Seed Release Timing for Riparian Plant Species on the

Seed Release Timing for Riparian Plant Species on the Lower Yuba River

Prepared by the South Yuba River Citizens League

October, 2016

List of preparers: Chris Friedel, Gary Reedy, Adele Rife, and Rachel Hutchinson

Suggested Citation:

SYRCL. 2016. Seed Release Timing for Riparian Plant Species on the Lower Yuba River.

Seed Release Timing for Riparian Plant Species on the Lower Yuba River 1

Introduction

The Lower Yuba River (LYR) riparian corridor has been altered by a series of human-caused physical

and hydrologic disturbances, including gold mining and dam construction. These disturbances have

altered the movement of water, sediment, large wood, and anadromous fish populations. The cumulative

effect of these historical and ongoing alterations is a reduction in the quantity and quality of riparian and

aquatic habitats in the LYR (cbec et al. 2010). Substrate remains coarse, especially in the upper reaches of

the LYR, due to the presence of Englebright Dam. Flows are altered by upstream dams, diversions and

hydropower operations, and altered flows diminish the potential for riparian plant species to recruit and

establish in riparian areas (Mahony and Rood 1998, Stella 2006). The condition of riparian habitat in the

LYR corridor has been an area of long-standing concern for fisheries managers (CDFG 1991, LYRFTWG

2005, NMFS 2012). The Recovery Plan for Central Valley salmon and steelhead (NMFS 2014) calls for

the development of programs and the implementation of projects that promote natural river processes,

including projects that add riparian habitat and instream cover. While some recovery has occurred

naturally since the end of the hydraulic mining era, processes that create diverse and complex riparian

habitat continue to be affected by upstream dams which alter the supply of wood and sediment, as well as

hydrologic characteristics of flow magnitude, timing, and duration.

Remnant riparian communities on the LYR are dominated by willow and cottonwood species, namely

dusky willow (Salix melanopsis), narrowleaf willow (Salix exigua), Fremont cottonwood (Populus

fremontii), and arroyo willow (Salix lasiolepis) (Watershed Sciences, Inc. 2011). In California, over 90%

of riparian and wetland habitat was lost during the late 1800’s and early 1900s (Dahl 1990) and while

historical aerial photographs show that cover of riparian habitat along the LYR has increased since 1937

(YCWA 2012), current conditions lack species diversity, structural heterogeneity, and connectivity

among forest patches (cbec et al. 2010) A high proportion the riparian corridor is unvegetated (WSI and

Fremier 2012, YCWA 2013) and woody riparian species that grow higher than 30 ft. are uncommon (cbec

et al. 2010, WSI and Fremier 2012). Fremont cottonwood’s, which can grow to over 120 ft., are largely

absent within the river corridor, where they could be recruited as large wood during channel migration or

bank undercutting (cbec et al 2010). The largest cottonwoods are typically found high on floodplain

terraces at the toe of levee and training wall slopes, with some young seedlings found in unvegetated

areas along the channel margins (YCWA 2012)

In unregulated river systems in California, flows typically recede as the snow melts with snowmelt driven

flood events that attenuate slowly until they reach summer baseflow. This slow recession in the spring

and early summer exposes pockets of freshly deposited sediment that are available as nursery sites for

newly dispersed seeds (Scott et al. 1997; Rood et al. 1998; Cooper et al. 1999; Karrenberg et al. 2002).

Woody plant species occupy different spatial niches based on the timing of each species’ seed dispersal

and the corresponding availability of suitable habitat as determined by river flow levels and recession

rates (Rood et al. 2003). The present-day managed flows on the LYR differ significantly from an

unregulated river system in that winter and spring flows are stored upstream to meet water demands later

in the year. Thus, there is less frequent inundation of the floodplain to disperse seeds and increased

drought-induced mortality of seedlings that germinate due to flow recession rates faster than maximum

seedling root growth rates.

The natural recruitment of many riparian species, including willow and cottonwood, is dependent on the

timing of seed dispersal in relation to a specific set of hydrologic and climatic conditions (Mahoney and

Rood 1998). This is referred to as the riparian recruitment box, wherein riparian species dispersal

naturally occurs within a period of hydrologic recession, allowing those species to germinate and

establish without being re-inundated or experiencing drought conditions. Most riparian tree and shrub

species release seed during the spring (e.g., Fremont cottonwood) when flows most often support seed


germination and seedling establishment. In managed river systems, the timing of seed release may no

longer coincide with a natural spring snowmelt flow pulse and recession (Fenner et al. 1985).

Large trees like Fremont cottonwood and Gooddings willow are important contributors of aquatic as well

as riparian habitat benefiting primiary productivity on the floodplain (Ahearn et al. 2006), bird diversity

(Gardali and Holmes 2011), floodplain development (Stillwater Sciences 2006), and finally as juvenile

salmonid habitat through the recruitment of large wood into the river channel. Restoring floodplains

through flood reconnection and managing the spring snowmelt recession is key to rehabilitating riparian

areas and the ecological health of the river ecosystem. The development of flow related strategies for

increasing riparian habitat and improving aquatic habitat through the recruitment of woody riparian

species depends on accurate information regarding the timing of peak seed release in relation to recession

timing.

Methods

In 2013, the South Yuba River Citizens League (SYRCL) produced an assessment of riparian conditions

on the Lower Yuba River (LYR). The report, “Riparian Surveys of the Lower Yuba River to Supplement

Rehabilitation Planning,” summarized methods and results for riparian surveys conducted in April-

August of that year, and included data on seed release timing for 10 woody riparian plant species

(SYRCL 2013). Four sites along the LYR between Hwy 20 and the Teichert Hallwood Plant were

selected for seed dispersal survey, based on accessibility, access permission, and presence of a

representative sample of woody plant species (Figure 1). In March-August 2014, 8 of the 10 species were

resurveyed using the same methodology (Table 1). These 8 species are the focus of this report.

Figure 1. Study locations on the lower Yuba River


Table 1. Number of species measured at each of the four sites in the seed dispersal study. Asterisk (*)

indicates species that were not surveyed in 2014.

Common

Name

Species Site 1

(Parks Bar)

Site 2

(Hallwood)

Site 3

(Upper

Teichert)

Site 4

(Lower

Teichert)

Total

white alder Alnus rhombifolia

(ALRH)*

3 4 5 1 13

Oregon ash Fraxinus latifolia (FRLA) 1 3 5 9

California

sycamore

Platanus racemosa

(PLRA)*

2 4 4 10

Fremont

cottonwood

(POFR) 4 5 1 10

narrowleaf

willow

Salix exigua (SAEX) 3 3 3 9

Goodding’s

willow

Salix gooddingii (SAGO) 1 2 2 5

Red willow Salix laevigate (SALAE) 2 2

Arroyo

willow

Salix lasiolepis (SALAS) 5 3 2 10

Shining

willow

Salix lasiandra (SALU) 4 3 7

Dusky

willow

Salix melanopsis (SAME) 2 4 3 9

Total 16 32 23 13 84

Sites were sampled 14-19 times during each growing season, from March through August of 2013 and

2014. Visits were timed one week apart, with less frequent visits towards the end of the survey period

when less seed dispersal activity was occurring. During each survey, seed production was quantified for

each sample shrub or tree by visual estimates of 2 variables: percent of the canopy containing catkins, and

percent of visible catkins actively dispersing seeds. To understand the relative production of each species

on a given date, a “Seed Production Index” was calculated as the product of the two percentages that were

collected in the field.

To quantify seed release timing for each species, the mean (average) seed production index value was

calculated byspecies at each survey date. Average Seed Production Index values were then plotted to

create seed production index curves. From these curves, the integral of seed production index was

generated to represent cumulative seed production for each species over each season (Sokal and Rohlf

1995). These integral curves were then interpolated using best-fit standard curves (asymmetric sigmoidal,

5PL) to find the 20th and 80

th cumulative quantile day-of-the-year values (referred to hereafter as DY20

and DY80).1

The DY20 is used as representative of the first observed occurrence of seed release, following the logic

used by Stillwater Sciences et al. (2006) in their analysis of riparian woody plant seed dispersal in the San

Joaquin basin. Unlike traditional measures of first occurrence, an intermediate quantile such as the 20th is

not temporally sensitive to the tails of the distribution, and the 20th quantile provides a logical

1 These analyses were performed using GraphPad Prism 7 for Mac OS X, a biostatistics, curve fitting, and scientific graphing software program.


management target for timing restoration flows to coincide with highest propagule density. Similarly, the

80th quantile is used as representative of the end of the peak seed release period.

Results

Start and end dates (DY20 and DY80) for the peak seed release period for all species in both years are

shown below, along with the duration of the period and the mean duration for all years (Table 2). Graphs

are presented for each species showing the Seed Production Index (blue line) and Integral of Seed

Production Index (orange line) curves, along with DY20 and DY80 (red dashed lines) and DY50 (yellow

dashed line) r (Figures 2-9).

Table 2. Peak seed release periods for selected woody riparian plant species.

Patterns of seed release timing differ for the 8 species in this study. Arroyo willow (Salix lasiolepis) and

red willow (Salix laevigata) begin releasing seeds in April or earlier. The peak seed release period for

Fremont cottonwood (Populus fremontii), narrowleaf willow (Salix exigua), and shining willow (Salix

lasiandra) begins in late April or early May. For dusky willow (Salix melanopsis) and Goodding’s willow

(Salix gooddingii) the peak seed release period begins in late May or early June and continues through

June. Oregon ash showed a wide variation in start of peak seed release period between years, and surveys

were not conducted late enough to establish the duration of the period.


The duration of peak seed release period is about 30 days or less for most of the species in this study. The

notable exception is dusky willow, which showed a peak seed release period of about 55.5 days, lasting

until the end of July. Of all the species surveyed, dusky willow is the only species for which the period of

peak seed release coincides with summer baseflows.

Figure 2. Arroyo willow (Salix lasiolepis) seed release timing. The blue line indicates the Seed

Production Index.

Figure 3. Red willow (Salix laevigata) seed release timing. Lines are as follows: The Seed Production

Index (blue line), Integral of Seed Production Index (orange line), DY20 and DY80 (red dashed lines) and

DY50 (yellow dashed lines).

Figure 4. Fremont cottonwood (Populus fremontii) seed release timing. Lines are as follows: The Seed

Production Index (blue line), Integral of Seed Production Index (orange line), DY20 and DY80 (red dashed

lines) and DY50 (yellow dashed lines).


Figure 5. Narrowleaf willow (Salix exigua) seed release timing. Lines are as follows: The Seed



Figure 6. Shining willow (Salix lasiandra) seed release timing. Lines are as follows: The Seed Production



Figure 7. Dusky willow (Salix melanopsis) seed release timing. Lines are as follows: The Seed Production




Figure 8. Goodding's willow (Salix gooddingii) seed release timing. Lines are as follows: The Seed



Figure 9. Oregon ash (Fraxinus latifolia) seed release timing. The blue line indicates the Seed Production

Index.

Discussion

Peak seed release initiation and duration found in this study compares similarly to results from a study on

the Tuolumne and San Joaquin Rivers (Stillwater Sciences et al. 2006), however the start of seed release

for some species appears to be earlier on the Yuba River and the duration of seed release is often shorter.

These results can be directly applied to flow management to improve the potential for riparian tree and

shrub recruitment. For example, while mean start dates for Goodding’s willow in each year are similar

between the Yuba (May 24–June 3) and the San Joaquin basin (May 21–June 6), mean duration for all

years is shorter on the Yuba (26 days versus 45). Narrowleaf willow exhibited an earlier mean start date

on the Yuba (April 17–May 9) than in the San Joaquin basin (May 17-June 20) in each year. Mean

duration of narrowleaf willow peak seed release period is slightly shorter on the Yuba (33.5 days) than in

the San Joaquin basin (39 days). Fremont cottonwood exhibited a slightly earlier range of mean start dates

on the Yuba for all years (April 28-May 6) than those in the San Joaquin basin (May 2-May 18), but the

ranges are overlapping and standard error could account for the difference. Mean duration of cottonwood

peak seed release period is similar on the Yuba (32 days) as in the San Joaquin basin (27 days).

One outcome of the current flow regime is that the channel margins are dominated by dusky willow, as

the species has an elongated dispersal period which allows it to establish along the summer baseflow

channel margins which are wet well into the summer months. Interestingly, narrowleaf willow is also


prevalent in these vegetation bands, even though its period of peak seed release has ended by early June.

The ability of narrowleaf willow to propagate vegetatively may be one explanation for its prevalence

despite a lack of seed propagule availability during summer baseflows.

The establishment of large trees near the channel margin and on the floodplain surface is of particular

interest due to their potential recruitment into the channel as large wood and their ability to improve

overall ecosystem health in the region. Both of the larger trees included in this study (Fremont

cottonwood and Goodding’s willow) have peak seed release periods that end by early or late June. If

recession rates are not favorable for seedling establishment in May and June, recruited seedlings will

experience increased mortality and may have difficulty establishing. Managing flows for increased flood

inundation area during periods of seed release for large tree species, and reduced rates of flow recession

to follow, could significantly increase the successful recruitment of these ecologically valuable tree

species on the LYR.


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Documents

Seed Release Timing for Riparian Plant Species on the