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Strong recruitment from sparse plug plantings of native California bunchgrasses

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Strong recruitment from sparse plug plantings of native California bunchgrasses

Vol. 25, No. 1 Winter 2015
9 | GRASSLANDS Winter 2015
Introduction
Grassland restoration efforts in California include plug planting or
direct seeding of native perennial grasses. These plantings are usually
installed at densities intended to achieve full cover of mature plants
from the planted individuals, not relying on recruitment of their
progeny. Recruitment of future generations would both be reassuring
for the long-term success of projects and an indication that lower
density plantings might be sufficient, but restoration monitoring
rarely looks for recruitment in later years (see Morgan 1999).
The extent of recruitment of new individuals into restoration
plantings of native perennial bunchgrasses is poorly known for
several reasons. First, most restoration projects simply monitor
planted individuals for 1–3 years after planting (Kettenring and
Adams 2011) and might miss new recruits. Second, most restoration
plantings of grasses are installed at relatively high densities, which if
successful could both render new recruits more difficult to recognize
and also be less likely to establish them in stands with high cover.
Third, restoration practitioners in California grasslands are aware
that it is not uncommon for initial perennial grass planting to be
cryptically successful; that is, potentially high densities of planted
individuals establish at very small sizes in the first year or two and
only become evident in subsequent years (Vaughn and Young,
forthcoming). The identification of new recruits may be more
difficult under these conditions.
Nonetheless, there are some indications that additional recruitment
from the progeny of planted native grasses in restoration settings
occurs. For example, Rayburn and Laca (2013) reviewed the success
of strip-seeding and seed islands in restoration projects where target
species are planted over only a fraction of the landscape in separated
strips or in small patches, with the intention that future recruitment
would fill in the unplanted areas. However, there appears to be a lack
of such studies in the highly invaded grasslands of California, where
Mediterranean annuals appear to prevent the recruitment of native
species (Stromberg et al. 2007). However, even in California
grasslands there are indications of such recruitment. We have found
native California grasses recruiting beyond the boundaries of seeded
plots in our restoration experiments (Porensky et al. 2012, Young et
al., forthcoming, Kurt Vaughn, pers. comm.; see also Dyer 2003).
Restoration practitioners and native seed producers also have seen
recruitment of several native California grass species between drilled
seeded rows, especially after several years (John Anderson, Andrew
Fulks, Emily Allen, Kurt Vaughn, Chris Rose, pers. comm.).
We report here results from a study site that provided a test of how
very low planting densities of native California bunchgrasses can
spread and recruit over a long interval (11 years). A small number of
purple needlegrass (Stipa pulchra) plugs at this site led to the
recruitment of many hundreds of additional individuals.
Methods
The study site is an experimental valley oak woodland restoration
site in research fields of the University California, Davis. The site
was planted with several hundred valley oak (Quercus lobata) acorns
and seedlings in the winter of 1999 as part of a separate restoration
experiment (Young and Evans 2005, Holmes et al. 2008, 2011). The
Strong Recruitment from Sparse Plug Plantings
of Native California Bunchgrasses
by Truman P. Young1, Professor and Restoration Ecologist, Dept. of Plant Sciences, UC Davis, CA 95616, and
Kari E. Veblen2, Assistant Professor, Dept. of Wildland Resources, Utah State University, Logan, UT 84322.
1Truman Young has studied grassland and
savanna rangelands in California and Kenya
for over 20 years. 2Kari Veblen has studied
grasslands in Kenya, California, Oregon,
Utah, and Colorado, the latter being where
both authors were raised.
continued next page
Winter 2015 GRASSLANDS | 10
Figure 1. One of multiple stands of volunteer Stipa pulchra bunchgrasses that recruited from a handful of plugs planted very sparsely 8 years
previously at the restored oak woodland site in Davis, California. This patch contains over 20 reproductive individuals. Note the bare ground
between bunchgrasses. Planted valley oaks are in the background. Photo: Truman Young
original experiment was divided into 54 plots of 9 oaks each, planted
at 2-m spacing, with 5 m between rows of subplots. In March 2003,
following successful establishment of valley oaks, we planted in each
subplot 2 plugs each of Stipa pulchra and blue wildrye (Elymus
glaucus) at 2-m spacing in the interstices of the 9 oaks, for a total of
108 plugs of each species. One-third of the subplots were subject to
controlled burns in June 2003 and another third in May 2004
(Holmes et al. 2008, 2011), after which there remained alive 48 S.
pulchra and 63 E. glaucus individuals in May 2006 (Veblen et al.
2007). There was yearly mowing in the rows between the subplots,
but no other management interventions.
In July 2014, we surveyed the entire study area, counting all
individuals of S. pulchra and E. glaucus, which tended to occur in
dense patches (Fig. 1). When these patches overlapped with known
planting locations, we were not able to determine the fates of original
plantings, but we could confirm the loss of original plantings from
sites with no surviving individuals. Several stands of S. pulchra were
so dense that distinguishing individual tussocks was difficult, so our
counts may be underestimates.
Results
By the time of the 2014 survey, 15 of the 54 original subplots had
been destroyed by new research projects. The remaining 39 subplots
had been planted with a total of 78 S. pulchra and 78 E. glaucus plugs
in 2003, of which 34 and 48 (respectively) were still alive in May
2006. We did not do a broader survey on 2006, and so we do not
know whether some volunteers were already present at that time. By
2014, most of the original plugs were no longer present, but there
had been recruitment of new individuals of these two native
perennial grasses. We found at least 39 individuals of E. glaucus, of
which not more than 12 were original plantings. More strikingly, we
counted at least 1,153 individuals of S. pulchra within the study area,
of which the vast majority were clearly not planted. This represented
at least a 30-fold increase in the needlegrass population over a period
of 11 years.
Sparse Plug Plantings continued
11 | GRASSLANDS Winter 2015
Several stands of S. pulchra were sufficiently dense that virtually no
other vegetation was present (see Fig. 1). Although there was usually
virtually no understory vegetation (including grasses) under the
densest oak overstories, there was otherwise no striking pattern of S.
pulchra with respect to canopy and intercanopy locations.
Discussion
Practicing restorationists have also reported native grass recruitment
from planted individuals in California restoration sites, as well as
cases where no such recruitment was seen (Rayburn and Laca 2013
and pers. comm.). However, all of these reports are from restoration
plantings (or seed-increase fields) at fairly high density. Our data
uniquely show that large-scale recruitment past the planted
generation can occur even at very low effective planting densities of
2-m spacing, in contrast to typical plug spacings of 20–40 cm
(Anderson 2001, Huddleston and Young 2004)
This observation begs the question: Why do natural (remnant)
populations of California native grasses not similarly increase
dramatically when left alone? One possibility is that the moderate
disturbance at our study site (partial mowing) may have provided
opportunities for needlegrass recruits. We have also seen needlegrass
recruitment past the planted generation in seeded restoration
research plots nearby (Porensky et al. 2012 Young et al.,
forthcoming). Similarly, rangeland researchers have found a diversity
of responses of native grasses to different management actions. In
particular, disturbance (grazing, clipping, fire, mechanical soil
disturbance) is sometimes associated with increases in Stipa pulchra,
and sometimes not (reviewed in George et al. 2013).
In any case, this case provides a hopeful example of how even very
low densities of planted native grasses may serve as nuclei for more
substantial recruitment. Experiments currently under way by
Rayburn and Laca will formally test this possibility in a restoration-
style setting.
References
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Sparse Plug Plantings continued
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