Biology Reference
In-Depth Information
therefore not surprising that deer browsing should effect the overall abundance
of species known to be preferred by deer. Less clear is which aspect of herbivory
is most important.
Leaf removal did not affect flower production in any of the species, which is
consistent with other studies [7,16,17]. Ehrlen demonstrated that flower num-
bers were predetermined the previous fall by budding in Lathyrus vernus[18].
The same may be true in all of our species because removing leaves did not im-
pact their flower numbers. Nonetheless, high levels of leaf removal did negatively
impact the seed weight in all of the study species which produced seeds (Figure
3b). These results suggest that stored resources are available for flowers and seeds
before the onset of flowering [19,20] and changes in current year resources have
a negligible effect on flower number. However, leaf removal appears to reduce the
amount of carbon available for allocation to developing seeds in Phlox, Rosa, and
Rhus, which causes a decrease in the overall seed mass produced by an individual
plant.
Though we found a negative relationship between seed mass and percent leaf
damage in Phlox, Rosa, and Rhus, we did not detect a threshold level of leaf
removal that had to be reached before seed mass was impacted. Other studies,
which have attempted to quantify the point where defoliation begins to impact
reproduction, have yielded widely variable results [1,4-6], though these studies all
found significant results at 50% or lower levels of defoliation.
Additionally, defoliation may have differential effects on seeds depending on
when it occurs. In this study, all treatments were administered within a few weeks
of flowering. One study [5,20] found that when leaves were removed several
months before the time of flowering, the plant suffered a large loss in reproductive
output. When the same treatment was administered just before flowering, there
was no response [5,20]. Timing, then, may be a key in determining how well a
plant copes with herbivory [11].
Meristem removal was more harmful to the reproductive output of Phlox and
Rosa than leaf removal alone (Figures 4a &4b). With meristem removal, Phlox
had fewer flowers than in the control and leaf removal treatments, but its seed
mass was not affected. Meristem removal more strongly impacted Rosa, which
had fewer flowers and a lower seed mass than either the leaf removal or control
groups.
Because Phlox is a small herbaceous plant with terminal flowers, it often suf-
fered complete flower loss and substantial leaf removal under the meristem re-
moval treatment. The individuals in this treatment that did produce seeds sent
up a side shoot after the meristem was nipped off. In contrast, Rosa produced
many flowers and never suffered a complete flower loss with meristem removal.
