Seed rain under tree islands planted to restore degraded lands in a tropical agricultural landscape

Background

Planting native tree seedlings is the predominant restoration strategy for accelerating forest succession on degraded lands. However, planting tree “islands” is less costly and labor intensive than establishing larger plantations and simulates the nucleation process of succession. Tree islands can attract seed dispersers to gradually spread restoration patterns from the islands. Restoration design can be informed by assessing the effect of potential planting arrangements on seed dispersal by birds and bats and determining the influence of surrounding forest cover.

Research goals & methods

This study tests how three restoration approaches affect the arrival of forest seeds at 11 experimental sites spread across a gradient of surrounding forest cover in a 100-km² area of southern Costa Rica. Each site had three 50x50 m treatments: control (natural regeneration); island (planting tree seedlings in patches of three sizes: 16 m², 64 m², and 144 m²); and plantation. Four tree species were used in planting (Terminalia amazonia, Vochysia guatemalensis, Erythrina poeppigiana, and Inga edulis). Seed rain was measured for 18 months beginning 2 years after planting.

Conclusions & takeaways

Plantations received the most animal-dispersed tree seeds while islands were intermediate and controls were lowest. Greater tree seed deposition in the plantations was due to birds, not bats. Seed rain was primarily small-seeded, early-successional species. Large and medium islands received twice as many animal-dispersed tree seeds as small islands and areas away from island edges, suggesting there is a minimum island size necessary to increase animal-born seed deposition. Planting design was more important for seed deposition than amount of forest cover within the surrounding 100- and 500-m radius areas.

Establishing plantations and large islands facilitates the arrival of early-successional tree seeds and represents a broadly applicable strategy for increasing seed rain on abandoned agricultural lands.