Getting Around on a College Campus: Sapling Dispersal and Spatial Patterning

A picture of my team’s study site. Flags mark the outlying corners of the study site. The site sits between two academic buildings at Grinnell College and is planted with shade trees, conifers, and shade dwelling ground covers. Emerging saplings are left in the site by the landscapers and so pose an excellent opportunity to observe tree dispersal on campus.

An individual’s dispersal ability greatly affects its chances of success. Animals move when habitat proves unsuitable.  Plants do not have this luxury and must establish in a single location. Thus it follows that plants would be more sensitive to microhabitats, or variation on a smaller spatial scale, and that this would affect their spatial arrangement in a site. Such spatial arrangements would also be sensitive to dispersal barriers, requiring plant communities to develop another way to disperse offspring into suitable microhabitats or tolerate a higher density of individuals within the currently occupied space. The resulting separation between populations stems from a larger issue of isolation. This has particular consequences for restoration efforts within the urban setting because of the extreme fragmentation of natural areas and the barriers created by manmade structures.

Recruitment from established source populations often aids the succession of restoration and reclamation projects.  However, this process requires source communities to be spatially close to the project to work, a frequent limitation in urban settings where intervening buildings and pavements hamper dispersion. Robinson and Handel (2000) experimented with artificial habitat islands in an attempt to attract dispersal agents, namely frugivore birds, to the region. By attracting dispersal agents, established populations could overcome dispersion barriers and influence the spatial patterning of the site, e.g. succession from bare ground to woodland. With a total of 24 plots, they planted varying sized, native trees and shrubs in an old, barren landfill in New Jersey, and kept track of the population over several years. While they examined several aspects, they explicitly analyze the effect of plant size, finding that larger plants had an initial advantage in plant recruitment, but that this advantage diminished over time. While these results suggest smaller starting plants would provide the best benefit for the cost of restoration, they additionally note recruitment in seriously isolated urban settings may never rise to the species diversity levels associated with historic communities.

While my team’s project does not look at a patch undergoing primary succession, we are looking at the effect of microhabitat on dispersion, particularly of saplings. Understanding source population influences and animal dispersal agents will underscore the processes of secondary succession occurring in the site and will inform the interpretation of its spatial patterning. Because we are in an urban setting, we may have some of the issues of isolation. The adult trees in the site may contribute the most to new individuals. However, nearby green spaces may act as additional source populations. We focus on sapling abundance in relation to microhabitat (overhead cover and slope steepness) in our study. If microhabitat works in concert with dispersion to influence spatial patterning of saplings, this could have implications for other human landscape management.

For the full article by Robinson and Handel (2000):

http://www.esajournals.org/doi/full/10.1890/1051-0761%282000%29010%5B0174%3ADSPORD%5D2.0.CO%3B2

Figure 2

 (a) A diagram of the experimental study site of Robinson and Handel (2000) shows the spatial pattern of plots. Each shaded box represents one of the experimentally planted plots, whereas the open and dashed boxes represent the plots that tested for the effects of fencing in attracting birds. Open boxes have fencing but no intentional plantings, whereas dashed boxes have no fencing or intentional plantings. (b) Shows an up close diagram of planting patterns. The tilled strips were used to evaluate the dispersion of species from the planting plots to outlying areas, but are not discussed in this blog.