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Impact of Drought Severity and Topography on Tropical Forests of Costa Rica
Kaiya Weatherby1,2, Gang Zhao2, Huilin Gao2, Georgianne Moore3, Kelly Brumbelow2
1Department of Earth and Environment, Boston University, Boston, MA 022152Zachry Department of Civil and Environmental Engineering, Texas A&M University, College Station, TX 77845
3Department of Ecosystem Science and Management, Texas A&M University, College Station, TX 77845
Acknowledgements:• Funding for this Research Experiences for Undergraduate program is provided by the National Science
Foundation’s Division of Earth Sciences (EAR-1659848).
• Eugenio Gonzalez – Director at Texas A&M Soltis Center
Introduction
• Understanding tropical rainforest responses to extreme climate events is
critical given the amount of ecosystem services these forests provide for
their flora and fauna as well as humans.
• However, these response mechanisms are still not very well understood,
especially when it comes to tropical montane rainforests.
• Characterized with large variations of vegetation properties, precipitation
amount, and topographical gradients, Costa Rica serves as an ideal test
bed for evaluating the impacts of drought conditions on tropical
rainforests.
Fig 1. Costa Rica trends in various parameters. a) Time series of the monthly average PDSI (Palmer Drought
Severity Index) values for Costa Rica. b) map of Costa Rican forest types c) 50-year average precipitation
map using TerraClimate data. d) 20-year average EVI (Enhanced Vegetation Index) map using MODIS data.
e) Elevation map using SRTM Digital Elevation data.
c) d)
Data & Methods• Various remotely sensed satellite and meteorological datasets available on
Google Earth Engine were utilized.
• EVI (Enhanced Vegetation Index) quantifies forest greenness
• LST (Land Surface Temperature) temperature of the canopy
• ET (Evapotranspiration) evaporation plus plant transpiration data from
MODIS sensor
• PDSI (Palmer Drought Severity Index) index for drought severity based
on precipitation and temperature data from the TerraClimate dataset
• *Data from the months of January to April (dry months) were used for
analysis, with the intention of avoiding seasonal variation
Categories and Regions• The country was divided up into regions based on drought severity (PDSI)
during the drought years. Region 1 (dark gray) represents the least severe
drought, Region 2 (light gray) moderately severe drought, and Region 3
(white) most severe drought
• To account for elevational differences, elevation categories were created.
For simplicity, the categories consist of ‘High’ (black), ‘Middle’(gray), and
‘Low’ (white) elevation.
e)
Fig 2. a) Regions based on PDSI values from drought years. Region 1, dark gray, has a PDSI range of -2.5 to
-3.5; Region 2, light gray, ranges from -3.5 to -4.5; Region 3, white, is less than or equal to -4.5 b) High
elevation, black, is defined to be greater than 1200m; Middle elevation, gray, is between 300m and 1200m;
Low elevation, white, is less than or equal to 300m . c) The non-forested area of Costa Rica was excluded
for data collection/analysis
a) b) c)
Results• The PDSI yearly average clearly drops during 2012 or 2013 and does not
return to ”normal” conditions until 2017.
• EVI seems to gradually decline during these years, and increases notably in
2017.
• Comparison of trends suggest the possibility of a lagged-effect
Results
Conclusion
References• Jadin, I, et al. “International Trade, and Land Use Intensification and Spatial Reorganization
Explain Costa Rica’s Forest Transition.” Environmental Research Letters, vol. 11, no. 3,
2016, p. 035005., doi:10.1088/1748-9326/11/3/035005.
• Yang, Yan, et al. “Post-Drought Decline of the Amazon Carbon Sink.” Nature
Communications, vol. 9, no. 1, 2018, doi:10.1038/s41467-018-05668-6.
b)
Objective
The primary objective of this study is to explore the responses of Costa
Rican forests to drought conditions while taking differences in severity of
drought and elevation into account.
• Figure 4 shows EVI changes by elevation for Costa Rica as a whole, and
by drought region (as defined in the method section)
• Most cases demonstrate a gradual decline in EVI during 2013 – 2016,
followed by a resurgence in 2017
• Notably, the lowest elevation of region 3 has the smallest EVI values, and
the highest elevation has the greatest EVI values. This is most likely due
to the different vegetation types that are present in this region (further
discussed in the “Discussion” section)
Fig 3. EVI and PDSI Trends in Costa Rica
Fig 4. a) EVI time series for all forested areas in
Costa Rica by elevation. b) EVI time series for
drought region 1 by elevation. c) EVI time series for
drought region 2 by elevation. d) EVI time series for
drought region 3 by elevation.
Fig 5. Trends in LST and ET compared to EVI trend in drought region 3, lowest elevation.
• Figure 6 shows LST was poorly correlated to EVI in all elevations.
• Differences in EVI trends were seen when zooming into each drought
region and elevation category.
• While there was an overall decline in EVI for all areas, some drought-
elevation subcategories showed irregular patterns (like the example
above).
• As shown in Figure 5 and 6, EVI and ET appear to display similar
behavior in lower elevations. This correlation between the two
parameters was found to be weaker in other elevations.
Fig 6. Scatter plot showing correlation between EVI and ET
Discussion
• Due to the variation in topography and precipitation patterns, Costa Rican
forest types are diverse.
• Region 3, according to PDSI patterns, experienced the most severe
drought conditions, yet the EVI trends are among the least affected.
• This most likely has to do with the fact that region 3 includes the tropical
dry forest type (among other forest types), coupled by the fact that this
part of the country receives less rainfall on average
• It is possible that although this region experienced the most severe
drought conditions, the vegetation here may not be susceptible to such
conditions in the same way that tropical wet/moist forests are.
• As for the highest elevation EVI being highest for drought region 3, this
can be a result of having tropical dry forests in lower elevations and
having tropical wet/moist forests in higher elevations all within region 3.
• EVI trends show a gradual overall decrease for all elevations during the
years 2013-2016, when PDSI values are well below normal conditions
(possibility of a lagged-effect of drought on forest greenness)
• EVI and ET appear to be more correlated in lower elevation tropical
forests, compared to other elevations.
a)
b) c)
d)
a)
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