Western North American Naturalist Volume 62 Number 2 Article 5 5-2-2002 Flood flows and population dynamics of Arizona sycamore (Platanus wrightii) Juliet C. Stromberg Arizona State University, Tempe Follow this and additional works at: https://scholarsarchive.byu.edu/wnan Recommended Citation Stromberg, Juliet C. (2002) "Flood flows and population dynamics of Arizona sycamore (Platanus wrightii)," Western North American Naturalist: Vol. 62 : No. 2 , Article 5. Available at: https://scholarsarchive.byu.edu/wnan/vol62/iss2/5 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 Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Western North American Naturalist 62(2), © 2002, pp. 170–187 FLOOD FLOWS AND POPULATION DYNAMICS OF ARIZONA SYCAMORE (PLATANUS WRIGHTII) Juliet C. Stromberg1 ABSTRACT.—Platanus wrightii is a pioneer tree species of warm-temperate riparian deciduous forests in southwest- ern United States and northern Mexico. Dendrochronological analysis of populations in central and southern Arizona indicated that P. wrightii seedlings establish episodically. Long intervals (10–40 years) elapsed with no apparent estab- lishment. Seedling establishment years were positively associated with winter flood size and annual flow rate, and weakly negatively associated with summer flood size. Large floods sometimes preceded multi-year establishment waves. During the past 2 decades an abundance of winter floods and very wet springs has allowed frequent establishment of P. wrightii. For example, plants established at many sites during the winter flood years of 1993 and 1995, particularly along channels scoured and widened by flood waters. Platanus wrightii also reproduces asexually. Ramets were more abun- dant than genets in all populations, and ramets established more frequently than seedlings. Ramet density (mean num- ber per genet) varied widely between populations, from 2 (Huachuca Canyon) to 9 (Haunted Canyon), and increased with tree size within 4 of 9 populations. Population size structure varied across a gradient of watershed area. Populations along streams draining the largest watersheds had an abundance of small trees (mean trunk diameter of <10 cm), while those at headwater sites were dominated by mature trees with trunk diameter ≤190 cm and age ≤235 years. These observations suggest that population structure of P. wrightii is influenced by temporal as well as spatial differences in stream flow regimes. Key words: Platanus wrightii, floods, riparian habitat, seedling establishment, age structure, ramet. Population dynamics of many riparian tree well understood. Both P. fremontii and S. good- species are influenced by flood disturbance dingii disperse short-lived seeds in spring and (Duncan 1993, Hughes 1994, Scott et al. 1996, have narrow windows of opportunity for Cordes et al. 1997, Timoney et al. 1997, Rood seedling establishment (Horton et al. 1960, et al. 1998, Sakai et al. 1999). Patterns of ripar- Shafroth et al. 1998). Establishment of these ian tree mortality and establishment vary species depends on winter floods to erode or widely depending on frequency, magnitude, deposit alluvium, high spring stream flows to timing, and duration of floods (Poff et al. moisten seed beds, and slowly receding flood 1997). In arid regions flood patterns are highly waters to allow seedling roots to remain in variable between and within years, and flood contact with declining groundwater. Large flows can be very large relative to base flows. winter floods of long duration can extensively Large floods cause mortality of riparian trees widen channels and facilitate large-scale re- by eroding sediments from root zones and plenishment of Populus-Salix stands (Stromberg uprooting trees, breaking plant stems or bury- 1997, Stromberg et al. 1997, Mahoney and ing them with sediment, and causing mass Rood 1998). Along some perennial to intermit- wasting of flood plains. Floods also intermit- tent alluvial rivers in the Sonoran Desert, tently create conditions that allow establish- flood sequences that allow establishment of ment of pioneer species by changing channel P. fremontii occur approximately once every and flood plain geomorphology, clearing com- 7–10 years. These generally do not occur reg- peting vegetation, and moistening frequently ularly over time (Stromberg et al. 1991, dry flood plain surfaces. Stromberg 1998). Establishment frequency Hydrologic and geomorphic controls on varies among rivers, and some populations are establishment of Populus fremontii and Salix dominated by a single age cohort (Everitt 1995). gooddingii, the dominant pioneer species of Rates of population turnover appear to be Sonoran riparian deciduous forests, are fairly higher on lower reaches of rivers, as suggested 1Plant Biology Department, Arizona State University, Tempe, AZ 85287-1601. 170 2002] FLOOD FLOWS AND POPULATION DYNAMICS 171 by patterns of decreasing tree age with de- production has been shown to be stimulated creasing elevation (Stromberg and Patten 1992, by a variety of factors including disturbance Stromberg 1998). and changes in resource availability (Ischinger Less is known about establishment patterns and Shafroth 1995). of Platanus wrightii, a dominant species of My objective was to determine the in- warm-temperate riparian deciduous forests in fluence of temporal and spatial variability in the southwestern United States and northern stream flow regimes on establishment pro- Mexico (Brown 1994). Platanus wrightii occurs cesses and population size structure of P. in Arizona, New Mexico, western Texas, and wrightii. I hypothesized that P. wrightii seed- northern Mexico at elevations of approxi- lings would establish episodically in response mately 400–2000 m (Kearney and Peebles to flood disturbance and that establishment 1960). At lower elevations P. wrightii co-occurs would occur in years with large winter floods with Populus fremontii and Salix gooddingii. At and high spring flows, similar to patterns for higher elevations it grows with Populus angus- P. fremontii and S. gooddingii. I expected that tifolia and Alnus oblongifolia, among other recruitment frequency and abundance of species. Floods are a primary disturbance factor young trees would increase along a down- that structures these warm-temperate riparian stream gradient as drainage area, flood magni- plant communities (Campbell and Green 1968). tude, and channel instability increased. I also Floods occur during late summer after convec- hypothesized that ramets would establish tive thunderstorms, during winter and spring more frequently than seedlings and that pro- after Pacific frontal rainstorms and snow, and duction of ramets would increase during years occasionally in fall as a result of dissipating with above-average stream flow. tropical storms. Life history traits of P. wrightii suggest that MATERIALS AND METHODS it is adapted to disturbance. Each year trees Study Sites produce many small seeds (achenes) with tufted hairs that are suited to wind dispersal. Nine Platanus wrightii study sites were Seeds persist in clusters of 2–4 round balls selected along 7 rivers in southern and central (multiple achenes), each about 2.5 cm in diam- Arizona (Table 1). Each site spans a river eter, that gradually become detached from the length of 1–3 km. At 7 of the sites, data were tree throughout fall, winter, and spring. Seeds collected on tree age and population size ripen in November, germinate in April or May structure; at 2 sites (Sycamore Creek–Round following several weeks of winter stratifica- Valley and Huachuca Canyon), data were col- tion, and lose viability about 6 months after lected only on size structure. The Haunted maturation (Zimmerman 1969, Bock and Bock Canyon and Pinto Creek sites were combined 1989, Brock 1994). Platanus occidentalis, a for some analyses, as were the Sycamore species that occurs in eastern North America, Creek–Bushnell and Sycamore Creek–Work germinates at high rates on bare mineral soil Station sites. Some sites were selected because exposed to high light intensities (Fowells 1965, they allowed investigation of other biohydrol- Sigafoos 1976), and the same may be true of P. ogy relations of P. wrightii, including relation- wrightii. ship of groundwater level to bioproductivity Platanus wrightii reproduces vegetatively (Stromberg 2001a). Other sites were added to as well as by seed. Shanfield (1984) suggested provide a wide range of watershed sizes and that Platanus racemosa, a California species elevations. that is closely related to P. wrightii, may main- The streams mainly flow through alluvial tain its population size by relying on vegeta- sediments that are readily reworked by flood tive reproduction. However, factors that stim- flows; there also are areas within sites where ulate production of vegetative sprouts (ramets) channel morphology is controlled by exposed by P. wrightii are not known. Along one stream bedrock. Platanus wrightii is a dominant species in southern Arizona, Glinski (1977) found no at all sites. Riparian ecosystems at the low-ele- correlation between P. wrightii sprout densities vation study sites are bordered by Sonoran and several parameters including percentage desertscrub and interior chaparral; those at of canopy dieback, soil texture, and distance to higher elevations