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The Role of Vegetative Reproduction in the Recruitment and

Survival of Populus fremontii and Salix spp. in a

Californian Riparian System

Mandy Tu

Section of Evolution and Ecology, One Shields Avenue,

University of California, Davis, CA 95616

E-mail: imtu@ucdavis.edu; Phone 530-752-1092; FAX 530-752-1449

Current Address:

The Nature Conservancy’s Invasive Species Initiative

821 SE 14th Avenue Portland, Oregon 97214

imtu@tnc.org, 530-802-8100

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ABSTRACT

The importance of vegetative reproduction of woody species in the family

Salicaceae has been assumed for a long time. Most studies of reproduction in

this family however, have concentrated on seed dispersal and seedling survival,

and scientific studies on the success of vegetative propagules are largely lacking.

The initial recruitment and establishment of riparian woody vegetation on a

newly-formed sandbar along the lower Cosumnes River, located in California’s

Central Valley, was followed from 1996 to 1998. The establishment and survival

of woody species populations were monitored using permanent belt-transects.

Cottonwoods (Populus fremontii ssp. fremontii) and willows (Salix exigua, S.

gooddingii, S. laevigata, S. lasiolepis) were the only woody colonizers. Seed

viability and dispersal were not limiting factors in their regeneration.

Nevertheless, the successful recruitment of P. fremontii from broken-off branches

is six times more important than recruitment from seeds at this site. Salix spp.

reproduced exclusively vegetatively from deposited branches.

Keywords riparian succession, Populus fremontii, Salix ssp., seedling

establishment, vegetative propagation

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INTRODUCTION

Vegetation succession following regular disturbance events such as large

floods has received much attention within the last 30 years (Bliss and Cantlon

1957, Leopold et al. 1964, Nanson and Beach 1977, Malanson 1993, Naiman and

Décamps 1997). In riparian systems, fluvial geomorphic processes have been

documented to continually create, change, and destroy environments suitable for

the establishment of riparian plant species (McLeod and McPherson 1973, Fonda

1974, Sands 1977, Frye and Quinn 1979, Warner and Hendrix 1984, Asplund and

Gooch 1988, Auble and Scott 1998). On sandbars, as well as on gravel bars and

point bars, the processes of sedimentation and erosion may continually alter the

colonization substrate (McBride and Strahan 1984, Taylor et al. 1999). The degree

of water drawdown may also influence the survival of established plants (Fenner

et al. 1984, 1985, Stromberg and Patten 1991, Rood et al. 1994, 1998). Most studies

of vegetation succession along rivers have looked at natural rivers or stream

systems (Frye and Quinn 1979, Fyles and Bell 1986, Dunham 1989). In controlled

river systems, several studies have concluded that certain rates of water flow are

necessary for the successful establishment of native riparian species (Fenner et al.

1985, Stromberg and Patten 1991, 1996, Segelquist et al. 1993, Braatne et al. 1996,

Begg et al. 1998, Mahoney and Rood 1998, Rood et al. 1994, 1998).

In California’s Central Valley, riparian forests once extended up to three

miles wide along each river in pre-European settlement times (Thompson 1961).

Since the early 1900’s however, California rivers and streams have been

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especially highly impacted by anthropoegenic forces such as agriculture,

damming, and urban development (Mount 1995). Hunter et al. (1999) deduced

that only 3.3% of once-existing riparian areas remain in the Central Valley, and

are mostly much degraded. Since the Cosumnes River remains as the only

undammed river in the Central Valley and still retains yearly flooding regimes, it

provides a unique opportunity to study the early (pioneering) stages of plant

succession following disturbance. Previous studies have examined plant

establishment on sandbars in the southwestern United States (Shafroth et al.

1994, 1998, Taylor et al. 1999) and in other parts of California (McBride & Strahan

1984, Shanfield 1984, Strahan 1984, McCarten 1989, Stromberg et al. 1991), but

none have studied these events along an uncontrolled river in California’s

Central Valley. By understanding some aspects of the reproductive ecology of

woody pioneering species, recommendations for the restoration of these highly

degraded environments, by simulating natural processes, can be implemented.

The goal of this paper is to describe the early recruitment of woody tree

species on a sandbar at the Cosumnes River Preserve, where the most pristine

patches of riparian forests remain in California’s Central Valley (Reiner 1996,

Wicinas 1998). This paper addresses the early colonization stage of plant

establishment and survival on a newly deposited substrate. I hypothesized that

the primary means of recruitment of woody trees would be from seeds, as the

seed production by the studied species is prolific and most of the published

literature from the western and southwestern United States have focused on the

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successful establishment of woody seedlings (McBride & Strahan 1984, Strahan

1984, Shafroth et al. 1994, 1998, Stromberg et al. 1991). At the same time, records

on vegetative reproduction are mostly anecdotal. Only very few studies have

determined the relative importance of sexual versus asexual propagules of

Salicaceae in native stands (Braatne et al. 1996). The present study quantifies the

magnitude of these two modes of Salicaceae reproduction.

METHODS

Study Site

The Cosumnes River Preserve is a Nature Conservancy preserve located

at 38˚ 15'N 121˚ 23'W, approximately 20 miles south of the city of Sacramento in

California’s Central Valley. The Cosumnes River is the only river in California's

Central Valley that has been left undammed (Mount 1995, Reiner 1996) and still

experiences seasonal flooding. This flooding may last between one to six months

in duration (Reiner, pers. comm.). The vegetation in this area is structured by

both stand age as well as by elevation in relation to the river floodplain (Tu,

unpubl. data, Conard et al. 1977). Dominant trees in this area are P. fremontii in

the lower wetter and younger areas, Fraxinus latifolia, Acer negundo and Salix

ssp. in the mid-layers, to mature Quercus lobata trees in the more upland and

older sites.

In the fall of 1995 a levee was intentionally broken by The Nature

Conservancy just south of a 15-year-old cottonwood forest (hereafter referred to

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as the Accidental Forest). In the ensuing winter floods, a new sandbar

established perpendicular to where the levee was breached, on an old abandoned

farm-field.

Study Species

Populus fremontii S. Watson ssp. fremontii – This common tree of

streamsides and alluvial bottomlands is widespread from California throughout

the West to the Rocky Mountains (Hickman 1993). P. fremontii was prominent in

the pristine riparian forests of the Sacramento Valley (Thompson 1961), and is

still abundant in wet areas throughout California’s Central Valley (Griffin and

Critchfield 1972). This species has seedlings that are easily recognized by their

two almost cordate-shaped cotyledons. P. fremontii is known to recruit easily

from both seeds (Fenner et al. 1984) and from branches (Pope et al. 1990). For the

duration of the study, it could easily be determined if new individuals that

arrived on the sandbar were derived from either branches (by their initial large

size) or from seeds.

Salix exigua Nutt., Salix gooddingii C. Ball, Salix laevigata Bebb, Salix

lasiolepis Benth. – These dioecious trees and shrubs of moist habitats are also

widespread throughout California and the West in lowland areas (Hickman

1993). For the purposes of this study, all species of this genus were combined for

ease of analysis. Salix species have been documented to recruit via both seeds

(Niiyama 1990, Shafroth et al. 1994) and by clonal growth (Douglas 1987).

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Seedling Recruitment and Survival

On the newly-created north-south oriented sandbar, five east-west

running belt-transects which run perpendicular to the water line (parallel to the

river) were established during the early summer of 1996. The five belt-transects

varied in length from 8 to 12 meters, and were 0.5 meters wide. These transects

were monitored for seed germination and survival of all woody tree species four

times from 1996-1998. A total area of 23 square meters was sampled for

seedlings.

Vegetative Branch Recruitment and Survival

In addition to these transects, within the boundaries of the entire transect

area (1,000 m2), vegetative growth from pieces of broken-off branches of both P.

fremontii and Salix spp. were tagged, mapped and monitored for survival from

1996 to 1998. These sprouting branches were measured four times between 1996

to 1998. Stem diameter was measured at the soil surface. Height was measured

to the apical meristem. Survival of seedlings and branches was calculated from

both the first and second cohort years (1996 and 1997).

RESULTS

Seedling Recruitment and Survival

The germination of woody species growing on this sandbar began in late

May of 1996. Seeds of P. fremontii have a short viability time of 2-3 weeks (Tu,

unpub. data; Fenner et al. 1984) in which there is almost complete viability and

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the tendency to germinate immediately after dispersal (McBride and Strahan

1984). Seeds of P. fremontii were dispersed over the entire surface of the

sandbar, but were more concentrated in the moist zones either at the edge of the

outflow or in small depressions which were scattered across the sandbar surface.

In all three years of this study, no seedlings of any Salix spp. were found.

In the first field-season (1996) after the levee breach, I mapped and

followed the fates of 7,898 P. fremontii seedlings that had germinated within the

five belt-transects. By the end of the first field-season, all but 37 of the seedlings

had died. There was no significant correlation of surviving seedlings with

relative elevation on the sandbar (Tu, unpub. data). In the following year only 7

seedlings of the original seedling cohort remained, and by the summer of 1998,

only 4 seedlings had survived. Figure 1 shows the survival of the first cohort of

P. fremontii seedlings, the actual numbers of germinated seedlings from the first

initial year of establishment, and the total numbers of survivors over the first

three years.

Branch Recruitment and Survival

In contrast with the low numbers of survivors of P. fremontii seedlings

and complete absence of Salix spp. seedlings, there were many surviving plants

from re-sprouting branches of both P. fremontii and Salix spp. which had been

washed down from the river within the boundaries of the studied area (Figure 1).

These branches lay either on top of, or embedded up to 0.3 meters deep (pers.

obs.) on the deposited sandbar. The proportions of survivors from the initial

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year of colonization from seeds is extremely low (Table 1). For P. fremontii, only

0.004 seed/m2, or one surviving seedling per every 250 m2, survived. In

comparison, the number of surviving branches of P. fremontii was much higher,

at 0.025 branches/m2, or one surviving branch per 40 m2. No seedlings of Salix

were found during the three years of this study, but there were relatively high

numbers of Salix from broken-off branches (0.032/m2, or one surviving

branch/31.25 m2) surviving over three years. The second establishment cohort

year (1997) similarly concluded that vegetative branches had high rates of

survivorship (Figure 2).

Growth Rates of Seeds versus Branches

Throughout the duration of this research, it could easily be determined if

the origin of P. fremontii plants were from seeds or from branches. Both the

height and the diameter of plants derived from branches were significantly

larger than those derived from seeds (Figure 3). In the initial year of plant

establishment, all seedlings were observed to have been derived directly from

seeds. It was similarly easy to determine which plants came from branches,

because most plants from branches were quite large (over 0.3 meters tall, unpub.

data) at the initial colonization period. By the third year of observation, all new

plants on the sandbar are assumed to have been derived from branches, as all

new inputs were sizable (over 0.3 meters tall, unpub. data). No new seeds or

seedlings were observed in any of the following years of the study.

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DISCUSSION

The natural regeneration of native woody riparian species on this sandbar

was not wholly unexpected. In fact, this levee breach was instigated in hopes of

“naturally creating” a young forest dominated by P. fremontii. Historical photos

show that the Accidental Forest, the nearby 15-year-old stand of trees dominated

by P. fremontii, initially established following an accidental levee breach during

the high floods of 1983. What was unexpected however, was the rejection of our

hypothesis that the primary means of recruitment of woody tree seedlings would

be from seeds. On this sandbar, millions of seeds of P. fremontii had washed-up

along the edges and into small depressions on the sandbar. The exact locations

of approximately 8,000 P. fremontii seedlings were recorded and mapped. After

the long, hot summer of 1996, the majority of seedlings had died, apparently

because of desiccation. This was evident as the remnants of many young

seedlings remained as dried little pieces of plant material. By the end of that first

year, only 37 seedlings remained. After three years, only 4 of the initial 8,000 P.

fremontii seedlings had survived. McBride and Strahan (1984), Mahoney and

Rood (1998) and Rood (1998) have all noted that the survival of P. fremontii is

highly dependent upon sufficient soil moisture during the early seed

germination and seedling establishment stages. McLeod and McPherson (1973)

have also documented this dependency for Salix. If the roots of P. fremontii

seedlings are not sufficiently long enough at the time of water drawdown, or if

there is not enough sufficient soil moisture at the time of seed dispersal and

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germination, all seedlings will die. At this site, grazing by herbivores did not

appear to have an affect on the recruitment and survival of woody species (pers.

obs.). The leading cause of mortality appears to have been lack of moisture as

most dead seedlings remained intact and could be accounted for in the following

years.

In contrast to the low survival of seedlings, there were much greater rates

of survival for plants resprouting from branches of P. fremontii and Salix spp.

This is a phenomenon that has been eluded to by many botanists and restoration

ecologists in riparian systems (Henderson 1991, Shafroth et al. 1994, McBride and

Strahan 1984). Frequently in the the riparian literature (Malanson 1993), mention

is made of vegetative reproduction playing some part in vegetation succession

(Douglas 1989, Shafroth et al. 1994). Rood et al. (1994) even excavated several

hundreds of small cottonwoods to determine their method of origin. They found

that clonal reproduction was able to account for approximately half of the

regeneration efforts in several cottonwood forests in Alberta. Fay et al. (1999)

concluded similarly that Populus euphratica in Spain was exclusively clonal.

Even with this knowledge however, sexual reproduction is still commonly cited

as the primary mode of natural regeneration in Salicaceae (Bradley et al. 1986,

Niiyama 1990, Stromberg 1996), and studies with hard evidence of vegetative

reproduction (rates of survival, rates of growth) remains lacking. Studies of

aspens (Populus trichocarpa, P. grandidentata) however, are the exception in that

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it has been shown conclusively that vegetative reproduction is common (Barnes

1966, Braatne et al. 1996).

In this study, it was easy to determine whether propagules were of sexual

or of vegetative origin (Figure 3). We emphasize that the primary mode of

woody regeneration, at least in this Mediterranean climate, is from vegetative

propagation, not from seeds. In comparing the total numbers of survivors of

both branches and seeds on this sandbar, to those of the neighboring Accidental

Forest and to a nearby 35-year-old mixed-riparian forest, there are a few

noticeable differences. In regards to Salix, there were slightly higher numbers of

Salix individuals in the Accidental Forest (Table 1). However, this higher

number may be a result of additional root or stem suckers. There were no

individuals of Salix found in any of the 35-year-old mixed-riparian forested sites.

Nor were there any seedlings of Salix found on the sandbar. Hence it appears

that Salix reproduces almost solely by vegetative means.

P. fremontii also appears to reproduce primarily by vegetative means.

Even though the number of survivors after three years from branches outnumber

those which survive by seed almost 6 to 1, recruitment by seed has been shown

to be a valuable source of propagules in most other studies in the riparian

literature (McBride and Strahan 1984, Stromberg and Patten 1991, Taylor et al.

1999) and should not be discounted completely. However, this study shows that

broken-off branches were the primary source of propagules for recruitment. In

comparing the total numbers of branches and seeds from this sandbar to the

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Accidental Forest, there are almost three times as many young trees of P.

fremontii per square meter in the Accidental Forest as compared to the 3-year-

old sandbar. We speculate that this may be partially explained by the much

higher flood peak flows of 1983 (801.8 m3 in January through June) versus the

floods of 1996 (352.9 m3 in January through June; data from CA Dept. of Water

Resources). The higher water flows may have washed in more branches, and

may have retained a higher water table for a longer time, which would have

benefited the survival of P. fremontii seedlings. Another aspect of P. fremontii

regeneration to be considered is their natural suckering ability or the ability to

put-out multiple stems (Rood et al. 1994, Fay et al. 1999). It is not known how

many genetic individuals comprise the Accidental Forest. However, it is known

that the branches from the 3-year-old sandbar were each washed-down

individually. How related those branches are to one another is not known.

In the Accidental Forest, there are no indications of the P. fremontii trees

dying of old age, windthrow, or other natural disturbances. There is however,

evidence of disturbance and herbivory by beaver (unpub. data). In the 35-year-

old forest, the large trees of P. fremontii are starting to die, presumably from old

age and/or windthrow. Thus explaining the low total numbers of P. fremontii

individuals in the 35-year-old forest.

Additionally, new inputs from branches may occur following the initial

creation of the sandbar habitat. In the second year of monitoring, 17 new

branches of P. fremontii and 6 new branches of Salix appeared on the sandbar.

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Furthermore, there was very low mortality of these additional branches. P.

fremontii had complete survival of the second cohort of branches and Salix had

60% survival of the branches (4 of 6 survived). No new seedlings of either genus

were found after the initial year of establishment. This may be due to increased

competition for light and nutrients from other species.

Being able to reproduce vegetatively may be a selective advantage for

both Populus and Salix spp. Both genera produce copious amounts of small,

wind-dispersed, short-lived seeds. If these seeds do not land in a safe-site (sensu

Harper) and have adequate moisture at the seedling stage, there is a very narrow

window of opportunity for successful establishment. By reproducing

vegetatively from branches, there is an increased chance of success because of

increased amounts of stored nutrients, increased competitive ability for light (by

being taller), and perhaps increased water availability (by being buried deep in

the soil).

The recruitment of woody tree species in riparian systems is not well

understood. By rejecting our initial hypothesis that the primary means of

recruitment of woody trees would be from seeds, shows us that there are many

complexities involved in tree recruitment. Much emphasis in the literature has

focused on woody seedling establishment, and how conditions for establishment

may be altered (Stromberg and Patten 1991, Mahoney and Rood 1998). By

acknowledging that natural regeneration of some species may occur primarily by

vegetative propagules, opens the door for many restoration possibilities.

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ACKNOWLEDGEMENTS

Many thanks to Marcel Rejmánek for his unending insightfulness and

guidance during this research. Many additional thanks to E. Grotkopp, S.

Brewer, and R. Reiner for comments, and the staff at the Cosumnes River

Preserve for logistical assistance. Cheerful field assistance was provided by R.

Nelson, R. Mølholm and L. Lopez. This work was supported by funds from The

Nature Conservancy, The Nature Conservancy’s Rodney Johnson/Katherine

Ordway Student Grant Fellowship Fund, The California Native Plant Society,

and The Davis Herbaria Society.

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Table 1. The total number of surviving individuals per square meter from the

initial recruitment cohort, as compared to two other different-aged stands.

Sampled stands P. fremontii Salix spp.* 3-year-old sandbar

Seeds 0.004 0.000

Branches 0.025 0.032 15-year-old Accidental Forest 0.090 0.048 (2,700 m2) 35-year-old forest 0.009 0.000 (1,800 m2) *Includes Salix exigua, S. gooddingii, S. lasiolepis and S. laevigata.

25

Figure 1. Survival rates of the initial (1996) cohort of seedlings and plants from

branches on the sandbar over three years time. The numbers above each bar

represent the actual number of plants per 1,000 m2. No seedlings of Salix spp.

were found.

Figure 2. Survival rates of the second (1997) establishment cohort of plants from

woody branches on the sandbar over two years time. The numbers above each

bar represent the actual number of plants per 1,000 m2. No new seedlings of

either P. fremontii or Salix spp. were found in 1997.

Figure 3. Height and diameter growth of P. fremontii plants from branches and

from seeds from 1996 to 1998. Vertical bars represent standard errors of mean.

June 96 Sept 96 July 97 Aug 98.0001

.001

.01

.1

1

10

Populus seedsPopulus branchesSalix branches

DATE

SUR

VIVO

RSH

IP

7,898 36 32

37

33 31

7

31 29

4

28 29

June 96 Sept 96 July 97 Aug 98.0001

.001

.01

.1

1

10

June 96 Sept 96 July 97 Aug 98June 96 Sept 96 July 97 Aug 98.0001

.001

.01

.1

1

10

.0001

.001

.01

.1

1

10

Populus seedsPopulus branchesSalix branches

Populus seedsPopulus branchesSalix branches

DATE

SUR

VIVO

RSH

IP

7,898 36 32

37

33 31

7

31 29

4

28 29

Figure 1. Survival rates of the initial (1996) cohort of seedlings and plants from branches on the sandbar over three consecutive years. The numbers above each bar represent the actual number of plants per 1,000m2. No seedlings of Salix spp. were found.

.

July 1997 Aug 1998.01

.1

1

10

Populus branchesSalix branches

DATE

SUR

VIVO

RSH

IP

17 6

4

17

.

July 1997 Aug 1998July 1997 Aug 1998.01

.1

1

10

.01

.1

1

10

Populus branchesSalix branchesPopulus branchesSalix branches

DATE

SUR

VIVO

RSH

IP

17 6

4

17

Figure 2. Suvival rates of the second (1997) establishment cohort of plants from woody branches on the sandbar over two years time. The numbers above each bar represent the actual number of plants per 1000m2. No new seedlings of either P. fremontii or Salix spp. were found in 1997.

June 96 Sept 96 July 97 Aug 980

10

20

30 BranchesSeeds

DATE

DIA

MET

ER A

T B

ASE

(mm

)

June 96 Sept 96 July 97 Aug 980

50

100

150

200 BranchesSeeds

HEI

GH

T (c

m)

June 96 Sept 96 July 97 Aug 980

10

20

30 BranchesSeeds

DATE

DIA

MET

ER A

T B

ASE

(mm

)

June 96 Sept 96 July 97 Aug 98June 96 Sept 96 July 97 Aug 980

10

20

30

0

10

20

30 BranchesSeedsBranchesSeeds

DATE

DIA

MET

ER A

T B

ASE

(mm

)

June 96 Sept 96 July 97 Aug 980

50

100

150

200 BranchesSeeds

HEI

GH

T (c

m)

June 96 Sept 96 July 97 Aug 98June 96 Sept 96 July 97 Aug 980

50

100

150

200

0

50

100

150

200 BranchesSeedsBranchesSeeds

HEI

GH

T (c

m)

Figure 3. Height and diameter growth of P. fremontii plants from branches and from seeds from 1996 to 1998. Vertical bars represent standard errors of mean.