WHY MONITOR THE BIOMETEOROLOGY OF TROPICAL DRY FORESTS?
Figure B1: Tropical Dry Forest distribution in central and south America (left) and annual rainfall across the Neotropics (right) demonstrates the close relationship between water availability and these semi-arid tropical ecosystems. Sources: Mahli and Phillips, 2005; WorldClim, 2004.
Current UN Global Forest Resource Assessment estimates that 10,000 ha of forested land worldwide is lost each day considering even the net gain in reforestation (FAO, 2012), and vanishing with them are their valuable ecosystem services from food production, carbon sequestration, water storage, and other climate buffering functions. Tropical forests and woody savannas account for approximately 50% of global forest cover and are responsible for an estimated 60% of terrestrial photosynthesis (Mahli and Grace, 2000). Seasonally Dry Tropical Forests are the most productive of all semi-arid ecosystem supporting high rates of species endemism and often more diverse mammal communities than rain forests (Quesada et al., 2009) yet only 4.5% of these important, yet often overlooked, ecosystems are currently protected in the Americas. Understanding the bio-meteorological dynamics of secondary TDFs and how they may respond to climate change is important for biophysical, geochemical, and energy cycling models in earth system sciences as well as the food and water security of local communities for years to come (Dixon et al, 1994; Sanchez-Azofeifa et al., 2003).
Phenology refers to the timing of life-history events and vegetative phenology pertains to the cyclic patterns of growth and senescence of deciduous foliage. Plant community phenology is a widely studied phenomenon due to the dependence of landscape productivity on phytomass (Fenner, 1998). Plant physiology and growth processes are very responsive to micrometeorological conditions; as a result, changes in plant phenology can act as important bio-indicators of climate change and other environmental stress. Numerous climate models and recent meteorological observations strongly support predictions for changing continental precipitation patterns with increases drought risk and subsequent changes in terrestrial primary productivity (Melillo et al. 1993, Ponce Campos et. al, 2013) . As tropical dry forests are water limited and highly responsive to moisture regimes, these changes in water availability should produce observable transitions in TDF growing season length and long-term sustainability, however, the extent of TDF resilience to water stress is not well understood over longer periods.
Phenology refers to the timing of life-history events and vegetative phenology pertains to the cyclic patterns of growth and senescence of deciduous foliage. Plant community phenology is a widely studied phenomenon due to the dependence of landscape productivity on phytomass (Fenner, 1998). Plant physiology and growth processes are very responsive to micrometeorological conditions; as a result, changes in plant phenology can act as important bio-indicators of climate change and other environmental stress. Numerous climate models and recent meteorological observations strongly support predictions for changing continental precipitation patterns with increases drought risk and subsequent changes in terrestrial primary productivity (Melillo et al. 1993, Ponce Campos et. al, 2013) . As tropical dry forests are water limited and highly responsive to moisture regimes, these changes in water availability should produce observable transitions in TDF growing season length and long-term sustainability, however, the extent of TDF resilience to water stress is not well understood over longer periods.
PHENOLOGY TRENDS IN NEOTROPICAL DECIDUOUS FORESTS
Figure B2: Study Sites. Looking at 25 years of growing season and dry season lengths as detected from MODIS and AVHRR satellite sensors indicate that tropical dry forest phenology has been changing to different extents, in different regions of the Americas. The dry season has been getting shorter on average in northern pacific and Yucatan Mexico, parts of Venezuela, Columbia, Bolivia, Paraguay and northern Argentina while the dry season length has been increasing in central pacific mexico and across a large region of eastern Brazil. Trends are based on start of season and end of season parameters as derived from the Savitsky-Golay time series smoothing using the TimeSat software package (Jonsson & Eklundh, 2004) and significant trend analysis is detected using the Mann-Kendall non-parametric time-series test (IAI Science SnapShot 2010). Changes in growing season length as derived from vegetation indices are known to reflect changes in above ground net primary productivity (Ponce Campos, 2013). Near-surface validation of these remote sensing observation are needed to support these trends and provide insights to the potential causes of such landscape-scale transitions in TDFs. Two sites of potentially decreasing primary productivity are highlighted on the map, one a continental dry forest in Brazil and the other a coastal dry forest in Mexico. These two TDF sites located at 15° north and south of the equator have the same ecosystem and general climate but very different local weather patterns due to the large difference in proximity to the ocean. Little is known about the meteorological triggers that control phenology in tropical deciduous forests across a latitudinal gradient and how they might differ. While most conservation efforts for TDFs focus of habitat loss and fragmentation there is a serious lack of knowledge as to how climate change may impact the long term sustainability of these semi-arid ecosystems.
RESEARCH OBJECTIVES
Using ground-based monitoring platforms to validate remote sensing observations i want to better characterize the seasonal phenology of Tropical Dry Forest leaf expression with respect to the local meteorological factors that control water availability. Do all TDFs in the Americas respond the same way to water availability? Does atmospheric water vapor influence canopy stress to the same extent as soil moisture in TDFs?
More specifically:
Using on canopy greenness as a proxy for leaf expression and canopy productivity, how do spectral greenness indices of a continental and coastal tropical dry forest canopy relate to air temperature, relative humidity, atmospheric vapor pressures, and soil moisture throughout the entire growing season and during different stages of leaf development?
Since TDFs are deciduous in response to seasonal drought, we might expect that water availability will be highly influential for when TDF develop and maintain leaves. Furthermore, within the growing season we would expect to changes in vegetative greenness due to water stress in the canopy as a result of high temperatures or low vapor pressures. How these relationships might vary between the two study sites n Mexico and Brazil are difficult to predict and warrant further investigation so that modelling efforts for climate change impacts on TDFs have solid evidence for differential ecophsiological responses of these ecosystems based on their local geography and current bio-meteorological signatures.
More specifically:
Using on canopy greenness as a proxy for leaf expression and canopy productivity, how do spectral greenness indices of a continental and coastal tropical dry forest canopy relate to air temperature, relative humidity, atmospheric vapor pressures, and soil moisture throughout the entire growing season and during different stages of leaf development?
Since TDFs are deciduous in response to seasonal drought, we might expect that water availability will be highly influential for when TDF develop and maintain leaves. Furthermore, within the growing season we would expect to changes in vegetative greenness due to water stress in the canopy as a result of high temperatures or low vapor pressures. How these relationships might vary between the two study sites n Mexico and Brazil are difficult to predict and warrant further investigation so that modelling efforts for climate change impacts on TDFs have solid evidence for differential ecophsiological responses of these ecosystems based on their local geography and current bio-meteorological signatures.
REFERENCES
Dixon, R.K., S. Brown, R.A. Houghton, A.M. Solomon, M.C. Trexler, and J. Wisniewski. 1994. Carbon pools and flux of global forest ecosystems. Science, 263: 185-190.
Fenner, M. 1998. The phenology of growth and reproduction in plants. Perspectives in Plant Ecology, Evolution and Systematics, 1(1): 78-91.
Jonsson, P. and Eklundh, L. 2004. TIMESAT - a program for analysing time-series of satellite sensor data, Computers and Geosciences 30: 833-845.
Mahli, Y, and J Grace. 2000. Tropical forests and atmospheric carbon dioxide. TREE 15: 332-337.
Melillo, J. M., A. D. McGuire, D. W. Kicklighter, B. Moore III, C. J. Vorosmarty, and A. L. Schloss. 1993. Global climate change and terrestrial net primary production. Nature 363: 234–240.
Ponce Campos et. al. 2013. Ecosystem resilience despite large-scale altered hydroclimatic conditions. Nature, 494: 349-352.
Quesada, M, G.A. Sanchez-Azofeifa, M. Alvarez-Anorve, K.E. Stoner, L. Avila-Cabadilla, J. Calvo-Alvarado, A. Castillo, M.M. Espirito-Santo, M. Fagundes, G.W. Fernandes, J. Gamon, M. Lopezaraiza-Mikel, D. Lawrence, L.P. Morellato, J.S. Powers, F.S. Neves, V. Rosas-Guerrero, R. Sayago, and G. Sanchez-Montoya. 2009. Succession and management of tropical dry forests in the Americas: Review and new perspectives. Forest Ecology and Management, 258: 1014-1024.
Sanchez-Azofeifa, G.A., K.L. Castro, B. Rivard, M.R. Kalacska, and R.C. Harriss. 2003. Remote sensing research priorities in tropical dry forest environments. Biotropica, 35: 134-142.
Fenner, M. 1998. The phenology of growth and reproduction in plants. Perspectives in Plant Ecology, Evolution and Systematics, 1(1): 78-91.
Jonsson, P. and Eklundh, L. 2004. TIMESAT - a program for analysing time-series of satellite sensor data, Computers and Geosciences 30: 833-845.
Mahli, Y, and J Grace. 2000. Tropical forests and atmospheric carbon dioxide. TREE 15: 332-337.
Melillo, J. M., A. D. McGuire, D. W. Kicklighter, B. Moore III, C. J. Vorosmarty, and A. L. Schloss. 1993. Global climate change and terrestrial net primary production. Nature 363: 234–240.
Ponce Campos et. al. 2013. Ecosystem resilience despite large-scale altered hydroclimatic conditions. Nature, 494: 349-352.
Quesada, M, G.A. Sanchez-Azofeifa, M. Alvarez-Anorve, K.E. Stoner, L. Avila-Cabadilla, J. Calvo-Alvarado, A. Castillo, M.M. Espirito-Santo, M. Fagundes, G.W. Fernandes, J. Gamon, M. Lopezaraiza-Mikel, D. Lawrence, L.P. Morellato, J.S. Powers, F.S. Neves, V. Rosas-Guerrero, R. Sayago, and G. Sanchez-Montoya. 2009. Succession and management of tropical dry forests in the Americas: Review and new perspectives. Forest Ecology and Management, 258: 1014-1024.
Sanchez-Azofeifa, G.A., K.L. Castro, B. Rivard, M.R. Kalacska, and R.C. Harriss. 2003. Remote sensing research priorities in tropical dry forest environments. Biotropica, 35: 134-142.