title: Seasonal Climate and Environmental Conditions Associated with Physiological Variations of Southwestern United States Pines
Creators: name: Brewer, Taylor
Publisher: name: University of Nevada, Las Vegas
publisherIdentifier: https://ror.org/0406gha72
publisherIdentifierScheme: ROR
Year: 2024

description: 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.</p><p>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 [<em>ψ<sub>p</sub></em>: 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.</p><p>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 <em>Pinus edulis</em> (Colorado pinyon; Colorado, Arizona) and <em>Pinus monophylla</em> (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 <em>P. edulis</em> and <em>P. monophylla</em>. 10-year background mortality ranged from 4-6% across locations and was lower than previously reported for <em>P. monophylla</em>. Adult <em>P. edulis</em> 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.