Relative Importance of Seed-bank and Post-disturbance Seed Dispersal on Early GapRegeneration in a Colombian Amazon Forest

Luis S. Castillo1 and Pablo R. Stevenson

Universidad de Los Andes, Laboratorio de Ecologıa de Bosques Tropicales y Primatologıa, Centro de Investigaciones Ecologicas La Macarena,

Carrera 1a No. 18a- 12, Bogota, Colombia

ABSTRACT

Early forest gap regeneration may be generated by postdisturbance seed rain and by seed, seedling or bud banks (i.e., resprouting). The relative importance of eachprocess may depend on several factors (e.g., fruit/seed production, abundance and behavior of seed dispersers, gap characteristics, etc.). We experimentally comparedthe importance of seed-bank and seed-rain affecting early recruitment of seedlings in an Amazonian forest (Zafire Biological Station, Colombia), using soil transplantsfrom forests to gaps and seed rain enclosures. We found that, during the 8-mo study, the seed-bank contributed with a larger number of individuals and species thanseed-rain. The low seedling recruitment rates may be associated with reduced populations of animal seed-dispersers due to hunting and/or low levels of forest fruitproduction.

Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp

Key words: pioneer plants; secondary succession; seed dormancy; seed rain; Zafire Biological Station.

NATURAL GAPS MAY PLAY IMPORTANT ROLES in forest regeneration,

floristic composition, and diversity in tropical forests. Canopy gaps

contribute to environmental heterogeneity and, consequently, pro-

vide more niches to be exploited by organisms (Grubb 1977, Con-

nell 1978, Denslow 1987, Wright 2002). The intermediate

disturbance hypothesis describes how frequency, magnitude andtime since disturbance may affect plant diversity, and predicts that

the highest number of coexisting species should be found at inter-

mediate magnitudes of these processes (Connell 1978). Gap regen-

eration is a complex process since dynamics are affected by a variety

of factors associated with gap size and age, the surrounding plant

and animal communities, weather, and human intervention,

among others (Schupp et al. 1989).

Gap regeneration arises from four different sources: (1) theseed bank which refers to the seeds dormant or transient in the soil

(Swaine & Hall 1983, Lawton & Putz 1988, Dalling & Hubbell

2002, Martins & Engel 2007); (2) the seedling bank, including re-

cently germinated plants that were present in the site before gap

formation (Teketay 1997, Frey et al. 2007); (3) the bud bank, rep-

resenting damaged plants able to sprout after gap formation (Bond

& Midgley 2001, Klimesova & Klimes 2007); and (4) the post-

disturbance seed rain that corresponds to the seeds arriving to thegap after disturbance (Schupp et al. 1989, Gorchov et al. 1993,

Estrada-Villegas et al. 2007). Few studies have made experimental

approaches to quantify regeneration dynamics in gaps (Dalling &

Hubbell 2002), and to our knowledge only Lawton and Putz

(1988) have compared the importance of seed bank and postdis-

turbance seed rain in this process in a Costa Rican cloud forest. The

aim of this study was to experimentally evaluate the relative impor-

tance of seed banks and seed rain in early seedling recruitment in

natural Amazonian forest gaps, using a novel approach, with some

elements from Lawton and Putz (1988).

METHODS

STUDY SITE.—Field work was conducted in a continuous, unloggedterra firme Amazonian forest in Zafire Biological Station

(410000000 S, 6915305700 W, 130 m asl) between September 2007

and April 2008. The station is 27 km north of Leticia city (Colom-

bia). Temperature is on average 261C, with low variation over the

year, and mean monthly precipitation is 280 mm, without consis-

tent dry seasons (minimum monthly precipitation ca 160 mm), al-

though the months with less rainfall tend to occur between June and

August (IDEAM Institute 2001). We conducted the experiments inten recently formed natural gaps (range: 2–24 mo old) distributed in

25 ha of forest, each one with an area of 101–309 m2 (174.4� 67.2

SE) and average canopy openness of 21.7 percent (8.2–39.5%).

SOIL SAMPLES AND TREATMENTS.—We used four different treatments

in each gap: seed-bank (SB), seed-bank plus seed-rain (SB1SR),

seed-rain (SR), and a control (C). The SB treatment consisted of

the transport of soil from primary forest to the gap and the ob-struction of seed-rain with nylon nets (0.5� 0.5 mm pores). Soil

samples were collected from 0–10 cm deep, a depth at which many

seeds are deposited by secondary seed dispersers (Andresen & Levey

2004) and are able to form the seed bank. The litter present in the

soils was not excluded from the samples. In this treatment, all re-

cruits came only from the seeds that were present in the primary

forest soil bank. The SB1SR treatment differed from the SB treat-

ment because it did not have the exclusion net. In this treatment,recruitment included seedlings emerging from the seed bank and

the seed rain. The SR treatment consisted of sterilized primary

forest soil, which then was carried to the gaps. The sterilization

Received 13 April 2009; revision accepted 5 September 2009.1Corresponding author; e-mail: santiagocastillom@yahoo.com

BIOTROPICA 42(4): 488–492 2010 10.1111/j.1744-7429.2009.00605.x

488 r 2009 The Author(s)

Journal compilation r 2010 by The Association for Tropical Biology and Conservation

consisted in placing the soil sample during 2 h in a closed pot that

was inside another pot with boiling water (Bell & Williams 1998).

In the SR treatment, all seeds were killed so that recruits came only

from the seed rain. The C treatment differed from the SR treatmentbecause it had a net that prevented the arrival of seeds, thus we also

excluded recruitment from the seed rain. The estimated volume of

soil for each treatment was about 45 L (100� 50 cm wide, 8–10 cm

deep). All soil samples were taken from primary forest at least 30 m

away from the corresponding gap and were situated randomly in-

side holes with the same dimensions and at least 2 m away from gap

edge. The nets were box-shaped and situated upside down at 5 cm

above the ground to allow the movement of small seed and seedlingpredators. This nylon net had larger dimensions than the area with

translocated soil samples (110� 60 cm wide, 60 cm high), to re-

duce seed rain contamination through the space between the net

and the ground. For the SB, SR, and SB1SR treatments we per-

formed two repetitions in each gap and the average was used in

subsequent analyses as replicates. Our methodology resembles the

protocol used by Lawton and Putz (1988), since both studies in-

cluded translocation of primary forest sample soils to gaps, exclo-sures to isolate the effect of the seed rain, and soil sterilization. This

study differs in two main aspects: (1) they spread the mature forest

samples (1000 cm2) in a larger area (3600 cm2) inside gaps; and (2)

in their study, the soil was sterilized with carcinogenic methyl-bro-

mide application. As a result, chemical residuals might be affecting

soil microorganism colonization and germination of seeds that

come from postdisturbance dispersion.

MEASURED VARIABLES AND STATISTICS.—Recruited vascular and non-

vascular seedlings 4 1 cm were identified to morphospecies and

marked to assess their fate over the next 8 mo. For each treatment in

each gap we recorded the number of individuals, the cumulative

number of individuals, species richness and calculated Simpson’s

reciprocal index of diversity [index = 1/Sp2, where p represents the

proportion of individuals of morphospecies i in the population

(Krebs 1999)]. Measurements were taken weekly for the first 2 mo,fortnightly for the next 2 mo, and a final reading 4 mo later. To

evaluate the statistical differences among treatments over time, and

since the assumption of sphericity of the variances (i.e., that differ-

ences of variances between any time period are the same) could not

be met (Mauchly’s test for all variables; W = 0, Po 0.001), we used

two alternatives to a repeated measures (RM) test: (1) we adjusted

the RM F-ratio using a Greenhouse–Geisser (G–G) analysis; and (2)

we summarized the responses for each treatment/time as a singlevalue: the slope obtained in a regression analysis using log-trans-

formed data (Quinn & Keough 2002, Gotelli & Ellison 2004). The

calculated treatment slopes were measured in each gap as replicates

and their means and variances were compared with a Kruskal–Wallis

(K–W) test and, subsequently, with a Tukey multiple comparisons

test. The significance level used was 0.05.

RESULTS

New seedlings first emerged during the second week after the treat-

ments were set up in all gaps. During the following 6 weeks, the

seedling emergence rate was relatively high and similar in the SB

and the SB1SR treatments, but it gradually declined there after. In

contrast, seedling emergence was very low in the SR and C treat-

ments throughout the study (Fig. 1). Significant differences wereobserved between two groups of treatments (SB = SB1SR4 SR

= C) for all parameters (G–G RM test: number of individuals:

Ftreat = 19.4, Ftreat�week = 11.0; cumulative number of individuals:

Ftreat = 20.1, Ftreat�week = 15.0; number of morphospecies: Ftreat

= 19.6, Ftreat�week = 10.7; and Simpson’s reciprocal index of diver-

sity: Ftreat = 14.1, Ftreat�week = 5.68, P values o 0.001; and K–W

test of slopes: w2 = 29.0; w2 = 29.9; w2 = 28.7; w2 = 22.6, respectively,

P values o 0.001; Table S1). The most abundant taxa recruitingwere mainly pioneer species: Cecropia spp. (Urticaceae), some

members of Melastomataceae, Apeiba sp. (Malvaceae), and mem-

bers of Dilleniaceae, Solanaceae, Annonaceae and Poaceae. The

representation of shade tolerant species (e.g., Iryanthera sp.) was

small in all treatments (6.6% of individuals in week 32).

DISCUSSION

In our study, the seed-bank played an important role in early gap

regeneration compared with that from the seed rain. This pattern

has been found in other studies (e.g., upland evergreen forest in

Ghana: Swaine & Hall 1983; cloud forest in Costa Rica: Lawton &

Putz 1988; moist tropical forest in Panama: Dalling & Hubbell

2002; and seasonal semideciduous tropical forest in Brazil: Martins

& Engel 2007), where the recruiting seedling community tightly

reflected the seed bank composition. In our two seed bank treat-ments (SB and SB1SR), seed germination of pioneer species began

in the first 2 weeks as a consequence of favorable conditions to

which the seeds were exposed (Bazzaz & Pickett 1980, Garwood

1983, Kozlowski 2002). At the end of the experiment, these two

seed bank treatments had more than six times the number of indi-

viduals and accumulated number of individuals, and three times the

morphospecies richness and diversity, than those in the seed rain

treatment and control. Seed rain after gap formation played a mi-nor role in the early natural regeneration process in our site during

the study period.

The data showed a nonsignificant trend for higher seedling

emergence in the SB treatment compared with the SB1SR treat-

ment (Figs. 1A and B). Possibly, independent of the species, the

nylon net could reduce seedling predation, reduce seed removal (by

predators and rainfall), and increase seedling survival (i.e., reducing

the variability in humidity, temperature, and radiation). The ster-ilization method used in the SR and C treatments could reduce

microorganism populations in the soil potentially affecting seedling

emergence, growth, and survival. Nevertheless, since there were no

physical or chemical barriers, we assumed that microorganisms

could colonize in these soils again. We also assumed that the soil

sterilization method did not considerably change the soil physical

and chemical characteristics. The control treatment showed a re-

duced number of germinated seedlings, similar to the SR treat-ments. All those seedlings are of pioneer plants. We assume that the

great proportion of those seedlings came from horizontal dispersion

(e.g., earthworms: Willems & Huijsmans 1994; ants: MacMahon

Seed-Bank vs. Seed-Rain in Early Gap-Dynamics 489

et al. 2000; dung beetles: Andresen 2001; and abiotic factors such aswind and runoff), rather than ineffective sterilization or nylon net

seed permeability.

At the end of the study seedling mortality was close to 30

percent in all treatments. It is possible that during the gap regen-

eration process, at some point, the recruitment rates from seed

rain will become higher than those from the seed bank, due to a

continuous seed input from the seed rain. Then, in the long run,

the relative importance of these sources of recruitment willdepend on the size of gaps and seed dispersal processes (Hubbell

et al. 1999).

The differences between seed rain and seed bank recruitment

in early regeneration have many explanations that are not exclusive.

(1) The seed bank has more time to build up than the seed rain

(Swaine 2001), and a great proportion of these seeds (i.e., pioneer

species), can be dormant for a long period of time. (2) In contrast to

the seed bank, seed rain could depend more closely on plants fruit-ing during the study period and on biotic and abiotic dispersal

agents. Thus, dispersal limitation may affect the recruitment of

plants from the seed rain (Hurtt & Pacala 1995, Hubbell et al.1999, Nathan & Muller-Landau 2000), and not so strongly from

the seed bank. (3) Young gaps may not be attractive to biotic dis-persers like birds and bats, since in gaps these organisms may face

high predation risks and low levels of resources (Schupp et al.1989). Nevertheless, bats continue to be the most important agents

of dispersion in open areas because they use these sites and they can

defecate in flight (Gorchov et al. 1993, Medellın & Gaona 1999).

Finally, (4) the Amazonian forest, like the one at Zafire Station,

grows on old, nutrient-poor soils associated with sedimentation of

black-water rivers (Irion 1978). Soil conditions limit forest produc-tivity and, as a consequence, the community of seed dispersers

shows low densities (Palacios & Peres 2005). The relationship be-

tween forest productivity and the abundance of dispersers has been

documented in primates (Stevenson 2001, 2007b), birds (Loiselle

& Blake 1991) and bats (Medellın et al. 2000).

The low representation of recruits from shade-tolerant plants

may also be linked to the role and abundance of large seed dispers-

ers (Peres & van Roosmalen 2002). We witnessed some degree oflocal hunting (L. S. Castillo pers. obs.), which is known to affect the

densities of some important seed dispersers like primates and large

birds (Peres & Palacios 2007). In addition, large seeds are transient

in the seed bank because they do not have long dormancy periods

FIGURE 1. Effects of experimental treatments (seed-bank plus seed rain [SB1SR], seed bank [SB], seed rain [SR] and control [C]) through time on four parameters

of early gap regeneration dynamics: (A) mean number of individuals; (B) mean cumulative number of individuals; (C) mean number of morphospecies; and (D) mean

reciprocal Simpson’s index. Sampling was performed in 10 different gaps. Each gap contained one 0.5 m2 sample of each treatment in the Zafire Biological Station,

Colombian Amazon. The ‘a’ group differs statistically from the ‘b’ group of treatments (Kruskal–Wallis test of Slopes and Greenhouse-Geisser RM analysis,

Po 0.001). Error bars correspond to 95% CI.

490 Castillo and Stevenson

(Swaine 2001, Martins & Engel 2007) and, as a result, their num-

ber in the soil seed community is reduced (Dalling & Hubbell

2002). Another potential factor affecting the low representation of

large seeds is the higher predation risk that these face in gaps com-pared with mature forest (Schupp et al. 1989). This may result since

they are easier to find, more nutritious than small seeds, and may be

prone to predation in an impoverished seed environment (Steven-

son 2007a). We predict that in sites where forest productivity is

higher and animal dispersers are abundant, regeneration rates will

be faster since the seed bank and seed rain will be enriched, and

shade tolerant seeds will be less limited by dispersal. Nevertheless, it

is necessary to undertake other studies in different sites to evaluateour predictions.

ACKNOWLEDGMENTS

We are grateful to M. C. Penuela and the indigenous communities

to allow the study in Zafire Biological Station. We thank the mem-

bers of LEBTYP (Laboratorio de Ecologıa de Bosques Tropicales

y Primatologıa), C. Escallon and D. Cadena for their comments.We also thank the referees for their useful comments on an earlier

version of this manuscript.

SUPPORTING INFORMATION

Additional Supporting Information may be found in the online

version of this article:

TABLE S1. Slopes from the relationship between log time and fourvariables of early seedling regeneration.

Please note: Wiley-Blackwell Publishing are not responsible for

the content or functionality of any supporting materials supplied by

the authors. Any queries (other than missing material) should be

directed to the corresponding author for the article.

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