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

摘要

Planting native tree seedlings is the predominant restoration strategy for accelerating forest succession on degraded lands. Planting tree "islands" is less costly and labor intensive than establishing larger plantations and simulates the nucleation process of succession. Assessing the role of island size in attracting seed dispersers, the potential of islands to expand through enhanced seed deposition, and the effect of planting arrangements on seed dispersal by birds and bats informs restoration design. Determining the relative importance of local restoration approach vs. landscape-level factors (amount of surrounding forest cover) helps prioritize methods and locations for restoration. We tested 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-km_2area of southern Costa Rica. Each site had three 50 X 50 m treatments: (1) control (natural regeneration), (2) island (planting tree seedlings in patches of three sizes: 16 m~2, 64 m ~2, and 144 m~2), and (3) plantation (planting entire area). 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. Plantations received the most zoochorous tree seeds (266.1 ± 64.5 seeds-m~(-2')yr~(-1) [mean ± SE]), islands were intermediate (210.4 ± 52.7 seeds.m ~(-2)yr~(-1)), and controls were lowest (87.1 ± 13.9 seeds.m~(-2)yr~(-1)). Greater tree seed deposition in the plantations was due to birds (0.51 ± 0.18 seeds'm~(-2)'d ~(-1)), not bats (0.07 ± 0.03 seeds-m~(-2)'d ~(-1)). Seed rain was primarily smallseeded, early-successional species. Large and medium islands received twice as many zoochorous tree seeds as small islands and areas away from island edges, suggesting there is a minimum island size necessary to increase seed deposition and that seed rain outside of planted areas is strongly reduced. 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. However, more intensive restoration approaches may be necessary for establishment of dispersal-limited species.