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

Forest persistence amid shifting climate conditions requires sufficient tree recruitment for sustaining populations. In dryland ecosystems where precipitation can be episodic and variable, changes in precipitation pulse regimes may be critically important for seedling establishment, as some populations may be locally adapted to variable resource availability. 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 endpoints of range-wide gradients of climatic water deficit and summer precipitation. Seedlings 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. Seedlings demonstrated substantial trait differences among seed sources but few differences among water pulse treatments. Multivariate analysis of seedling traits suggested that seed source climate was related to two trade-offs in resource-use strategies: (i) acquisitive leaf traits versus seedling biomass and (ii) water use efficiency versus nitrogen use efficiency. A better understanding of intraspecific trait variation (ITV) can facilitate predictions of drought and temperature responses and help to identify suitable seed sources for P. monophylla restoration. While observed levels of ITV provide indirect evidence of local adaptation, this variation was not related to different precipitation pulse responses. However, the observed relationship between seed source climate and patterns of ITV suggests that local resource-use strategies underlying successful tree regeneration may constrain responses to a changing climate.