Rainfall and cloud-water interception in tropical montane forests in

Author's personal copy

Rainfall and cloud-water interception in tropical montane

forests in the eastern Andes of Central Peru

Daniel Gomez-Peralta a,*, Steven F. Oberbauer a,b,Michael E. McClain c, Thomas E. Philippi a

a Department of Biological Sciences, Florida International University, 11200 SW 8th Street, Miami, FL 33199, USAb Fairchild Tropical Botanic Garden, 11935 Old Cutler Road, Miami, FL 33156, USA

c Department of Environmental Studies, Florida International University, Miami, FL 33199, USA

Received 28 June 2006; received in revised form 9 October 2007; accepted 20 October 2007

Abstract

Cloud-water and rainfall interception are hydrological processes of particular interest in Tropical Montane Cloud Forests (TMCF). Studies in

these systems have shown important contributions of cloud/fog water to the hydrological balance. To evaluate the importance of cloud/fog water to

montane forests of the western slope of the Cordillera Yanachaga in the eastern Andes of central Peru, we monitored bulk precipitation (gross

rainfall), cloud water and net precipitation weekly over one year at two elevations, 2468 and 2815 masl. Bulk precipitation was greater at the upper

site (2753 mm) than at the lower site (2222 mm). Annual net precipitation was 92.4% and 70.4% of rainfall at the upper and lower sites,

respectively. Net precipitation was primarily composed of throughfall; stemflow was negligible, contributing less than 0.2% of annual rainfall at

both sites. Apparent annual rainfall interception losses by the canopy were 7.7% and 29.6% of the bulk precipitation at the upper and lower sites,

respectively. Apparent weekly rainfall interception at the upper site was sometimes negative and lower compared to that of the lower site. Apparent

cloud-water interception occurred at least during weeks when negative rainfall interception was recorded, a contribution of 21 mm (0.8% of the

annual rainfall) at the upper site. However, the apparent low rainfall interception losses at the upper site suggest that canopy wetting and subsequent

saturation by cloud/fog water during periods with apparent positive rainfall interception may have contributed a difference of ca. 22% of apparent

rainfall interception losses between sites. These contributions were supported by maximum apparent fog interception recorded by a fog gauge at

the upper site. Although other equally important factors like topography, crown exposure and mossiness were not evaluated, the quantity of

apparent fog interception at the upper site was found related to the canopy leaf area index. This important relationship has been previously

described but mostly overlooked in studies of TMCF.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Cloud-water interception; Rainfall interception; Fog; Tropical montane cloud forests; Tropical montane forests; Cordillera Yanachaga; Peru

1. Introduction

Tropical Montane Cloud Forests (TMCF) occur in humid

tropical upland areas with rugged topography where cloud belts

originating from moist ascending air masses frequently form

fog or mist (Stadtmuller, 1987; Zadroga, 1981). The average

annual rainfall is generally above 2500 mm and humidity

remains near the saturation point (Zadroga, 1981).

TMCF are some of the most endangered and rapidly

disappearing ecosystems as a result of human activities and

climate shifts (Bubb et al., 2004). Conversion of forests to

grazing and agricultural lands and climate warming are among

the strongest pressures affecting TMCF. Considered extremely

fragile ecosystems, the annual deforestation rate in TMCF is

estimated to be at least 1.1% (Doumenge et al., 1995).

Furthermore, the microclimate and hydrology of TMCF can be

negatively affected by global climate change and adjacent

lowland deforestation, which reduce moisture input and

interception of cloud water (Foster, 2001; Lawton et al.,

2001; Loope and Giambelluca, 1998; Nair et al., 2003; Still

et al., 1999). Negative impacts of climate change in TMCF have

already been found among species of epiphytic plants,

amphibians, and bats (La Val, 2004; Lips, 1998; Pounds

et al., 2006; Pounds and Crump, 1994; Pounds et al., 1999).

Consequently, monitoring of TMCF for climate shifts and

changes of indicator species is a high priority.

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Forest Ecology and Management 255 (2008) 1315–1325

* Corresponding author. Tel.: +1 305 348 6047; fax: +1 305 348 1986.

E-mail address: [email protected] (D. Gomez-Peralta).

0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2007.10.058


The hydrology of TMCF has been of particular recent

interest, initiated by the work of Zadroga (1981). Zadroga

(1981) found a contrast in runoff coefficients of catchments

with and without cloud-water interception, defined as the

movement of cloud or fog water on the surfaces of tree leaves,

stems and epiphytes by direct contact of the water droplets and

subsequent dripping (Bruijnzeel and Proctor, 1995; Doumenge

et al., 1995). Studies have shown that this input of cloud water

significantly enhances the net precipitation beyond rainfall,

possibly adding from two percent to as much as 154% of annual

rainfall (Bruijnzeel, 2001, 2005; Bruijnzeel and Proctor, 1995).

Stadtmuller (1987) included this additional water input in

his definition of cloud forests. However, it is uncertain if all

cloud forests, in the broad sense, receive significant cloud-

water input. To better understand the role of cloud-water

interception in cloud forest hydrology, case studies are

necessary across a series of TMCF, where the different types

of precipitation (gross and net), rainfall interception, and cloud

water are quantified, allowing an accurate calculation of the

quantity of cloud-water interception. Because cloud-water

interception may depend on forest structure, as well as crown

exposure to wind and fog, it should be quantified along with the

components of precipitation.

In Peru, TMCF areas are located within the eastern slopes of

the Andes (a region also known as Andean Amazon), the semi-

isolated mountain ranges east of the Andes, and some areas of

the Andean western slopes (Leo, 1995; Young and Leon, 1995).

Therefore, the term Andean cloud forests has also been used

(Gentry, 1993). In central Peru, the forests of the cordillera

Yanachaga from 2000 to 3900 m have been mapped as cloud

forests (translation of bosques de neblina, from original

classification by Drewes, cited and modified by Brack (1987)).

Given its description, such classification seems to be based on

qualitative forest and climate observational appearance

(‘‘cloudy, mossy, and rainy’’), and in some ways lacks

geographic detail and hydrologic definitions. Even with those

aforementioned drawbacks, Brack’s review (1987) still remains

as the only comprehensive document of the cordillera

Yanachaga, despite its administrative origins rather than

scientific.

The present study offers a further insight in the unknown local

forest hydrology of the cordillera Yanachaga. Our objectives

were to quantify the importance of cloud water as an additional

input of water to the forest, and to quantify the importance of

forest structure on the capture of cloud water in the TMCF of the

cordillera Yanachaga in central Peru, using correlation and

regression analyses as major tools. We hypothesized that:

(1) cloud-water interception provides significant water input to

the cloud forest of the Cordillera Yanachaga,

(2) forest structure affects how much cloud water is inter-

cepted.

2. Methods

2.1. Study site

The study was conducted at Yanachaga-Chemillen National

Park in central Peru, which protects a semi-isolated mountain

range known as the Cordillera Yanachaga (Fig. 1). This

cordillera follows a Northwest-Southeast orientation within the

western Amazon River Basin (Brack, 1987). It has also been

classified as one of the eastern cordilleras of the larger Andes

complex (Brack, 1992; Young, 1991, 1992).

Two study sites were established on the western (leeward)

slope near the crest of the Cordillera Yanachaga at 2815 (upper

Fig. 1. (a) Location of study area in central Peru. Adapted from: Bubb et al. (2004) and Young (1991). (b) Location of sample points within the two elevation sites

(50 m-contour intervals).

D. Gomez-Peralta et al. / Forest Ecology and Management 255 (2008) 1315–13251316


site) and 2468 (lower site) masl, corresponding to the forest

types lower montane cloud forest and lower montane rain

forest, respectively (data from this study was used to distinguish

forest types, following classification by Bruijnzeel (2001,

2005)). Diversity of tree species found in the area was 156

spp. ha�1 (dbh � 10 cm, elevation 2450 m (Gomez-Peralta,

2000)). Tree height averaged 14.3 m � 3.7 and 15.5 m � 2.8,

at the upper and lower sites, respectively (n = 20 at each site,

data from this study). Although moss was a conspicuous feature

at both elevations, it appeared to be more prominent at the upper

site (personal obs., D. Gomez-Peralta).

Long-term climatic data were only available from a

meteorological station 13 km west of the study sites at

1800 masl, with annual rainfall averaging 1452 mm (range

from 986 to 1961 mm, from 1963 to 2001). May through

September are markedly drier than the remaining months and

mean temperatures are 10–19 8C, with higher values during the

wet season (Saunders, 2004). The geology of the area is poorly

studied, classified as sedimentary, Paleozoic (Permian-Carbo-

niferous) rocks (Brack, 1987). Hillslopes are generally steep

and unstable, with slopes of 45–908, and soil profiles have a

thick 10–50 cm O-horizon followed by B and C horizons

(Saunders, 2004).

2.2. Rainfall and apparent cloud-water interception

Bulk precipitation, net precipitation and fog-water inputs

were measured at both study sites. Cleared areas provided

sufficient space adjacent to the forest to record gross rainfall

(bulk precipitation). One Tru-Chek manual rain gauge (orifice

size 36.29 cm2, Edwards Manufacturing Co., Albert Lea, MN)

was placed on each open area at approx. 1 m above the ground.

Twenty throughfall and 20 stemflow collectors were placed at

each study site under randomly chosen trees along a fixed

elevation (Fig. 1). Creeks and tree gaps were avoided,

consequently, throughfall is likely underestimated at the stand

level. Throughfall gauges consisted of U-shaped troughs, 1.15 m

long and 10 cm wide, made of split PVC pipes. The troughs were

positioned 308 above the horizontal level, therefore having a

surface area of 0.1 m2, draining into 18.9-l water containers.

Additional throughfall gauges were also positioned within the

open areas to account for splash loss (see data analysis).

Stemflow collectors were gutter-like custom-made plastic collars

attached at approximately 1.30 m above ground level around the

trunk of individual trees and connected to 7.6-l water containers.

Leakage was prevented by scratching the stem bark and

tightening the collars with plastic wrap. Cumulative water input

from each collector and gauge was recorded weekly between 29

July 2003 and 10 August 2004 (54 weeks).

Rainfall interception was calculated as the difference

between bulk and net precipitation, as follows:

rainfall interception

¼ bulk precipitation� ðthroughfallþ stemflowÞ; (1)

where throughfall and stemflow are the components of net

precipitation. Given the admixture of cloud water during

periods of low and negative rainfall interception, the term

‘‘apparent rainfall interception’’ will be preferably used

throughout this study.

Fog-water inputs were estimated within each open area

using an open-ended fog catcher placed above an electronic

tipping-bucket rain gauge (330 cm2 orifice, 0.25 mm per tip,

Rainwise1 Inc., Bar Harbor, ME), which recorded the total

weekly amount of water input using a digital tip counter. These

gauges collected both rainfall and cloud water. The fog catchers

were made of nylon harp cylinders adapted from the design of

Gonzalez (2000). Harp cylinders were 20 cm in diameter and

40 cm tall, employing 0.5 mm diameter vertically strung nylon

that provided a surface interception area of 2327 and 2426 cm2

for the upper and lower study sites, respectively. Electronic

gauges were calibrated with the Tru-Chek gauges before

installation. Differences of water volume between the paired

gauges (tipping-bucket and manual gauges) were divided by the

harp cylinders interception area to give precipitation depth

amounts contributed by clouds. Because the surface area of the

harp cylinders only represent an index of the fog-trapping

ability of the canopy, the term ‘‘apparent fog-water inputs’’ will

be used throughout this study. Fog water was measured during

36 and 35 weeks for the upper and lower elevation sites,

respectively, out of the total 54-week sampling period.

Malfunctioning of equipment prevented fog-water measure-

ments during the remaining weeks.

2.3. Canopy openness and leaf area index

To estimate leaf area index (LAI, the ratio of the total one-

sided leaf area to the projected ground area, Parker, 1995), and

percent canopy openness, hemispherical photos were taken

directly upwards above throughfall and stemflow collectors,

using Nikon 950 and 4500 Coolpix digital cameras and a Nikon

FC-E8 fisheye lens adapter (Nikon USA, Melville, NY). The

images were then downloaded and imported into the program

Gap Light Analyzer (GLA) v. 2.0 (Frazer et al., 1999), to

compute 4-ring LAI and percent canopy openness for the entire

hemispherical views (openness1808). A second analysis of the

fish-eye photos was conducted considering only the projected

area directly above the throughfall collectors. The images were

divided into 108-zenith regions (from 08 to 908) and the central

308-zenith region was re-analyzed with GLA to compute

percent canopy openness (openness308).

2.4. Data analysis

Precipitation recorded by the throughfall collectors placed in

open areas was used to account for weekly splash water losses

by the collectors placed beneath the forest canopy. At the upper

site, during a 7-week period, open-site throughfall trough and

rain gauge weekly recordings were plotted and linearly

regressed as follows:

throughfall collector ðmmÞ ¼ 0:90 rain gauge ðmmÞ;

R2 ¼ 0:99:(1)

D. Gomez-Peralta et al. / Forest Ecology and Management 255 (2008) 1315–1325 1317


At the lower site, during a 26-week period, the regression

equation was as follows:

throughfall collector ðmmÞ ¼ 0:94 rain gauge ðmmÞ;

R2 ¼ 0:99:(2)

Hence, recordings from throughfall collectors were corrected

by dividing by 0.90 and 0.94 for the upper and lower sites,

respectively. It is acknowledged that throughfall drops have

higher kinetic energy than rainfall, therefore these corrections

may not fully account for throughfall splash losses. Hall and

Calder (1993) noted that throughfall drops and their subsequent

impact-splash effects differ among tree species. However, we

assume species-dependent splash losses had little effect on the

results of this study.

Monthly rainfall regimes were classified following Richards

(1996). Months recording less than 100 mm were considered

dry; months recording 100–200 mm were considered wet,

whereas months recording more than 200 mm were classified as

very wet.

Absolute amounts and percentages of annual bulk pre-

cipitation, throughfall, stemflow, and apparent rainfall inter-

ception totals were calculated for each site. Linear regressions

were used to evaluate the relationships between weekly

apparent fog interception and apparent rainfall interception

and between weekly bulk precipitation against throughfall,

stemflow, weekly apparent rainfall interception, and percentage

of apparent rainfall interception.

3. Results

3.1. Total rainfall

Over the 54-week observation period, open site bulk

precipitation totals were 2753 mm and 2222 mm at the upper

and lower elevation sites, respectively. A distinctive very wet

season was observed at both elevations from December through

March. At the upper site, all the remaining months were wet

(above 100 mm). At the lower site, wet months were observed

in May–August and November; dry months occurred in

September, October, and April (Fig. 2a). Weekly bulk

precipitation was greater at the upper site during 91% of the

sample weeks (Fig. 2b). Weekly rainfall ranged from 0 to

154 mm at the upper site, and from 0 to 130 mm at the lower

site. At both sites, the greatest weekly amounts occurred from

December to March.

3.2. Apparent fog-water inputs (fog catcher)

At the upper site, apparent fog-water inputs were evaluated

during 36 weeks. Apparent fog-water input values were found

for all 36 weeks, and amounted to 221.0 mm when normalized

by the surface area of the harp cylinder. Absolute (non-

normalized) quantities of fog-water inputs were sometimes

much greater than bulk precipitation; for example, during the

weeks of August 26th and September 2nd, 2003, bulk

precipitation totaled 19 and 3 mm, whereas fog-water catches

Fig. 2. (a) Monthly rainfall during study period at upper and lower study sites

(vertical bars), and long-term average records of rainfall and temperature

(1963–2001) from Ordonez (2001). Thresholds of dry, wet, and very wet

months are indicated. (b) Weekly variation in rainfall at upper and lower study

sites during August 2003–August 2004.

Fig. 3. Weekly rainfall and fog-water inputs: (a) upper site, (b) lower site.



were 29 and 64 mm, respectively. This can be interpreted as a

good evidence of fog inputs above bulk precipitation at the

upper site (Fig. 3a). Apparent fog-water inputs were evaluated

during 35 weeks at the lower elevation site (Fig. 3b). During 19

weeks the rain gauge with the fog catcher collected less water

than the rain gauge due to the fog catcher’s interference with the

normal collection of rainfall. Fog water during the remaining 16

weeks was slightly above bulk precipitation. Weekly bulk

precipitation was linearly related to apparent fog-water inputs

at the upper site (R2 = 0.40, F1,36 = 23.0, P < 0.0001, Fig. 4),

but not at the lower site (R2 = 0.004, F1,35 = 0.139, n.s.).

3.3. Net precipitation (throughfall and stemflow)

At the upper site, average total annual net precipitation was

2542 mm (92.4% of bulk precipitation). At this site, avera-

ge � standard deviation total throughfall was 2540 � 562 mm

(92.3%) and stemflow was 2.1 � 2 mm (0.08%, Table 1). At the

lower site, average total annual net precipitation was 1564 mm

(70.4%), composed of 1561 � 401 mm (70.3%) of throughfall

and 3.3� 4.8 mm (0.15%) of stemflow (Table 1). Both annual

net precipitation amounts and percentages were significantly

greater at the upper site than at the lower site (t38 = 6.34 and

t38 = 3.61, respectively, P < 0.001).

Weekly variation in throughfall and stemflow followed

similar patterns to that of bulk precipitation at both sites

(Fig. 5). Bulk precipitation was strongly related with through-

fall at both the upper (R2 = 0.99) and lower (R2 = 0.98) sites

(Fig. 6a). At the upper site, R2 values for individual throughfall

collectors averaged 0.94 and slopes averaged 1.01 � 0.21

(mean � standard deviation); 11 of 20 collectors had regression

slopes greater than 1.0. At the lower site, regression R2 values

averaged 0.95 and slopes were 0.82 � 0.19. Five of 20

collectors had regression slopes greater than 1.0, indicating

greater throughfall than bulk precipitation.

Mean weekly throughfall averaged 47 mm per collector

(CV = 0.22) at the upper site, and 29 mm per collector

(CV = 0.26) at the lower site. Weekly frequency of throughfall

exceeding bulk precipitation averaged 17 weeks per collector

(31% of events, CV = 0.72) at the upper site, and 5 weeks per

collector (9% of events, CV = 1.13) at the lower site.

The relationships between bulk precipitation and overall

stemflow were also strong at both upper (R2 = 0.88) and

lower (R2 = 0.84) sites (Fig. 6b). Linear regressions for

individual stemflow collectors showed a very small

contribution 0.001 � 0.001 mm of stemflow for every mm

of bulk precipitation at the upper site. At the lower site, those

Fig. 4. Relationship between weekly bulk precipitation and fog-water inputs at

upper site.

Fig. 5. Weekly distribution of bulk precipitation, throughfall, and stemflow. (a)

Upper site, (b) lower site.

Table 1

Calculated annual water balance for the two working sites

Working site (elevation) Bulk

precipitation

Throughfall Stemflow Rainfall

interception

mm % mm % CV (%) mm % CV (%) mm %

Upper (2815 m) 2753 100 2540 � 562 92.3 22.1 2.1 � 2.0 0.08 93.3 211 7.7

Lower (2468 m) 2222 100 1561 � 401 70.3 25.7 3.3 � 4.8 0.15 142.1 658 29.6

Percentages are relative to bulk precipitation. Throughfall and stemflow averages �1 S.D.



contributions were slightly greater, 0.002 � 0.003 mm. This

indicates not only very small contributions from stemflow to

net precipitation, but also high variability, especially at the

lower site.

3.4. Apparent canopy rainfall interception

Over the 54-week study period, the canopy intercepted only

7.7% of bulk precipitation at the upper site, whereas 29.6% was

intercepted at the lower site (Table 1), i.e. a difference of 22% of

apparent annual rainfall interception between the two sites. For 9

of 54 weeks at the upper site and 2 of 54 weeks at the lower site,

net precipitation exceeded bulk precipitation, that is, apparent

rainfall interception was negative, indicating the presence of fog

precipitation (Fig. 7). Negative rainfall interception during those

weeks amounted to 21 mm and 0.5 mm at the upper and lower

elevations, respectively. These events were most common at the

upper site during the very wet season (December–March),

coinciding with the highest recordings of the artificial fog

catcher. At the upper site (Fig. 8), apparent rainfall interception

and apparent fog were negatively related (R2 = 0.21, F36 = 8.93,

P = 0.005). This relationship was not found at the lower site

(R2 = 0.01, F36 = 0.34, n.s.).

Plots of bulk precipitation against apparent rainfall inter-

ception (Fig. 9a and b) showed saturating-type relationships

and greater variability than plots against both throughfall and

stemflow. Plots of bulk precipitation against percentage rainfall

interception showed logarithmic relationships at both sites

(Fig. 9c and d).

3.5. Canopy properties and precipitation components

Leaf area index at the upper site averaged 2.5 � 0.7 and

canopy openness1808 averaged 12.1% � 6.2. At the lower

site, LAI averaged 2.9 � 0.2 and canopy openness1808

averaged 9.3% � 1.6. The upper site exhibited greater

variability in both LAI and canopy openness1808. The

observed mean LAI at the lower site was significantly

greater than at the upper site (t test for unequal variances,

t25.4 = 1.83, P = 0.04). Mean canopy openness1808 at the

upper site was greater than at the lower site (t test for unequal

variances, t25.4 = 1.82, P = 0.04).

The relationship between canopy structure and apparent

cloud-water interception was tested by regressing weekly

negative rainfall interception values calculated from each

throughfall collector against their respective LAI. At the

upper site, LAI explained a very small proportion of the

variation of the weekly negative rainfall interception

(R2 = 0.05, F1,337 = 16.10, P < 0.0001), whereas this rela-

tionship was not significant at the lower site (R2 < 0.01,

F1,97 = 0.87, n.s.). To further analyze the significant pattern

found at the upper site, the minimum negative values from

Fig. 6. (a) Relationship between weekly bulk precipitation vs. throughfall. (b)

Relationship between weekly bulk precipitation vs. stemflow.

Fig. 7. Weekly rainfall interception over the study period.

Fig. 8. Relationship between fog-water and rainfall interception at upper site.



each throughfall collector (as a surrogate of maximum

amount of intercepted fog) were plotted against their

respective LAI (Fig. 10), explaining much more variation

in apparent interception (R2 = 0.43, F1,17 = 11.5, P = 0.004).

4. Discussion

4.1. Total rainfall

Total annual rainfall and net precipitation increased with

elevation on the western slope of the Cordillera Yanachaga.

Monthly precipitation totals at both sites were much greater

than the existing long-term records at 1800 masl (Fig. 2a). Dry

months were almost nonexistent at both elevations with peak

rainfall occurring from December through March. Long-term

observations from different meteorological stations located

within a 245 km radius from the upper site, between elevations

200 and 1800 masl around Cordillera Yanachaga, showed

similar seasonality (Ordonez, 2001).

Rainfall data from rainfall interception studies in Tropical

Andean Forests were used for comparison of seasonal patterns

(Fig. 11). In Cordillera Central, Colombia (Veneklaas and Van

Ek, 1990), sites at elevations 2550 m and 3370 m, showed a

decrease in total annual precipitation with elevation, from

2115 mm to 1452 mm, respectively. In that study, the upper site

showed two dry seasons, from January to April, and from July

to September; at their lower site, dry months were only

observed in January, March, and August. At their both sites,

peak rainfall was observed in October and June. In Zamora-

Chinchipe, Ecuador (Fleischbein et al., 2005), at 1952 m

Fig. 9. Relationship between bulk precipitation and rainfall interception: (a)

upper site, (b) lower site. Relationship between bulk precipitation and percent

rainfall interception: (c) upper site, (d) lower site.

Fig. 10. Relationship between minimum negative rainfall interception and LAI

above throughfall collectors at upper site.

Fig. 11. Comparison of monthly rainfall with studies of rainfall interception in

Tropical Andean Forests. Legend shows location, elevation, and total annual

rainfall. Data from *1present study and adaptations from *2Veneklaas and Van

Ek (1990) and *3Fleischbein et al. (2005).



elevation, November was the only dry month observed, and

peak rainfall was observed from April to July. Thus, among the

compared studies, the upper elevation site of the present study

is not only the wettest in terms of total annual rainfall, but also

in terms of seasonality, followed by Zamora-Chinchique

(Ecuador), and our lower elevation site.

Despite the large rainfall amounts of this study, it is

possible that bulk precipitation measurements have been

underestimated due to the interactive effects of wind and

sloping ground (Holwerda et al., 2006; Sharon, 1980).

Unfortunately, no wind data were collected. During dry and

foggy periods, conditions were windier at the upper site than

at the lower site; however, windy-rainy conditions were

observed at both sites (personal obs., D. Gomez-Peralta).

Wind effects may contribute to 2–10% of rainfall loss

(Holwerda et al., 2006). Therefore, wind speed and direction

must be accounted in future studies.

4.2. Apparent fog-water inputs (fog catcher)

Findings observed in Figs. 3 and 4 suggest a clear difference

of climatic conditions between the two sites. Apparently, clouds

at the upper site not only produced more orographic rainfall, but

also affected fog occurrence itself through a greater extent.

Qualitative observations of wind-driven fog at the upper site

were greater than at the lower site.

Observed weekly apparent fog-water inputs at the upper site

were somewhat greater during the very wet season (December–

March), but appeared to have no clear distinction between wet

and very wet seasons. Net precipitation events exceeding bulk

precipitation ascribed to fog precipitation at the upper site

during the very wet season may be explained by availability of

moist rising air during such months. The seasonal pattern

differs from that of Holder (2004) who found greater

contributions of fog water in the dry season. As mentioned

above, a dry season was not observed at the upper of the present

study. Such seasonal differences may indicate a relationship

between season and fog availability distinct for cloud forests of

different regions.

Weekly measurements of apparent fog precipitation were a

logistical necessity because of the remoteness of the sites but

were suboptimal for quantifying individual fog interception

events. More frequent measurements (daily or hourly) may

have shown more definitive occurrence of fog precipitation

during rainless periods.

Interference of fog catchers with collection of rainfall as

noted in this study for the lower site has been previo-

usly reported by Vogelmann et al. (1968) and Vogelmann

(1973). This undesired effect is explained by drops being

scattered by the nylon harp. During weeks with both rain and

fog events, it is possible that weekly amounts of fog at both

sites have been underestimated due to the scattering effect.

However, it might be expected that fog gauges could

overestimate fog catch because of the inclusion of

horizontally-driven rainfall. This reinforces the importance

of detailed measurements to distinguish rain and fog events in

future studies.

4.3. Net precipitation (throughfall and stemflow)

Both throughfall and stemflow equations were linearly

related with bulk precipitation (Fig. 6) showing their

occurrence in the same order of magnitude, as described by

Jackson (1975) for heavy storms. Stadtmuller and Agudelo

(1990) and Cavelier et al. (1997) determined that cloud

interception occurred when throughfall events exceeded bulk

precipitation in both wet and dry seasons. Veneklaas and Van

Ek (1990) found two of their weekly throughfall measurements

exceeding bulk precipitation, but considered it ‘‘presumptu-

ous’’ to ascribe these observations to cloud interception, and

suggested canopy saturation after large storms, splash loss, hail

events, and random errors as possible causes. Juvik and Nullet

(1995), in a Hawaiian cloud forest, found throughfall related

with bulk precipitation but considered open-site precipitation

alone a poor predictor of throughfall when the latter is a mixture

of rainfall and cloud-water interception.

We found stemflow values to be very low, less than 1% of

bulk precipitation, as has been reported for some montane

forests. Other studies in montane forests have not measured

stemflow, considering it insignificant, while some have found

much higher stemflow values, as high as 5–12% of bulk

precipitation (Bruijnzeel, 2001; Bruijnzeel and Proctor, 1995).

4.4. Apparent canopy rainfall interception

Both depth (mm) and percentage (%) of apparent rainfall

interception by the forest canopy were smaller at the upper site

than at the lower site (Table 1). Variability of plots of bulk

precipitation against apparent rainfall interception (Fig. 9a and

b) can be explained by the grouping of two or more storms

together within a week (Jackson, 1971). Plots of bulk

precipitation against percentages of apparent rainfall intercep-

tion (Fig. 9c and d) can be extrapolated beyond maximum

weekly precipitation of 154 mm (upper site) and 130 mm

(lower site), since the apparent rainfall interception percentage

is unlikely to exhibit further decrease (Jackson, 1975).

The negative relationship between apparent fog availability

and apparent rainfall interception (Fig. 8) at the upper site

shows that apparent rainfall interception decreases when more

fog was available, and negative rainfall interception occurred

when apparent fog inputs were greater than 7 mm. This

indicates the presence of fog water in some weeks both before

and after the canopy attained saturation.

Apparent rainfall interception percentages at the upper and

lower sites were within the range of previous studies (Table 2).

Vis (1986) and Holder (2003) also found a decrease in rainfall

interception percentage with elevation in Colombia and

Guatemala, respectively. However, in the latter study, bulk

precipitation was considered constant for different elevation

sites, an assumption not justifiable in the present study. Weaver

(1972) found a large decrease in rainfall interception with

elevation from windward slope to ridge (from 1000 to 1015 m)

and from leeward slope to ridge (from 930 to 1015 m) slopes. In

Weaver’s study (1972), rainfall interception at the leeward site

was positive, whereas it was negative at both windward and



ridge sites. The extreme exposure to wind-driven clouds was

considered the possible cause for such low rainfall interception

on the windward and ridge sites (Weaver, 1972). These reasons

may also explain the different apparent rainfall interception

values found in our study. Veneklaas and Van Ek (1990) also

found an increase in depth and relative rainfall interception

with elevation in Colombian forests.

The apparent rainfall interception found at the upper

site (7.7%) comes close to the average interception of 12%

for lower montane cloud forests with significant fog as

derived by Bruijnzeel (2001, 2005). On the other hand, the

apparent rainfall interception at the lower site (29.6%) is

close to the interception of 25% for lower montane forest

without significant fog (Bruijnzeel, 2001, 2005). Therefore,

such distinction confirms differences in forest type not

previously detected by the Holdridge classification (Hol-

dridge, 1967), which is widely used in Latin America. Thus,

the large difference in apparent rainfall interception percen-

tages between sites (22%) can be attributed to cloud-water

inputs at the upper site and warmer temperatures at the lower

site.

Part of the differences in overall apparent rainfall intercep-

tion between the elevations may also be attributed to canopy

storage capacities, which are different among forest types. The

forest at the lower site had higher LAI and lower canopy

openness1808, characteristics that should lead to higher canopy

storage capacity (Leonard, 1967). Additionally, the forest at the

lower elevation has trees reaching up to 25 m, and three forest

storeys (Gomez-Peralta, 2000), whereas at the upper site the

forest is noticeably shorter, more stunted, and probably one or

two-layered. Holder (2004) ascribed differences in rainfall

interception from 2100 to 2550 masl to both cloud interception

and different forest canopies. Cavelier et al. (1997) found 37%

rainfall interception, the highest value reported for a tropical

montane rainforest, which they attributed to a tall (20–30 m)

forest canopy and large epiphyte load.

4.5. Canopy properties and precipitation components

The canopy differences between sites, although significant,

are small compared to the large difference in apparent rainfall

interception (22% difference). Considering the canopy open-

ness as a surrogate of the free throughfall coefficient, our

overall results are in contrast to smaller interception changes

with different elevations that were observed by Fleischbein

et al. (2005) in a lower montane rain forest (Zamora-Chinchipe,

Ecuador), supporting the idea that decreasing rainfall inter-

ception at the upper site was caused by cloud precipitation.

The theoretically predicted influence of the canopy structure

on cloud-water interception (Stadtmuller, 1987; Zadroga, 1981)

was observed at the upper site, but not at the lower site. The

relationship seemed to be linear, with greater LAI intercepting

more cloud water. LAI values estimated from hemispherical

photos did not distinguish between leaves and moss, and had

fields of view much larger than the actual projected areas of the

collectors. A fog-exposed ridge site in the Luquillo mountains,

Puerto Rico recorded an average rainfall interception of�26%,

with an LAI of only 1.99 (Weaver, 1972; Weaver et al., 1986),

smaller than the LAI of the present study. Although this study

cannot definitely pinpoint the basis of the relationship, it shows

that when sufficient fog interception is present, a relationship

with canopy structure can be observed. To our knowledge, this

is the first study to demonstrate such relationship, in contrast to

the works of Fleischbein et al. (2005) and Holscher et al. (2004)

based solely on rainfall interception.

4.6. Evidence for fog precipitation

Several lines of evidence point toward the incidence of fog

precipitation in this study, primarily at the upper site.

Foremost among them was the occurrence of negative weekly

rainfall interception values. Greater fog-gauge water inputs at

the upper elevation were also indicative of fog precipitation

Table 2

Rainfall interception differences among montane cloud forest sites

Location Period of

measurement

Elevation

(masl)

Forest type Precipitation

(mm)

Rainfall

interception (mm)

Rainfall

interception (%)

Panama1 1 year 1200 LMRF 3510 1306 37.2

Honduras2 1 year 1720–1870 LMCF 1469 �453.6 �31

Colombia3 16 months 1700 LMRF 3968 978 24.6

Colombia3 16 months 1950 LMCF 2779 420 15.1

Colombia3 16 months 3000 UMCF 2123 243 11.4

Colombia4 1 year 2550 LMRF 2115 261.5 12.4

Colombia4 1 year 3370 UMRF 1453 215.2 18.3

Peru5 54 weeks 2468 LMRF 2222 657.7 29.6

Peru5 54 weeks 2815 LMCF 2753 210.5 7.5

Guatemala6 44 weeks 2100 LMCF 2559 918 35.0

Guatemala6 44 weeks 2550 UMCF 2559 307 1.3

Puerto Rico5 8 months 930 DCF 1646 11.9 1.0

Puerto Rico7 8 months 1000 DCF 1456 �284 �20.0

Puerto Rico7 8 months 1015 DCF 1228 �435.6 �35.0

Sources: 1Cavelier et al. (1997); 2Stadtmuller and Agudelo (1990), forest types were adapted following classification by Bruijnzeel (2001, 2005); 3Vis (1986), forest

types were adapted following classification by Bruijnzeel (2001, 2005); 4Veneklaas and Van Ek (1990); 5this study; 6Holder (2004); 7Weaver (1972).

DCF: dwarf cloud forest; LMCF: lower montane cloud forest; LMRF: lower montane rain forest; UMCF: upper montane cloud forest; UMRF: upper montane rain

forest.



and these values were significantly and negatively related to

the apparent rainfall interception values. The scatter in the

relationship between bulk precipitation and rainfall inter-

ception for the upper site was also indicative of contributions

by fog precipitation. Also, wetting caused by fog and

subsequent canopy drip was observed repeatedly during visits

to the field sites, mostly at the upper site preceding rain

events.

Additional evidence was found in the difference between

apparent rainfall interception percentages between sites. As

both are lower montane forests, this could distinguish the upper

site as cloud-affected and the lower site as cloud-free, and

potentially attribute up to 22% of apparent rainfall interception

differences to fog precipitation at the upper site, during the year

of study.

At the upper site, the maximum magnitude of negative

rainfall interception values were significantly related to canopy

LAI as determined by hemispherical canopy photography,

suggesting that plots with higher leaf surface area intercepted

greater amounts of fog, as predicted by Stadtmuller (1987) and

Zadroga (1981). Wind, exposure, and moss cover are additional

important factors that were unmeasured in this study that may

have contributed to the relationship.

In conclusion, our year-long analysis of the components of

precipitation at two Peruvian montane forests indicate that

only one site, the upper elevation forest, is apparently receiving

significant inputs from fog interception and can actually be

described as a ‘‘cloud forest’’ in the sense that not only is fog

present, but also apparent capture of fog water by the

vegetation occurs. Those inputs were observed mostly as

reduced canopy interception losses (only 7.7%) compared to

the lower site where interception losses were 29.6%, the latter

similar to reported 25% for cloud free sites by Bruijnzeel

(2001, 2005). In the Cordillera Yanachaga, cloud-water input

at forests below 2500 m may be insignificant. These lower-

elevation forests would not fall in the definition of cloud forests

in strict sense, at least during the study period. Canopy

structure was related to apparent fog interception at the upper

site. The results suggest that forest canopies in ‘‘true’’ TMCF

with greater leaf area index will capture more cloud water. The

lower site, probably in a fog-free zone, did not show apparent

fog interception despite greater leaf area index and larger trees

than the upper site, providing further evidence of the

importance of climatic over canopy controls on fog intercep-

tion. This can also be supported by Troll’s altitudinal

classification (Stadtmuller, 1987), where our lower site would

be situated at the bottom of the cloud base.

Lowland deforestation and global climate change has been

reported to cause an upward movement of the cloud belt with

associated consequences in other TMCF (Gandu et al., 2004;

Lawton et al., 2001; Loope and Giambelluca, 1998; Nadkarni

and Solano, 2002; Nair et al., 2003; Still et al., 1999). The

forests of the Cordillera Yanachaga may already have

experienced an upward cloud cover shift, hence the different

apparent cloud-water interception amounts for the upper and

lower sites, although a long-term monitoring will be necessary

to examine such hypothesis.

Acknowledgements

This work was funded by the Andrew W. Mellon

Foundation, the Inter-American Institute for Global Change

Research, and the Graduate Student Association at Florida

International University. We thank Dr. William Anderson at

Florida International University; Dr. Leo Sternberg at

University of Miami; Dr. Carlos Reynel, Dr. Carlos Llerena,

and Carlos Garnica at Universidad Nacional Agraria La

Molina. We also thank the local support of the Andean Amazon

Research Station, Bonnie Hall, Pilar Verde, Jorge Noguera-

Tapia, Guido Casimiro, Gino Arteaga, Jaime Guerovich, Denis

Orihuela, Nilton Bedrinana, the Yanachaga-Chemillen

National Park agency INRENA, and Damien Catchpole. This

paper is contribution 132 of the Tropical Biology Program at

Florida International University.

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