunzmann, R. Johnson, est to a lO-year return flood. Great Basin Naturalist and P.S. Bennett, technical coordinators, Tamarisk 53,1l8-130. control in southwestern United States. Special Report WARREN, D.K., AND RoM. TURNER. 1~75. Salt cedar (Tamarix 9, Cooperative National Park Resources Studies Uuit chinensis) seed production, seedling establishment STEVENS, L.E., AND C.L. WARI:;.lG. 1985. The effects of pro and response to inundation. Journal of the Arizona longed flooding on the riparian plant community in Academy of Science 10:135-144. Grand Canyon. USDA Forest Service General Tech nical Report RM-43:81-86. Received 6 August 1996 STHOMBERG, le., J. FHY, AND D. PATTEK, 1997. Marsh Accepted 24 Mal"ch 1997 development after large floods in an alluvial, arid land river. Wetlands: 17:292-300.