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54 F. ZAMORA-ARROYO ET AL Ground transects In July and August 1999, we established nine ground transects to document floodplain geomorphology, soil salinity, depth and salinity of ground-water and distribution of plant species by percent cover and plant density. Sampling methods were adapted from those used elsewhere on the Colorado River by others (Busch & Smith, 1995; Ohmart et al., 1988). Transect locations were preselected before visiting the river to ensure that they were placed without bias towards particular vegetation conditions. Lines were marked on a topographic map of the river at 10 kIn intervals starting 5 kIn south of Morelos Dam and ending near the junction of the Hardy River and Colorado River. The nearest vehicular access point to the river on either bank was then taken as the starting point for establishing a field transect for each line marked on the map. In some cases the predetermined spot on the map could be accessed in the field, by driving along the levee banks and using GPS, whereas other transects were established as far as 1 kIn from the predetermined spot due to lack of access to the river. The anchor point for each transect was established by walking from the vehicular access point to the river channel, then pacing a random distance "(0-300 paces by random number selection) upstream or downstream, determined by coin toss. A baseline was then established, running perpendicular to the river from the anchor point to the levee, road or agricultural field at the back of the floodplain. A stratified sampling method for surveying vegetation was used (Cook & Bonham, 1977) in which each transect was divided into different strata based on based on plant species composition and elevation with respect to the river channel. This method allowed us to sample as intensively within the native tree stands as within the much more common T. ramosissima areas. We recognized a low-zone stratum, consisting of a beach sloping to a narrow, low terrace, at sites where the river had not incised; this stratum was characterized by stands of the emergent species, Phragmites australis, nearest the water with narrow strands of native trees and other vegetation behind. Behind the low-zone was a mid-zone stratum, constituting the major terrace of the flood plain at all sites; this stratum was generally dominated by salt tolerant shrubs (T. ramosissima and P. sericea) but in some cases native trees were also present as isolated specimens scattered over the terrace. Finally, we recognized a back-zone stratum, where flood water had washed seeds against the containing levees to produce a narrow strand of native trees along the inside bank of the levee. Not all transects had all three strata present. The length of each transect and of each stratum was measured by tape or for long transects, GPS. Each transect was surveyed by theodolite to determine elevation of each zone relative to the bottom of the channel (river flow was minimal during surveys). In each stratum, up to five plots, 2 x 30 m, were established at random intervals along the transect baseline. The 30 m lengths of plots ran upstream or downstream, deter- mined by coin toss, parallel to the river. Canopy cover (% of the transect occupied by each plant type) was recorded by height class for each perennial species along the 30-m length of plot nearest the river using the line-intercept method, and plant density was determined by counting individual plants within each plot (Curtis & Cottam, 1962). Height classes were: 0-2,0 m (understory); 2.1-6.0 m (midstory); and> 6'0 m (over- story). Since annual plants were scarcely present, the percent of bare soil along the transect was estimated by summing the percent cover of individual species and subtract- ing from 100. When a stratum was longer than 100 m, plots were located in the 100 m of the zone nearest the river. When strata were too short to support 5 non-overlapping plots, fewer were established with a minimum of two, one upstream and one down- stream in very narrow zones. One transect (Pescaderos) consisted of a nearly impenetrable monoculture of T. ramosissima; cover and density were estimated along the baseline at this site without establishing side plots. In total, 52 plots in 14 strata were completed. To estimate the % cover of species over the entire study area, means REGENERATION OF TREES IN RESPONSE TO FLOOD RELEASES 55 and variances of plant composition in each stratum were weighted according to their length compared to the total length of all strata using methods in Cook & Bonham (1977). Tree cenSUS data C~) c;~ c,...) r.:> Popu~us fremontii and S. gooddingii trees were not numerous enough in the transect plots to gall~ an a~curate estimate of their distribution by species, size and age class. We did more Intensive sampling near three transects (Nos 2, 6 and 9) that contained well- developed stan~$. of trees. We select~d a starting point along the baseline within a .stratum. contaInIng trees, then determIned the species composition, height and trunk diameter lust above the basal swelling of the .first 50 trees ( > 4 m height) encountered ~pstream and downstream of the starting point, by selecting the nearest tree to the one lust mea~ured .as the next one t~ saml?le. Tree height was estimated by a triangulation method In which a 2-m measuring sock was held near the tree and projected visually uP, the length of the tree by an observer standing several tree lengths distant. We esomated age of trees from their trunk diameters by taking core samples from a sub- sample ?f trees to corr:late number of annual rings with length of core ( x 2 to project to trunk diameter assummg cor~s re~resent radii ~f trees), using methods in Stromberg (1998a). Ho~ever, we found It easier to count rmgs without sanding cores first. These trees h,ave .dlffuse pores, making rings difficult to distinguish, so ages are only approxlmaoons. At total of 264 trees were measured (50 trees were not available at some sites). (';"':"J. 00 Comparison of native tree cover on U.S. and Mexico river stretches The U~ited Sta~es Bur~au. of Recl~ma~on (BOR) maps vegetation by aerial photogra- phy USIng a seml-quanotaove classlficaoon system based on vertical structure complex- ity and percent .of na~ve trees. (Ohmart et al., 1988; M. Balough, BOR, Boulder, Nevada, unpublished mformaoon sheets accompanying 1997 aerial survey data). We. used the sa~e general system to classify the 63 aerial images taken along the na~ve ~ee zone In the delta. BOR classifies riparian vegetation in I-ha mapping urnts usmg a two-tier system. First the mapping unit is classified by dominant plant type. In general the dominant plant type must constitute > 50% of plant cover, b~t BOR ~ounts a plot that has > 10% P. fremontii + S. gooddingii as cottonwood- WIllow habitat because even a few trees are considered to improve habitat value over shrub monocultures. Each mapping unit is then classified into one of six vertical structure classes based on the percent cover by overstory,' midstory and understory plan~s. For example, a p!ot with 35-80% cover of native trees over 5 m height is conSidered cottonwood-WIllow, open gallery forest habitat, while a plot with > 80% trees is classified as closed gallery forest. We classified eaeh aerial image (0'67 ha) having > 10% of the vegetation in the tree category as native tree habitat, then used the percentage of groundcover, shrub and trees in each image as rough equivalents of the three height classes of the Bureau of Reclamation to classify those images into gallery forest or shrub vertical structure types. Our height classes are not exactly the same as theirs, however. They consider understory plants as everything < 1 m height, but we used 2 m as the cutoff because juvenile plants of all major species were within this range. We used 6 m rather than 5 m as the minimum height for overstory plants, because this cutoff separated mature native trees from T. ramosissima and other shrubs. Hence, we tend to underestimate overstory, native tree density compared to their methods.