Role of remnant trees in seed dispersal through the matrix: Being alone is not always so sad

Background/Question/Methods Scattered remnant trees in non-forested matrix-habitats are common structures in many fragmented landscapes worldwide. These trees are frequently perching sites for seed-disperser animals, and accumulate seed rain beneath their canopies. Thus, remnant trees may ultimately act as stepping stones for seed dispersal services throughout fragmented landscapes, and may serve as foci for forest recovery in matrix habitats. Despite the recognized role of remnant trees in seed deposition, no previous study has explicitly compared seed rain patterns in remnant trees relative to trees embedded in the forest.
During 2004 and 2005, we examined the patterns of seed deposition by frugivorous birds and post-dispersal seed predation by rodents beneath the canopies of hawthorn (Crataegus monogyna) trees embedded in forest fragments (i.e patch trees) and beneath trees in a non-forested matrix (i.e. remnant trees), in a Cantabrian mountain landscape in northern Spain. We compared seed rain between patch and remnant trees both before and after seed predation. Interannual sampling enabled us to compare two different scenarios of fruit availability at the focal trees and at the landscape scale.

Results/Conclusions Hawthorn and holly (Ilex aquifolium) seeds dominated the seed rain beneath both tree types. The density of dispersed seeds beneath trees differed strongly between tree types, with higher values in patch trees relative to remnant ones. Dispersed seed density also varied between years. The effect of tree type depended, however, on the year (stronger in 2004) and the seed species (stronger in holly). Seed predation rate was higher under remnant trees than under patch trees in both years and for both species (i.e. hawthorn and holly). Estimated density of dispersed seeds surviving to predation differed between tree types in 2004, but not in 2005; an effect demonstrated for both seed species. Finally, fruit abundance was found to be much higher in 2004 than in 2005; both at the level of the focal trees and at the landscape scale.
Our study suggests that scattered remnant trees not only maintain an ecological function as seed dispersal foci in fragmented scenarios, but also that this role may even be equitable to that of forest trees. Moreover, the interannual differences in the relative role of remnant trees for seed dispersal depended on the changes in fruit availability at the landscape scale, and the concomitant variations in foraging patterns by seed dispersers.