50%) seedling emergence, although high variability within treatments resulted in no statistically significant differences. These results show that the needle layer hinders germination and/or emergence of seedlings from the seed bank. its removal may be a recommended technique to accelerate natural restoration in pine plantations. Rev. Biol. Trop. 59 (3): 1071-1079. Epub 2011 September 01]]>
50%), debido a la alta variabilidad de los tratamientos no se produjeron diferencias estadísticamente significativas. Estos resultados muestran que la capa de hojarasca impide la germinación y/o la emergencia de las plántulas del banco de semillas del suelo. Su eliminación puede ser una técnica recomendada para acelerar la restauración natural en las plantaciones de pino.]]>
Soil seed bank and the effect of needle litter layer on seedling emergence in a tropical pine plantation
Andrea Bueno & Zdravko Baruch
Laboratorio de Ecología Vegetal. Departamento de Estudios Ambientales. Universidad Simón Bolívar. Aptdo 89000. Caracas, Venezuela; abuenog@gmail.com, zbaruc@usb.ve
The soil seed bank is the basis for community establishment and permanence and plays a primary role in natural restoration of degraded or altered ecosystems. As part of a restoration project, this study aimed to quantify the soil seed bank and to evaluate the effect of the ]]>
2 from 17 morphotypes. in the field, the number of morphotypes and seedlings was only 9% and 6% respectively, of those emerged in the ]]>
Epub 2011 September 01.
El banco de semillas del suelo es la base para el establecimiento y la permanencia de una comunidad y desempeña un papel fundamental en la restauración natural de los ecosistemas degradados o alterados. Como parte de un proyecto de restauración, este estudio tuvo como objetivo cuantificar el banco de semillas del suelo y ]]>
2 de 17 morfotipos. En el campo, el ]]>
Palabras clave: germinación, Pinus caribaea, ]]>
Viable seeds stored in the soil at a given time make up the soil seed bank, a source of propagules that contributes to the long term permanence of individual species, and the plant community as well, and to the processes of succession and restoration after disturbances ]]>
et al. 2006, Csontos 2007). Soil seed bank studies are of great importance for the understanding of the secondary succession and it is considered as a necessary first step for the design of ecological restoration plans (Baskin & Baskin 1998, Abella et al. 2007, Bossuyt & Honnay 2008).
Several factors may induce or ]]>
et al. 1999). However, environmental variables, as well as the standing vegetation of a site, can induce or inhibit germination from the soil seed bank. For example, canopy species modify understory conditions in several ways, e.g.: by altering water and nutrient availability, by modifying microclimate, and by determining the quality of the litter layer that covers the soil (Grime 1979, ]]>
et al. 2006). This litter layer can prevent germination by altering the intensity and quality of sunlight reaching the soil, and by forming a mechanical barrier for the access of seeds to the soil and the emergence of seedlings (Facelli & Pickett 1991). In some cases, it may also limit germination and establishment of seedlings due to allelophatic effects (Del Moral & Cates 1971, Crawley 1986, Fernández et al. 2006, van Andel 2006). Conversely, this layer may favor recruitment by maintaining soil humidity and by hiding the ]]>
et al. 2003).
In conifer woods, it has been found that the needle layer can favour or inhibit germination of seeds from the seed bank (McAlpine & Drake 2002, Sánchez et al. ]]>
Materials and methods
The plantation is located in the natural area of Universidad Simón Bolívar, Caracas, Venezuela (10º24’36’’ N - 66º53’11’’ W) at 1 300m.a.s.l. it extends on ~50 ha composed mainly by Caribbean Pine (Pinus caribaea Morelet). The plantation ]]>
Seed bank ]]>
et al. 2005). in January 2008, 15 sampling points were selected at random along a transect in a representative area of the plantation. At each point, two 400cm2 plots (20x20cm) were established: in one of them (F-N) all pine needles were carefully removed. The other (F+N) was left intact. in both, seedling emergence was monitored every two weeks for 6 months. in addition, at each point two sets of 15 soil samples ]]>
any remaining seeds. This treatment (Gh+N) evaluated the effect of pine needles on emergence under controlled conditions. The second soil sample set was placed in similar trays, but left uncovered with needles. This treatment (Gh-N) evaluated composition and abundance of ]]>
Trays were placed in the greenhouse under a shading net to simulate conditions in the plantation understory, and their location in the ]]>
]]>
Temperature, relative humidity and rainfall were recorded inside and outside of the plantation with loggers (Onset Hobo H08-032-08) and rain gauges. Depth and dry weight of the needle layer was measured at 15 random points. In the greenhouse, maximum and minimum temperatures (Taylor 5458 thermometer) and radiation (LI-COR LI-188 radiometer) were recorded. Understory vegetation was sampled in 15 circular plots (2m radius; 12.67m2) next to each seed bank sampling point. Species ]]>
Density of each species or morphotype was obtained by adding the number of individuals that emerged in each tray and is presented on a square meter basis. Percentage frequency was also calculated. The index of Value importance (IVI) for each species was calculated by adding relative density and frequency. Shannon diversity (H) and evenness (EH) ]]>
et al., 2006). A Mantel test (Mantel 1967) was employed to compare the two matrices from the greenhouse treatments (Gh-N and Gh+N) (McCune & Mefford 2001). ]]>
Results
]]>
Environment: Mean maximum temperature was ~4ºC lower below the pines than outside (25.5 vs 29.3ºC, while mean minimum temperature was slightly higher (15.6 vs 15.5ºC) which resulted in mean minimum relative humidity higher inside the plantation than outside (41% vs 28%). Rainfall could not be measured from January to March, but ]]>
2. Air temperature in the greenhouse was close to that in the field ranging between 15±2ºC and 29±3ºC. Photosynthetically active radiation was 20.8%±1.7% of that outside the greenhouse which was close to that recorded in the pine understory (16-19%, unpublished data). ]]>
Soil seed bank composition and the effect of needles
Greenhouse: The potential maximum ]]>
Table 1), whose density was 1 222/m2. There was a high variability among trays (number of seedlings ranged from 0 to 36 seedlings per tray) but Emilia coccinea and Melinis minutiflora were the most important ]]>
Appendix). Only 23 seedlings from three morphotypes emerged from seeds retained in the needle layer (treatment SN) Table 1, where E. coccinea was also the absolute dominant (91% of all seedlings) (Appendix). Contamination in the greenhouse was minimal. Only two morphotypes emerged from control ]]>
The presence of the needle layer (Gh+N) decreased the number of seedlings by >50%, but the number of morphotypes decreased by only 8% resulting in increased diversity (H) and evenness (EH) (Table 1). Here, E. coccinea and
M. minutiflora were also dominant (53% of emerged seedlings, Appendix). Due to high variability, (in some trays no seedlings emerged at all, but in others up to 12 seedlings emerged) the difference between treatments (Gh-N vs Gh+N) was not statistically significant (PERMANOVA, p>0.05). The Mantel test (r=0.113 and p=0.128) which compared the Gh+N and Gh-N treatments, showed that the matrices were similar. ]]>
Both treatments had 10 morphotypes in common with a Ss of 62.1%. in all greenhouse treatments the emergence sequence was similar: first M. minutiflora, E. coccinea and dicot E and lastly Melastomataceae D and dicots I and H. Only 16% of the seeds emerged from the soil seed bank were retained by the needles in treatment SN).
Field
In the field, the number of morphotypes and seedlings, in F+N and F-N treatments, was only 9% and 6% respectively of those emerged in the ]]>
Table 1). Similarity between field treatments was 37.7% with almost absolute predominance of M. minutiflora (Appendix). Again, differences between treatments were not significant (PERMANOVA, p>0.05). The standing vegetation next to the sampling plots comprised 31 species, of which 52% were woody (Appendix). The dominant species were the graminoids Scleria sp. ]]>
Laciasis sp., while Melastomataceae was the dominant family (Appendix). Species richness was 45% higher than that of the seedlings emerged in the greenhouse (Gh-N), but H y EH were similar (Table 1).
Since it was ]]>
Appendix). E. coccinea, the dominant in the soil seed bank, was the fourth most important in the understory, while M. ]]>
, second in the seed bank, was the 13th in the understory. Scleria sp., the dominant of the understory, appeared only twice in the seed bank (Appendix).
Discussion ]]>
The soil seed bank was dominated by E. coccinea and M. minutiflora which produce numerous, light-weight and wind-dispersed seeds that characterize early-successional or ruderal species (Fenner & Kitajima 1999, Gurevitch et al. 2006). Seed density (1 222/m2) was within the wide range reported for pine plantations in temperate zones ]]>
et al. 2009). Unfortunately, these figures are unavailable for tropical pine plantations. The high variability between replicates found here is frequent in this type of studies (Thompson 1986, Moles & Drake 1999, Olano et al. 2002), and may suggest a non-random spatial distribution of seeds in the soil of the pine plantation. Despite the relatively favorable conditions under the pine canopy (lower air temperature and higher humidity) which would favor survival and growth ]]>
The needle layer may retain seeds, which depends on their size and shape thus preventing seeds from reaching soil surface and germinate ]]>
E. coccinea and M. minutiflora although species richness was similar in both treatments. ]]>
et al. 2010). In this study, this suggests that the qualitative effect of the needles was more important than the quantitative one. In the field, pine needles inhibited seedling ]]>
Some studies have shown that the ]]>
et al. 2009, 2010). These variables were not considered in this study, but it would be important to include them in future researches, as they also indicate potential negative effects of the presence of the litter layer.
]]>
Since it was not possible to identify all seedlings emerged from the soil seed bank and some understory species, the correspondence between them was established only partially. Morphotype richness in the understory was higher than that of the seed bank, suggesting the importance of seed bank sampling throughout the year. However, it is also possible that limited time for germination allowed in this study underestimated the real composition of the seed bank. E. coccinea and ]]>
M. minutiflora turned out to be less important in the understory that in the seed bank. It is possible that shading under the canopy inhibited their growth after emergence. On the other hand, Scleria sp., the most important species in the standing vegetation had a frequence of only 13% in the seed bank. This lack of correspondence has been found numerous times (Korb et al. 2005, Sakai et al. 2005, Hopfensperger 2007, Lang & Halpern 2007, Zobel
et al. 2007, Bossuyt & Honnay 2008) and has been attributed to biological differences among species (seed viabilidad and/or likelihood to be predated) or to methodology (germination technique, time of sampling, number or volume of soil samples) which may limit quantification of the soil seed bank (Brown 1992, Malo 2000, Csontos 2007).
The results obtained here suggest ]]>
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Received 03-IX-2010. Corrected 01-II-2011. Accepted 01-III-2011.