Plant Succession on Merced River Dredge Spoils[1]
Thomas H. Whitlow and Conrad J. Bahre[2]
Abstract.—One hundred and nine species of vascular plants were collected from 22 stands at six sites in the 2,800 ha. (7,000 ac.) of dredge spoils along the Merced River near Snelling, California. Five sites were dredged, one per year, in 1910, 1928, 1938, 1941, and 1950; one was not dredged. Association analysis of the stand data identified four species groups closely related to dredge-spoil topography and moisture availability. In addition, the program ordinated stand data according to floristic affinities. The ordination showed no age-dependent patterns.
Introduction
Nearly 24,000 ha. (60,000 ac.) of floodplains and terraces in the northeastern part of California's Central Valley were mined by huge gold dredges from 1898 to 1968 (fig. 1). In all, 12 major gold fields were dredged between Butte Creek and the Merced River (Clark 1970; Wagner 1970). The dredge spoils consist of wormlike ridges of unsorted boulders and cobbles with intervening swales of fine-textured soils and standing water (fig. 2). Except for a few areas of limited extent leveled for housing or used as a source of aggregate, the spoils now serve as little more than poor grazing lands and wildlife habitat. Nevertheless, they offer plant ecologists a novel means of studying plant succession because the spoils are of similar structure and can be dated to the week of deposition as far back as 70 years. Summarized here is a preliminary investigation of successional patterns in the Snelling dredge field (fig. 3).
The Snelling dredge field, mined between 1907 and 1951 (Aubury 1910; Davis and Carlson 1952; Clark 1970), consists of 2,800 ha. (7,000 ac.) of spoils paralleling the Merced River near the town of Snelling (fig. 4). Snelling (79 m. [259 ft.] above sea level) has an average annual precipitation of 840 mm. (33.2 in.), most of which falls between November and April (California Department of Water Resources 1980). Local vegetation is valley oak woodland (sensu Griffin 1977), grassland (sensu Heady 1977),
Figure l.
Yuba Dredge No. 2. This dredge, once owned by Yuba
Goldfields, was brought to Snelling from Montana in 1935.
Only 23 m. (75 ft.) long, it was one of the smallest dredges
ever used at Snelling. It was dismantled in 1939 and
taken to Chico.
Figure 2.
Dredge spoil mound tops. Note cobbles and boulders
as well as sparse vegetation cover. Trees in the swales
are Salix spp. and Populus fremontii . and riparian forest
(sensu Conard et al . 1977).
[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].
[2] Thomas H. Whitlow is Research Associate, Urban Horticulture Institute, Cornell University, Ithaca, N.Y. Conrad J. Bahre is Assistant Professor, Department of Geography, University of California, Davis.
Figure 3.
Map of east-central California. Note the Snelling field.
The gold is mostly flour gold derived from the pocket belt of the Mother Lode in Mariposa Country, which is traversed by the upper course of the Merced River. It is found in the Quaternary gravel deposits of the modern river which range in depth from 5.4 to 11 m. (18 to 36 ft.)
Bucket-line gold dredges, constructed chiefly of steel, were so heavy and cumbersome they had to be assembled in a dry pit. Water from a canal was turned into the pit, allowing the dredge to float (Aubruy 1910). The dredges were only capable of digging 7.6 to 9.1 m. (25 to 30 ft.) below the surface of the dredge pond. Their earth-moving capacity, depending on their size, ranged from 153,000 to 260,000 cu. m. (200,000 to 340,000 cu. yd.) per month.
Individual dredge piles usually differ in slope and elevation. Fines may or may not be found on the surfaces. Whereas some dredge piles are in neat rows. most are rather haphazard. Differences in spoil topography reflect dredging depths, the particulars ways in which the tailing
Figure 4.
Vertical aerial photograph of part of the Snelling dredge field. Note tree-lined swales and sparsely
vegetated spoil tops. The oldest spoils do not necessarily have tree-lined swales. In some cases, the
spoils were stacked so that no swales were left for trees to colonize (photograph taken in 1959).
stackers were operated, and whether overburden was sent directly to the stacker without sorting.
Methodology
Five study-sites in the Snelling dredge field were selected and dated using USDI Geological Survey 1:24,000 topographic maps, large-scale aerial photographs, mining progress maps, interviews with local residents, and records in the Merced County Recorder's Office (table 1). These sites were dredged, one per year, in 1910, 1928, 1938, 1941, and 1950. One undredged site was selected as a control.
Stands on mound tops, high swales, and low swales at each dredge site were subjectively chosen to conduct a complete floristic inventory. This inventory was needed to detect floristic distinctions between sites of different ages. The sharp gradients and irregular depositional patterns of the spoils made random and systematic sampling techniques impractical and highly sensitive to artifacts of sample location. Edge effects were minimized by placing the stands no closer than 100 m. (300 ft.) from the edges of the spoils. The stands, which varied in size and shape, were inventoried using the Braun-Blanquet relevé method (Mueller-Dombois and Ellenberg 1974). High swales were inventoried intoto , while mound tops and low swales were inventoried along belt transects whose lengths were designed to include all species in a particular habitat. The sampling method was designed to eliminate minimum area problems arising from the use of discrete sample sizes, to represent the total flora, and to establish within-stand homogeneity while maximizing the possibility of between-stand differences.
The stand data were analyzed using the association analysis program of Ceska and Roemer (1971). This program, which identifies associational relationships between species using pre-established phyto-sociological criteria, arranges species into groups of co-occurrence and ordinates stands according to floristic similarities. Of the program's five different "inside-outside" rules, the 50%-inside: 20%-outside rule was chosen because it included the highest percentage of the entire flora and hence accounted for more variations than the other rules. According to this rule, a species belongs to a species group if it occurs in 50% of the stands in its group and does not occur in more than 20% of the stands outside of its group. A stand belongs to a species group when it contains 50% of the species in the group. Field work was carried out between March and May 1980, during the peak bloom of the annual flora.
Results and Discussion
One hundred and nine species of vascular plants were collected from 22 stands at the six study-sites in the dredge field. The association
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analysis program organized the stand data into four species groups (table 2). Only 36 of the 109 species were sufficiently well associated to be included in these groups. From left to right in table 2, the stands range from dry mound tops with sparse annual grass and herbaceous cover, to high dry swales dominated by thickets of Salix spp., to low mesic swales with forests of Salix spp. and Populusfremontii surrounding
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shallow ponds. The four high-swale stands at the far right of the table do not fit within the topographic-moisture gradient of the rest of the ordination or within any single species group. This fact, plus their affinities to Groups 1, 2, and 4, are due to small-scale habitat variation. Stand 2, a high swale covered by grass, also falls within the xeric mount tops, whereas Stand 10, another high swale, contains two species groups.
The nesting of the species groups according to moisture availability and topography is readily apparent in table 2. Group 1, composed primarily of introduced annual species, grows exclusively on mound tops and corresponds floristically and physiognomically to the "California annual type", a climax annual grassland described by Heady (1956, 1958, 1977). Heady's climax annual grassland depends on the development of organic mulch; here it has developed on largely unweathered dredge spoil and completely lacks a litter layer.
Group 2, made up entirely of native tree species, occupies the bottom of nearly every swale with fine-textured soils. It overlaps little with Group 1; only Stand 10 includes extensive representation from both groups. Group 2 species are common pioneers in disturbed riparian zones in the Central Valley, and except for Quercuslobata all have wind-dispersed seeds (Munz and Keck 1959; Thompson 1961; Conard et al . 1977). Except for two large individuals at the 1910 site, the oaks are seedlings growing in the shade of willows. Since no parent oaks grow nearby, the oaks probably germinated from acorns dispersed by animals.
Group 3, consisting of woody perennials and emergent aquatics, occupies the edges of forested sites and shares most of the low mesic swales with Group 2. Group 3 reflects the greater habitat diversity of the low swales because of the presence of perennial ponds and a forest canopy. Vitiscalifornica and Rubusprocerus occupy the landward margins, whereas Typhalatifolia and Juncuseffusus occupy the pond margins.
Group 4 is made up largely of herbaceous annuals growing in semi-shaded, moist places in the swales containing standing water. The species in this group range from emergent and floating aquatics (Cyperusalternifolius , Alisma sp., and Marsileavestita ) to plants usually found in open, droughty habitats (Centaureamelitensis , Brizaminor , and Cynodondactylon ). Group 4, like Group 3, is also associated with a forest canopy.
Sixty-two percent of the species collected in the dredge field are native, 38% are introduced. According to Heady (1977), the percentage of native plants in species lists for individual stands in California's annual grassland ranges between 71% at Hastings Reservation (White 1967) and slightly less than 20% at Hopland (Heady 1956). Our findings indicate that the Snelling dredge field has a relatively high proportion of native species.
In addition to identifying associational groups, the Ceska and Roemer program ordinated the stand data according to floristic affinities (table 2). However, the ordination shows no age-dependent patterns. In some cases, closer affinities occur between stands of the oldest and youngest sites than between stands of the same age.
Seventy-three species were not included in any of the four species groups because they were too frequent (>66% is the threshold value of the program) or non-faithful. Although we lack the quantitative data to fully interpret the significance of these rare species, two conclusions are drawn. First, our sampling was adequate because it included so many rare species. Large stands usually include a high proportion of rare species (Preston 1948). Secondly, rare species are continually being added to the flora for at least 50 years after spoil deposition.
Table 3 summarizes the percentage of rare species in proportion to the total flora of the six study sites. Vicia benghalensis was excluded because its occurence exceeded the 66% threshold of the program. The other species had frequency values of 18% or less. Note that the rare species made up 55% or more of the total species in the undredged and 1910 sites, and only 30–39% of the species in the other sites. A similar trend has been documented by Bazzaz (1975) in his study of old field succession in Illinois. There, he found that species diversity increased most rapidly during the first 15 years after field abandonment and that the species colonization curve maintained a positive slope for at least 40 years. Since the youngest spoils at Snelling are 30 years old, we have probably missed the rapid colonization phase that occurred shortly after dredging. White (1966) in his studies of abandoned fields at Hastings Reservation noted that only 18 to 28 years were needed after initial disturbance for the dominant species of the climax grassland to re-establish themselves.
Changes in species diversity were not quantifiable because the stands were of different sizes. Nevertheless, the 1910 and 1928 sites were floristically richer than the other sites (table 3). Of particular interest is the fact that Chlorogalum grandiflorum , Brodiaeacalifornica , and B . multiflora , all native perennials, were only found at the 1910 site. The occurrence of these perennials and greater species diversity at this site may not be entirely age-dependent. The site, which is periodically flooded by the Merced River, has some alluvium and is exposed to flood-borne propagules. The undredged grassland site contained most of the Group 1 species plus three native perennials: Stipapulchra , Brodiaeahyacinthina , and Calochortus luteus . In general, species
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diversity was highest in the forests of the mesic swales and lowest in the grasslands of the mound tops.
Conclusions
Only small changes in species composition were noted in the dredge-spoil sequence investigated at Snelling. The dominant species probably colonized rapidly after spoil deposition. The four species groups identified by the Ceska and Roemer (1971) program occur on all sites regardless of age. The groups correlate most closely with dredge-spoil topography and moisture availability. In general, the slow weathering of the dredge spoil has not resulted in enough soil development to affect the vegetation. Only in a few swales with moist, shallow soils and standing water is the vegetation very diverse or structurally complex. Several immature Quercuslobata in the swales and a Quercus wislizenii at the 1910 site are the only evidence of direct species replacement in the study-sites. The data suggest that species richness increases with successional age and that 50 years or more are required for the accumulation of well-developed flora. Structural changes will be much slower, correlating with the slow development of soil.
Acknowledgments
Financial support was provided by a Faculty Research Grant from the University of California, Davis. We thank the following residents of Snelling for their assistance and information: Robert Peirce, H.G. Kelsey, Ed Romero, and Kermit Robinson. Thanks must be extended also to Jack Major and Marlyn Shelton who read and commented on the manuscript.
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