Graduation Date

Summer 2020

Document Type



Master of Science degree with a major in Environmental Systems, option Geology

Committee Chair Name

Dr. Jasper Oshun

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Dr. Lucy Kerhoulas

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Dr. Andrew Stubblefield

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Dr. Bandon Browne

Fourth Committee Member Affiliation

HSU Faculty or Staff

Subject Categories

Environmental Systems


Millions of people live in or depend on ecoregions dominated by Tropical Dry Forests (TDFs), but due to their high accessibility, convenient topography and mild climate conditions their distribution is fragmented with less than 10% of their original extent remaining in many countries. Despite the vast ecosystem services provided by TDFs, including vital water resources in water limited environments, ecohydrological research in this biome has been limited to a small number of short-term investigations. Similar to worldwide trends, the TDF surrounding Bahía de Caráquez (Bahía), Ecuador, has been severely deforested over the past 400 years. The land use history in Bahía, which has resulted in TDFs of different age and disturbance, provides a valuable setting to study the relationship between forest age and TDF hydrology. This thesis, conducted in the Cordillera del Balsamo (a local landowner-managed bio-corridor of protected TDF) presents the results of one year of frequent monitoring to explore subsurface moisture dynamics and species-specific water use strategies across TDFs of different ages. We 1) captured snapshots of changes in subsurface water content with direct measurements of shallow subsurface moisture and measurements of predawn and midday plant water potential, and 2) identified seasonal patterns in tree water use by analyzing the stable isotope composition of bulk soil and bulk saprolite moisture, rainwater, groundwater, and tree xylem water. Our results over the transition from wet season to dry season in 2018 show that moisture is held at greater tensions, and thus is likely less available to trees in disturbed TDFs (old) than in old growth TDFs. We found there was insufficient seasonally dynamic moisture storage in the top meter to sustain expected rates of TDF transpiration, and that trees relied on moisture stored in weathered bedrock. The results of our stable isotope monitoring uncovered age-specific differences in tree water source and confirmed our hypothesis that younger trees growing in disturbed, secondary forests (old) must rely on deeper water sources below the soil to maintain physiological function into the dry period. In the secondary forest, the combined results of subsurface moisture data, predawn and midday water potentials, and stable isotopes allowed us to interpret three distinct water use strategies in three native TDF species: (1) the deeply rooted Ceiba trichistandra accesses deep sources of water held at relatively low water potentials through the dry season; (2) Sideroxylon celastrinum accesses shallow water in the early dry season and deep water in the late dry season via a dimorphic rooting system; and (3) Tabebuia chrysantha achieves the lowest midday water potentials and accesses soil and saprolite moisture late into the dry season, but also accesses deeper sources of moisture either directly or indirectly via hydraulic lift of neighboring trees. The results presented here may inform site specific or regional studies that quantify the effects of land use history on transpiration, subsurface water storage, groundwater recharge, and forest water yield in order to guide forest regeneration while achieving water security for human communities.

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