Islas Project
Research Description
In 2004, Dr. Rakan (Zak) Zahawi (Director of the Charles Darwin Foundation) and Dr. Karen Holl (UC Santa Cruz, [email protected]) started a long-term, large-scale tropical forest restoration studying applied nucleation (planting small patches or "islands" of trees) as a low-cost method to encourage seed dispersal and shade out pasture grasses. We established three treatments – natural regeneration, applied nucleation, and full tree planting (plantation) – at several sites. We have also been investigating the effect of proximity to seed sources on forest recovery in these sites. We and collaborators have measured seed rain, seedling establishment, litterfall, birds, bats, insects, mycorrhizae, and many other variables in these sites for over 20 years. Our results show that planting tree islands is an effective and comparatively cheaper approach to accelerate forest recovery that better simulates the natural recovery process than a traditional plantation-style restoration approach (Holl et al. 2020). In addition, results to date show that the amount of surrounding forest cover is not correlated with animal-dispersed seed rain and native woody species establishment in the first decade of succession, but distance to seed sources of individual tree species strongly predicts abundance of recruits. For more information on our results and outreach see our summaries and publications below. You can also watch our 7-min summary video on the project (Spanish version). As we move into the third decade of this study, the results continue to become increasingly interesting, given that we have one of the few well-replicated, long-term studies of tropical forest recovery. Recently, we have been focusing on understanding the mechanisms that limit the establishment of later successional trees species and conducting multi-year analysis. We are extremely appreciative of multiple grants from the U.S. National Science Foundation and our numerous collaborators in Costa Rica, the US, and other countries that have allowed us to continue this research.
Recent General Audience Publications
San José, M. 2025. Proyecto Islas: más de 20 años investigando cómo traer de vuelta el bosque
Wilson, S. J., N. S. Alexandre, K. D. Holl, J. L. Reid, R. Zahawi, D. Celentano, S. Sprenkle-Hyppolite, and L. Werden. 2021. Applied nucleation guide for tropical forests. Conservation International.
Holl, K.D. & Reid, J.L. 2020. Do we really need to plant a trillion trees? Trees islands are an ecologically and economically sound strategy for tropical forest recovery. The blog discusses our Journal Applied Ecology paper which summarizes 15 years of results from our long-term experiment in Costa Rica.
Holl, K. D. 2019. Rules of thumb for predicting tropical forest recovery. This blog provides a general summary of our 2018 Applied Vegetation Science paper.
Teaching Case Studies
Holl, K.D. and R. A. Zahawi 2023. This module for upper-division colleges students provides a broad introduction to both ecological and social aspects of tropical forest restoration. It focuses on our long-term study comparing three tropical forest restoration strategies in southern Costa Rica. The module is available in English, Spanish, Portuguese, French, and Bahasa Indonesia.
Blebea, C. B., and F. H. Joyce 2024. This module for college students explores seed dispersal by animals and how this ecological process can be harnessed to facilitate forest recovery through a forest restoration experiment in Costa Rica. The module includes an exercise to apply this knowledge to a forest restoration project. The module is also available in Spanish and Portuguese.
Selected Scientific Publications and Data
Joyce, F. H., R. A. Zahawi, and K. D. Holl. 2025. Lower-intensity restoration interventions drive greater seedling establishment for later-successional tree species. Journal of Applied Ecology.
Schubert, S. C., R. A. Zahawi, F. Oviedo-Brenes, J. A. Rosales, and K. D. Holl. 2025. Active restoration increases tree species richness and recruitment of large-seeded taxa after 16–18 years. Ecological Applications 35:e3053.
Quirós Cedeño, G. J., F. H. Joyce, M. A. Zúñiga Amador, and K. D. Holl. 2025. Differing Agents of Physical Damage to Artificial Seedlings in Remnant and Restored Forests. Biotropica 57:e13421.
Joyce F.H., Rosales J.A., Holl K.D., Zahawi R.A., Bui A., Reid J.L. (2024) Active restoration accelerates recovery of tropical forest bird assemblages over two decades. Biol Conservation 293, 110593. Data
Joyce, F. H., B. M. Ramos, R. A. Zahawi, and K. D. Holl. 2024. Vertebrate seed predation can limit recruitment of later-successional species in tropical forest restoration. Biotropica 56:e13381.
Kulikowski II, A. J., R. A. Zahawi, L. K., Werden, K. Zhu, and K. D. Holl. 2023. Restoration interventions mediate tropical tree recruitment dynamics over time. Philosophical Transactions of the Royal Society B 378: 20210077. Data
Werden, L. K., S. Zarges, K. D. Holl, C. L. Oliver, F. Oviedo-Brenes, J. A. Rosales, and R. A. Zahawi. 2022. Assisted restoration interventions drive functional recovery of tropical wet forest tree communities. Frontiers in Forests and Global Change 5. Trait data
San-José, M., L. K. Werden, F. H. Joyce, J. L. Reid, K. D. Holl and R. A. Zahawi (2022). "Effects of landscape structure on restoration success in tropical premontane forest." Scientific Reports 12: 13452. Data
Kulikowski II, A. J., R. A. Zahawi, and K. D. Holl. 2022. Effects of insect herbivory on seedling mortality in restored and remnant tropical forest. Restoration Ecology 30:e13467. Data
Reid, J. L., R. A. Zahawi, D. A. Zárrate-Chary, J. A. Rosales, K. D. Holl, and U. Kormann. 2021. Multi-scale habitat selection of key frugivores predicts large-seeded tree recruitment in tropical forest restoration. Ecosphere 12:e03868. Data
Zahawi, R. A., L. K. Werden, M. San-José, J. A. Rosales, J. Flores, and K. D. Holl. 2021. Proximity and abundance of mother trees affects recruitment patterns in a long-term tropical forest restoration study. Ecography 44:1826-1837. Data
Werden, L. K., K. D. Holl, J. M. Chaves-Fallas, F. Oviedo-Brenes, J. A. Rosales, and R. A. Zahawi. 2021. Degree of intervention affects interannual and within-plot heterogeneity of seed arrival in tropical forest restoration. Journal of Applied Ecology 58:1693-1704. Data
Holl, K. D., J. L. Reid, R. J. Cole, F. Oviedo-Brenes, J. A. Rosales and R. A. Zahawi 2020. Applied nucleation facilitates tropical forest recovery: Lessons learned from a 15-year study. Journal of Applied Ecology 57, 2316-2328.
Kulikowski II, A. J. 2020. Ant–scale mutualism increases scale infestation, decreases folivory, and disrupts biological control in restored tropical forests. Biotropica 52:709-716. Data
Werden, L. K., K. D. Holl, J. A. Rosales, J. M. Sylvester, and R. A. Zahawi. 2020. Effects of dispersal- and niche-based factors on tree recruitment in tropical wet forest restoration. Ecological Applications 30:e02139. Data
Holl, K. D., J. L. Reid, F. Oviedo-Brenes, A. J. Kulikowski, and R. A. Zahawi. 2018. Rules of thumb for predicting tropical forest recovery. Applied Vegetation Science 21:669-677. Data
Lanuza, O., F. Casanoves, R. A. Zahawi, D. Celentano, D. Delgado, and K. D. Holl. 2018. Litterfall and nutrient dynamics shift in tropical forest restoration sites after a decade of recovery. Biotropica 50:491-498. Data
Holl, K. D., J. L. Reid, J. M. Chaves-Fallas, F. Oviedo-Brenes & R. A. Zahawi. 2017. Local tropical forest restoration strategies affect tree recruitment more strongly than does landscape forest cover. Journal of Applied Ecology 54:1091-1099. Data
Cole, R. J., K. D. Holl, R. A. Zahawi, P. Wickey & A. R. Townsend. 2016. Leaf litter arthropod responses to tropical forest restoration. Ecology and Evolution 6:5158-5168. Data
Reid, J. L., J. M. Chaves-Fallas, K. D. Holl & R. A. Zahawi. 2016. Tropical forest restoration enriches vascular epiphyte recovery. Applied Vegetation Science 19: 508-517. Data
Holste, E. K., K. D. Holl, R. A. Zahawi, and R. K. Kobe. 2016. Reduced aboveground tree growth associated with higher arbuscular mycorrhizal fungal diversity in tropical forest restoration. Ecology and Evolution 6:7253-7262. Data
Reid, J. L., K. D. Holl, and R. A. Zahawi. 2015. Seed dispersal limitations shift over time in tropical forest restoration. Ecological Applications 25:1072-1082. Data
Reid, J. L., C. D. Mendenhall, R. A. Zahawi, and K. D. Holl. 2015. Scale-dependent effects of forest restoration on Neotropical fruit bats. Restoration Ecology 23:681-689. Data
Zahawi, R.A., J.P. Dandois, K. D. Holl, D. Nadwodny, J. L. Reid, and E.C. Ellis. 2015. Using lightweight unmanned aerial vehicles to monitor tropical forest recovery. Biological Conservation 186:287-295.
Reid, J. L., C. D. Mendenhall, J. A. Rosales, R. A. Zahawi, and K. D. Holl. 2014. Landscape context mediates avian habitat choice in tropical forest restoration. Plos One 9:e90573. Data
Holl, K. D., and R. A. Zahawi 2014. Factors explaining variability in woody above-ground biomass accumulation in restored tropical forest. Forest Ecology and Management 319:36-43. Data
Holl, K. D., V. M. Stout, J. L. Reid & R. A. Zahawi. 2013. Testing heterogeneity-diversity relationships in tropical forest restoration. Oecologia 173:569–578.
Zahawi, R. A., K. D. Holl, R. J. Cole & J. L. Reid. 2013. Testing applied nucleation as a strategy to facilitate tropical forest recovery. Journal of Applied Ecology 50:88-96. Data
Reid, J. L. & K. D. Holl. 2013. Arrival ≠ Survival. Restoration Ecology 21:153-155.
Lindell, C. A., R. J. Cole, K. D. Holl, and R. A. Zahawi. 2012. Migratory bird species in young tropical forest restoration sites: effects of vegetation height, planting design, and season. Bird Conservation International 22:94-105.
Holl, K. D. R. A. Zahawi, R. J. Cole, R. Ostertag, and S. Cordell. 2011. Planting seedlings in tree islands versus plantations as a large-scale tropical forest restoration strategy. Restoration Ecology 19: 470-479.
Holl, K.D., Aide, T.M., 2011. When and where to actively restore ecosystems? Forest Ecology and Management 261: 1588-1563.
Cole, R.J., Keene, C., Zahawi, R.A., and Holl, K.D. 2011. Direct seeding of late successional trees to restore tropical montane forest. Forest Ecology and Management 261:1590-1597.
Cole, R.J., Holl, K.D., Zahawi, R.A. 2010. Seed rain under tree islands planted to restore degraded lands in a tropical agricultural landscape. Ecological Applications 20: 1255-1269.
Collaboration and Data Sharing
We have had numerous collaborators to date and welcome additional collaborators on our research project. To request more information on collaborating send an introductory email with your background, interests, and request to Dr. Karen Holl (kholl at ucsc.edu) and Dr. Zak Zahawi (rakan.zahawi at gmail.com). Many data for this project are available at the links next to the paper above and on Dryad. We also have additional data on soil nutrients, reference forest vegetation data, site descriptions, surrounding forest cover, and other topics. If you are going to use our data for synthetic analyses we ask that you email us to let us know how you are using the data. For queries about additional data sets please contact Drs. Holl or Zahawi.
In 2004, Dr. Rakan (Zak) Zahawi (Director of the Charles Darwin Foundation) and Dr. Karen Holl (UC Santa Cruz, [email protected]) started a long-term, large-scale tropical forest restoration studying applied nucleation (planting small patches or "islands" of trees) as a low-cost method to encourage seed dispersal and shade out pasture grasses. We established three treatments – natural regeneration, applied nucleation, and full tree planting (plantation) – at several sites. We have also been investigating the effect of proximity to seed sources on forest recovery in these sites. We and collaborators have measured seed rain, seedling establishment, litterfall, birds, bats, insects, mycorrhizae, and many other variables in these sites for over 20 years. Our results show that planting tree islands is an effective and comparatively cheaper approach to accelerate forest recovery that better simulates the natural recovery process than a traditional plantation-style restoration approach (Holl et al. 2020). In addition, results to date show that the amount of surrounding forest cover is not correlated with animal-dispersed seed rain and native woody species establishment in the first decade of succession, but distance to seed sources of individual tree species strongly predicts abundance of recruits. For more information on our results and outreach see our summaries and publications below. You can also watch our 7-min summary video on the project (Spanish version). As we move into the third decade of this study, the results continue to become increasingly interesting, given that we have one of the few well-replicated, long-term studies of tropical forest recovery. Recently, we have been focusing on understanding the mechanisms that limit the establishment of later successional trees species and conducting multi-year analysis. We are extremely appreciative of multiple grants from the U.S. National Science Foundation and our numerous collaborators in Costa Rica, the US, and other countries that have allowed us to continue this research.
Recent General Audience Publications
San José, M. 2025. Proyecto Islas: más de 20 años investigando cómo traer de vuelta el bosque
Wilson, S. J., N. S. Alexandre, K. D. Holl, J. L. Reid, R. Zahawi, D. Celentano, S. Sprenkle-Hyppolite, and L. Werden. 2021. Applied nucleation guide for tropical forests. Conservation International.
Holl, K.D. & Reid, J.L. 2020. Do we really need to plant a trillion trees? Trees islands are an ecologically and economically sound strategy for tropical forest recovery. The blog discusses our Journal Applied Ecology paper which summarizes 15 years of results from our long-term experiment in Costa Rica.
Holl, K. D. 2019. Rules of thumb for predicting tropical forest recovery. This blog provides a general summary of our 2018 Applied Vegetation Science paper.
Teaching Case Studies
Holl, K.D. and R. A. Zahawi 2023. This module for upper-division colleges students provides a broad introduction to both ecological and social aspects of tropical forest restoration. It focuses on our long-term study comparing three tropical forest restoration strategies in southern Costa Rica. The module is available in English, Spanish, Portuguese, French, and Bahasa Indonesia.
Blebea, C. B., and F. H. Joyce 2024. This module for college students explores seed dispersal by animals and how this ecological process can be harnessed to facilitate forest recovery through a forest restoration experiment in Costa Rica. The module includes an exercise to apply this knowledge to a forest restoration project. The module is also available in Spanish and Portuguese.
Selected Scientific Publications and Data
Joyce, F. H., R. A. Zahawi, and K. D. Holl. 2025. Lower-intensity restoration interventions drive greater seedling establishment for later-successional tree species. Journal of Applied Ecology.
Schubert, S. C., R. A. Zahawi, F. Oviedo-Brenes, J. A. Rosales, and K. D. Holl. 2025. Active restoration increases tree species richness and recruitment of large-seeded taxa after 16–18 years. Ecological Applications 35:e3053.
Quirós Cedeño, G. J., F. H. Joyce, M. A. Zúñiga Amador, and K. D. Holl. 2025. Differing Agents of Physical Damage to Artificial Seedlings in Remnant and Restored Forests. Biotropica 57:e13421.
Joyce F.H., Rosales J.A., Holl K.D., Zahawi R.A., Bui A., Reid J.L. (2024) Active restoration accelerates recovery of tropical forest bird assemblages over two decades. Biol Conservation 293, 110593. Data
Joyce, F. H., B. M. Ramos, R. A. Zahawi, and K. D. Holl. 2024. Vertebrate seed predation can limit recruitment of later-successional species in tropical forest restoration. Biotropica 56:e13381.
Kulikowski II, A. J., R. A. Zahawi, L. K., Werden, K. Zhu, and K. D. Holl. 2023. Restoration interventions mediate tropical tree recruitment dynamics over time. Philosophical Transactions of the Royal Society B 378: 20210077. Data
Werden, L. K., S. Zarges, K. D. Holl, C. L. Oliver, F. Oviedo-Brenes, J. A. Rosales, and R. A. Zahawi. 2022. Assisted restoration interventions drive functional recovery of tropical wet forest tree communities. Frontiers in Forests and Global Change 5. Trait data
San-José, M., L. K. Werden, F. H. Joyce, J. L. Reid, K. D. Holl and R. A. Zahawi (2022). "Effects of landscape structure on restoration success in tropical premontane forest." Scientific Reports 12: 13452. Data
Kulikowski II, A. J., R. A. Zahawi, and K. D. Holl. 2022. Effects of insect herbivory on seedling mortality in restored and remnant tropical forest. Restoration Ecology 30:e13467. Data
Reid, J. L., R. A. Zahawi, D. A. Zárrate-Chary, J. A. Rosales, K. D. Holl, and U. Kormann. 2021. Multi-scale habitat selection of key frugivores predicts large-seeded tree recruitment in tropical forest restoration. Ecosphere 12:e03868. Data
Zahawi, R. A., L. K. Werden, M. San-José, J. A. Rosales, J. Flores, and K. D. Holl. 2021. Proximity and abundance of mother trees affects recruitment patterns in a long-term tropical forest restoration study. Ecography 44:1826-1837. Data
Werden, L. K., K. D. Holl, J. M. Chaves-Fallas, F. Oviedo-Brenes, J. A. Rosales, and R. A. Zahawi. 2021. Degree of intervention affects interannual and within-plot heterogeneity of seed arrival in tropical forest restoration. Journal of Applied Ecology 58:1693-1704. Data
Holl, K. D., J. L. Reid, R. J. Cole, F. Oviedo-Brenes, J. A. Rosales and R. A. Zahawi 2020. Applied nucleation facilitates tropical forest recovery: Lessons learned from a 15-year study. Journal of Applied Ecology 57, 2316-2328.
Kulikowski II, A. J. 2020. Ant–scale mutualism increases scale infestation, decreases folivory, and disrupts biological control in restored tropical forests. Biotropica 52:709-716. Data
Werden, L. K., K. D. Holl, J. A. Rosales, J. M. Sylvester, and R. A. Zahawi. 2020. Effects of dispersal- and niche-based factors on tree recruitment in tropical wet forest restoration. Ecological Applications 30:e02139. Data
Holl, K. D., J. L. Reid, F. Oviedo-Brenes, A. J. Kulikowski, and R. A. Zahawi. 2018. Rules of thumb for predicting tropical forest recovery. Applied Vegetation Science 21:669-677. Data
Lanuza, O., F. Casanoves, R. A. Zahawi, D. Celentano, D. Delgado, and K. D. Holl. 2018. Litterfall and nutrient dynamics shift in tropical forest restoration sites after a decade of recovery. Biotropica 50:491-498. Data
Holl, K. D., J. L. Reid, J. M. Chaves-Fallas, F. Oviedo-Brenes & R. A. Zahawi. 2017. Local tropical forest restoration strategies affect tree recruitment more strongly than does landscape forest cover. Journal of Applied Ecology 54:1091-1099. Data
Cole, R. J., K. D. Holl, R. A. Zahawi, P. Wickey & A. R. Townsend. 2016. Leaf litter arthropod responses to tropical forest restoration. Ecology and Evolution 6:5158-5168. Data
Reid, J. L., J. M. Chaves-Fallas, K. D. Holl & R. A. Zahawi. 2016. Tropical forest restoration enriches vascular epiphyte recovery. Applied Vegetation Science 19: 508-517. Data
Holste, E. K., K. D. Holl, R. A. Zahawi, and R. K. Kobe. 2016. Reduced aboveground tree growth associated with higher arbuscular mycorrhizal fungal diversity in tropical forest restoration. Ecology and Evolution 6:7253-7262. Data
Reid, J. L., K. D. Holl, and R. A. Zahawi. 2015. Seed dispersal limitations shift over time in tropical forest restoration. Ecological Applications 25:1072-1082. Data
Reid, J. L., C. D. Mendenhall, R. A. Zahawi, and K. D. Holl. 2015. Scale-dependent effects of forest restoration on Neotropical fruit bats. Restoration Ecology 23:681-689. Data
Zahawi, R.A., J.P. Dandois, K. D. Holl, D. Nadwodny, J. L. Reid, and E.C. Ellis. 2015. Using lightweight unmanned aerial vehicles to monitor tropical forest recovery. Biological Conservation 186:287-295.
Reid, J. L., C. D. Mendenhall, J. A. Rosales, R. A. Zahawi, and K. D. Holl. 2014. Landscape context mediates avian habitat choice in tropical forest restoration. Plos One 9:e90573. Data
Holl, K. D., and R. A. Zahawi 2014. Factors explaining variability in woody above-ground biomass accumulation in restored tropical forest. Forest Ecology and Management 319:36-43. Data
Holl, K. D., V. M. Stout, J. L. Reid & R. A. Zahawi. 2013. Testing heterogeneity-diversity relationships in tropical forest restoration. Oecologia 173:569–578.
Zahawi, R. A., K. D. Holl, R. J. Cole & J. L. Reid. 2013. Testing applied nucleation as a strategy to facilitate tropical forest recovery. Journal of Applied Ecology 50:88-96. Data
Reid, J. L. & K. D. Holl. 2013. Arrival ≠ Survival. Restoration Ecology 21:153-155.
Lindell, C. A., R. J. Cole, K. D. Holl, and R. A. Zahawi. 2012. Migratory bird species in young tropical forest restoration sites: effects of vegetation height, planting design, and season. Bird Conservation International 22:94-105.
Holl, K. D. R. A. Zahawi, R. J. Cole, R. Ostertag, and S. Cordell. 2011. Planting seedlings in tree islands versus plantations as a large-scale tropical forest restoration strategy. Restoration Ecology 19: 470-479.
Holl, K.D., Aide, T.M., 2011. When and where to actively restore ecosystems? Forest Ecology and Management 261: 1588-1563.
Cole, R.J., Keene, C., Zahawi, R.A., and Holl, K.D. 2011. Direct seeding of late successional trees to restore tropical montane forest. Forest Ecology and Management 261:1590-1597.
Cole, R.J., Holl, K.D., Zahawi, R.A. 2010. Seed rain under tree islands planted to restore degraded lands in a tropical agricultural landscape. Ecological Applications 20: 1255-1269.
Collaboration and Data Sharing
We have had numerous collaborators to date and welcome additional collaborators on our research project. To request more information on collaborating send an introductory email with your background, interests, and request to Dr. Karen Holl (kholl at ucsc.edu) and Dr. Zak Zahawi (rakan.zahawi at gmail.com). Many data for this project are available at the links next to the paper above and on Dryad. We also have additional data on soil nutrients, reference forest vegetation data, site descriptions, surrounding forest cover, and other topics. If you are going to use our data for synthetic analyses we ask that you email us to let us know how you are using the data. For queries about additional data sets please contact Drs. Holl or Zahawi.