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HYDROLOGICAL PROCESSESHydrol. Process. 25, 465498
(2011)Published online 30 December 2010 in Wiley Online
Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.7974
Hydrometeorology of tropical montane cloud forests:emerging
patterns
L. A. Bruijnzeel,1* Mark Mulligan2 and Frederick N. Scatena31
Faculty of Earth and Life Sciences, VU University, De Boelelaan
1085, 1081 HV Amsterdam, The Netherlands
2 Environmental Monitoring and Modelling Research Group,
Department of Geography, Kings College London, Strand, London
WC2R2LS, UK
3 Department of Earth & Environmental Science, Hayden Hall,
University of Pennsylvania, 240 South 33rd Street, Philadelphia, PA
19104, USA
Abstract:Tropical montane cloud forests (TMCF) typically
experience conditions of frequent to persistent fog. On the basis
of thealtitudinal limits between which TMCF generally occur
(8003500 m.a.s.l. depending on mountain size and distance to
coast)their current areal extent is estimated at 215 000 km2 or 66%
of all montane tropical forests. Alternatively, on the basis
ofremotely sensed frequencies of cloud occurrence, fog-affected
forest may occupy as much as 221 Mkm2. Four hydrologicallydistinct
montane forest types may be distinguished, viz. lower montane rain
forest below the cloud belt (LMRF), tall lowermontane cloud forest
(LMCF), upper montane cloud forest (UMCF) of intermediate stature
and a group that combines stuntedsub-alpine cloud forest (SACF) and
elfin cloud forest (ECF). Average throughfall to precipitation
ratios increase from072 007 in LMRF (n D 15) to 081 011 in LMCF (n
D 23), to 10 027 (n D 18) and 104 025 (n D 8) in UMCFand SACFECF,
respectively. Average stemflow fractions increase from LMRF to UMCF
and ECF, whereas leaf area index(LAI) and annual evapotranspiration
(ET) decrease along the same sequence. Although the data sets for
UMCF (n D 3) andECF (n D 2) are very limited, the ET from UMCF (783
112 mm) and ECF (547 25 mm) is distinctly lower than thatfrom LMCF
(1188 239 mm, n D 9) and LMRF (1280 72 mm; n D 7). Field-measured
annual cloud-water interception(CWI) totals determined with the
wet-canopy water budget method (WCWB) vary widely between locations
and range between22 and 1990 mm (n D 15). Field measured values
also tend to be much larger than modelled amounts of fog
interception,particularly at exposed sites. This is thought to
reflect a combination of potential model limitations, a mismatch
between thescale at which the model was applied (1 1 km) and the
scale of the measurements (small plots), as well as the inclusion
ofnear-horizontal wind-driven precipitation in the WCWB-based
estimate of CWI. Regional maps of modelled amounts of
foginterception across the tropics are presented, showing major
spatial variability. Modelled contributions by CWI make up lessthan
5% of total precipitation in wet areas to more than 75% in
low-rainfall areas. Catchment water yields typically increasefrom
LMRF to UMCF and SACFECF reflecting concurrent increases in
incident precipitation and decreases in evaporativelosses. The
conversion of LMCF (or LMRF) to pasture likely results in
substantial increases in water yield. Changes inwater yield after
UMCF conversion are probably modest due to trade-offs between
concurrent changes in ET and CWI.General circulation model
(GCM)-projected rates of climatic drying under SRES greenhouse gas
scenarios to the year 2050are considered to have a profound effect
on TMCF hydrological functioning and ecology, although different
GCMs producedifferent and sometimes opposing results. Whilst there
have been substantial increases in our understanding of the
hydrologicalprocesses operating in TMCF, additional research is
needed to improve the quantification of occult precipitation inputs
(CWIand wind-driven precipitation), and to better understand the
hydrological impacts of climate- and land-use change. Copyright
2010 John Wiley & Sons, Ltd.
KEY WORDS cloud forest; cloud-water interception; fog;
evaporation; rainfall interception; stemflow; throughfall;
transpiration;wind-driven rain
Received 2 November 2010; Accepted 3 December 2010
INTRODUCTIONTropical montane cloud forests (TMCF) are
typicallyfound in foggy, wet and often windy environments
whoseecological and hydrological functioning have puzzled
andchallenged investigators for decades. Apart from being
* Correspondence to: L. A. Bruijnzeel, Faculty of Earth and Life
Sci-ences, VU University, De Boelelaan 1085, 1081 HV Amsterdam,
TheNetherlands. E-mail: [email protected] This
paper is partly derived from a chapter previously published
asBruijnzeel LA, Kappelle M, Mulligan M, Scatena FN. 2010.
Tropicalmontane cloud forests: state of knowledge and
sustainability perspec-tives in a changing world. In Tropical
Montane Cloud Forests. Sciencefor Conservation and Management,
Bruijnzeel LA, Scatena FN, Hamil-ton LS (eds). Cambridge University
Press: Cambridge, UK; 691740(www.cambridge.org/9780521760355).
amongst the worlds most valuable terrestrial ecosystemsin terms
of species richness and levels of endemism[see Bruijnzeel et al.
(2010a,b) for a recent overview],headwater areas with TMCF also
provide a stable supplyof high-quality water that is indispensable
for maintainingirrigation, hydro-electric power generation and
drinkingwater (Zadroga, 1981; Brown et al., 1996; Tognetti et
al.,2010). Although cloud forests are often referred to asa single
category, it is helpful to distinguish between(1) tall-statured
lower montane cloud forest (LMCF),(2) upper montane cloud forest
(UMCF) of intermediatestature and (3) stunted sub-alpine (SACF) and
elfincloud forests (ECF). The rationale for making sucha
distinction lies in the wetter and cooler conditions
Copyright 2010 John Wiley & Sons, Ltd.
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466 L. A. BRUIJNZEEL, M. MULLIGAN AND F. N. SCATENA
generally encountered as one moves from the lowermontane to the
upper montane and sub-alpine belts, andwhich are known to affect
the hydrological and ecologicalfunctioning of the respective forest
types (Grubb, 1977;Silver et al., 1999; Bruijnzeel, 2001; Gerold et
al., 2008;Benner et al., 2010; Roman et al., 2010).
The wet and generally remote and difficult terrainof the worlds
TMCF has not only made them hydro-logically and ecologically unique
but also given themsome de facto protection in the past compared to
tropi-cal forests situated in more accessible areas. However, inthe
late 1970s and early 1980s, it became apparent thatin many parts of
the world TMCF were rapidly beingconverted and in need of more
formal forms of pro-tection (LaBastille and Pool, 1978;
Stadtmuller, 1987).Indeed, between 1981 and 1990, montane forests
acrossthe tropics were being lost at a faster rate than low-land
tropical forests (11% vs 08% year1, respectively;Doumenge et al.,
1995). Two recent inventories estimatedthat around the year 2000
about 4555% of all cloud-affected forests located between 235 N and
35 S hadbeen converted to other forms of land use (Mulligan,2010;
Scatena et al., 2010). Conversions to agriculturaland grazing
lands, excessive timber harvesting, invasionsby exotic species,
road ingressions and various types ofdevelopment have been
identified as threats to TMCFin all regions, whereas mining, fire,
forest clearing fordrug cultivation and other activities like golf
courses orcommunication facilities can be locally important
(Hamil-ton et al., 1995; Bruijnzeel and Hamilton, 2000; Kap-pelle
and Brown, 2001; Bubb et al., 2004; Hemp, 2005a;Asbjornsen and
Garnica-Sanchez, 2010; Mulligan, 2010).In recent years, climatic
warming and drying relatedto global or regional climate change have
become anincreasingly important factor that can potentially
threatenTMCF hydrological functioning (Lawton et al., 2001;Hemp,
2005a; Ray et al., 2006), in addition to havinga devastating effect
on particularly vulnerable plant andanimal groups like mosses and
amphibians (Pounds et al.,1999, 2006; Nadkarni and Solano, 2002;
Williams et al.,2003).
Whilst it is broadly recognized that all of these threatscan
impact the hydrological functioning of headwaterareas with TMCF,
the scientific information required toquantify these impacts and to
help manage these uniquebut vulnerable ecosystems was largely
lacking until com-paratively recently. In 1993, the First
International Sym-posium on TMCF was held in San Juan, Puerto Rico,
theproceedings of which (Hamilton et al., 1995) containedthe first
overview of what was known hydrologicallyof TMCF at the time
(Bruijnzeel and Proctor, 1995)as well as one of the first
physically based studies ofcloud-water interception (CWI) in a TMCF
setting (Juvikand Nullet, 1995a). Certain aspects of CWI and
TMCFhydrology have been considered at a series of Confer-ences on
Fog and Fog Collection (held every three yearssince 1998;
Schemenauer and Bridgman, 1998; Scheme-nauer and Puxbaum, 2001;
Rautenbach and Oliver, 2004;Biggs and Cereceda, 2007; Climatology
Working Group,
2010). Arguably, however, the San Juan Symposiummarked the start
of increased research activity in thefields of TMCF hydrology,
hydrometeorology and eco-physiology. Thus, whilst Bruijnzeel and
Proctor (1995)were able to list only eight studies of crown drip
andoccult precipitation in TMCF environments, plus a meresix
studies estimating overall evaporation loss throughindirect methods
and none quantifying transpiration ratesin TMCF or the impact of
TMCF conversion on stream-flow amounts and seasonal distribution,
at the follow-upSymposium on Science for the Conservation and
Man-agement of TMCF held in 2004 in Waimea, Hawaii(Bruijnzeel et
al., 2010a), some 25 presentations reportedon hydrometeorological
and plant physiological work thathad been conducted since 1993.
Quantitative evidence onthe effects of TMCF conversion to pasture,
as well as onthe impacts of climatic variability and change were
givenin another ten presentations.
The presence of cloud forest is widely assumed toincrease
streamflow volumes, not only because of theextra amounts of water
captured from passing fog,beyond that provided by precipitation,
but also becauseof reduced evaporative losses under the prevailing
lowradiation levels and high atmospheric humidity (cf.Zadroga,
1981; Calvo, 1986; Jarvis and Mulligan, 2011).In addition, the
forest helps to reduce the number ofshallow landslides and prevents
surface erosion, therebymaintaining better water quality (Sidle et
al., 2006;cf. Bruijnzeel, 2004). Such considerations lie at
theheart of many payment for ecosystem services (PES)schemes in
which downstream users pay a certain fee for(mostly hydrological)
services rendered by cloud forestto compensate upstream forest
owners who conserve theircloud forests instead of converting them
to economicallymore profitable forms of land use such as grazingor
cropping (Pagiola, 2002; Rodriguez-Zuniga, 2003).Given the great
pressure on the worlds remaining cloudforests, and the growing
recognition of their value astreasure houses of biodiversity and as
providers of high-quality water, an array of PES-initiatives aimed
at TMCFconservation has emerged in recent years (Asquith andWunder,
2008; Munoz-Pina et al., 2008; Porras et al.,2008; Garriguata and
Balvanera, 2009; Tognetti et al.,2010). Needless to say, such PES
schemes and land-and forest managers and policy-makers in general
needto determine which cloud forests under their
jurisdictionprovide the best water supplies (and to whom), which
arethe most vulnerable to climate change or most threatenedby
encroachment, and what are the hydrological impactsassociated with
forest conversion or climate change. Inshort, there is a great need
for site-specific informationon TMCF hydrological functioning for
incorporation intoconservation and management plans at various
spatialscales (Bruijnzeel et al., 2010b).
After first defining the various types of cloudforests and
exploring their global distribution, thisarticle summarizes the
currently available knowl-edge on the hydrometeorology of TMCF and
provides
Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 25,
465498 (2011)
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HYDROMETEOROLOGY OF TROPICAL MONTANE CLOUD FORESTS 467
Table I. Summary of key structural characteristics marking the
chief tropical (montane) forest types distinguished in the
presentpaper (based on Frahm and Gradstein, 1991; Whitmore,
1998)
Forest formationa LERF LMRF/LMCF UMRF SACF
Canopy height 2545 m 1533 m 1518 m 159 mEmergent trees Up to 67
m tall Often absent, up to 37 m Usually absent, up
to 26 mUsually absent, up
to 15 mCompound leaves Abundant Occasional Rare AbsentPrincipal
leaf size classb Mesophyllous Meso-/notophyllous Microphyllous
Nanophyllous
Leaf drip-tips Abundant Present Rare or absent AbsentButtresses
Frequent and large Uncommon, and small Usually absent
AbsentCauliflory Frequent Rare Absent AbsentBig woody climbers
Abundant Usually absent Absent AbsentBole climbers Often abundant
Frequent to abundant Very few AbsentVascular epiphytes Frequent
Abundant Frequent Very rareNon-vascular epiphytes
(mosses, liverworts)Occasional Occasional/Abundant
80%
a LERF, lowland evergreen rain forest; LMRF/LMCF, lower montane
rain/cloud forest; UMCF, upper montane cloud forest; SACF,
sub-alpine cloudforest.b Leaf sizes according to the Raunkiaer
classification system: mesophyllous, 450018 225 mm2; notophyllous,
20254500 mm2; microphyllous,2252025 mm2; nanophyllous,
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468 L. A. BRUIJNZEEL, M. MULLIGAN AND F. N. SCATENA
As elevation continues to increase, the trees become grad-ually
smaller, moss cover on the stems increases from
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HYDROMETEOROLOGY OF TROPICAL MONTANE CLOUD FORESTS 469
Figure 1. Modelled distribution of cloud-affected tropical
montane forests, with UNEP-WCMC listed cloud forest sites indicated
in red. The colourscale indicates the approximate fractional cover
of forest within the 1-km pixel
variation in the actual elevations at which the cloudforests
occur (Table II) and in their spatial extent indifferent
continental regions (Table III and Figure 1).In general, the
distributions depend on the upper andlower bounds of the cloud belt
(Table II) and on theglobal, regional and local factors that
influence cloudformation. As stated previously, the transition
fromLMCF to UMCFas well as the thickness of the cloudforest belt
itselfis primarily governed by the levelof persistent cloud
condensation (Grubb and Whitmore,1966; Frahm and Gradstein, 1991;
Kitayama, 1995). Thelatter, in turn, is determined by the moisture
content and
temperature of the atmosphere such that the more humidthe
uplifted air, the lower will be the altitude at whichit condenses
(Foster, 2010). With increasing distancefrom the ocean, the air
tends to be less humid and willrequire lower temperatures, and thus
higher elevations, toreach condensation. Consequently, the
associated cloudbase, and thus the presence of TMCF, will occur at
ahigher elevation as one is moving away from the ocean.Similarly,
for a given atmospheric moisture content, thecondensation point is
reached more rapidly for coolair than for warm air (Foster, 2010).
Hence, at greaterdistance from the equator, the average
temperatureand
Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 25,
465498 (2011)
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470 L. A. BRUIJNZEEL, M. MULLIGAN AND F. N. SCATENA
therefore the altitude at which condensation and TMCFoccurwill
be lower (Nullet and Juvik, 1994; Jarvis andMulligan, 2011).
In addition to the elevation of the cloud base, thedistribution
and extension of the TMCF belt is alsogoverned by the upper limit
of cloud formation, whichis also influenced by global-scale
atmospheric circulationfeatures, such as the Hadley cell. In the
latter, heated airrises to great elevations in the equatorial zone,
and flowspolewards and eastwards in the upper atmosphere as
itcools. The cool dry air then descends in a broad belt in theouter
tropics and sub-tropics from where it returns to theequator. This
subsidence reaches its maximum expressionat oceanic sub-tropical
high-pressure centres and alongthe eastern margins of the oceanic
basins. As the airdescends and warms, it forms a temperature
inversionthat separates the moist layer of surface air (being
cooledwhilst rising) from the drier descending air above.
Thisso-called trade wind inversion (TWI) forms a surfacethat
generally rises towards the equator and from east towest across the
oceans (Riehl, 1979). Over the easternPacific Ocean, the TWI occurs
at only a few hundredmetres above sea level, for example, off the
coast ofsouthern California. It rises to about 2200 m near
Hawaii(Cao et al., 2007) and dissipates in the equatorial
westernPacific (Nullet and Juvik, 1994). The consequences ofthe TWI
for the occurrence of the upper boundary ofTMCF are profound and
are another reason why thevegetation zonation on mountains situated
away fromthe equator tends to be compressed. For instance,
somewindward slopes in the Hawaiian archipelago receivemore than
6000 mm of rain year1 below the inversionlayer. However, above the
inversion, montane cloudforest suddenly gives way to dry sub-alpine
scrub becausethe inversion prevents clouds moving upward and
bringmoisture to those areas (Kitayama and Muller-Dombois,1994a,b;
cf. Loope and Giambelluca, 1998).
Superimposed on these global-scale moisture and tem-perature
gradients are more local processes influencingthe temperature of
the air column and thus the start-ing point for air subject to
cooling by lifting. Theseinclude the influence of offshore sea
surface tempera-tures, landsea interactions involving the coastal
plain,the size of a mountain and its orientation and expo-sure to
the prevailing winds (Malkus, 1955; Van Steenis,1972; Stadtmuller,
1987; Jarvis and Mulligan, 2011). Theinteractions of these local
and regional influences on thedistribution of TMCF can be quite
pronounced. The sheermass of large mountains exposed to intense
radiation dur-ing cloudless periods is believed to raise the
temperatureof the overlying air sufficiently to decrease the
lapserate and enable plants to extend their altitudinal range.This
effect is commonly referred to as the mass ele-vation or
telescoping effect and has been recognizedfor many decades
(Schroter, 1926; Van Steenis, 1972;Whitmore, 1998). More recent
research has indicated thatlow-statured, mossy forests occurring at
relatively lowelevations (
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HYDROMETEOROLOGY OF TROPICAL MONTANE CLOUD FORESTS 471
still remains and that some 272 Mkm2 of CAF havebeen converted
(Mulligan, 2010). Whilst there is rea-son to believe that the
hydro-climatic approach slightlyover-estimates the area with TMCF
[see detailed discus-sion by Mulligan (2010)], the results were
tested witha high level of success against the more than 560
trop-ical sites listed by WCMC-UNEP as having confirmedcloud forest
presence (Aldrich et al., 1997). The best fitbetween actual and
modelled cloud forest presence wasobtained when using a threshold
value for ground-levelcloud occurrence (i.e. fog) of at least 70%
of the time(Mulligan, 2010; Figure 1). It is recognized that this
isa relatively high level of fog occurrence, but the use ofeither
higher or lower values of fog frequency resulted insignificant
reductions in the proportion of observed cloudforests being
correctly modelled as CAF [see Mulliganand Burke (2005b) for
details on the models sensitivity].
CLOUD FOREST HYDROMETEOROLOGY
General climatic conditionsThe more than 560 tropical sites with
confirmed cloud
forest presence (albeit unspecified in terms of cloudforest
type; Aldrich et al., 1997; Figure 1) represent awide range of
climatic conditions (rainfall and temper-ature, wind) and landscape
settings (altitude, exposure,mountain size, distance to sea,
bedrock geology). Jarvisand Mulligan (2011) employed spatial data
sets derivedfrom the WorldClim data-base (Hijmans et al., 2004),
todescribe the climate at 477 cloud forest sites as identi-fied by
UNEP-WCMC. Further, comparisons were madebetween the climate of
cloud forest sites and that of ran-domly generated sites covering
forested areas throughoutthe montane tropics, with the aim of
identifying the cli-matic variables most important in
distinguishing TMCFfrom other tropical forests. TMCFs were found to
be wet-ter (by 184 mm year1 on average), cooler (by 42 C onaverage)
and less seasonally variable than other mon-tane forests. The most
statistically significant differencesin climate between TMCFs and
other montane forestswere: maximum temperature, mean temperature,
rainfalland rainfall seasonality (in order of significance).
Cloudforests also tend to be located closer to the coast
(particu-larly in Asia) and at higher altitudes than montane
forestsnot affected by cloud. Furthermore, cloud forests occupymore
topographically exposed areas than do other mon-tane forests.
Interestingly, cloud forest sites in Africa tendto be drier
(average annual rainfall 500 m;Pruppacher and Klett, 1978), and CWI
is fundamental toassessing the hydrological importance of intact
and con-verted cloud forest areas. Because it is difficult to
distin-guish drizzle from rain in precipitation records, the
termprecipitation is used in this article to denote either.
Like-wise, the term wind-driven precipitation (WDR) refersto either
form of near-horizontal precipitation, whereasthe term occult
precipitation (HP) is used to denotethe sum of CWI and WDR without
making a distinctionbetween the two (cf. Frumau et al., 2011a). The
impor-tance of occult contributions is illustrated by the
resultsobtained by several early studies from Central Amer-ica that
arguably contributed greatly to the reputationof TMCFs as suppliers
of high amounts of streamflowthroughout the year. Zadroga (1981)
compared the rain-fall and streamflow regimes for two groups of
catchmentsin northern Costa Rica, one located on the (wetter)
wind-ward Atlantic side of the Continental Divide and theother on
the (drier) leeward Pacific side. Annual stream-flow from the
Pacific catchments amounted to 34% ofthe rainfall and showed a
clear seasonal flow patternthat followed that for rainfall, whereas
annual streamflowfrom the Atlantic catchments roughly equalled
rainfall(102%), and even exceeded rainfall inputs for seven outof
12 months (Figure 2). Whilst acknowledging that thehigh runoff
coefficient derived for the Atlantic catch-ments was partly due to
underestimation of rainfall inputsin the higher, rainier parts of
the catchments that lackedrainfall measurement stations, Zadroga
(1981) attributedthe very high streamflows primarily to unmeasured
inputsof CWI. He also emphasized the fact that months withexcess
streamflow over precipitation coincided with thedominant occurrence
of moisture-laden clouds broughtin from the Caribbean by the trade
winds. In addi-tion, Zadroga recognized that evaporative losses
fromthese fog-ridden slopes should be low. These contentionswere
subsequently confirmed by measurements of rain-fall, streamflow and
climatically based estimates of evap-otranspiration for another
Atlantic catchment located fur-ther south in Costa Rica (Calvo,
1986). Although bothof these early investigations must be
considered blackbox studies that did not quantify the underlying
hydro-logical processes, further support came from
comparativeobservations of rainfall and throughfall (TF) in
Atlanticcloud forests in Puerto Rico (Weaver 1972), Costa
Rica(Caceres, 1981) and Honduras (Stadtmuller and Agudelo,1990).
These studies indicated that annual TF at exposedlocations could
attain values of as much as 110180%of measured rainfall.
To what extent are these early observations exemplaryfor the
hydrological behaviour of TMCF in general?And how reliable are such
direct comparisons of rain-fall and TF in view of such potentially
disturbing factorsas wind-induced precipitation losses around rain
gauges(e.g. Frland et al., 1996; Yang et al., 1998; Nespor
andSevruk, 1999) and the effect of inclined precipitationfalling
onto steeply sloping terrain as opposed onto a
Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 25,
465498 (2011)
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472 L. A. BRUIJNZEEL, M. MULLIGAN AND F. N. SCATENA
Figure 2. Contrasting rainfall and streamflow regimes for
catchments situated on the Atlantic and Pacific slopes of northern
Costa Rica (afterZadroga, 1981)
horizontal gauge orifice (Sharon, 1980; Herwitz and Slye,1992),
relative to unmeasured contributions by CWI orWDR? Earlier reviews
of the hydrometeorological litera-ture on TMCF (Bruijnzeel and
Proctor, 1995; Bruijnzeel,2001, 2005) lacked sufficient data for a
meta-analysisbut the proliferation of local studies of net
precipitation(or at least of TF)many of which are reported
byBruijnzeel et al. (2010a)now allows an analysis ofsome of these
questions and whether different types ofTMCF do indeed exhibit
different net precipitation frac-tions.
Table IV lists net precipitation data for lower montanerain
forests that are little or not affected by fog and lowcloud (LMRF,
n D 15), tall LMCF subject to moderatefog incidence (n D 23), UMCF
of intermediate staturesubject to frequent fog incidence (n D 18)
and stuntedSACF and ECF (n D 8). Figure 3 shows the averageamounts
of rainfall (P), the fraction of rainfall becomingTF, and the leaf
area index (LAI) for the respectivemontane forest types, whereas
Figure 4 shows scatterplots of annual TF versus P at individual
study sitesgrouped per forest type.
On the basis of the more than 60 local studies listedin Table
IV, the following patterns emerge: (1) averageLAI values per forest
type decrease from 554 181in LMRF through LMCF (467 111) and
UMCF(396 125) to 310 121 in SACFECF; (2) P atSACFECF sites tends to
be higher on average thanat sites representing the other three
forest categories forwhich differences between groups were
comparativelysmall and (3) averaged ratios of TF to P increase
steadilyfrom LMRF to SACF, viz. from 72 7% (SD) in LMRF,to 81 11%
in LMCF, 100 27% in UMCF, and 104 25% in SACFECF (Figure 3).
Rigorous comparisons of the statistical differences inTF/P
between the different forest types is limited by thesmall and
uneven sample sizes as well as by differencesin the sampling
methodologies used in the studies of
both TF and P. Nevertheless, comparisons of means andmedians
using t-tests, MannWhitney rank sum testsand analysis of variance
(ANOVA) where appropriate,do support the patterns observed in
Figure 3. Moreover,there are significant differences (at p D 005)
betweenthe means or medians of TF/P for UMCF and bothLMCF and LMRF
(but not SACFECF). The medianTF/P value for LMCF is also
significantly higher thanthe median value for LMRF.
Comparison of the slopes of the P versus TF graphsper forest
type (Figure 4) also indicates that TF exceedsprecipitation as
measured in the open at SACFECFsites, whereas the two are nearly
equal at UMCF sites.To these TF fractions the fraction of P
reaching the forestfloor as stemflow (SF) should be added.
Unfortunately,not all studies of net precipitation have measured
SFbut values observed in LMRF and LMCF are typicallyvery low (
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HYDROMETEOROLOGY OF TROPICAL MONTANE CLOUD FORESTS 473
Tabl
eIV
.Thr
ough
fall
(TF)
,ste
mflo
w(S
F)an
dap
pare
ntra
infa
llin
terc
eptio
n(E
i)fra
ctio
ns(%
of
inci
dent
prec
ipita
tion)
and
appa
rent
clou
d-w
ater
inte
rcep
tion
(CW
I,m
mye
ar1
;c
Dv
alue
corr
ecte
dfo
rwin
dan
dto
pogr
aphi
cef
fect
s)as
mea
sure
din
diffe
ren
tty
pes
oft
ropi
calm
on
tane
rain
fore
st
Loca
tion
and
fore
stty
peEl
evat
ion
(m.a.
s.l.)
MA
Pa(m
m)LA
I/H/
(m)
TFSF
(%o
fP)
Ei
CWI
(mm
year
1)
Rem
arks
Mon
tane
rain
fores
tslit
tlea
ffect
edby
fogB
oliv
ia,Y
un
gasb
1850
2310
/20
79