Date

2017

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Felzer, Benjamin S.

Other advisers/committee members

Booth, Robert K.; Yu, Zicheng

Abstract

While the individual effects of temperature and soil moisture on individual plants are well explored, the associated effects on forest ecosystems are more complex. Gross primary productivity (GPP), ecosystem respiration (ER) and net ecosystem productivity (NEP) are the major components of carbon flux between the land and atmosphere. I correlate summer temperature and drought indices with GPP, ER and NEP from eddy covariance forest sites across the U.S. It is not always true that GPP and ER are correlated with temperature positively and with drought negatively as expected. The combinations of correlation signs between temperature, drought and either GPP or ER can be: a) negative, negative; b) negative, positive; c) positive, negative; and d) positive, positive. An explanatory model is developed to interpret these combinations. These combinations imply a) soil moisture limited; b) temperature surpasses the optimal; c) expected response; and d) temperature dominated, respectively. Forest ecosystems in the contiguous US occur in three climate regions, which is the primary factor determining the category of their response. For humid continental climates, ER responds to temperature and drought as expected; and GPP responds to temperature and long-term drought as expected, while GPP is temperature dominated with short-term drought. For Mediterranean climates, ER responds to temperature and short-term drought as expected, while ER is temperature dominated with long-term drought; GPP is limited by short-term droughts. For humid subtropical climates, ER is temperature dominated with both long-term and short-term droughts and GPP with short-term drought, while GPP responds to temperature and long-term drought as expected. Changes in NEP are the result of changes in GPP and ER. GPP is the dominant flux for the monthly variation in NEP within the growing season. The inter-annual change in NEP from one growing season to the next is dominated by ER in Dfb and Csb climates, and by GPP in Cfa climates.

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