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22 J. C. STROMBERG RESTORATION OF RIPARIAN VEGETATION OF FLOW REGIME 23 summer rains (Scott er al., 2000a). With sufficient data on transpiration rates b C ~iltypes and stratigraphy (soil water holding capacity, clay lenses and perched water plant assemblage and with vegetation maps, one c~ estimate total wa~er use rates fo lllyers) and other hydrologic modifiers (inundation frequencies). long river reaches (O'Keefe & Davies, 1991; Goodnch er al., 2000). With a know~edg Mahoney & Rood (1998) have developed a model, coined the recruinnent box, that of the water requirements for high-quality states of each assemblage, one can esUInat bounds the requirements for Populus seedling establishment with respect to post-flood the amount of water necessary to restore or maintain ecosystem integrity. recession rate, river stage, and flood timing. Recruinnent also depends on high-energy In addition to knowing quantities of water needed, it is important to know how wat nOoods to perform the geomorphic 'work' necessary to create recruinnent sites and should be distributed in time and space. A plant's requirement for a particular spati reiriitiate community succession. Small winter or spring floods result in only a small and temporal distribution of water can be determined by exam~ng ho~ popu,Iatio . IUl.1ount of sediment movement, erosion, and deposition, and produce only small demographic processes such as recruinnent, growth, andmortabty vary 10 relauon t recruinnent bands (Stromberg et al., 1993a). Large, long winter floods, such as occurred hydrologic factors such as flood timing or ground water depth. One can th~n defin,e during the 1992-1993 E1 Nino rainy season, form new channels and widen existing thresholds and quantify relationships between plant processes and hydrolog1~ condl channels (Huckleberry, 1994). Extensive stands of Populus arid Salix can establish on tions. Spatial and temporal components of water needs also can be d~termmed b the expanses of bare mineral soil (Stromberg, 1997). analysing water samples for stable isotopes of oxygen and hydrogen. This can help t The period of water recession following the winter flood peaks can extend well into identify the source and depth of water used by the plant, be it the flood plain aquifer sUmmer. This creates recruinnent opportunities for species with a range. of temporal regional aquifer, stream channel, r~ ~a~er, or some ~ombination there.of.. ., regeneration niches, including spring-seeding Populus and Salix and summer-seeding Water requirements can not be mdlvldually quanufied for all species 10 npana l3accharis salicifolia, Tessaria sericea, and Tamarix spp. Late-summer monsoon floods ecosystems (Sparks, 1995). Riparian ecosystems are 'hot-spots'. of biodi.versity. and lllso directly create recruinnent opportunities. Sonoran panic-grass (Panicum sonorum), hundreds of different plant species can occur along perenmal and. mtermlttent, . (or example, depends on summer floods to complete its life-cycle. Once common along streams (Naimanetal., 1993; Brown, 1994; Wolden.et aI., 1994;.Tabac~hl~tal., 1~96;( . the l~wer Colorado River, the plant has been locally extirpated, due partly to sup- Dixon & Johnson, 1999). However, it may be pOSSible to claSSify species mto gwlds,: pressIOn of summer floods (Nabhan, 1985). Many other plants also belong in this guild select indicator or focal species within each guild, and thereby quanufy the water needs; of summer-flood dependent pioneer species. for a large number of species. Each focal species could define 'differe?t spatial ~nd. . Prl:sopis velutina, Prosopis glandulosa, Prosopis pubsecens, Sporobolus wrightii and compositional attributes that must be present in a landscape and their appropnate, Sporobolus airoides are among a guild of seral, warm-season species that establish in management regimes' (Lambeck, 1997). .' . . ...... response to summer rains or summer floods. They are deep-rooted, facultative Salix gooddingii and Populus fremontii are representanve of a guild of Sonoran npanan: phreatophytes that can utilize deep ground-water and shallow soil moisture. In wetter species that depend on shallow ground water and periodic winter/spring flooding. Dense.. ; parts of their range, they can survive solely on precipitation (Scott et al., 2000a; productive stands of these trees occur where ground water av~rages less than about three. ; Tiller, unpublished d.ata~.. Biomass struc?-1re varies greatly for P. velutina depending meters deep (Busch et al., 1992; Stromberg er al., 1996; Snuth et al., 1998). ~fthe two on ground-water avallability. Canopy heights of 4 m are typical for P. velurina trees species, P. fremontii seems to be able to utilize water from unsaturated sollla!~rs to' in Sonoran desert uplands. Larger size is attained for trees growing along ephemeral a greater degree then S. gooddingii, a more strict phreatophyte (Snyder & Williams,' , stream courses that receive periodic flooding. Trees reach greatest heights (up to 12 m) 2000). Both are quite sensitive to drought (Rood et al., 2000; Leffler et al., 2000). when growing along rivers where ground-water is at a depth of less than about 10 m Annual growth rate of Populus and Salix species declines in ye~ with low stre~m flows (Str,omberg et al., 1 ?92; Stromberg et al., 1993b). Small, low-energy floods that deposit and deep ground-water tables, with seasonal or annual declines of.1 m haVIng been, sediment aroun~ plOnee~ plants serve to create aggraded fluvial surfaces upon which observed to kill adults and juveniles (Stromberg & Patten, 1995; Willms. et aI., 1998; . these seral species establish. . Scott et al. 1999' Scott et al., 2000b; Shafroth et al., 2000). Other woody pioneer plants Natural flood regimes help to maintain high biodiversity not only by creating a diver- of low-ele~ation' south-western streams that appear to depend on similarly shallow sity of temporal regeneration niches but also by creating micro-habitats that vary ground-water include Baccharis salicifolia, Salix exigua, and Salix bonplandiana (Gary, spatially in depth to the water table, light availability, and soil properties. For example, 1963). .. . ... . flood deposits vary in depth, texture, and nutrie?t content, and support different Continued establishment of Populusfremontn and SallX gooddmgn depen~s on pen- assemblages of plants (Marks, 1950). Deep depOSits of sand and gravel, toci dry for odic occurrence of years with appropriately timed flood flows, high growmg-season . marsh plants or for Populus-Salix seedlings, favor a guild of drought-tolerant shrubs stream flows and very shallow water tables (Everitt, 1995). Along free-flowing streams . including Atriplex linearis, Bebbia juncea, Chrysothamnus nauseosus, Hymenoclea mono- in the Sono~ Desert, regeneration floods occur about once every 5-10 years (S,trOm- gyra, Petalonyx thurberi and herbs including Cleome lutea, Dicoria cansescens, Eriogonum berg, 1998b; Stromberg et al., 1991). .~ecruinnent patterns are irregular, varymg to spp., Euphorbia hyssopifolia, Heterotheca 1!sar:',mop.hila, Polanisia dodecandra, Sporobolus a large degree with frequencies ofEl Nmo years (Webb & Betancourt, 1992; Swetn~mCllntractus, Sporobolus cryptandrus, and Taqullla plJeata (Rea, 1983; Wolden et al., 1994; & Betancourt 1998). In recent decades, there has been a high frequency of years With Stromberg et al., 1996, 1997). Nutrient-rich soils with high content of silt and clay abundant ~ter rains and floods and consequently many new cohorts of Populus favour Lycium andersonii, Lycium fremontii, Viguaera dentata, Panicum obtusum, fremontii and Salix gooddingii have established. . ' 'Plueraphis mutica, Sporobolus wrightii, Sporobolus airoides, Ziziphus obtusifolia and many After a flood pulse, survivorship of Pop~l~s and Salix seed1ing~ depends on thelf other species.. Salty. areas, in floo? pla,ins ~upp~rt a, gliild of halophytes including ability to maintain root contact with the declining water table. Yearhngs are able to tap.. l1enrolfea OCCtdentallS, Atnplex lentiformlS, Dutlchlu spJeata, and Suaeda ton-eyana. water at a depth of about a metre or so by summers' end (Stromberg et al., 1991;, Floods also influence biodiversity patterns by causing river channels to relocate and Mahoney & Rood 1998). Along the Green River in Colorado, however, some.lmeander, creating abandoned channels and backwater depressions, and inducing chan- P. deltoides subsp. ~islizenii did not become phreatoph~c until they were seve!al years nel widening and subseq,uent re-narrowing. Areas with standing water or near-surface old (Cooper et at., 1999). Between-site differences 10 ground water ~equlrementslwater ~bles, such as might develop after floods ero~e ~errace.s or form off-~hannel may arise due to differences in climate (rainfall, temperature, evaporauve stresses), (epresslons, support marshlands vegetated by species mcludmg Juncus artlCUlatus, ~'- .- ' o W r. "'. <..::;; I.':;