Biology Reference
In-Depth Information
species. According to the second hypothesis, trees grow more slowly and
die more easily when they are close to a conspecific tree than when they
are close to trees of other species.
To test for the first hypothesis, the growth of seedlings of a species was
compared with abundance of conspecific adults. Analysis showed that, on
the scale of the whole plot, abundance of adult trees had little effect. This
hypothesis was further tested by comparing growth in each species with
abundance and average size within the same size class. The results showed
that growth was never correlated with abundance. The hypothesis was
also tested by relating abundance of adults with mortality of conspecific
seedlings. No correlation was found. Furthermore, 44 out of 48 tests for
distance to first and second nearest neighbor did not show an effect. If
there was an effect, its direction was not consistent. So, no further tests for
the effect of species of neighbor were made.
To test for the second hypothesis, growth rates of individuals with
conspecific neighbors were compared with growth rates of individuals
with neighbors of different species. In one, 11 out of 12 comparisons and
in the other 8 out of 12 comparisons were negative. However, a distinct
compensatory effect was found in the four comparisons of the two most
recent seedling classes in the tropical (northern Queensland) rainforest
area: growth was significantly faster when the first or second nearest
neighbor was a different species. Connell et al. concluded that a com-
pensatory mechanism may operate but only at a very early stage of
growth. In 13 out of 20 comparisons of seedlings and saplings in the
tropical rainforest, and in 12 out of 24 comparisons in the subtropical one,
mortality was greater when the nearest neighbor was a conspecific.
Mortality was never greater when the nearest neighbor belonged to a
different species. The second hypothesis was further tested by comparing
the effects of increasing density of common species on growth and
mortality of seedlings and saplings. The results showed practically no (or
at best a very weak) effect on growth and mortality. The authors also
conducted field experiments to test the hypothesis, using two plant
species. Seeds were placed on the ground either densely or sparsely. No
significant differences were found between the two treatments, although
almost all plants disappeared over the observation period of 9.5 months.
The second hypothesis was also tested at the scale of individual trees by
calculating mortality of seedlings and saplings in the inner circle, the outer
ring around each conspecific adult tree, and in the area beyond. The inner
circle is the area within 1.5 times the crown radius of the adult, the outer
ring is an annulus concentric to and of width equal to the radius of the
