Tropical Forest Community Ecology

258 A. Elizabeth Arnold

such as those of xylariaceous species and various
Eurotiomycetidae (see Holbet al.2004).
Inoculum volume plays an important role
in determinin gthe infection success of plant-
associated fungi (Agrios 1997), but until recently,
the rate of fungal propagule deposition in tropi-
cal forests was not known. Gilbert (2002), Arnold
(2002), and Arnold and Herre (2003) found that
typical leaves in the forest understory at Barro
Colorado Island (BCI), Panama, receive≥10–15
viable fungal propagules per cm^2 per hour dur-
in gthe mid- and late wet seasons. When adjusted
for mean leaf area and extrapolated to 24 hours,
these data suggest that the average leaf receives
more than 15,000 viable fungal propagules per
day (average based on mean leaf area for 28 tree
species in the understory at BCI; Arnold 2002).
These values consider only fungi capable of grow-
in gon one nutrient medium (malt extract a gar)
and likely underestimate total deposition.
Fungi in the forest air column represent
pathogenic, saprophytic, and endophytic species,
as well as numerous species of unknown eco-
logical roles. The proportion that can form
endophytic symbioses has not been quantified,
but the abundance of viable propagules in air
is positively associated with the frequency of
endophyte infections (Arnold and Herre 2003).

Propagules of fungi at BCI are highly abun-

dant in the air column immediately following

rainfall events, and then decrease as a curvi-

linear function with increasin gtime since si g-

nificant rainfall (Figure 15.2). Both ultraviolet

(UV) radiation and desiccation play an impor-

tant role in the mortality of fungal propagules:

Arnold and Herre (2003) found that the depo-

sition and persistence of livin gpropa gules on

leaves was greater under the forest canopy than

in the laboratory clearin gat BCI. After 1 week of

exposure, sterile plants placed in the lab clearing

accumulated significantly fewer endophyte infec-

tions than plants placed in the forest understory

(Arnold and Herre 2003).

ENDOPHYTE COLONIZATION AND

ABUNDANCE IN TROPICAL

LEAVES

The high abundance of inoculum in the air col-

umn, and the apparently universal receptivity

of tropical plants to colonization by endophytic

fungi (Arnold 2002, Van Bael et al.2005),

leads to high infection rates in mature foliage

of tropical trees. Arnold (2002) recovered endo-

phytic fungi from 100% of mature leaves sampled

5

10

15

20

25

020406080

R^2 = 0.967

Hours since rainfall

CFU per cm

2

per hour

Figure 15.2 Deposition of livin gfun gal

colony-formin gunits (CFU; hyphal fra gments

and both sexual and asexual spores) per cm^2 of

surface area per hour, as a function of hours

since significant rainfall in the understory of

secondary forest at BCI. Each point represents

the mean (±standard error) of six sampling

stations (data from Arnold 2002) and

underestimates total deposition: only fungi

capable of growing on a single medium

(2% malt extract agar) at a given temperature

(23◦C) are represented.