one-way, downward transfer by stemflow and through-
fall. Spores or hyphae might be dislodged during rain
events and swept down branches and trunks in stem-
flow, inoculating mats or epiphytes below. The trap-
ping of even one spore (Daft and Nicolson 1969) on
the moist underside of a root might be sufficient to
form mycorrhizae and to begin the inoculum accre-
tion process. That some epiphytes have moderate to
high numbers of VAM suggests that the inoculum
potential of some canopy mats is adequate for mycor-
rhizal formation. In ecosystems with stable canopy
mats, epiphyte species that specialize on such mats
could be obligately mycotrophic (Janos 1980). Con-
sequently, a canopy inoculum mosaic could influence
the distribution of epiphytic species in a way similar
to that suggested for terrestrial habitats by Janos (1980).Acknowledgments We thank Erin Addison, Norman
Dill, Wayne Ferren, Anne Gripp, Ian Ross, and Carolyn
Stange for their support. This work was completed as
a partial requirement of the Master of Arts degree of
University of California, Santa Barbara. We acknowl-
edge National Science Foundation Grants BSR 87-
14935 and BSR 90-18006 for support.FACTORS AFFECTING THE INITIATION AND GROWTH OF ABOVEGROUND
ADVENTITIOUS ROOTS IN A TROPICAL CLOUD FOREST TREE:
AN EXPERIMENTAL APPROACH
Nalini 1VL Nadkarniboveground adventitious roots (AAR) charac-
terize many species of tropical and temper-
ate wet forest trees (Lanner 1966, Jenik 1973,
Moore 1989) and have been termed "canopy roots"
(Nadkarni 1981), "aboveground adventitious roots"
(Herwitz 1991), and "apogeotropic roots" (Sanford
1987). They are structured much like their below-
ground counterparts and possess similar capacities to
absorb nutrients and water (Nadkarni and Primack
1989). These organs are often directly associated with
and/or penetrate accumulations of epiphytes and ac-
companying humus suspended in host tree crowns.
Aboveground adventitious roots may enhance nu-
trient use efficiency by retrieving leached nutrients
from stemflow (Herwitz 1991) or function as a "short-
cut" in nutrient cycles by allowing host trees to ex-
ploit the abundant nutrients contained in organic
matter suspended within their own crowns (Nadkarni
1981). Little is known about the anatomy of AAR,
their absorptive capacities of water and/or nutrients
under natural conditions, or the physiology of their
initiation and growth. Epiphytes and crown humus-
produced or related factors possibly responsible for
inducing AAR include darkness, moisture, inorganic
nutrients, hormones, or other organic exudates. I car-
ried out an experiment to identify stimuli that may
promote the growth of AAR in the crown of Senecio
cooperi (Compositae), a tropical cloud forest tree
(Nadkarni 1994).
Experiments were performed between May and
September 1982 in the MCFP. The 2-ha study area was
located on the windward side of the area known as
"La Ventana," which straddles the Continental Divide.
The area experienced a landslide 12-15 yr previous
to the study (W. Guindon, pers. comm.). Soils in the
area are derived from volcanic rhyolites (Vance and
Nadkarni 1990), contain very small amounts of or-
ganic matter, and are extremely clayey. Local vege-
tation consists of a thick cover of arborescent shrubs
with a uniform 3-5 m canopy dominated by S. co-
operi. These shrubs are multibranched, with all of
the foliage on the ends of branches, leaving the crown
interiors unobstructed and easily accessible. Other
common landslide-following plants such as Gunnera
insignis (Gunneraceae) and Bocconia frutescens (Papa-
veraceae) ringed the study area.
Senecio cooperi was used because of its uniform
height, low stature, and propensity for forming AAR
under natural conditions (pers. obs.). An aggressive
pioneer following major disturbances, this tree can
reach 6 m in height. Thirty-nine S. cooperi trees of
uniform height and structure were chosen. Sixty stem
segments adjacent to accessible branch nodes were
marked on the experimental trees. Each designated
stem segment was subjected to one of the following
six treatments and the number of roots initiated and
their rates of growth were recorded. Factors tested
were the presence of (1) wet epiphytes (live moist
mosses, liverworts, filmy ferns, and crown humus
collected from three Didymopanax pittieri trees col-
lected from the adjacent primary elfin woodland), (2)
dry epiphytes (air-dried epiphytes and crown humus),
(3) wet sponges (foam sponges saturated with distilled
water), (4) inorganic nutrient solution sponges (foam
sponges, water, and dissolved commercial nutrient
solution, equivalent to a Hoagland's solution), (5) dry339 Ecosystem Ecology and Forest DynamicsA