Direct seeding of late-successional trees to restore tropical montane forest
Research highlights
▶ Recruitment of large-seeded tropical trees is limited by lack of seed dispersal. ▶ Direct seeding increased recruitment under pastures, secondary forests, and plantations. ▶ Seedling survival and growth was highest under fast-growing N-fixing trees. ▶ Direct seeding is a low-cost restoration approach for degraded tropical lands.
Introduction
Natural regeneration of late-successional trees in fragmented and degraded landscapes can be strongly limited by a lack of seed dispersal into successional habitats (e.g. Duncan and Chapman, 1999, Holl, 1999, Rodrigues Da Silva and Matos, 2006, Wijdeven and Kuzee, 2000); this limitation is particularly acute for larger-seeded animal-dispersed trees. Studies across a range of locations in the tropics show that movement of larger seeds beyond the edges of forest fragments is rare (Cole, 2009, del Castillo and Rios, 2008, Dosch et al., 2007, Duncan and Chapman, 1999, Holl, 1999, Ingle, 2003) and establishment of these species in natural regeneration is limited even after many decades of succession (Aide et al., 2000, Finegan, 1996). The lack of natural recruitment of these species has led to concerns over their persistence in fragmented and degraded landscapes, and more aggressive restoration efforts such as enrichment planting have been suggested as a necessary step to augment severely dispersal-limited species in future forests (e.g. Dosch et al., 2007, Martinez-Garza and Howe, 2003, Zimmerman et al., 2000).
The predominant method used to restore degraded tropical lands is to plant nursery-raised tree seedlings (Chazdon, 2008, Lamb et al., 2005). Although this can be an effective technique for quickly establishing forest cover (Holl et al., 2010, Montagnini, 2001, Wishnie et al., 2007), there are drawbacks that make this method less useful for the restoration of most mature-forest species. First, the selection of species available from nurseries is often limited to trees that have commercial or agricultural value, and for which propagation techniques are known (Sautu et al., 2006). Second, studies of the seed biology of mature-forest trees suggest that a majority of larger-seeded species are recalcitrant (sensitive to desiccation and temperature, and germinate rapidly; Daws et al., 2005, Sautu et al., 2006, Vazquez-Yanes and Orozco-Segovia, 1990) making them difficult to store or establish in nurseries. Finally, planting nursery-raised seedlings can be quite costly and labor intensive (Engel and Parrotta, 2001, Hardwick et al., 1997, Zahawi and Holl, 2009) particularly when it involves the use of native species about which little is known (Blakesley et al., 2002, Sautu et al., 2006).
One potential alternative is to collect seed from local sources and plant them directly into the habitats targeted for restoration. Although direct seeding has not been widely adopted in forestry practice, it has been successfully used for establishment of some tropical and subtropical tree crops such as Acacia, Anacardium occidentale, Gmelina arborea, and Pinus (Engel and Parrotta, 2001, Evans, 1982). Only a handful of studies, however, have tested directly seeding late-successional, large-seeded trees as a restoration strategy (Bonilla-Moheno and Holl, 2009, Camargo et al., 2002, Doust et al., 2006, Hooper et al., 2002) and there is a need for better information on the habitat types and stage of succession in which direct seeding can be applied effectively.
The few previous direct seeding studies suggest that it is a promising restoration strategy for larger-seeded tree species (Camargo et al., 2002, Doust et al., 2006, Hardwick et al., 1997, Hooper et al., 2002, Nepstad et al., 1991) but results among habitat types have generally yielded divergent trends. For example, Hooper et al. (2002) found that larger-seeded, shade tolerant trees successfully germinated and grew under Saccharum pasture grasses in Panama, whereas Sampaio (2007) reported low levels of establishment from seeds planted into pastures in seasonal deciduous forest in Brazil. Camargo et al. (2002) reported that larger-seeded species could grow on degraded, bare ground and in pasture, but that seedlings survived less well in successional and mature forest in the Central Amazon. Conversely, Bonilla-Moheno and Holl (2009) found similar levels of survival and growth in 8–15 years and >50-year-old forests in the Yucatan but two of the three species (Brosimum alicastrum Sw., Enterolobium cyclocarpum (Jacq.) Griseb. compared to Manilkara zapota (L.) Royen) performed less well in recently abandoned (<5 years) fields. Other authors have suggested introducing a mixture of seedlings and seeds, or introducing later-successional species after a canopy of early-successional species has been established (e.g. Camargo et al., 2002, Hardwick et al., 1997, Ray and Brown, 1995). Thus far, it is not clear whether direct seeding can be usefully applied in both early and later stages of succession or used as a supplementary step in more intensive restoration planting. Finally, although direct seeding is often considered to be a low-cost approach (see Engel and Parrotta, 2001), there have been few actual cost comparisons between direct seeding and planting seedlings in a restoration setting.
The goal of this study was to evaluate the potential of direct seeding under a range of possible restoration settings. We planted seeds of five, large-seeded, late-successional trees into three habitat types that represented different successional stages and restoration approaches: in recently abandoned pasture; under naturally established young (8–10 years) successional forest; and under 3-year-old tree plantations established to facilitate tropical forest recovery. Specifically, we aimed to (1) assess how planting seeds into different successional stages and restoration treatments affected seed germination and seedling establishment, survival, and growth, and (2) compare the logistics and costs of direct seeding using locally collected seeds with planting nursery-raised seedlings.
Section snippets
Study area
The study was carried out at sites spread between the Las Cruces Biological Station (8°47′7″N, 82°57′32″W) and the town of Agua Buena (8°44′36″N, 82°58′04″W) in Coto Brus County in southern Costa Rica. The forest in this region is classified as a tropical montane rain forest by Holdridge et al. (1971). Study sites range from 1110 to 1290 m a.s.l. and the mean annual rainfall is ca. 3500 mm with a distinct dry season from December to March. The soils are a mix of ultisols and andisols, and the
Germination and survival
Of the 8880 seeds planted, 43% germinated and of these 87% survived as seedlings by the end of the first year. A total of 64% of germinated seeds (not including the seedlings that were harvested for measurements in year 1) survived as seedlings by the end of the second year. Percent germination varied significantly among species (F = 17.8, df = 4, p < 0.0001) but not across habitat types (F = 2.3, df = 2, p < 0.1086), and there were no interaction effects (p > 0.05; Fig. 1). Garcinia had the highest
General overview
Our results support several previous studies showing that late-successional, large-seeded trees can be successfully introduced into early stages of succession (Camargo et al., 2002, Hooper et al., 2002). We found surprisingly consistent trends among the five species tested and our results lead to several general conclusions regarding the efficiency of direct seeding under different restoration approaches. First, we found that although germination under recently abandoned pastures, young
Conclusions
Direct seeding is a viable, low-cost approach for including late-successional species that do not colonize naturally into early stages of forest recovery. Seeding resulted in reasonable levels of seedling establishment (25–82% of germinated seeds) in pastures and young secondary forests and high levels of establishment under plantations (62–89%). Because of the great variability in species performance, testing and selection of local trees suitable for planting in targeted habitats prior to
Acknowledgements
We are grateful for field research and logistical help from C. Keene, R. Gomez, F. Obando, J. A. Rosales, G. Sady, and D. Cole-Christensen. F. Oviedo provided valuable assistance with species identification. Support for this project was provided by a NSF grant (DEB 0515577), the Marilyn C. Davis Memorial Foundation, the Earthwatch Foundation, and the UCSC Environmental Studies Department, as well as a UCSC writing fellowship for the senior author. We thank the landowners who permitted us to
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