One goal of this research was to characterize shoot tissue-level responses in loblolly pine to soil moisture limitation in combination with fertilization as well as to more severe soil moisture limitation. We found that neither fertilization alone, nor fertilization in combination with soil moisture limitation resulted in changes to shoot tissue water relations parameters classically characterized in drought response studies. More severe water limitation was necessary to elicit responses, and those responses had not been fully described previously. The more severe water limitation resulted in increased capacitance beyond turgor loss, increased relative water content at turgor loss, a more negative turgor loss point, an increased bulk modulus of elasticity, more negative osmotic potential at 100% relative water content, and an increased apoplastic water fraction. As there were indications of reduced water use and moisture stress in the absence of shoot level responses under less severe drought, such parameters are insufficient alone to characterize moisture stress in fertilized and in less severely water limited loblolly trees. Additionally, we sought a morphological or physiological explanation for the reduced transpiration and increased water use efficiency reported for fertilized trees in the Virginia Piedmont. Our characterizations of the responses of root distribution and hydraulics to limited soil moisture here complement existing research, which demonstrated changes to root distribution and hydraulics in response to fertilization. The responses we discovered in fertilized trees that accompanied reduced transpiration and increased water use efficiency that differed from responses to reduced soil moisture alone were primarily large decreases to shallow root presence. We found this to be readily quantified using measures of root length density. Decreases to whole-tree hydraulic conductivity were also shown to occur with fertilization and were shown not to occur in shoot tissue, suggesting limitation via rhizosphere or root xylem conductance. Our results support the supposition that fertilization narrows hydraulic safety margins and potentially predisposes loblolly trees to moisture stress, particularly prolonged, severe water limitation following fertilization. Finally, we tested the validity of throughfall exclusion for simulating reduced rainfall using a greenhouse 'split-pot' study, which applied spatially fixed heterogeneous soil moisture to young, well-watered loblolly pines. The 'split-pot' experiments demonstrated that spatially fixed soil moisture heterogeneity does not confound drought effects; needle area specific transpiration was not decreased, nor was water use efficiency increased. This supports the validity of inferences taken from drought simulation experiments with loblolly pine where throughfall exclusion troughs reduce soil moisture content in a consistent, spatially heterogeneous manner. / Doctor of Philosophy / We investigated various effects of soil moisture limitation alone, and in combination with common fertilization practices in loblolly pine production. Responses at the shoot and needle level to different levels of soil moisture limitation produced new findings concerning how tissues respond to more severe water limitation. A 30% decrease in throughfall precipitation alone, or in combination with fertilization did not elicit drought related shoot tissue responses despite the presence of other indications of moisture stress and reduced water use. We also sought to explain why fertilized trees experiencing water limitation had environmental sensitivities that were different from unfertilized tree receiving ambient rainfall amounts or from trees only experiencing water limitation without fertilization. We found that changes to shallow root presence, especially root length density, accompanied the different patterns of environmental sensitivity and water use. Also, the water conducting ability of roots changed unevenly in soil with uneven moisture levels. The ability of roots to resist loss of conductivity to water did not change unevenly in the same way. We did another set of experiments to determine if using impervious troughs to catch rain is a valid approach to reducing soil moisture for the purpose of testing how loblolly responds to water limitation. These throughfall exclusion troughs create uneven soil moisture reduction, which can have effects on plant water use that are separate from water limitation alone. We found that in well-watered young trees, uneven soil moisture alone did not produce responses that could be confused with the effects of water limitation. This finding indirectly validates the use of throughfall exclusion troughs to simulate reduced rainfall.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/102387 |
| Date | 27 August 2019 |
| Creators | Russell, Edward Morgan |
| Contributors | Forest Resources and Environmental Conservation, Seiler, John R., Thomas, R. Quinn, Nilsen, Erik T., Maier, Christopher A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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