Research Plan
Project Summary
This project aims to improve our understanding of the ecological and hydrological
functioning of cloud forests in Puerto Rico’s El Yunque national forest. These unique
ecosystems are tightly coupled to regular cycles of cloud inundation, and thus are likely
to be quite sensitive to global changes like tropical ocean warming and land use/cover
changes. Water isotope tracers can provide a powerful constraint on cloud inputs to these
forests and local watersheds, since the isotopic signature of cloud water is typically
distinct from that of rainfall. Samples collected from cloud water, rain water, soil water,
and streamwater, as well as tree and epiphyte water, will be collected and analyzed for
their hydrogen and oxygen isotopic composition. The goal of this research is to answer
the following questions:
1) How much of the cloud forest water budget comes directly from cloud
inundation on a seasonal and interannual basis, and how does this vary by
forest component (canopy epiphytes, trees, understory vegetation)?
2) How much does cloud water contribute to the water budget of local
watersheds and communities, and Puerto Rico as a whole?
3) Can isotopic tracers provide quantitative evidence for changes in cloud
inundation cycles and cloud forest hydrology associated with either ocean
warming or lowland deforestation?
4) How will climate change and land use/cover change affect the cycles of
cloud inundation, and how does this ramify through the forest and human
communities?
Isotopic sampling of cloud water, precipitation and streamflow will be compared
extensively with remote sensing data provided by the suite of instruments on NASA’s
earth observing system (EOS) satellites (Terra, Aqua, and TRMM). Several of these
sensors (MODIS, CERES, and MISR) provide useful cloud products, including cloud
amount, altitude, and thickness. The MISR cloud products will be especially useful, as
the multi-angle imaging provides unprecedented information on cloud structure that can
be related to cloud water inputs and precipitation over Puerto Rico. Vertical profiles of
temperature and humidity from the AIRS/AMSU/HSB family of sensors will be
compared with orographic cloud formation patterns at El Yunque. Ocean surface
temperature, water vapor, and cloud liquid water content over the Caribbean basin
derived from AMSR-E will be compared with cloud water variations (amounts and
isotopic ratio) at El Yunque to better understand synoptic conditions affecting the cloud
forest.
The education component of this proposal will execute a three-year program of
undergraduate and graduate education at UCSB and UPRM, along with aggressive
community outreach through UPRM and the US Forest Service El Portal Tropical Forest
Center. An integral aspect of the education will be developming a short course at UCSB
on cloud forest ecology, climate change, and isotope hydrology. Ideally this will include
development of a Spanish language curriculum on global change and cloud forest
ecosystems with specific focus on the Caribbean and Central America, for eventual use at
UPRM. Another emphasis will be to work with the US Forest Service El Portal Tropical
Forest Center to design an exhibit on cloud forests, cloud water, and climate change for
visitors to El Yunque.
1. Introduction and Background
Tropical montane cloud forests (TMCFs) are unique among terrestrial ecosystems
in their tight coupling to the atmospheric hydrologic cycle. This coupling is accomplished
partly through regular cycles of inundation by orographic cloudbanks at the forest
elevation, and the moisture inputs from such cloud inundations are a significant fraction
of annual rainfall in many cloud forests (Bruijnzeel and Proctor 1995; Clark et al. 2000).
The unique environment produced by these cloudbanks has contributed to the high
species diversity and endemism found in these forests, which are critically endangered
throughout the tropics (Stadtmuller 1987; Hamilton et al. 1995; Haber 2000). Because
orographic cloud formation is determined by such processes as ocean evaporation and
vertical atmospheric profiles of temperature and humidity, it is strongly sensitive to
climate change (Still et al. 1999; Pounds et al. 1999). Acceleration of the tropical
hydrological cycle via enhanced ocean temperatures is expected to change these profiles,
with concomitant impacts on lapse rates and freezing surfaces (Diaz and Graham 1996).
Indeed, enhanced atmospheric warming with height (decreasing lapse rate) has been
observed over the tropics (Gutzler 1992), and another analysis suggests an enhancement
of the tropical hydrological cycle in recent decades (Flohn and Kapala 1989). Gaffen et
al. (2000) showed decreasing trends in the lapse rate over 1960-1997, as derived from
radiosonde observations, and consistent with enhancements to the tropical hydrological
cycle, although the reverse trend occurs from 1979-1997. Finally, climate model
simulations driven by recent tropical sea surface temperatures (SSTs) reproduce observed
tropospheric warming via an enhancement in the tropical hydrological cycle and
increased latent heat releases (Graham 1995).
Climate model simulations of doubled CO2 conditions also suggest an
enhancement in the tropical ocean evaporation, with impacts on vertical profiles of
temperature and humidity. Taking the grid-box average relative humidity surface at
current cloud forest elevations as a proxy for cloud formation height, Still et al. (1999)
showed that the elevation of this surface increases hundreds of meters under doubled CO2
conditions in the winter season for four regions containing well-known cloud forests. If
this proxy is reasonable, the height of cloud formation would also rise, and thus adversely
affect cloud forests, in particular those already located on mountaintops or ridgetops.
Indeed there is already evidence at the well-studied cloud forest in Monteverde, Costa
Rica, of a lift in cloud base height during the dry season. This has driven a drying trend,
which has been linked to anuran extinctions, and is strongly correlated with tropical sea
surface temperature variations (Pounds et al. 1999). In addition to climatic effects
accompanying tropical ocean warming, lowland deforestation and consequent changes in
the surface energy balance and evapotranspiration may also contribute to the cloud base
rise and drying trend observed at Monteverde (Lawton et al. 2001). These authors show
an effect on both convective and orographic cloud formation resulting from deforestation,
such that these clouds have lower cloud water mixing ratios and higher cloud bases. They
also present satellite data (Landsat and GOES) showing reduced cumulus cloud
development over upwind, lowland deforested areas in Costa Rica (Lawton et al. 2001).
Despite the significant water inputs from cloud inundation and the potential
changes in cloud formation regimes from tropical ocean warming and deforestation, the
reliance by forest vegetation on cloud water is poorly understood. To date, the vast
majority of studies in cloud forests examining cloud water inputs have constructed water
budgets of the inputs and outputs (Bruijnzeel and Proctor 1995). The inputs from cloud
water are typically given as a fraction of the precipitation amount, either on an annual or
seasonal basis. The cloud inputs are most important in the dry season, when convective
rainfall is suppressed. However, very few studies have demonstrated or attempted to
quantify how cloud water inputs contribute directly to the water status of forest
vegetation, on either a seasonal or interannual basis. The study of Field and Dawson
(1998) in the Monteverde cloud forest is a notable exception. This work illustrated the
power of isotopic tracers in delineating the reliance on fog water by different forest
components. This is possible because of the different isotopic composition of fog and
mist (so-called ‘horizontal precipitation’) versus rainfall. They showed that early life
stages of a forest pioneer hemiepiphytic species rely almost exclusively on fog water,
while later stages rely on soil water.
Another well-studied cloud forests exists in El Yunque Caribbean National Forest
of Puerto Rico, which contains the Luquillo Experimental Forest, an NSF Long-term
Ecological Research (LTER) site. The cloud water contribution to forest hydrology is
significant, accounting for 10% of mean annual rainfall, which is 4.9 m/year at the
highest elevations (Scatena LTER site). The cloud water contribution is undoubtedly
much greater during the dry season. The contribution of cloud water to downstream
watersheds and Puerto Rico as a whole is unknown. However, El Yunque is critical to the
water supply of Puerto Rico. At present, 20 % of the island’s population depends on
water from the forest (USFS web site), and this is likely to increase as development and
population pressures increase. Data on cloud water inputs to forests and streams in this
system will be used to better understand regional hydrological cycles and to predict the
impacts of climate change that affect cloud formation over El Yunque’s forests.
2. Project Objectives and Strategy
The objective of this research is to use water isotope tracers to enhance our
understanding of the ecological and hydrological function of the cloud forest at El
Yunque. This includes understanding the contribution of cloud water to forest
components (epiphytes, trees, understory vegetation) and to downstream watersheds used
by human communities. Critical to this effort is obtaining samples over several years to
understand how seasonal and interannual dynamics in cloud processes related to synoptic
atmospheric and oceanic conditions influence cloud water inputs. This understanding will
be employed to predict changes to this ecosystem resulting from the suite of global
changes including climate change and land cover conversions.
This project will focus on the cloud forest within the Luquillo Experimental
Forest (LEF). Isotopic data will be collected from the various inputs (cloud water and
precipitation), stocks (vegetation water and soil water), and outputs (streamflow). Mixing
models will be used to partition cloudwater usage by vegetation components (epiphytes,
hemiepiphytes, understory plants, and trees) on seasonal and interannual bases. This
project will take advantage of the numerous ancillary data collected within LEF,
including information on forest ecology and biogeochemistry, as well as meteorological
and streamflow data collected in LEF and in adjacent watersheds by the LTER
researchers and by the USGS Water Energy and Biogeochemical Budgets program.
3. Field Sampling
Isotopic tracers in the hydrological cycle will play a central role in this research.
Horizontal (wind-blown mist, fog and cloud water) and vertical precipitation, vegetation
water, streamwater, and soil water will be collected to assess water inputs, outputs and
stocks. Clouds and mist will be sampled on a weekly basis with collectors built in the
laboratory (e.g., Dawson 1998; Scholl et al. 2002). Precipitation will be sampled on a biweekly basis using a bottle and attached funnel, filter, and venting tube designed to
minimize evaporative enrichment (Dawson 1998; Riley et al. 2003). Vegetation water
(epiphyte tanks, tree stem water, and understory stem water) will be collected during
intensive field campaigns throughout the year. During these campaigns, soil water will be
collected from soil cores or soil pits dug in the forest floor. Local streams and springs
will be sampled with lower frequency (2-3 times/year). All water samples will be
analyzed for their hydrogen and oxygen isotopic composition at the UPRM Geology
department stable isotope facility.
4. Relation of Water Isotope Data to Modeling of Puerto Rico Hydrology
Although streamwater and soil water will be sampled for isotopic analysis and
compared with cloud water inputs to look at their contribution to downstream watersheds,
this will not be a primary focus of the isotopic work. However, streamwater isotopic data
offer potential constraints on the modeling, which will simulate the hydrological cycle on
the entire island, including groundwater flow. This approach was demonstrated by Scholl
et al. (1996, 2002), who used isotopic tracers in interpreting regional hydrology and
quantifying cloud water inputs on the islands of Hawaii and Maui.
5. Remote Sensing Data
This project will benefit from NASA’s intense focus on remote sensing of cloud
properties and dynamics for reducing uncertainty attending the role of clouds in the
climate system. The array of sensors onboard NASA EOS satellites, primarily Terra,
Aqua, and TRMM, will provide a large-scale, top-down context for understanding cloud
inputs to El Yunque. As the higher-level products become available, we will incorporate
them into our analyses. We anticipate that several MODIS cloud products will be
especially useful, including cloud amount, altitude, and thickness. The MISR cloud
products will be especially useful, as the multi-angle imaging provides unprecedented
information on cloud structure that can be related to cloud water inputs and precipitation
over Puerto Rico.
Dry season cloud formation in El Yunque is driven by evaporation over the ocean
and subsequent orographic uplift as air is advected over the island. The lifting
condensation level (LCL) of these orographic clouds is determined by the water content
of this air, along with vertical atmospheric profiles of temperature and humidity. All of
these variables are obtainable from the sensors on Aqua. Vertical profiles of temperature
and humidity will be taken from the AIRS/AMSU/HSB family of sensors. Ocean surface
temperature (including regions under cloud cover), water vapor content over the ocean,
and cloud liquid water content will be taken from AMSR-E products. These quantities
will be used to better understand synoptic conditions that contribute to cloud water
variations (amounts and isotopic ratio) sampled in El Yunque.
6. Relevance of Research to Earth Science Enterprise Research Questions
This proposed research is directly responsive to at least two of the questions
advanced in the NRA:
 How are global ecosystems changing?
 How are variations in local weather, precipitation and water resources
related to global climate variation?
Cloud forests are especially vulnerable to global change, as SST increases and
regional land use both may impact the quantity and quality of cloud formation over these
ecosystems (Pounds et al. 1999; Still et al. 1999; Lawton et al. 2001). Diagnosing these
impacts is especially promising using remote sensing, since clouds are a major focus of
numerous earth-observing sensors. This suite of sensors promises to provide an
incredibly rich data trove for exploring variations in cloud amounts, locations, and
properties. This research will enhance our understanding of cloud forests in the earth
system and in particular their role in tropical hydrology. It will help us to forecast how
land use and climatic changes will affect these unique ecosystems and their relationship
to local communities. With this information, we will better understand the complex
nature of interactions determining the response of cloud forests to the suite of ongoing
global changes.
Education and Outreach Plan
The objectives of the education component in this proposal are:
1) To integrate local Puerto Rican students and scientists into the research
and with their collaboration develop a Spanish curriculum on global
change and Caribbean cloud forests.
2) To create an innovative short course for upper-level undergraduate and
beginning graduate students at UCSB and UPRM in collaboration with R.
Williams, J. Gonzalez, and D. Erickson that directly involves students in
the knowledge-discovery process. Ideally, this course will introduce
students to multidisciplinary and complex systems thinking by
incorporating atmospheric science, hydrology, isotope biogeochemistry,
and forest ecology.
The educational component of this project includes a Puerto Rico element and a
UCSB element. As part of this component, I will develop new courses at UCSB that
integrate the research directly into course curricula. These courses will be at the
undergraduate and graduate levels. At the undergraduate level, I plan to offer a freshman
seminar on climate change and tropical islands. At the upper-division undergraduate and
graduate level, I will develop a short course course on cloud forest ecology and climate
change.
Beyond the university environment, I strongly believe in community outreach,
since the public funds almost all of my work. For this outreach, I will take advantage of
the numerous opportunities afforded by the US Forest Service El Portal Tropical Forest
Center, including the creation of an exhibit on cloud forests, cloud water, and climate
change El Yunque’s visitors.
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Outline of work plan
Year 1 (Research). Build fog water and precipitation collectors for amount
and isotope ratio measurements; install collectors in El Yunque cloud forest;
sample water isotopes along a transect from the Caribbean coast of Puerto Rico to
El Yunque; collect first vegetation, soil, and stream water samples; instrument El
Yunque canopy with portable sensors and dataloggers to monitor variations in
temperature, relative humidity, and light associated with cloud inundation cycles;
analyze water samples for deuterium and oxygen-18 content at UPRM-Geology
facility; obtain and analyze processed cloud data fields from Terra, Aqua, and
TRMM
Year 1 (Education). Develop short course on cloud forests and climate
change at UCSB
Year 2 (Research). Continue collection of fog and precipitation samples;
conduct intensive field campaigns in dry and wet seasons to sample vegetation
and soil water; sample water isotopes along a transect from the Caribbean coast of
Puerto Rico to El Yunque; sample stream water (1-2 times); analyze water
samples for deuterium and oxygen-18 content at UPRM-Geology facility; obtain
and analyze processed cloud data fields from Terra, Aqua, and TRMM
Year 2 (Education). Meet with UPRM collaborators to adapt short course
for UPRM, including development of a Spanish language version
Year 3 (Research). Continue collection of fog and precipitation samples;
conduct fewer intensive field campaigns in dry and wet seasons to sample
vegetation and soil water; sample stream water (1-2 times); analyze water samples
for deuterium and oxygen-18 content at UPRM-Geology facility; obtain and
analyze processed cloud data fields from Terra, Aqua, and TRMM
Year 3 (Education). Work with UPRM collaborators and El Portal
Tropical Science Center to develop a small display on cloud forests and climate
change in the Caribbean basin
References (to come!)