Annual and Spatial Variation in Seedfall and Seedling Recruitment in a Neotropical Forest

Abstract

Abstract. An economy of scale may lead to selection to increase interannual variation in seed production when the per seed probability of seedling establishment increases with seed production. Variable annual seedfall will, however, reduce this probability when postdispersal seed fate is negatively density dependent on the local density of seeds, and seed dispersal and density dependence act identically across years. Intuitively, more variable annual seedfall causes the representative seed to experience a greater density of conspecific seeds and suffer greater density-dependent effects. This handicap must be overcome for the per seed probability of recruitment to be greater in years with greater seed production.We quantified spatial and annual variation in seedfall and seedling recruitment, evaluated density dependence and economies of scale during the seed-to-seedling transition, and investigated the synergistic consequences of density dependence and variable annual seedfall for seedling recruitment on Barro Colorado Island (BCI), Panama. Weekly censuses of 200 0.5-m 2 seed traps documented seedfall for 15 years and 108 plant species. Annual censuses of 600 1-m 2 seedling plots documented recruitment for nine years and 32 species. The density of seedling recruits tended to increase with the density of seeds; however, the per seed probability of recruitment invariably decreased with seedfall density. Negative density dependence characterized the seed-to-seedling transition. Observed levels of spatial and interannual variation in seedfall density would reduce long-term recruitment by up to 28% if negatively density-dependent survival acted identically across years; however, the strength of negative density dependence varied significantly among years for 12 of 32 species. Negative density dependence occurred in all years for these species but was significantly weaker during the one or two years of greatest seedfall than during the remaining years of lower seedfall. The per seed probability of recruitment increased significantly with annual seedfall for eight of these species. These eight species realized postdispersal economies of scale despite the reduction in long-term recruitment expected from the synergism between variable annual seed production and negatively density-dependent seed fate.

Nonrandom, diversifying processes are disproportionately strong in the smallest size classes of a tropical forest

“…Whether differences in the strength of nonrandom mortality (regardless of its effect on diversity) across size classes are consistent in other forests has yet to be determined. Accordingly, our results help justify initiatives to incorporate the smallest size classes into several large-scale, long-term forest monitoring projects (38)(39)(40), and call for their more widespread inclusion elsewhere. Similar processes are likely to occur in all communities of sessile organisms in which recruitment primarily occurs through dispersed propagules, including marine communities (41).…”

Understanding strategies for seed dispersal by wind under contrasting atmospheric conditions

“…Gaps permit rapid regeneration when low light limits seedlings because bright light penetrates to the forest floor for a short time after a canopy tree death opens a gap and before rapid herb, seedling, shrub, and sapling growth closes the gap at ground level. Compared with Tabebuia, Jacaranda produces more (89 seeds cm Ϫ2 basal area yr Ϫ1 vs. 18) and smaller seeds (dry mass of endosperm plus embryo averages 2.16 vs. 18.1 mg) whose survival to the seedling stage is lower (0.051% vs. 1.3%) due to its dependence on gaps (14,15). Jacaranda seeds are also recalcitrant (germinate immediately), which is unusual among small-seeded, gapdependent species.…”

“…We evaluated reproductive status in Ն2 yrs between 2004 and 2007 for 252 Jacaranda and 115 Tabebuia, including all individuals larger than 20 cm DBH in 70 ha. We used weekly censuses of 250 seed traps to quantify the seasonal timing of seed dispersal (14,15) and 20-min censuses of seed traps and above-canopy sonic anemometry to quantify non-random seed release in response to wind conditions (henceforth, seed release). We counted seeds at 20-min intervals from dawn to dusk for 22 traps for 3 d for Tabebuia and 29 traps for 7 d and another 20 traps for 5 d for Jacaranda.…”

Seedling recruitment factors in low‐diversity Hawaiian wet forest: towards global comparisons among tropical forests

“…Within the 160 3 160 m central area of the 4 ha Laupahoehoe FDP, we established a grid of 64 seed and seedling census stations. Following standard CTFS seedling plot protocols (Wright et al 2005), each census station comprised one 0.5 m 2 seed trap with a fine mesh bag suspended approximately 80 cm above the ground (N ¼ 64) and three 1 3 1 m seedling plots within 2 m of each seed trap (N ¼ 192; Fig. 2).…”