Dynamics of Fremont Cottonwood ( Populus Fremontii) and Saltcedar ( Tamarix Chinensis) Populations Along the San Pedro River, Arizona

Joumal of Arid Environments (1998) 40: 133-155 Article No. ae980438

Dynamics of Fremont cottonwood ( Populus fremontii) and saltcedar ( Tamarix chinensis) populations along the San Pedro River, Arizona

J. Stromberg Department ofPlant Biology, Arizona State University, Tempe, AZ 85287-1601, U.S.A.

(Received JO February 1998, accepted 26 June 1998)

Woodlands of the exotic saltcedar ( Tamarix chinensis) have replaced forests of native Fremont cottonwood (Populus fremondi) and willow (Salix spp.) along many rivers of the American South-west. In the middle basin of the San Pedro River, salt cedar dominates only at the drier sites where the surface and ground-water conditions no longer support cottonwood-willow forests. At sites with perennial (or near-perennial) stream flow, saltcedar is co-dominant with Fremont cottonwood. However, saltcedar has been declining in importance at these sites, perhaps due to recent occurrence of conditions that favour cottonwood establishment (frequent winter flooding, high rates of stream flow during spring, exclusion of livestock). This shift provides evidence of capacity for self-repair in degraded Sonoran riparian ecosystems. In the upper basin, in contrast, saltcedar has increased in relative abundance at sites that show evidence of ground-water decline, signaling a need for vigilance in river management. Saltcedar is generally sparse in the upper basin. probably due to the combination of cool temperatures and persistence of perennial or near-perennial stream flows in most areas. Throughout the San Pedro River, saltcedar and cottonwood both have been influenced by changing flood patterns. Expansion of Fremont cottonwood populations and initial colonization by saltcedar both correlate with post-1960 increases in fall and winter flood frequency and decreases in summer flood size. © 1998 Academic Press

Keywords: population dynamics; Populus fremontii; Tamarix chinensis; hydrologic regimes

Introduction

Hydrologic regimes strongly influence the structure and function li\E~ and npanan vegetation (Bedford, 1996; Poff et al., 1997). Relationships between flow regimes and dynamics of cottonwood (Populus) populations are iJl'l,lij;iqJlFl.lJnW'll known, mainly because hydrologic threats to cottonwood-domirnl'i'm ~~'It.Ive stimulated extensive research (Braatne et al., 1997). Studies of the Aigeiros section of Surface Water Division 0140-1963/98/020133 + 23 $30.00/0 © 1998 Academic Press

ADWR13E_0001 134 J STROMBERG cottonwoods (Fremont cottonwood. P. fremontii and Plains cottonwood, P. deltoides) have revealed the importance of flood timing to seedling establishment. identified flood magnitude thresholds for recruitment, described recruitment frequencies in relation to flood flow frequency. quantified tolerance ranges of seedlings for water-table decline rates. and assessed effects of ground-water decline on tree survivorship (Fenner et al., 1985; Stromberg et al., 1991; Segelquist et al.. 1993; Hughes, 1994; Everitt, 1995; Rood et al.. 1995; Rood & Mahoney, 1996; Scott et al.. 1996, 1997, in press; Cordes et al., 1997; Shafroth et al., in press). Much has also been written about the hydrologic relationships of saltcedar ( Tamarix chinensis and related species), a Eurasian native that has been expanding along rivers of the U.S. South-west since the 1800s, while native cottonwood-willow forests have been declining. Saltcedar is known to be reproductively opportunistic, to have high water-use efficiency and deep roots, and to be tolerant of drought, flooding, and salinity (Merkel & Hopkins, 1957; Horton. 1977; Everitt, 1980; Graf, 1982; Brock, 1994; Busch & Smith, 1995). Thus, reduced stream flows, lowered water-tables, altered flood timing. and increased salinities may give saltcedar a competitive advantage over cottonwoods on managed rivers. However, we still have much to learn about the factors that allow saltcedar to establish and persist relative to cottonwoods. On some rivers the presence of saltcedar may be a legacy of past ground-water pumping, overgrazing, or cottonwood-willow clearing for alleged water salvage, with the population persisting via biologic inertia until flood-scour creates opportunities for species replacement. On other rivers, there may be ongoing. active modification of ecosystems processes that continue to favor saltcedar. Studies that examine how various flow regimes affect the relative abundance of saltcedar vs. cottonwood can lay the foundation for using flow management as a tool to restore the native species as dominants and relegate the exotic to subdominant status, and thereby provide an alternative to control measures such as physical removal (bulldozers, chain saws, hand pulling), chemical treatments (herbicides), fire, or insect biocontrol (Brock, 1994). The objective of this research was to examine patterns of establishment and abundance of saltcedar and Fremont cottonwood along a free-flowing river in a semi-arid region. Primary goals of this case study were to: (1) determine how the relative abundance of saltcedar and Fremont cottonwood have changed over time in response to changing stream flow conditions; (2) compare these patterns between river reaches with different hydrologic conditions; and (3) increase understanding of how hydrologic regimes influence establishment and survivorship of these species. A secondary goal was to assess the influence on populations of these two species of other factors that varied among sites (notably livestock grazing, elevation, and water salinity). Ultimately, the objective was to put more items in the tool-box of ecosystem managers who wish to restore riparian ecosystems, and to increase understanding of the capacity for self-repair of degraded riparian ecosystems of the American South-west.

Site description

1 The San Pedro is a low gradient (0-002 mm_, to 0·005 mm - ) alluvial river that flows northward from Sonora, Mexico to the Gila River in southern Arizona. U.S.A. It drains a watershed (1900 km2 at the U.S./Mexico border, 11,700 km 2 near the Gila River confluence) vegetated primarily by Sonoran and Chihuahuan desert scrub, semi-desert grassland, and Madrean evergreen woodland (Brown, 1982). The San Pedro River during the 1800s was described as having marshy, treeless reaches intermixed with forested reaches. Cooke (1938, in Davis, 1982) describes the upper San Pedro in the 1880s as having a 'marshy bottom' surrounded mainly by grasses and a

ADWR13E_0002 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 135

few ash trees. Clarke (1852, in Davis, 1982) reported that 'trees are becoming common on the river, they are principally cottonwoods, with some sycamore, willow, and mesquite'. Near the site of present Benson, the river banks again were 'devoid of timber' (Parke 1855, in Davis, 1982). A major episode of arroyo cutting occurred about 100 years ago, and perhaps as early as the 1850s, during which the San Pedro River channel incised by several meters (Hastings, 1959). The incision likely was caused by extreme climatic events compounded by human stressors. For example, silver mining during the 1880s to 1910s put pressure on the timber resources of the San Pedro River and uplands and required the diversion ofriver water from a temporary dam site (Bahre, 199 I). Cattle were very abundant, and over 50% of the herds in south-eastern Arizona died during an extended drought in the 1890s. Formerly abundant beavers were locally extirpated by trappers by the 1880s. About 3500 acres of San Pedro River land were under cultivation by 1899, with 10 canals diverting irrigation water from the river. The proximate cause of the incision appears to be a series of large floods in the 1890s and 1900s. The character of the river changed markedly after incision, and much of the marshland vegetation was lost (Hendrickson & Minckley, 1984). Today, sacaton (Sporobolus wrightii) grasslands, mesquite ( Prosopis velutina) woodlands, and a few cottonwood trees persist on the pre-entrenchment terrace of the San Pedro River. The new floodplain supports a continuous corridor of Fremont cottonwood, Goodding willow (Salix gooddingii), and/or saltcedar, intermixed with young mesquite woodlands and various types of shrubland. The change from intermittent to continuous forest may be related, in part, to reduced fire frequency. Fuel-rich grasslands in the uplands have converted to shrublands, thereby decreasing the frequency of fires in the riparian zone as well; thus, marshland and grassland are no longer favored over forest and woodland (McClaran & Van Devender, 1997). Absence of beaver further contributes to homogeneity of the floodplain landscape, and to a scarcity of marshland (Naiman & Rogers, 1997).

Study sites

Eighteen study sites were established along the San Pedro River. Seven of these were in the middle hydrologic basin (Cascabel area; elevations from 950 m to 1015 m) on private, state trust, or federal land [Bureau of Land Management (BLM) I. Mean annual flow rate in the middle basin is 1·26 m3 s- 1 (Redington gage, 906 m elevation). In much of this basin, the amount of ground-water flowing from the regional aquifer towards the river is insufficient to maintain alluvial aquifer levels at the elevation of the streambed. Tributary and ground-water inflow serve to maintain short sections of perennial flow in some reaches (Brown & Aldrige. 1973). Three of the middle basin sites were in a reach with quasi-perennial flow: surface flow was present 100% of the time at two of the three sites, but became intermittent in 1997 at one site after a drought. Ground-water was shallow throughout most of the floodplain in this reach, with levels remaining less than about 1 m below the thalweg throughout theJ!P.11llul;!in. Four of the middle basin sites were in a strongly losing, ephemeral reach (~l'fatt!YloW present from 10-50% of the time and ground-water ranging to more than 1 m below the level of the thalweg) (Fig !). MIIY '). 5 ?Qll Eleven sites were in the upper hydrologic basin of the river (Braun '1/,' 'Ma'daoc'li, 1992). All 11 were in the BLM's National Riparian Conservation A~~21,ftat elevations from 1130 to 1300 m (Stromberg et al., 1996). Mean annual flow rat~ll'I' t11€E!illl'i<¥~~ivision in the upper basin ranges from 0·90 m3 s- 1 (U.S./Mexican border, Palominas gage, 1295 m elevation) to 1·65 m3 s- 1 (Charleston gage, 1214 m elevationl. Of these, four

ADWR13E_0003 w ! 2(a) - S 2Cb) .... m ~ 1995 1996 1997 BS-S (50 ml j [ 1993 ~ 1994 Nl995 laoq1(4o ml I 1 1 ~ ". o 2:'/:~~~~~~~:° BS-13 (280 m) ,,!,. o --._ ■ §~1 -0- § t -1 BS-15 {440 m) ~ -1 s s

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$ 2{c) ._ 'ij 2(d) I r~~--~ ';:;' 1995 1996 1997 Dew-D (74 m) 'i::' I 1993 I 1994 I 1995 I Icontention I Bl .St • . ~ . O i --...... ____v Dew-M(252m) i of I "w V I ~ I ';;; ti, -1 ~ -1 --, .-2s .-2s "'0 ~ 3 LLL.J.Lu..J..LL.J.LWJ..LL..LI-.J.Ll-ULLl..LL ~ -3 ✓ I J I II I I I I l II II I I I I ! I I I t I II II ~ " - J AsoNoJ-l'1AM/ ASJloJ-l'1AMJJ As " Jl1AMiAso~Jl1AM/lo~JFMAM/A ~ ,..... (e) ____ Month 2 t ,[ 1995 I 1996 I 1997 I :-B (62 ml " L NA-D (109 ml g 0 {;, -1

! _2 Lr 1l. • --3 ,::, J ASONDJFMAM/ ASONDJFMAMJJ AS Month Figure 1. Recent ground-water depths (relative to thalweg) at the San Pedro River study sites. Distance from the wells to the stream channel is shown in parentheses. (a) Middle San Pedro-perennial; (b) Upper San Pedro-perennial; (c) Middle San Pedro-ephemeral: (d) Upper San Pedro-intermittent; (e) Middle San Pedro-ephemeral.

ADWR13E_0004 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 137 were in a gaining, perennial reach wherein shallow bedrock helps to maintain perennial flows and stable ground-water levels. Ground-water levels were above the level of the thalweg and have fluctuated annually by < 1 m (Fig. 1). The other seven sites were either upstream or downstream of this perennial reach, in areas where the river loses surface water to ground-water. This creates short periods in the summer dry season (May/June) with no stream flow and greater fluctuation in ground-water levels. Ground-water levels in this intermittent reach have fluctuated annually by up to about 2 m per year, frequently falling below the level of the thalweg during May/June and in dry years (e.g. 1990, 1996). 1 Mean annual precipitation in both basins is about 30 to 33 cm year- , about half to two-thirds of which falls in the late summer Ouly-September) storm season. Flood flows in the summer monsoon season generally have shorter duration than winter floods, which typically result from regional, stationary Pacific frontal storms. Large floods in fall and winter also result from rain events associated with tropical storms (Webb & Betancourt, 1992).

Methods

Vegetation data

A transect line was established perpendicular to the steam at each site. Each transect line was divided into homogeneous patch types, and the width of each patch type was recorded. Patch types were subjectively differentiated on the basis of visual inspection based on the apparent dominant woody plant species, tree age class, and fluvial geomorphic surface. One randomly located dimensionless study plot was established per patch type and marked with rebar (N - 114 plots in the middle basin, 158 plots in the upper basin). Patch type classifications were subsequently refined based on analysis of vegetation data. For example, the dominant species was defined as that with the greatest basal area, and age class was defined based on analysis of tree rings. In the middle basin, increment cores or slabs were collected from 103 Fremont cottonwood trees distributed among 20 of the 31 plots that contained cottonwood, and from 100 saltcedar at 34 of the 57 plots with saltcedar. In the upper basin, cores or slabs were obtained from 112 Fremont cottonwoods at 30 of 51 plots with cottonwood, and from 36 saltcedar at 9 of 15 plots. The cores were taken at 30-50 cm above ground-level using increment borers. Slabs were obtained with hand saws or chain saws from excavated root crowns (Scott et al., 1997). The cores and slabs were collected in 1995 (middle basin) or 1993 (upper basin). In the lab, cores were placed into wood mounts. Cores and slabs were sanded with a graded series of sand paper to 600 grit (saltcedar) or 900 grit (Cottonwood) and viewed with a dissecting microscope to count annual rings. Annual rings were cross-dated with populations to reduce error rates from intra-annual rings. Patch age (stand age) was defined as the age of the oldest tree cored or slabbed per patch. With only a few exceptions, each patch contained only a single age cohort. Age of uncared patches was determined either by counting bud scale scars for cottonwood saplings, or by using size-age regressions to estimate age of cottonwood and saltcedar trees (Fig. 2). At each study plot, one 5 x 20 m quadrat Uong axis parallel to the stream) was sampled for basal diameter and density of live and dead woody stems emR§m§Ilffili\ the ground surface, by species. For each site, density and basal area of each age cohort of saltcedar and cottonwood then were scaled up from quadrats (values l0f\Jlilv2 ~ti!: tlJ!llj flood plain (values ha - I), based on the percentage of the flood plain occuflll!d ~ ¥Jam cohort. Density, basal area, and patch width values were subsequently pooled for Surface Water Division

ADWR13E_0005 138 J. STROMBERG

300 (a)

250 • e 3 200 • "'A "' .ll" 150 • • •a • ·~ 100 • ;:;; • • I 50 • • ~ ··•·········• • ...... a•····•·ff····································· . 0 20 40 60 80 100 120

300 (b)

250 e 3 200 "'A "' "' .'l" 150 •a • I ·~ 100 ;:;; • • -··················· 50 • Q. ..•••...... •...... --·· ~ 0 20 40 60 80 100 120 Stand age (years) Figure 2. Tree size (dbh in cm of largest tree in the stand) as a function of stand age in years, for Populus fremontii (e) and Tamarix chinensis (o) in the (a) upper and (b) middle basins of the San Pedro River. Regression equations are: dbh = 1-79 X age, r 2 = 0·82, P. fremontii upper basin; dbh - 1·77 X age, r' - 0·71, P. fremontii middle basin; dbh - 0·25 x age, r' - 0·57, 2 T. chinensis upper basin; dbh =' 0·73 X age, r = 0-64, T. chinensis middle basin.

cohorts that established in the same decade. These pooled groups are referred to as decadal cohorts. Because many patches in the flood plain had no cottonwood or saltcedar, the site values for density and basal area of these decadal cohorts are fairly low. The site values were then averaged to provide means for the study reaches. Importance values were then calculated for each decadal cohort of Fremont cottonwood and saltcedar using a modified version of the Curtis & Macintosh (1951)

ADWR13E_0006 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 139

formula: importance value - (relative density + relative basal area + relative patch width) /3. Relative patch width was used as a surrogate for relative frequency. Because the focus of this paper was on Fremont cottonwood and saltcedar, the importance values for these species were calculated relative to each other rather than to all woody species in the quadrats, as is customary.

Site hydrology and land use

Data were collected on depth to ground-water and frequency of surface water presence. In the middle basin, steel sandpoint monitoring wells were hand-pounded into the floodplain alluvium at six of the seven transect lines in 1995 or 1996. In most cases, two wells were nested per transect, one within 25 to 75 m from the channel edge and the other at a distance of 100 to 300 m. Data for depth to ground-water were collected monthly through 1997. In the upper basin, depth to ground-water data were obtained from the BLM for one monitoring well per transect line (Stromberg et al., 1996). Well locations, transect lines, and plot locations were surveyed for elevation above the stream thalweg (low point in the channel) using an autolevel transit. During site visits, it was noted whether surface flow was present or absent during the growing seasons of 1995 and 1997 at the middle basin study sites. Surface flow data for the upper basin sites were obtained from the BLM. Long-term chronologies for daily stream flow and instantaneous peak discharge were developed using data from the USGS stream gages with the longest periods of records in each basin (Redington gage 094 72000 and Charleston gage 09471000). The Bureau of Land Management or private land owners were queried as to whether the sites were presently grazed by cattle, and if not, when the cattle were removed. Values for electrical conductivity of the San Pedro surface water and ground-water were obtained from USGS, literature review, and from direct measurements using a conductivity meter. Site elevation was determined from topographic maps.

Data analysis

The non-parametric Wilcoxon rank sum test (Mann-Whitney statistic) was used to test for significant differences in density, basal area, and patch width of cottonwood and saltcedar between the perennial and intermittent reaches of the upper basin ( N - 4 and 7 sites each), between perennial and ephemeral reaches of the middle basin ( N - 3 and 4 sites), and between middle and upper basins ( N - 1 and 11). The Wilcoxon rank sum test also was used to test for significant differences in importance values between cottonwood and saltcedar, within basins and by decade. Linear regression analysis was used to identify site factors significantly related to cottronwood and saltcedar abundance, using data pooled from the upper and middle basins ( N - 18 sites). Floodplain width (patch width) dominated by each species and the percentage of the floodplain dominated by each species were used as the dependent variables, because the two other measures of abundance (density and basal area) are strongly influenced by population age, which differed between sites. Independent variables were site elevation, maximum floodplain ground-water depth (relative to the thalweg) during the summer of 1995 or 1996, and width of the floodplain. RECEIVED Discriminant analysis was used to identify surface flow conditions associated with establishment years for cottonwood and saltcedar. The dependent variable was creat~d by assigning a value of 1 for non-establishment years and a value of 2 for esta."ti\118hii\eOt 2011 years. Establishment was not broken down into sexual (seed) vs. asexual (sprout) categories. Root crown excavations suggested that most of the cottonwo~a!>Wlm!r Division

ADWR13E_0007 140 J STROMBERG from seed, whereas saltcedar established from seeds and sprouts. Independent variables included total annual flow, monthly flow rates, and peak annual flow rate (all during the water year), and peak annual flow during winter (October-March) and summer (April-September). SPSS (PC) was used for all statistical analyses. Values for decadal cohort abundance and importance by decade were used as a basis for discerning temporal population patters. While emphasis is placed on the influence of establishment events in determining population trends, it is recognized that the field data reflect recruitment as well as mortality from flood scour. drought, and other factors. Attempts were not made to assess tree mortality patterns (e.g. by examining historical aerial photos or dating the few standing dead cottonwood trees present at the study sites to discern year of death; Stromberg & Patten, 1992). Mortality from floods can be substantial, but the rate of mortality from flood scour declines with cottonwood age, given that older trees grow on sites that become increasingly less flood prone (Stromberg et al., 1997). Drought also can be a major mortality factor In these arid region rivers where stream flow and precipitation vary widely between years.

Results

Spatial patterns

Patch width of saltcedar increased significantly with decreasing site elevation (r2 = 0-80, p < 0·01, df. = 17), while patch width of Fremont cottonwood increased significantly with decreasing depth to ground-water (r' = 0·24, p = 0·02, df. = 17). Density, basal area, and patch width for saltcedar were significantly greater in the middle basin than in the upper basin (p < 0-05, Mann-Whitney statistic), and did not differ significantly between hydrologic reaches within basins. Fremont cottonwood was equally abundant in the perennial and intermittent reaches of the upper basin, but had significantly lower density, basal area, and patch width in the ephemeral reach compared to the perennial reach of the middle basin ( p < 0-05, Mann-Whitney statistic). Cottonwoods were entirely absent at the ephemeral site (Narrows) with the deepest ground-water (0-5 to 2 m below the thalweg and about 2 to 4 m below the lower floodplain surfaces, Fig. 3) and were sparse at the other sites in the ephemeral reach. Depth to ground-water increased with age of cottonwood and saltcedar stands to a maximum of 3·2 m for cottonwoods and 7 · 1 m for the oldest (43 years) saltcedars (Fig. 4). Depth to ground-water differed significantly (at f = 0·07) between the saltcedar and cottonwood stands (analysis of covariance, df. = 44 . Thus, whereas saltcedar dominated at ephemeral sites in the middle basin, those in the perennial reach of the middle basin were a mosaic of patches dominated by saltcedar and of patches containing Fremont cottonwood (and sometimes Goodding willow) with an understory of shorter but same-aged saltcedar. The riparian forests in the upper basin were generally dominated by Fremont cottonwood and Goodding willow.

Establishment years

The flood of record in the San Pedro River occurred in September 1926, with an instantaneous discharge of > 2500 m3 s- 1 at the Charleston and Redington gages (Fig. 5). No Fremont cottonwood trees in the middle basin predate this 1926 flood (Fig. 6) In the upper basin, a few Fremont cottonwoods on the pre-entrenchment terrace date

ADWR13E_0008 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 141

(a) 12 E

3 B cD 13 1211•10 9 8 7 6

_3L___J.__ _j___ j_ _ ___,_ __L_ _ _j__ __JL___ _,__--1-__..__ _ _, 0 W ~ lW = ~ - - - - ~ ~

12 n 9 A 6 B

-3L___ L._ __ _jL._ __ _jL_ __ --1 ___ __,_ ____, w ~ ~ = - = - 15------,

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_3L______L ___ __JL______L ______. ____ --'------~ 0 W = ~ ~ B - Distance (m) Figure 3. Floodplain cross-sections of three sites in the middle San Pedro River basin showing study plots (numbers and letters), well sites (*), and minimum and maximum ground-water depths (extrapolated from well datal during the study period. (a) BLM Steinman; (b) Barb Clark; (c) Narrows. Note that plots are labeled alphabetically or numberically depending on which side of the channel they lie. BLM Steinman has quasi-perennial flow and stream flow is ephemeral at the other two sites. RECEIVED

MAY 2 5 2011

Surface water Division

ADWR13E_0009 142 J. STROMBERG

7 0 0

! 0 M .s 6 ~ "Ca 0 ~ t'. .s .a ~

~ Q"' 2

0 20 40 60 80 Stand age (years) Figure 4. Mean depth to ground-water at plots dominated by Populus fremontii (e) and Tamarix chinensis (o) along the middle San Pedro River. Regression equations are: ground-water - 0·034 X age + I ·24, df. - 18, r 2 - 0·65 for P. fremontii, and ground-water - 0·048 X age + 1·77, df.- 24, r 2 - 0·13 for T. chinensis.

to the 1890s (one patch) and 1900s (one patch). All of the younger Fremont cottonwood cohorts were on the post-entrenchment floodplain. Saltcedar were much younger than the cottonwoods, with the oldest dating from the 1950s (middle basin) or 1960s (upper basin) (Fig. 6). Large winter flood peaks, high flows during spring and early summer, and small summer floods correlate strongly with establishment years for Fremont cottonwoods in both basins (Table 1; Fig. 7). In the 1990s, for example, recruitment conditions were favorable in 1991, 1993, and 1995, all years with large winter floods, high spring flows, and negligible summer monsoon flows (Fig. 8). As the pattern of winter and summer floods has changed over time, so has the pattern of cottonwood recruitment. From about 1960 to the present, winter floods have been more frequent (i.e. one winter flood every 4 years in the 36-year period encompassing 1960 through 1995, compared to one every 9 years from 1924-1959; middle basin datal and summer floods have been smaller. Concomitantly, spring and early summer flows have been higher since the mid-1960s despite declines in annual flow rate over the past century (Fig. 9). Cottonwoods have established, on average, about once every 5 years in the past 36 years, compared to once every 12 to 18 years in the prior 36-year period (Fig. 6). The few cottonwood trees that persist on the pre-entrenchment terrace in the upper basin pre-date the period of stream gaging. Saltcedar establishment years generally overlapped with those for cottonwood, with the mixed stands of cottonwood and saltcedar dating back to the same year. However, there were several saltcedar patches that established in non-cottonwood years. Winter flow peaks and high spring flows were associated with saltcedar establishment, although

ADWR13E_0010 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 143

annual flow peaks (independent of season) show stronger correlations with saltcedar than for cottonwood (Table 1). Saltcedar has established, on average, about once every 5 years in the past 36 years.

Temporal patterns

Relative importance of Fremont cottonwoods and saltcedar has varied by decade (Figs JO~ 12). In the middle basin, cottonwoods from the pre-I 960 period were sparse, with patches from the 1920s and 1950s each occupying 1% of the floodplain (which averaged 301 m wide) (Fig. IO). Saltcedar were even less abundant, though, and cottonwood had greater importance during this period (Fig. 12). Cottonwood decadal cohorts from the 1960s, 1970s, and 1980s were abundant (cumulatively covering 10% of the floodplain areal, but occupied less area than the extensive, dense stands of saltcedar from these same decades (which covered 32% of the floodplain); thus saltcedar was more important in these three decades. The 1990s cottonwood decadal cohort had high density and occupied more floodplain area (10%) than any other decadal cohort, whereas 1990 saltcedar decadal cohorts occupied only 5% of the area; thus, cottonwood had greater importance in the 1990s. Generally. cottonwoods have been increasing in abundance relative to saltcedar from the 1960s to the present. In the perennial reach, cottonwoods in the 1990s were significantly more important than saltcedar for the first time since the 1920s (p < 0·05, Mann-Whitney statistic). In the ephemeral reach, cottonwoods have recovered only slightly relative to saltcedar in recent decades (Fig. 12). Similar to the middle basin, pre-1960 cottonwood cohorts in the upper basin also were sparse (Fig. 11). In contrast to the middle basin, the cottownood population in the upper basin was dominated by middle-aged rather than by young cohorts. The 1960s decadal cohort was the most abundant of the cottonwood cohorts, and occupied 24% (56 of 235 m) of the floodplain. Post-1960s cohorts occupied increasingly less floodplain area. Decadal cohorts from the 1970s dominated 15% of the floodplain (mainly along the active channel), whereas those from the 1980s and 1990s occupied only small bands at the channel edge or in overflow channels (6% and 4% of the floodplain, respectively). The most abundant saltcedar decadal cohort (1960s cohort) occupied only 3% of the floodplain. Cottonwood has had greater importance relative to saltcedar in all decades in both reaches of the upper basin. However, there have been subtle differences between the two reaches in relative importance of the two species (Fig. 12). In the intermittent reach, but not the perennial reach, cottonwood decreased in importance relative to saltcedar in the 1990s.

Discussion

This study elucidates subtle differences in biohydrology relationships of saltcedar and Fremont cottonwood. some of which could form the basis of site management plans. The study also describes recovery of Fremont cottonwood populations. and thus leads to an understanding of the natural processes that enable degraded Sonoran riparian ecosystems to self-repair. Notably. the increasing importance of Fremont cottonwood in the saltcedar-dominated perennial reach of the middle basin, attribul'erflDftJ)!(jO climate-based changes in flow regime and partly to livestock removal. suggesfs 'ffiat active management for saltcedar reduction may not be necessary in these areas. In contrast, the recent increase in importance of saltcedar in the cottonwo~lifottii!iail011 intermittent reach of the upper basin signals a need for vigilance in river management. These patterns reify a conclusion of Sprugel (I 991) that vegetation t¥fJtfgggifflr9:Yivision

ADWR13E_0011 144 ]. STROMBERG

(a) 1000

7• 750 ro_g

If 500 -ti" :a• ~ ~ 250

0 1915 1925 1935 1945 1955 1965 1975 1985 1995 1920 1930 1940 1950 1960 1970 1980 1990 Year

(b) l00crni",~---=--;."'------~.2521 1401

~ 750 'w ro_g g>,, " 500 -ti" -~-0 -" • 250 ~

0 Llli.u.LWllll 1925 1935 1945 1955 1965 1975 1985 1995 1930 1940 1950 1960 1970 1980 1990 Year Figure 5. (a) Instantaneous peak discharge in summer (April-September) and winter (October-March) in the upper (Charleston gage) and middle (Redington gage) San Pedro River. (a) Charleston, summer; (b) Redington, summer; (c) Charleston. winter; (d) Redington, winter.

long-lived trees and shrubs may be more unstable than they appear, with small or transient changes in environmental factors causing large and long-lasting vegetation changes. Understanding the proximate and ultimate factors that influence vegetation change is critical to riverine forest management. Proximate factors that influence the abundance of Fremont cottonwoods and saltcedar along the San Pedro River include flood magnitude, timing, and duration (Warren & Tumver, 1975; Everitt. 1980, 1995; Scott et al.. 1997); availability of ground-water and surface waters (Busch & Smith. 1995); salinity of surface and ground-water (Shafroth et al., 1995); grazing regimes (Stromberg. 1997); and temperature (Bowser, 1957). as elucidated below. Changes in flood regime and fluvial geomorphology have strongly influenced forest abundance and composition along the San Pedro River. Very large floods associated

ADWR13E_0012 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 145

(c)

; 750 ~ M_g gj,

~ 500 -5- -~ "'-" ~ 250 0..•

o U..O.u.l.cw.l.Jl..il.lilitl.uilll.u..Ll.U.Uili.Lu.l.cw.l.llU.u..11.il.U.U.UJJIU-U.IJII.U.>.w.JLI.U 1915 1925 1935 1945 1955 1965 1975 1985 1995 1920 1930 1940 1950 1960 1970 1980 1990 Year (d) 1000~------~

0 Lw.u.Lil..u.L.w.ill.u.uu.ucl.u.u..ll=LLu.LlliLl.LLLlli.L.Ll.illl.Lilll.u.ilcll.iW 1925 1935 1945 1955 1965 1975 1985 1995 1930 1940 1950 1960 1970 1980 1990 Year. RECEIVED Figure 5. Continued MAY 2 5 2011 with tum-of-the-century arroyo cutting and channel incision cl&ldl:lllt!JW!t~Hlili.\li$ion from the channel. Cottonwood and willow trees were uncommon in the slowly widening floodplain during the early 20th century, with the 1926 flood-of-record no doubt contributing to this sparseness (Lacey et al., 1975; Hereford, 1992; Friedman & Zube, 1992). After 1960, climatic fluctuations tied to El Nii\o Southern oscillation weather patterns (Webb & Betancourt, 1992) created a pattern of flood flows that was more favorable to riparian tree establishment. Increased activity of Pacific frontal winter storms and of dissipating late summer and fall tropical cyclones resulted in increased magnitude of fall and winter floods (favoring germination and growth of the tree seedlings), while activity of convective summer thunderstorms decreased (favoring seedling survivorship). Recovery of watersheds from heavy livestock grazing also may have contributed to smaller summer flood peaks (Hereford, 1992) and to increased seedling survivorship. These changed flood patterns facilitated the mid-century expansion of cottonwoods

ADWR13E_0013 146 J. STROMBERG

(a) (b) 3 3 • • ~ "u• • 2 ~ 2 " " !l." •"0. ~• 1 i 1 &" • "' 0 1890s 1910s 1930s 1950s 1970s 1990s 1900s 1920s 1940s 1960s 1980s

2 2

1 1

o~~~~~-~~~~~~=~ 0 1890s 1910s 1930s 1950s 1970s 1990s 1890s 1910s 1930s 1950s 1970s 1990s 1900s 1920s 1940s 1960s 1980s 1900s 1920s 1940s 1960s 1980s Figure 6. Number of establishment events by decade for (a) Populus fremontii and (b) Tamarix chinensis in the middle (bottom graphs) and upper (top graphs) basins of the San Pedro River. Data are derived from the age structure of the existing population. and willows along the San Pedro River, and perhaps allowed for the initial colonization of saltcedar. Large summer floods may have precluded extensive saltcedar establishment prior to 1960, raising the possibility that flood scour may be a useful management tool along regulated rivers, and underscoring the need for experimental studies that identify thresholds for survivorship of floor scour and shear stress by seedlings of native and exotic tree seedlings. Or, the sparseness of saltcedar prior to the 1960s may simply have been due to seed scarcity. Saltcedar was first observed along the nearby Gila River in

Table 1. Pooled within-group correlations between discriminating variables and canonical discriminant functions Populus fremontii Tamarix chinensis establishment years establishment years Upper Middle Upper Middle basin basin Basin basin (N-62) (N-48) (N- 41) (N- 41)

November mean discharge 0·26* 0·19 0·23 0· 17 December mean discharge 0·40* 0·32* 0·33* 0·28 January mean discharge 0·40* 0·27 0·31 * 0·23 February mean discharge 0·54* 0·48* 0·49* 0·42* March mean discharge 0·54* 0·46* 0·28 0·39* April mean discharge 0·39* 0·31 * 0-28 0·28 Winter flood peak 0-54* 0·41 * 0·44* 0.36* Summer flood peak -0·25* -0·20 -0·14 -0·09 Annual flood peak 0·03 0·07 0·22 0·23 * Correlation significant at p < 0·05.

ADWR13E_0014 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 147 6~------~ • 5- + 4-

3- +

2- + + + 1- + + 0 0 1~ ,.. I - ' 0 ~ 200 400 ~ 600 800 1000 3 1 Peak annual discharge (m s- ) Figure 7. Cottonwood establishment years in relation to peak annual discharge and average daily flow rate during spring (March) at Redington, San Pedro River. (o) = non-establishment year: (e) = establishment year: a bar through the symbol denotes a year in which the peak annual flood occurred in winter (October-March).

1916 (on a denuded floodplain after a large flood; Robinson, 1965) but did not became abundant there until the 1940s and 1950s (Turner, 1974). This may have provided the seed source for spread of saltcedar up the San Pedro River during the 1950s (middle basin) and 1960s (upper basin). Most of the cottonwoods and saltcedars along the San Pedro River date to ~*AP winter (October-March) floods in the post-1960 era. During this time, saltcedar and cottonwood established more synchronously than expected based on ,ijKtvl\i§!J!ry,OH differences between the species (Horton, 1977; Fenner et al., 1985). I'l~pil'li 'fue abundance of saltcedar seeds in late summer, saltcedar establishment was not strongly . . . associated with summer floods. Late-germinating seedlings may notSbrillaultali!ffl: D1v1s1on reserves to over-winter. Or, the flashy summer floods may not create appropriate fluvial surfaces for germination, with herbaceous plants rapidly colonizing the small patches of alluvium that are exposed. Late summer flood waters also may recede too rapidly to allow seedlings to keep pace with declining water levels (Mahoney & Rood, 1992). Winter floods, in contrast, tend to be of longer duration and thus may result in more channel movement, vegetation scour, and sediment reworking. They also are typically followed by high flows in spring and early summer (correlation of 0·50 between annual winter flow peak and average flow rate during Feb-May at the Redington gage; N = 48; p < 0·01). This provides the moisture essential to sustain young seedlings and exposes competition-free seed beds during the germination season for cottonwoods and willows and in many cases for saltcedar as well. Although

ADWR13E_0015 148 J. STROMBERG 35~------~ (a)

V1991 30 ■ 1993 □ 1995

35------~ (b) • 1987 30 - □ 1988 □ 1989 o 1990 • 1992 IUl 25 ._ ■ 1994 M5 ~ 201- 0 ~ C 2;> 151- ·@ ""O ~ 10~ ::,:

Month Figure 8. Mean daily flow rates. by month, during years (a) with and (b) without cottonwood recruitment during the late 1980s and early 1990s (Redington stream gage). saltcedar and cottonwood both responded positively to winter/spring high flows, subtle variations in timing, duration, and recession patterns of the winter floods can influence relative dominance of the two species (Stromberg et al.. 1993; Stromberg, 1997). Stream flow rates and ground-water levels exert a strong influence on riparian forests as they affect survivorship of seedlings and mature trees. Over the length of the San Pedro River, differences in water availability relate to differences in composition of the

ADWR13E_0016 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 149

3.5~------, (a)

3.0

0.5

0.0 L__ _L_ ___i__ .L__ _ _L_...... o---Jp~',N.::..'!~ .... ,._i~-l, ... _.._-::-,l 1900 1910 1920 1930 1940 1950 1990 2000 6.0 (b)

5.0

1.0

0 ·f900 1910 1920 1930 1940 1950 1950 1910 19ao 1990 2000 Year RECEIVED Figure 9. Mean daily flow rates during (a) spring and early summer (Feb-May), and (b) throughout the year (Oct-Sept), in the middle (Redington, e) and upper (Charleston, o) San Pedro River. MAY 2 !ii 2011

surface Water Division dominant tree species. The dominance of saltcedar at the ephemeral, deeper ground- water sites is consistent with studies showing it to have deeper roots, higher water-use efficiency, and greater capacity for utilizing water from unsaturated soils, compared to Fremont cottonwood (Graf, 1982; Busch & Smith, 1995). In the upper basin, recent increases in relative abundance of saltcedar at sites with large ground-water fluctuation (e.g. Contention well; with declines to 1 m below the thalweg in the dry season) may be a symptom of recent ground-water decline tied to municipal ground-water overdrafts and reduced recharge from the dewatered Babocomari basin (Schwartzman, 1990). Various studies suggest that Fremont cottonwood-Goodding willow forests are best

ADWR13E_0017 150 J. STROMBERG

60------~ 60------50 50 40 40 30 30 20 20 10 10 0 LalLL_ _l__l_-cL 0 L__L_ _L__L_ _u__u_1_ill._J_LL_l_J 1920 1930 1940 1950 1960 1970 1980 1990 1920 1930 194019501960 197019801990 3------3------

0 _ _L_ _L__L_ _u__Ll_L_Lll_j_JL-...J 1920 1930 1940 1950 1960 1970 1980 1990

~ 2000 ....~ 2000 !@ 1500 a 1sao ,'l I 1000 ~ 1000 .€ 00 500 :i 500 C § 8 Oc__J__ _J__c__J__ __!_._1---..L o o--'----_L__J__ _u__Ll_1_LU__LJ'_L__l_J 19201930194019501960 1970 1980 1990 1920 1930 1940 1950 1960 1970 1980 1990 Establishment decade Establishment decade Figure 10. Patch width, basal area, and stem density for decadal age cohorts of Populus fremontii (left) and Tamarix chinensis (right) in the middle basin of the San Pedro River. developed where ground-water is about I to 3 m below the flood plain surface (McQueen & Miller, 1972; Stromberg et al., 1991, 1996; Busch & Smith, 1995) whereas saltcedar stands retain high density and biomass at ground-water depths up to about 7 to 10 m (Gatewood et al., 1950; Horton & Campbell, 1974; Van Hylckama, 1974; Graf, 1982). There remains a need to refine threshold values for ground-water depths necessary to maintain Fremont cottonwoods in the plant community, given the complexity of influencing factors. These included soil type and stratigraphy (as they affect moisture-holding capacity, capillary rise, formation of perched water-tables, and lateral water movement), climate (e.g. rainfall and evaporative stresses), and population factors (e.g. stand density, stand age, and acclimation to site conditions). For example, although a few Fremont cottonwood centenarians persist on San Pedro River terraces where water-tables are estimated to now be at a depth of 5 m, cottonwoods in other areas have been killed by ground-water declines of 1 m (Scott et al., in press). Livestock grazing is a common land use along the San Pedro River. Among multi-species patches of seedlings, livestock prefer the more palatable cottonwoods and willows over saltcedar. Livestock thus favor saltcedar by reducing competition from the faster-growing cottonwoods and willows (Hughes, 1993; Stromberg, 1997). Exclusion of cattle since 1994 from the perennial sites of the middle basin (BLM land) probably has contributed to the recent recovery of Fremont cottonwoods (and of Goodding willows, data not shownl. In the upper basin, cattle have been excluded from all sites since 1987. Since that time, herbaceous vegetation along the river banks has increased substantially The fine roots of the grasses, rushes, and other plants have stabilized channel banks and contributed to stream channel narrowing (Krueper, 1993). Channel stabilization may be reducing availability of sites available for colonization by any pioneer tree seedlings, and contributing to the post-1960 trend for reduction in area colonized by young cottonwoods.

ADWR13E_0018 DYNAMICS OF FREMONT COTTONWOOD AND SAL TCEDAR 151

60 60 g 50 g 50 ,s 40 ,s 40 l 30 i 30 -" 20 20 !l ] 10 10 Ola• o'.;

15 15 ,. • •-" 12 '•-" 12 g 9 g 9 • 6 • 6 " •" • 3 3 •• •• • 0 dl 0 1890 1910 1930 1950 1970 1990 1890 1910 1930 1950 1970 1990 "' 1900 19~0 1940 1960 1980 1900 1920 1940 1960 1980 ]' 1500 ]' 1500 I 1250 I 1250 e 1000 8 1000 • ~ 750 f 1so l !~~ ~ o'--'-_l_--1._J'--'-..L-11-11='--'-~ 1890 1910 1930 1950 1970 1990 1900 1920 1940 1960 1980 Establishment decade Establishment decade Figure I I. Patch width, basal area, and stem density for decadal age cohorts of Populus fremontii (left) and Tamarix chinensis (right) in the upper basin of the San Pedro River.

(a) (b) 100 100 • •0 ~ 80 80 > •> - ~ 60 u 60 - 0 • t:• 40 40 - 0 l 0 0, 0, ,§ 20 e 20 - H ., ,l, 0 0 I I ' I I I 1920 1930 1940 1950 1960 1970 1980 1990 1890 1910 1930 1950 1970 1990 (c) (d) 1900 1920 1940 1960 1980 100 l00II""------.r--..------.---, 80 _E 80 i ~ • 60 • 60 I :: l I: 0 tI__tl_L_o_t,_l.I__t•a_c_•..i...wL'U..c,.·l.I_ '11 0 1920 1930 1940 1950 1960 1970 1980 1990 1890 1910 1930 1950 1970 1990 1900 1920 1940 1960 1980 Establishment decade EstablimJ\?EfJ Figure 12. Importance values (relative to each other) of Popa/us fremontii ( ■) and Tamarix chinensis (o) by decadal age cohort, in different reach types of the middle and u_pper l;>M,ips of the San Pedro River. (a) Middle San Pedro, perennial; (b) Upper San Pedij./Wer!l)iiai/.\Od)I Middle San Pedro, ephemeral; (d) Upper San Pedro, intermittent. Surface water Division

ADWR13E_0019 152 J. STROMBERG

Table 2. Mean values or ranges for electrical conductivity (Mil cm- 1 at 2S>C) in San Pedro River surface water and flood plain ground-water Ground-water Surface water* 1950s 1996/97 1960s and 1970s varying varying March July monthst months+ Upper basin Charleston 371 342 152-379 274- Middle basin Benson 692 490 Cascabel 638-945 Lower basin Winkleman 1162 776 387-1540

* USGS data 1 Halpenny et al. (1952) +This study.

Several variables that influence riparian forest structure vary continuously over the length of the San Pedro River. Among these are water quality (including salinity; Shafroth et al., 1995) and temperature, Zimmennan (1969) identified high salt content as a key factor favoring saltcedar abundance in the middle basin of the San Pedro. Low values for electrical conductivity in the upper basin (Table 2) may contribute to the continued dominance of Fremont cottonwood in the reach from the U.S./Mexican border to about Fairbanks, The increase in salinity with distance downstream may be due to accumulation of salts from irrigated agricultural fields, a major land use in the floodplain. The peak period for irrigated acreage in the middle basin (1960s) coincides with the period of greatest abundance for saltcedar along the San Pedro River (Lacey et al., 1975). Turner (1974) noted that saltcedar became abundant along the Gila River during a time period corresponding to increasing human use of the floodplain. The post-1960 decline in irrigated acreage (and thus of runoff of salty irrigation tail water) along the San Pedro may be contributing to the declining dominance of saltcedar in some areas, However, it is difficult to determine if salinities of surface and ground-water have changed appreciably over time, given differences in sampling protocols. Persistence of free-flowing conditions, allowing for frequent flushing of floodplain soils, will help to maintain ECs in the range that allows for at least co-dominance of Fremont cottonwood. Cool temperatures and frequent frosts may be one factor reducing the overall importance of saltcedar in the upper basin. Radial growth rate of saltcedar (but not Fremont cottonwood) was substantially lower at the higher elevation sites (Fig, 2), This resulted in a larger size differential between same-age cottonwoods and saltcedars in the upper basin than in the middle basin, and presumably a larger size differential in reproductive output as well, Lacey et al. (1975) reported that extensive stands of saltcedar extended along the San Pedro River as far upstream as Fairbanks (elevation of about 1350 ml, and this remains true today. This is consistent with Bowser's (1957) observation that although saltcedar grows in the South-west at elevations up to 3350 m, it does not spread rapidly above 1220 m.

I would like to thank Barb Clark, Dave Perino, the Dewells, the Bureau of Land Management, and the Arizona State Land Department for providing access to study sites. I also thank The Nature Conservancy for providing funding through their Ecosystem Research Program. Many thanks also to Caitlin Cornwall, Joelle Don De'Ville, Kim Fox, David Gori, Holly Harris,

ADWR13E_0020 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 153

Jennifer Hickman, Raenna Merrill, Ginny O'Connell, Duncan Patten, Susan Pierce, Troy Smith, and Ron Tiller for assisting with field sampling or laboratory analyses; and to Matthew Chew and Pat Shafroth for providing valuable review comments.

References

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ADWR13E_0021 154 J. STROMBERG

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ADWR13E_0022 DYNAMICS OF FREMONT COTTONWOOD AND SALTCEDAR 155

Stromberg, J.C., Richter, B.D., Patten, D.T. & Wolden, L.G. (1993). Response of a Sonoran riparian forest to a ID-year return flood. Great Basin Naturalist, 53(2): 118-130. Stromberg, J.C .. Tiller, R. & Richter, B. (1996). Effects of groundwater decline on riparian vegetation of semiarid regions: the San Pedro River, Arizona, USA. Ecological Applications, 6: 113-131. Stromberg, J.C., Fry, J. & Patten, D.T. (1997). Marsh development after large floods in an alluvial, arid-land river. Wetlands, 17: 292-300. Turner, R.M. (1974). Quantitative and historical evidence of vegetation changes along the Upper Gila River, Arizona. U.S. Government Printing Office 543-583/91. Van Hylckama, T.E.A. (1974). Water use by saltcedar as measured by the water budget method. United States Geological Survey Professional Paper 491-E, pp. 1-30. Warren, D.K. & Turner, R.M. (1975). Saltcedar (Tamarix chinensis) seed production, seedling establishment, and response to inundation. Joumal of the Arizona Academy of Science, 10: 135-144. Webb, R.H. & Betancourt, J.L. (1992). Climatic variability and flood frequency of the Santa Cruz River, Pima County, Arizona. U.S. Geological Survey Water Supply Paper 2379, pp. 1-40. Zimmerman, R.L. (1969). Plant ecology of an arid basin, Tres Alamos-Redington Area. U.S. Geological Survey Professional Paper 485-D, pp. 1-51.

RECEIVED

MAY 2 ~ 2011

Surface Water Division

ADWR13E_0023