Population-Level Differences in Pinus monophylla Whole-Plant Seedling Strategies Under Varying Precipitation Pulses
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.Seedlings demonstrated varied and substantial differences in morphological and physiological traits between seed sources, but few differences between treatments. The observed pattern of intraspecific trait variation obtained from principal component analysis (PCA) suggests that tradeoffs among resource-use strategies vary by seed source. A first PCA axis illustrated tradeoffs between acquisitive leaf traits and whole seedling structure that were related to climatic water deficit at the seed source. The second axis showed a gradient between maximizing water use efficiency and maximizing nitrogen use efficiency. The third axis demonstrated a tradeoff between root growth and stem diameter growth. The observed intraspecific trait variation provides indirect evidence of local adaptation, yet the observed trait variation was not related to precipitation pulse responses. Regardless, P. monophylla demonstrates morphological and physiological intraspecific trait variation that varies between populations and along climate gradients reflecting seasonal water availability. As such, future studies should further explore local adaptation using molecular approaches and investigate possible genotype by environment interactions. Knowing whether intraspecific trait variation is facilitated by phenotypic plasticity or genetic drivers can help to identify ideal seed sources for P. monophylla regeneration and restoration. As climate change shifts the adaptive landscape, phenotypic plasticity and gene flow may allow for unassisted regeneration. However, strong local adaptation could mean that some populations may be pre-adapted for projected climate change while others may be mismatched.
Common garden, Intraspecific trait variation, Pinus monophylla, Precipitation pulses, Resource-use strategies, Tree regeneration
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