When Noyce Hall was built in 2007, its courtyard
contained only two species of woody foliage: a couple hornbeam trees planted
near the edges and some serviceberry bushes. Looking at the same area in 2016,
however, I discovered an unexpectedly diverse assortment of trees: scrawny
sycamores popping up through the pavement, redbuds arcing up from the bottom of
the courtyard’s slope, even a tiny little pine tree wedged between two wobbly
stones in a retaining wall. The extent to which the seeds from these species –
most of them recognizable from elsewhere on campus – had managed to infiltrate
such a small and isolated habitat was impressive and confusing. How far,
really, do seeds usually spread from their source, and how well do they really
colonize microhabitats like this courtyard? Given the extent to which habitat
fragmentation has become a part of the world we live in – especially here in
Iowa, where the once-widespread prairie persists mostly as isolated patches –
the ability of seeds to travel long distances between habitat could be crucial
for species survival.
One
paper
by Amy B. McEuen and Lisa M. Curran, titled “Seed Dispersal and Recruitment
Limitation Across Spatial Scales in Temperate Forest Fragments,” attempts to
answer some of the questions about seed dispersal in fragmented habitats.
Noticing that most studies of plant seed dispersal focused either on single
trees or on dispersal within an individual, continuous area of habitat, McEuen
and Curran decided to study how seeds spread among multiple small, separate
patches of forest, and how this influenced species composition. In five small
forest remnants within an area in Southeastern Michigan, they sampled species
present and used traps to collect “seed rain” (the seeds that fall onto a given
area) over a two-year period. They found that many species did not disperse
well between sites, and those that did generally produced large amounts of
small, wind-dispersed seeds.
Image: Unsuccessful vs.
successful distribution for two species. This figure shows the seed trap
results for two species (Carpinus
caroliniana, American hornbeam, above, and Fraxinus pensylvanica, green ash, below), where an X represents a
trap where the species’ seeds where found and a dash represents a trap where
the species was not found (contour lines represent an approximation of
dispersal patterns). On the right – both sites where the species in question
was already common – the seeds of the species were abundant, but little
dispersal seems to be occurring in the images of Site B on the left. As Site B
was a location where neither of these two species was already well-established,
this supports the paper’s conclusions that species had difficulty dispersing
between sites.
Just
because a species had many seeds landing in a given site, however, didn’t
necessarily mean that the seeds were growing well – the authors noted that many
of the species abundant in their seed rain samples had few to no saplings in
the area, a reminder that colonization depends on more than just dispersal. It
seems likely, however, that increased habitat fragmentation can still have a
large impact on which species spread between areas, to the detriment of species
that depend on animals to spread their seeds, or that produce large seeds that
do not disperse very far.
This
paper seems to suggest an interesting avenue of inquiry that relates to my own
research on the tree colonization history of the courtyard in Noyce Hall: of
the woody species present in this microhabitat, how many are wind-dispersed as
opposed to dispersed via other methods? How large are their seeds? While my
research focuses on the colonization history of only a couple of the
courtyard’s most common invaders (Sycamores and mulberries, to be specific),
it’s important to keep in mind the question of how and why this particular set
of species ended up in this unusual microhabitat in the first place. Although
many of the species are not native to the area – having likely spread from
cultivated individuals elsewhere on campus – the small size and isolation of
this habitat makes its array of saplings and small trees surprising. The
narrower focus of my own studies doesn’t stop me from wondering why these
species are the most abundant courtyard trees in the first place – and what
that can tell us about which species, in our increasingly patchwork ecosystems,
are most likely to persist.
Work Cited
McEuen, A.B., &
Curran, L.M. (2004). Seed dispersal and recruitment limitation across spatial
scales in temperate forest fragments. Ecology,
85(2), 507-518. Retrieved from http://www.jstor.org/stable/3450214
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