Award Date

5-1-2024

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Life Sciences

First Committee Member

Matthew Petrie

Second Committee Member

Scott Abella

Third Committee Member

Paul Schulte

Fourth Committee Member

Marie-Odile Fortier

Number of Pages

118

Abstract

Juvenile and adult conifers in the southwestern United States are vulnerable to a range of different abiotic and biotic factors, yet it is unclear how their stress tolerance varies across environments. Predicted increases in the frequency and intensity of disturbance as well as greater climate and weather variability across the region highlight the potential for differential physiological responses across tree species and populations. This thesis focuses on two physiologic mechanisms, hydraulic vulnerability and mortality, that could be altered by climate change in two broadly distributed southwestern United States conifers: ponderosa pine and pinyon pine.

In chapter 2, I measured the hydraulic vulnerability of juvenile (~20-30 years old) ponderosa pines (Pinus ponderosa) in six climatically-varying locations (northern and central Arizona, southern and northern New Mexico, southern Colorado, and southern Nevada) of the southwestern United States (SWUS). I evaluated hydraulic vulnerability using the p50 value of stem conduits. P50 is the pressure (MPa) at which 50% of xylem conductance is lost, and lower p50 values correspond to lower tree vulnerability to soil moisture limitation. I then contrasted the climate and environmental conditions of our sampling sites from 1990-2020 using site characterization data, gridded meteorological estimates (DayMet), soil estimates (POLARIS), and soil water balance modeling (SOILWAT2). My objectives were to determine if hydraulic vulnerability varied between juvenile ponderosa pines growing in different subregional locations, and to determine if climate and environmental variables were associated with these differences. Juvenile ponderosa pines in southern Nevada had more negative p50 values (-5.05 ± 1.16 MPa) than juveniles in southern Colorado (-4.26 ± 0.79 MPa), southern New Mexico (-3.72 ± 0.86 MPa), and central Arizona (-4.06 ± 0.96 MPa; ANOVA, p < 0.05). Compared to all other SWUS locations, southern Nevada also had significantly more severe warm season (April-September) meteorological moisture deficit, lower cool season (October-March) and warm season soil water potential [ψp: MPa], and lower soil water holding capacity (p < 0.05). Thus, lower p, imparted by low warm season precipitation and physical soil characteristics, was associated with higher stress tolerance found in juvenile ponderosa pines growing in southern Nevada. Although future research is required to determine the reason for their higher stress tolerance – which could include acclimation, adaptation, and/or environmental control – my results provide new evidence for physiological differences at early stages of ponderosa pine development in the SWUS.

In chapter 3, I used the Cox Proportional Hazards (PH) model to evaluate background mortality hazard for adult pinyon pines in three climatically-varying locations (Colorado, Arizona, and Nevada) of the southwestern United States (SWUS). Life status (living/dead) and tree size were recorded for Pinus edulis (Colorado pinyon; Colorado, Arizona) and Pinus monophylla (single-leaf pinyon; Nevada) over multiple 10-year periods from 2001-2019 (USFS FIA). I then contrasted the climate and environmental conditions of each location using gridded meteorological estimates (DayMet) and soil simulations (POLARIS). My objectives were to determine if background mortality varied between adult pinyon pines growing in different sub-regional locations over a 10-year period, and to analyze associated variables and their potential time-dependent effects for P. edulis and P. monophylla. 10-year background mortality ranged from 4-6% across locations and was lower than previously reported for P. monophylla. Adult P. edulis in Arizona had the greatest cumulative hazard and lowest overall survival probability over a 10-year period. Based on my results, factors influencing background mortality varied between study locations. The probability of mortality significantly decreased with greater cool and warm season precipitation, % soil organic matter (30-100 cm), and % soil silt content (100-200 cm; p < 0.05). The probability of mortality significantly increased with greater tree size (height and diameter), soil pH (30-100 cm), and cool season maximum air temperature (p < 0.05). My results show that the mechanisms driving background mortality in adult pinyon pines differ between and among species occupying different climate regimes, and provide further evidence that greater tree size increases the probability of mortality pinyon pine.

Keywords

climate; hydraulic vulnerability; mortality; pinyon pine; ponderosa pine; southwest

Disciplines

Environmental Sciences | Medical Physiology | Physiology | Plant Sciences | Terrestrial and Aquatic Ecology

File Format

pdf

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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Available for download on Thursday, May 15, 2025


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