THE EFFECT OF SEASONALITY OF BURN ON SEED GERMINATION IN CHAPARRAL: THE ROLE OF SOIL MOISTURE

2005·7.1 MB·English

by  Danielle Le Fer

Madrono, Vol. 52, No. 3, pp. 166-174, 2005
THE EFFECT OF SEASONALITY OF BURN ON SEED GERMINATION IN
CHAPARRAL: THE ROLE OF SOIL MOISTURE
Danielle Le Fer' and V. Thomas Parker
San Francisco State University, Dept. of Biology, 1600 Holloway,
San Francisco, CA 94132
Abstract
Fire represents an important recruitment phase for many chaparral species. Prescribed burns are often
scheduled during winter or spring when soil moisture is high in order to minimize risks of uncontrolled
fire. However, chaparral wildfires typically occur in the summerorfall when soil moisture is low. Chang-
ing the seasonality of burn affects pre-burn soil moisture, burn temperature and timing of germination.
High soil moisture during winter or spring burns is hypothesized to lower germination rates ofchaparral
plants compared to fall burns. The purpose ofthis study was to evaluate the effects ofprescribed spring
burns on the germination ofchaparral species in the Mendocino National Forest, California.Weconducted
two experiments to test for effects of moisture on seed germination. In the soil heating experiment, soil
collected under chaparral was heated at several temperatures and soil moistures, and germinating seeds
were counted. In the seed heat treatment experiment, seeds of 13 species were heated moist and dry to
determine the moisture effect on heated seeds. Results indicate a differential response of seeds to heat
and soil moisture. Lotus humistratiis, Daiicus pusilliis and Penstemon heterophyllus were negatively af-
fected by temperature in both moist and dry treatments. Ceanothus ciineatus and Genista monspesiiUana
germination increased with temperature in both dry and moist treatments. Germination of six species
{Adeuostoina fasciculatwn, Camissonia coutorta, Emmenanthe pendiflora, Epilobiiun ciHatwn, Galium
apahue and Malacothrix clevelandii) decreased under moist heat treatments. These results suggest that
spring burns may lead to decreased diversity ofchaparral due to reduced seed survival and germination
ofcertain species.
Key Words: seasonal burns, chaparral, soil moisture, temperature, seed germination, prescribed burns,
seed bank.
Fire has become an important management tool, California that are characterized by seasonal
to recreate or maintain specific vegetation coin- drought (Christensen and Muller 1975a). The com-
munities (oak savanna, grassland, southeastern pine bination of dense shiub cover, severe summer
forest, tall grass prairie, chaparral), to maintain spe- drought and the accumulation of "fuel" in chap-
cies diversity and endangered or rare species, and arral lead to frequent fires, with frequencies ofonce
to control invasive species (Zedler and Scheid every 40 to 60 years in southern California (Chris-
1988; Minnich 1989; Keeley 1991). Fire as a bio- tensen and Muller 1975b). Fires represent an im-
logical process consists of a complex set of com- portant recruitment phase for many chaparral spe-
ponents (timing, intensity, duration, interval be- cies: Diversity and seedling numbers are highest in
tween burns), and "fire adapted'' vegetation is typ- the first and second growing seasons after fire
ically adapted to a specific set of ranges of these (Christensen and Muller 1975b). Recruitment may
components. Shifting the timing of fire can sub- be affected by changes in the fire regime, such as
stantially affect vegetation structure, vegetation fire exclusion or very frequent fires (Keeley 1995;
composition and the soil seed bank ofa site (Kauff- Zedler 1995; Keeley 2002).
man and Martin 1991). In chaparral, the previously In chaparral, winter and spring prescribed burns
cdrourimtamnetntsoislousreceed b(Kaenekleisyan19i8m7p)o.rtAantbeptotsetr-fuirnederre-- ctihcaulnagie; cshoamnegicnogintphoenesnetassoonfaltihteyfiorfe rtehgeiibneu.rnInfpraorm-
standing ofthe effect of seasonality ofburn on spe- the fall to the spring affects soil moisture, timing
cies diversity and seed bank coinposition would im- of regrowth, and fire intensity (Parker 1990). Since
prove the ability of land managers to incorporate seed response to heat and soil moisture varies ac-
fire into management plans. cording to seed water absorption, the seasonality of
Chaparral structure and composition is greatly burn and soil moisture content can greatly influence
influenced by fire (Moreno and Oechel 1991). germination patterns.
Chaparral is an evergreen sclerophyllous shrub veg- Timing of burns also has the potential to change
etation that doininates in moderately xeric sites of
the invasion regime of post-fire alien species
(Kauffman and Martin 1991). Forexample, burning
' Current address: U.S.F.W.S., San Francisco Bay Na- in early summer, just as plants begin to flower, pre-
tional Wildlife Refuge, RO.B. 524, Newark, CA 94560, vents seed production of yellow star-thistle {Cen-
email: [email protected]. taiirea solstitialis L. [Asteraceae]) in grasslands in
2005] LE PER AND PARKER: SEASONALITY OF BURN AND SEED GERMINATION 167
California. However, late spring and tall burns in- the chaparral vegetation ofthe Mendocino National
crease diversity of native species and decrease Forest, California. In order to examine seed bank
abundance of alien species (Hastings and Di- response to soil moisture and heat treatments, we
Tomaso 1996). In some cases, fire may allow in- conducted two experiments. In the first experiment,
vasive species like Cytisus scopariiis L. (Fabaceae) germination was measured in soil heated in the lab-
and Carpobrotus ediilis (Aizoaceae) to increase in oratory under several moisture conditions. In the
numbers (Zedler and Scheid 1988; D'Antonio et al. second experiment, seeds of selected species were
1993; Agee 1996). Thus, management strategies in- heated dry and after imbibing water to determine
volving fire must take into account the mix of in- moisture effects on specific species. Heat treat-
dividual species with different life histories and re- ments were selected to duplicate soil temperatures
sponses to fire and the phenological timing ofburns during a burn (70°C to 1 10°C). Soil moisture treat-
(Parker 1990; Glitzenstein et al. 1995; Spier and ments included expected summer soil moistures
Snyder 1998). (3% to 7%) and possible spring soil moistures (17-
Previous studies have examined the effect ofheat 30%).
on the seed response of chaparral species (Keeley
and Nitzberg 1984; Keeley et al. 1985), but few Methods
studies have examined the effect of changing the We obtained soil and seeds from the east slope
seasonality of burn and soil moisture on seed ger- of the Coast Range Mountains, in previously
mination at the time of fire. Understanding the ef- burned chaparral, in the Mendocino National For-
fect of soil moisture at the time of fire can help est, in northern Glenn County, near Highway 162,
predict the effect of burns on responses of individ- north-west ofElk Creek, California USA 122°7'W,
ual species and the resulting species composition of 39°72'N). The vegetation consisted primari(ly ofA<i-
the chaparral. euostoma fasciciilatwu Hook. & Arn. (Rosaceae)
An important step in managing vegetation with and Ceanothus ciineatiis var. cuneatus (Hook.)
fire is to classify taxa into ecological response Nutt. (Rhamnaceae). Soil samples were collected
groups, keeping in mind that each species has from the upper 5 cm ofsoil under mixed Ceanothus
unique life history characteristics (Parker 1990). and Adenostoma chaparral in August 1994. All
Two functional groups can be distinguished within
seeds except Genista nionspessulana (L.) L. John-
a seed bank: Species that do not require fire for son (Fabaceae) were collected in the Mendocino
germination (fire-independent), and species which
National Forest in 1994 and 1995. Genista seeds
are fire-stimulated either via heat, smoke chemistry were collected from Old Railroad Grade on Mount
or some other mechanism (Parker and Kelly 1989; Tamalpais in Marin and were selected for this ex-
Zammit and Zedler 1994). Fire-stimulated seeds are periment because its seeds are known to imbibe less
aadnadptreepdretsoensturavbiovuitng21th%e ohfeattheprtootdaulcseededinbaankfirien, than 5% of their dry weight, and the species is an
invasive in chaparral.
chaparral (Zammit and Zedler 1994). Fire-indepen-
dent seeds comprise almost 2/3 of the total seed Soil Experiment
bank in mixed chaparral; these species can poten-
tially recruit at any point in the fire cycle, including We moistened the soil, and placed it in ziplock
immediately after fire (Zammit and Zedler 1994). bags for one week to ensure even moistening. We
Seeds of obligate resprouters, geophytes, perennial conducted four heat treatments (control [unheated],
grasses and many introduced annuals are non-dor- 90°C, 100°C, and 110°C) and five moisture treat-
mant and fire-independent (Zammit and Zedler ments (3%, 15%, 22% and 30% ofsoil dry weight),
1994). with four replicates per treatment. The soil was
In California chaparral, higher rates of seed ger- heated for 10 minutes in an oven, using soil probe
mination occur following autumn fires than spring thermometers to measure soil temperature. Samples
fires (Parker and Kelly 1989). In laboratory exper- were stratified at 5°C to 8°C for two weeks then
iments, germination is negatively affected for many placed in the greenhouse and 50 ml of charate (as
species heated under moist conditions typical of per Keeley 1987) was added to each fiat (20 cm X
spring burns (Rogers et al. 1989). Seeds lacking 20 cm). We monitored germination every two
hard seed coats have lower heat tolerance after they weeks over the course of 120 days.
have absorbed moisture (Parker 1990). Spring and
winter burns, typically of lower heat intensity, may Seed Experiment
have a negative impact on species with hard seed We examined the effect of temperature on dry
coats that require high temperatures to germinate and moistened seeds of thirteen species: Adenos-
(e.g., Cecmothus) (Parker 1990). tomafasciculatwn, CeanotJuis cuneatus var. cunea-
The goal ofthis study was to examine the impact tus, Penstenum heterophyllus Lindley (Scrophular-
on the seed bank of spring prescribed burns in Cal- iaceae), Camissonia contorta (Douglas) Raven
ifornia chaparral. Our objectives were to determine (Onagraceae), Mentzelia dispersa S. Watson (Loas-
the effect of changes in soil moisture and temper- aceae), Galium aparine L. (Rubiaceae), Daucus
ature during spring burns on seed germination in pusillus Michaux (Apiaceae), Malacothrix cleve-
1
MADRONO
168 [Vol. 52
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2005] LE PER AND PARKER: SEASONALITY OF BURN AND SEED GERMINATION 169
Table 2. Percent Water Absorption ([Moist Weight Data Analysis
- Dry Weight]/Dry Weight) for Seeds Soaked 8
Hours in Distilled Water. We used a Kruskal-Wallis analysis followed by
a multiple range analysis on ranks to compare ger-
Percent water mination of soil burn treatments (SAS 2004). For
Species absorption each species, we compared percentage germination
Adeiiostomafasciciilatiiiii 49.7 for heat and moisture treatments with a two-way
Cainissonia contorta 70.0 Analysis ofVariance after arc-sine transforming the
Ceanothiis cuiieatus 4.2 data, followed by multiple pair-wise comparisons
Daiicus pusillus 43.0 (Tukey) to determine differences between moisture
Dicentrci chyscuitJici treatments and between control and heat treatments
Einmenanthependijiora 46.7 (SAS 2004).
Epilobium ciliaturn 187.0
Gallium apariue 65.5
Results
Genista monspesiillaiia 0.0
Lotus huiuistratus 81.0 Soil Experiment
Malacothrix clevelandii 86.1
Meutzelia dispersa 5.5 Germination in soil heated under dry conditions
Penstenion heteropliyllus 71.3 (i.e., 3% soil moisture) varied among species. At
3% soil moisture, Eriodictyon germination in-
creased significantly with heat (P < 0.01) (Table
1). Total density reflected the high germination of
(laHnodoiki,Aa.ndGrAaryn.)(AWstaelrpa.ce(aPea)p,avDeircaecneater)a,cEhpriylsochiinitihma Eriodictyon (Table 1 ). Eriodictyon germinated in
small quantities in the control and 90°C treatments
ciJiatiim ssp. ciliatum Raf. (Onagraceae), Emiueu-
(0.5 per flat); its densities increased significantly
aphnytlhleacpeeaned)u,liLfolotruasvhaunmipsetnrdaitiiliifsloEr.a BGernetehn.e(H(yFdarboa-- aanhdigwheroef 2v9ersyeehdliignhgsa/tfl1at00a°tC10a0n°dC1(1P0°<C,0.r0e1a)ch(iTnag-
c(eFaaeb)aceaaned).Genista mouspessulana (L.) L. Johnson ble 1 ). In contrast, Ceanothus andAdenostoma ger-
We completed two sets of seed heat treatments: Gminnaapthiaolniudimd cnaoltifinocrrneiacsuem wDiCth. h(eAastte(rTaacbelaee)1,). LFsoor-
a set with pre-soaked seeds (soaked in distilled wa- pyrum stipitatum A. Gray (Ranunculaceae), Navar-
ter for eight hours prior to heating) and a set with retiapiihescens (Benth.) Hook, and Arn. (Polemon-
dry seeds. Seeds were heated in glass tubes in an iaceae), and Verbascum thapsus L. (Scrophulari-
oil bath heater containing sand for 10 minutes at aceae), germination was too low to examine statis-
7hse0ua°rteCed,d.t9h0eA°fhCteearatnhidneat1th1e0t°rtCeu.abetsmT.eenmOtp,neerwacetounrptleraolcpersdoebtsewesaedsmsenaoi-tn twiecrEalerliyonde(igTcaattbyilovene,l1y)A.adfefneoctsetdombay haenadt tuontdalergemromiisntatcoino-n
aoPefqturWeihopaulstatmseaoslnu(t2ni5oo.nseo1efdfscihltpaererartpepa,lpaeptrer,)epabanerdtewdaepaepsnlipteewdroKalea5eylemerlys bdilteio1ns). (Ei.rei.,od1i5c%t,yo2n2g%eramnidna3t0io%nsowialsmosiisgtnuirfei)ca(nTtal-y
lower under moist conditions at soil temperatures
p(l1a9t8e7s),wetrheenesxtaramtiinfieedd sweeeedkslyinfoarn gienrcmuibantaotri.onPeftroir of 100°C and 110°C (P < 0.01) (Table 1). Aden-
ostoma germination decreased under moist condi-
up to 14 weeks, and germinated seeds were counted tions when the soil was heated (P < 0.01) (Table
and removed.
Seeds of different species were stratified at spe- 1).Total density decreased as temperature and soil
cific combinations oflight and temperature to inax- moisture increased (Table At low temperatures
imize germination (Keeley and Keeley 1987; V.T. (no heat and 90°C), total d1en).sity was not affected
Parker, unpublished data). The seeds of Adeiiosto- by soil moisture (P > 0.05) (Table 1). However,
ma, Ceaiiothus, Malacothrix, Dicentra, Eiiimeiiaii-
total density decreased significantly in moist soil at
the, Daiiciis, Galium and Epilobium were stratified temperatures of 100°C and above (P < 0.05) (Table
in the dark at 5°C for 3 weeks, then for 4 weeks
1).
with 12 hours oflight (20°C) and 12 hours ofdark- At 90°C, Ceanothus germination was higher un-
ness (5°C). This cycle was repeated once in order
to maximize germination rates. Cainissonia, Pen- tduerresm,oisositl cioTniodiisttiuorensdi(dTanbolte a1f).fecAttCoetahnerottheumspegrear--
stemoii and Mentzelia seeds were first stratified in mination. Ceanothus was the only species germi-
the dark (5°C) for 3 weeks, then with 12 hours of
nating in moist soil at temperatures above 90°C (Ta-
light (20°C) and 12 hours of darkness (5°C) for 4
weeks. For the second cycle they were placed at ble 1).
30°C (dark) for 1 week, then at a regime of 12 Seed Experiment
hours of light and 12 hours of dark for 2 weeks.
Genista and Lotus were stratified for4 weeks in the Seed water absorption, as percent of dry weight
dark at 5°C, then at 10 hours light (20°C) and 14 ([moist weight — dry weight]/dry weight) ranged
hours dark (5°C) for 3 weeks. from 4% to more than 100% of their dry weight
MADRONO
170 [Vol. 52
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MADRONO
172 [Vol. 52
(Table 2). Ceanothus, Genista, and Mentzelia seeds ditions {Adenostoma, Camissonia, Galium, Epilob-
imbibed 4% or less of their dry weight in water. ium, Malacothrix, Emmenanthe) had no seed coat
All other seeds absorbed 23% or more of their dry dormancy and absorbed water above 23% of their
weight. weight. These included seeds ofopportunistic "fire
Three species {Ceanothus, Genista and Camis- survivors" that do not require fire to germinate and
sonia) showed a positive temperature effect on ger- may survive low intensity heat (e.g., Adenostoma,
mination when heated dry (Table 3). Ceanothus Epilobium, Galium). A group offire-sensitive post-
germination increased from 0 at the control tem- fire colonizers was sensitive to any amount ofheat,
perature to 25% at 110°C. Genista germination in- moist or dry {Emmenanthe, Daucus).
creased from 15% at no heat to 23% when heated. Conditions during spring prescribed burns may
Germination of Camissonia seeds increased from be conducive to germination of fire-stimulated
6% when unheated to as much as 63%. seeds with seed coat dormancy (e.g., Ceanothus,
In eight species, Adenostoma, Emmenanthe, Epi- Genista). Hard-coated seeds absorbed 4% or less of
lobium, Galium, Daucus, Eotus, Malacothrix and their weight in water and did not exhibit a negative
Penstemon, at least one heat treatment under dry temperature effect under moist conditions. Ceano-
conditions negatively affected germination (Table thus, an obligate seeding shrub, may increase in
3). Heat tolerance of species varied, but germina- numbers after spring burns. However, some spring
tion of all of the former species except Emmenan- burns may not reach temperatures high enough to
the dropped at 110°C. Daucus and Penstemon were stimulate Ceanothus seed germination (Parker
most sensitive to heat: germination dropped at 1989). Assuming seed longevity of about 50 years,
70°C. Emmenanthe, Galium and Lotus germination any ungerminated seeds might die before the next
dropped at temperatures of 90°C. Epilobium was fire (Parker 1989; O'Neil 2002), leading to de-
most tolerant of heat: germination dropped signifi- creased seed bank diversity. Ceanothus reaches
cantly at 1 10°C. sexual maturity at 20 years of age, at which time
Germination offive species {Ceanothus, Genista, seed banks reach their highest levels (Zamitt and
Lotus, Daucus and Penstemon) was not affected by Zedler 1994). Ungerminated seeds are also suscep-
moisture (Table 3). Lotus, Daucus and Penstemon tible to predation (Mills and Kummerow 1989;
seeds were negatively affected by temperature in O'Neil 2002).
both moist and dry treatments (Table 3). Ceanothus Seeds of shrub species that would germinate un-
and Genista germination was higher when seeds der the low moisture conditions typical offall burns
were heated in both moist and dry treatments (Table (e.g., Eriodictyon and Adenostoma), are negatively
3). affected under conditions found during spring
Germination of six species {Adenostoma, Cam- burns. Adenostoma may continue to dominate post-
issonia, Emmenanthe, Epilobium, Galium, Mala- fire vegetation by resprouting. However, without
cothrix) decreased under moist heat treatments at the establishment ofyoungerplants by germination,
temperatures as low as 70°C (Table 3). shrub density will eventually decline.
Diversity in the chaparral community is retained
Discussion primarily within the seed bank (Sampson 1944;
Sweeney 1956; Keeley and Keeley 1987; Parker
Levels of heat tolerance vary among species and Kelly 1989). Spring prescribed burns may
(Keeley et al. 1985; Keeley 1987; Keeley and Kee- change the composition ofthe seed bank over time
ley 1987; Odion and Davis 2000), and high tem- by differentially promoting germination of species
peratures (150°C) can be lethal for seeds (Keeley with hard-coated seeds over those with soft-coated
et al. 1985; Cruz et al. 2003). For several fire-stim- seeds. High soil moistures during burns negatively
ulated species in this study, germination decreased affect germination for many species that would sur-
at 120°C. vive fall burns, and this would likely lead to shifts
The heat-stimulated seed germination (heat or in community composition. The impact of winter
chemical cues) observed in many species in this or spring burns may be particularly severe for fire-
study {Ceanothus, Camissonia, Lotus, Genista, Em- dependent herbaceous annuals with soft seed coats
menanthe) supports findings in other field and lab- (e.g., Emmenanthe, Epilobium), since their seeds
oratory studies (Keeley and Nitzberg 1984; Keeley wouldn't survive fires under wet conditions, and
et al. 1985; Keeley and Keeley 1987; Moreno and may be depleted from the seed bank.
Oechel 1991; Ferrandis et al. 1999). Also, Aden- Fire can promote invasion of nonnative species
ostoma has previously been observed to be sensi- (Zedler and Scheid 1988; D'Antonio et al. 1993).
tive to heat during winter burns (Moreno and Oech- A late spring fire leads to germination the following
el 1991). spring, leaving an opportunity fornonnative species
Soil moistures of 15%, equivalent to those oc- to disperse to and establish at the site (Parker
curring during spring burns in the Mendocino Na- 1989). The reduction in density and species rich-
tional Forest, had a negative impact on seeds with ness of plants germinating from dormant seed
no seed coat dormancy (Le Fer 1998). Seeds that banks also can give nonnative species an opportu-
were negatively impacted by heat under moist con- nity to become estabhshed (Parker 1989). Seeds of
2005] LE PER AND PARKER: SEASONALITY OF BURN AND SEED GERMINATION 173
some nonnative species (e.g.. Genista) may ger- north Florida longleaf pine savannas. Ecological
minate at high rates after spring fires, leading to Monographs 65:441-476.
increased eradication difficulties. The European in- Hastings, M. S. andJ. M. DiTomaso. 1996. Fire controls
vasive Genista can resprout after fire, and produces yatelSluogwarsltoaarfthRiistdlgeeinStCataeliPfaorrkn.iaRgersatsosrlaatnidosn:atnesdtMpalont-s
a large and persistent seed bank (Parker and Kers- agement Notes 14:124-128.
nar 1989), stimulated by moderate heat either from Kauffman, J. B. AND R. E. Martin. 1991. Factors influ-
fire or summer heat (Parker 1993; Fenandis et al. encing the scarification and germination of three
1999). montane Sierra Nevada shrubs. Northwest Science
Burns are an important component of chaparral 65:180-187.
management and many species require fire to ger- Keeley, J. E. 1987. Role of fire in seed germination of
minate. However, shifting the fire regime to the woody taxa in California chaparral. Ecology 68:434-
winter or spring affects soil moisture, fire intensity 443.
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dromes in the California chaparral. Botanical Review
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decreases these alterations to the ecosystem's dy- . 1995. Future of California floristics and system-
namics. If this is not feasible, selecting prescribed atics: wildfire threats to the California flora. Madrono
burn windows that minimize soil moisture may de- 42:175-179.
crease negative impacts. In addition, post-fire mon- . 2002. Fire management of California shrubland
itoring ofregeneration and community composition landscapes. Environmental Management 29:395-408.
can provide information that increases our under- AND M. E. NiTZBERG. 1984. Rolc ofcharred wood
standing of the effects of environmental variables in the germination of the chapan-al herbs Emmenan-
(e.g., moisture, heat, time of year) on specific spe- the pendidiflora (Hydrophyllaceae) and Ehophyllum
coufertiflorum (Asteraceae). Madroflo 31:208-218.
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Acknowledgments in the germination of chaparral herbs and suffrutes-
cents. Journal of Ecology 73:445-458.
Special thanks to the staff at the Mendocino National and S. C. Keeley. 1987. Role of fire in the ger-
Forest, especially David Isle for his project ideas and in- mination of chaparral herbs and suffrutescents. Ma-
spiration; the Hot Shot crew and Fire Captains. Research drono 34:240-249.
wFoarsesft.unCdoedlubsya: CUo.Su.ntFyorFeistshSearnvdiceG,aMmeendAodcviinsoorNyatCioonma-l KeeleWy.,JSo.hnCs,onJ.. E1.98K1e.ePloesyt,firSe. Msu.ccHeustsciohninosfont,heahnedrbAa.-
mission, Glenn County Fish, Game and Recreation Com- ceous flora in southern California chaparral. Ecology
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District Bureau ofLand Management, U.S. Forest Service Le Fer, D. 1998. Effect ofseasonality ofburn, soil mois-
Pacific Southwest Research Station and the Mt. Lassen ture and temperature on chaparral regeneration. M.S.
Chapter ofthe California Native Plant Society. Thanks to thesis. San Francisco State University, San Francisco,
two anonymous reviewers for their suggestions for im- CA.
proving the manuscript. Mills, J. N. and J. Kummerow. 1989. Herbivores, seed
predators and chaparral succession. Pp. 49-55 in S.
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