Publication Date

2012-12-03

Availability

Embargoed

Embargo Period

2014-12-03

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Biology (Arts and Sciences)

Date of Defense

2012-11-14

First Committee Member

Leonel S. Sternberg

Second Committee Member

David P. Janos

Third Committee Member

Don L. DeAngelis

Fourth Committee Member

Eric S. Menges

Abstract

Dry seasonal ecosystems are defined by a pronounced and consistent dry season, which reduces water availability in the dry season. Plants must adapt to water limitation in the dry season and high water availability during the wet season. In addition, the soil is nutrient-poor with the majority of nutrients concentrated in shallow soil. Considering the temporal drying of these layers, nutrient availability is dependent on the climatic conditions that govern water availability in the shallow soil. In this dissertation I address differences in fine root density, water source use, and nutrient uptake between deciduous and evergreen species in a dry seasonal ecosystem. My five study species are the deciduous Carya floridana and Quercus laevis and the evergreen Lyonia ferruginea, Q. geminata, and Q. myrtifolia. Chapter 2 compares the change in fine root density from dry to wet season (fine root turnover) and the distribution of fine root density with soil depth within dry and wet seasons. I tested the hypotheses that 1) root density would increase from dry to wet season and 2) that the increase in root density from dry to wet season would be smaller in deciduous species than in evergreen species. Fine root density decreased in all species and all root orders, except in the fourth order of C. floridana. High water availability appears to be the cue for high fine root production. Fine root turnover from dry to wet season was lowest in the deciduous C. floridana, possibly because aboveground dormancy reduced transpiration, which increased the amount of water available for shallow roots during the dry season. Chapter Three compares water uptake during the year between deciduous and evergreen species using stable isotopes. I examined the hypothesis that evergreen and deciduous woody species take up shallow soil water in the wet season, but when shallow soil water is unavailable evergreen species switch to deep soil water, while deciduous species remain dormant. Evergreen and deciduous species used the same water sources through the year. During the dry season, four of the five species took up water based on the distribution of available water, while during the onset of the wet season and wet season water source use was based on fine root distribution. During the dry season Lyonia ferruginea took up more shallow soil water than expected if uptake was based on the distribution of available water. Deep water (50-150cm) was the most important water source during the dry season and an essential water source throughout the year. Chapter Four compares PO43-, NH4+, and NO3- availability in the top 150cm of the soil profile using ion-exchange resins. I hypothesized that phosphorus and nitrogen availability would be highest in the shallow soil. This was true for PO43-, but NH4+ and NO3- did not differ with depth. Nitrogen uptake by evergreen and deciduous species was measured by using 15N as a tracer of nitrogen uptake. I hypothesized that nitrogen uptake rate would be highest among deciduous species because of the nitrogen demands of new leaf growth. During the late dry season 15N uptake rate was highest among evergreen species. Low N uptake during leaf-out may mean that deciduous species cannot take up sufficient N to meet the demands of leaf growth. This temporal uncoupling of nutrient demand and uptake may be why deciduous species are more common in fertile sites. From this research, it appears that the most important driver controlling rooting depth and distribution was water availability, while root density was controlled by nutrient availability.

Keywords

deciduous; evergreen; nutrient uptake and availability; water source use; fine root turnover

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