Monteverde : Ecology and Conservation of a Tropical Cloud Forest

1991). Such connections cannot form in the absence of

host growth at the site of attachment of the mistletoe.

Thus, limited host growth may prevent or constrain

mistletoe establishment. The nutrient status of the host

is also known to influence water-use efficiency of

mistletoes, suggesting that nutrient concentration in

xylem sap may limit mistletoe growth (Ehleringer et al.

1985). Although water availability may not often be lim-

iting in Monte verde, within- and between-year varia-

tion in rainfall may contribute to fluctuations in host

water potentials and the ability of mistletoes to become

established. A survey of the age structure of a desert

mistletoe population showed that mistletoes failed

to become established in years of summer drought

(Dawson et al. 1990). In Monteverde, trees associated

with barnyards or septic systems tend to have much

higher levels of mistletoe infections than others, suggest-

ing that both hosts and mistletoes thrive in these rich

conditions. Mistletoe seeds thus fare best on interme-

diate-sized twigs on fast-growing hosts. However, many

aspects of the puzzle of mistletoe distribution remain

unknown and warrant further research.

Figure 325, (above left) Proportion of Phoradendron robustissimum seedlings surviving for 1, 2, and 3 years on each

size class of host stern diameter, summed over replicate trees. Figure 3.26. (above fight) Fates of Phoradendron

robustissimum seedlings after 2 years, as a function of initial host stem diameter, reported as proportions of all seeds

planted in each size class of host stem, summed over replicate trees.

AN EXCEPTIONAL MISTLETOE, GAIADENDRON PUNCTATUM

Sarah Sargent

onteverde is a site of an unusual species of

mistletoe, the terrestrial Gaiadendron punc-

tatum (Loranthaceae). A large stand of it

grows on the uphill side of the road past "Windy Cor-

ner/^5 the sharp turn 200 m before the entrance to the

MCFP. Gaiadendron is a large shrub or small tree with

haustoria that are different from those of other mem-

bers of the family; they attach to the roots of other

plants, often smaller plants including ferns (Kuijt

1963), The seedlings go through a brief free-living

period, during which they form a tuber, before they

begin to parasitize neighboring plants. Gaiadendron

can grow on the ground, but it also grows high in trees

in epiphyte mats. Their locations are typically places

with canopy-like conditions, such as the steep ex-

posed bank near Windy Corner. When it grows off the

ground in epiphyte mats, it has never been found to

parasitize the supporting tree, only its neighboring

epiphytes. It is thus epiphytic with respect to the tree

but parasitic with respect to other epiphytes.

There is some debate about the origins of terres-

trialism in this species, whether it represents the

primitive condition of the Loranthaceae or whether

it is a secondary adoption of the habit after its ances-

tors had been true aerial parasites, A likely scenario

is that it is a secondary return to terrestrialism via

epiphyte mats (Kuijt 1969).

82 Plants and Vegetation

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