MISTLETOES AND WHERE THEY GROW
Sarah Sargentistletoes are unusual flowering plants in
several closely related families. The defin-
ing characteristic of mistletoes is that they
parasitize the aboveground stems of other plants, es-
pecially trees (Kuijt 1969). Unlike ordinary epiphytes,
mistletoes obtain all of their mineral nutrients and
water by forming direct connections with water-con-
ducting xylem inside host branches. Mistletoes are
unable to grow normal roots and depend completely
on host plants for survival. All mistletoes have pho-
tosynthetic pigments and produce their own sugars;
they do not parasitize the phloem of their hosts as do
other types of parasitic plants. Most people are famil-
iar with mistletoe because of the Christmas mistletoe
tradition that originated in Europe, which is based on
Viscum album (Viscaceae). However, the majority of
mistletoes are tropical. Mistletoes are found in four
plant families in the order Santalales. The largest of
these four are the Viscaceae and the Loranthaceae,
with about 370 and 700 species, respectively (Nickrent
1993). Most mistletoes in Monteverde belong to these
two large families, but Antidaphne viscoidea, in the
small family Eremolepidaceae (12 species), is also
present (Sargent 1994).
All mistletoes have extremely sticky seeds, which
enables them to adhere to the branches of potential
hosts rather than falling to the ground where they
would be unable to establish the host connections.
The stickiness results from a specialized tissue, the
viscin, attached to the seeds (Gedalovich-Shedletzky
et al. 1989). In all tropical mistletoes, the seeds are
dispersed by birds (see Sargent, "Specialized Seed
Dispersal," pp. 288-289).
One of the mysteries about mistletoes is why they
are so unevenly distributed among trees; some trees
are loaded with mistletoes while others have none.
What prevents mistletoes from growing on every tree?
Within a single host species, why do mistletoes occur
on some trees but not on others? One possibility is that
mistletoes are limited by seed dispersal; perhaps birds
rarely carry seeds to new hosts or to new branches
within hosts. Alternatively, perhaps seeds fail to es-
tablish the critical connections with the host xylem
once they get there.
To address the second possibility, I conducted a
seed planting experiment to determine the fate of
mistletoe seeds after they become established on a host
(Sargent 1995). Working with a common mistletoe
species, Phoradendron robustissimum (Viscaceae),
and its most common host species, Sapium glandu-
losum (Euphorbiaceae), I studied two aspects of the
way birds deposit seeds that might influence the abil-
ity of seeds to become established: (1) the size of the
host stem (i.e., twig or branch) onto which birds de-
posit seeds, and (2) the number of seeds deposited in
one place. I chose 10 trees, in each of which I marked
segments of branches in seven diameter classes (< 5
mm to > 80 mm). I stuck seeds at different diameters
on the marked twigs by hand. I checked on the seeds
over the next 3 years to determine which seeds estab-
lished as seedlings and which ones died, and to de-
termine why they died.
Few seeds survived: 10% were alive after 1 year,
4% after 2 years, and 3% after 3 years. Seeds in clumps
were no more likely to live than single seeds. Twig
diameter, however, had a strong impact on a seed's
chance of becoming established (Fig. 3.25). Smaller
twigs were better for initial establishment, but many
of the smallest twigs died (Fig. 3.26). This left the
intermediate twig sizes as the best for long-term sur-
vival of the mistletoe seedlings.
There was tremendous variation in survival of
seeds and seedlings even on twigs in the optimal size
range. Many seeds simply disappeared (Fig. 3.26). In
another experiment, I placed seeds on branches with
three different types of protection: complete protec-
tion (inside mesh bags), protection from crawling seed
predators (surrounded by Tanglefoot), and protection
from dislodgment (roofs of bag material, open on the
sides). Control seeds were placed on branches with-
out any protection. As expected, complete protection
was best; seeds in bags had the lowest rates of dis-
appearance (near 0%). The Tanglefoot barrier was
also effective (22% disappeared), suggesting that
many of the seeds that disappeared had been eaten
by seed predators that crawl along the host twigs
(e.g., ants and arboreal mice; see Timm and LaVal,
"Observations," pp. 235-236).
Another cause of variation in establishment success
of mistletoe seeds may have been differences in nutri-
ents and water available from the host trees. Formation
of new xylem connections between mistletoes and their
hosts is a passive process on the part of the mistletoe
(Salle 1983). Newly formed host vessels are tapped by
the mistletoe as they form, resulting in xylem-to-xylem
connections between host and parasite (Calvin et al.81 Plants and VegetationM