Date of Award

Spring 2013

Project Type

Dissertation

Program or Major

Natural Resources and Earth Systems Science

Degree Name

Doctor of Philosophy

First Advisor

Heidi Asbjornsen

Abstract

Understanding evapotranspiration (ET) variation associated with land use change is critical for assessing impacts on water resources and improving the applicability of climate models to predict water availability under future climate scenarios. The overall objective was two-fold: i) to examine ET variation associated to a land use change trajectory foreseen to increase in tropical mountains, and ii) to assess fog effects on tree transpiration (E t) and the potential implications of projected diminishing fog occurrence. An actively grazed pasture (PAS) represented the land use change baseline, and young (YREF) and mature (MREF) Pinus patula plantations represented typical reforestation efforts and subsequent local forest management. A combination of process-based measurements of near-surface climate and plant physiology were used to derive rates of E t and ET over a 1.5-year period in the seasonally dry tropical montane cloud belt of central Veracruz, Mexico. Four stand-alone articles are presented. In the first chapter, sap velocity radial profiles are studied in detail as a critical initial step in up-scaling point measurements to whole-tree sapflow and deriving reliable estimates of stand-level Et. The second chapter presents ET estimations for the three land cover types. Ranking of ET normalized by available energy (net radiation) was: PAS (0.80) = YREF (0.80) > MREF (0.42). Results suggest that in this high radiation and non-water lirriited environment, planting P. patula on former pasture uplands would not result in substantial increases in ET after >10 years of planting. Next, the Et suppression effect of fog was examined in the third chapter. Given relatively low fog frequency under current climate, if all fog occurrences are replaced by overcast conditions, the potential change in annual Et for YREF was estimated as a 2% increase; for sunny conditions, the increase was 17%. Lastly, in chapter 4, examination of nighttime Et in relation to the dynamics of fog occurrence revealed a slight increase in hourly rates during dry nights following fog events. These findings suggest that climate change-related decreases in dry-season precipitation, more than diminishing fog occurrence alone, presents a worrisome prospect for the study area due to potential reductions in soil water reserves.

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