Population-level differences in Pinus monophylla whole-plant seedling strategies under varying precipitation pulse regimes

Authors: ADKINS, JEREMY - University Of Georgia; URZA, ALEXANDRA - Us Forest Service (FS); Snyder, Keirith; WEISBERG, PETER - University Of Nevada

Submitted to: AoB Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2026
Publication Date: 5/29/2026
Citation: Adkins, J.S., Urza, A.K., Snyder, K.A., Weisberg, P.J. 2026. Population-level differences in Pinus monophylla whole-plant seedling strategies under varying precipitation pulse regimes. AoB Plants. 18(3). Article plag025. https://doi.org/10.1093/aobpla/plag025.
DOI: https://doi.org/10.1093/aobpla/plag025

Interpretive Summary: As climate change exacerbates forest mortality, subsequent tree regeneration depends on the presence of functional traits that allow seedlings to survive selective pressures from altered climate regimes. Many widely-distributed plant species exhibit intraspecific trait variation reflecting adaptation to local environments. Populations may vary in traits that regulate seedling growth and resource acquisition in response to resource pulse regimes, and some adaptations to historic conditions may become maladaptive under projected climate scenarios. Singleleaf pinyon pine (Pinus monophylla) is a dominant tree species in the Intermountain West, USA, that demonstrates intraspecific trait variation along multiple climatic gradients, most notably climatic water deficit and amount of summer precipitation. Climatic water deficit varies considerably across P. monophylla's range, increasing from north to south and decreasing with increased elevation. Along a gradient of precipitation seasonality, northwestern populations receive most precipitation in the winter and those in the southeast receive more in the summer. Anthropogenic global climate change continues to alter aridity and precipitation regimes which may require plants to change how they respond to the environment. Previous studies found trait variation organized along a summer precipitation gradient, but it is unknown whether this is in response to precipitation seasonality or to the pulsed nature of summer rainfall. Of particular interest is how seedlings from different seed sources will utilize water under altered precipitation regimes with varying pulse magnitudes and frequencies. We conducted a greenhouse common garden experiment to explore possible local adaptations associated with P. monophylla seedling responses to varying watering pulse regimes. We used seeds from four lower treeline sites representing different combinations of climatic water deficit and summer precipitation amounts. After the seedlings were established, they underwent a 56-day treatment period where they received the same amount of total water, distributed under three different pulse patterns (weekly small magnitude, bi-weekly intermediate, and monthly large magnitude). We compared morphological and physiological traits before and after treatment to evaluate overall responses to the pulsed watering regimes. We then evaluated the immediate pulse response by measuring photosynthesis and leaf moisture directly after a final watering pulse.

Technical Abstract: 1. Forest resilience to climate change-driven drought mortality necessitates sufficient tree recruitment for population recovery. Future tree recruitment depends upon the functional traits of individual tree seedlings in response to changing selective pressures, such as warming temperatures or increasing drought stress. Particularly in dryland ecosystems where precipitation can be episodic and variable, changes in precipitation pulse regimes may prove critically important for seedling establishment. Many dryland plant species balance resource acquisition strategies during pulse events with resource conservation strategies during intervening periods. Local adaptation to a specific precipitation pulse regime could be beneficial under a stable climate regime, but could become maladaptive under projected climate change scenarios. 2. We conducted a greenhouse common garden experiment to evaluate Pinus monophylla seedling responses to varying watering pulse regimes. We used seeds from four sites representing different combinations of high vs. low annual climatic water deficit and summer precipitation. Seedlings underwent a 56-day treatment period where they received the same amount of total water, but the water was applied in three different pulse patterns. We compared morphological and physiological traits in response to the three watering regimes, exploring whether responses were consistent with possible local adaptations. 3. Seedlings demonstrated substantial trait differences between seed sources, but few differences between water pulse treatments. The observed pattern of intraspecific trait variation suggested that tradeoffs among resource-use strategies are related to seed source climate. These tradeoffs can be summarized as 1) acquisitive leaf traits versus whole seedling biomass allocation, 2) water use efficiency versus nitrogen use efficiency, and 3) root growth versus stem diameter growth. 4. A better understanding of the mechanisms determining intraspecific trait variation can facilitate predictions of climate change responses and help to identify suitable seed sources for P. monophylla restoration. Strong local adaptation could mean that some populations may be pre-adapted for projected climate change while others may be maladapted. While our observed intraspecific trait variation provides indirect evidence of local adaptation, it was not related to different precipitation pulse responses.