Abstract
Juvenile tree survival will increasingly shape the persistence of ponderosa pine forests in the western United States. In contrast to severe pulse disturbances that induce widespread adult and juvenile tree mortality, moderate periods of low rainfall and warm temperatures may reduce forest persistence by killing juvenile trees at the seedling stage. Intensification of these periods in a changing climate could therefore increasingly restrict both natural regeneration and artificial regeneration of planted seedlings. We conducted a controlled field experiment at a single site in the Front Range of Colorado, USA, to determine the responses and survival of 3 Colorado subpopulations of <1 year old potted ponderosa pines to moderately dry conditions, variation in small rainfall events based on observed patterns, and shaded and unshaded microsite environments. Near surface soil moisture increased slightly following small rainfall events, but declined over the 45-day experimental period. Seedling transpiration and associated canopy cooling declined after ∼13 days, and further declines in transpiration and canopy cooling suggest that the majority of trees in lower rainfall treatments experienced hydraulic dysfunction between days ∼20–30. After 45 days, mortality across all subpopulations and treatments, inferred by relative water loss, exceeded 90–95%. Despite some uncertainty pertaining to the stress tolerance of nursery grown versus naturally germinated conifers, our results show that planted ponderosa pine seedlings <1 year old are unlikely to survive moderate dry periods of 20+ days relying on small rainfall events. Although microsite conditions and soil moisture availability shaped tree hydraulic functioning early in the experiment (days 1–13), later functioning was shaped predominately by the legacy of rainfall treatments. Our results illustrate the importance of moderate dry events that occur consistently as part of seasonal variation in climate, and show how their intensification may constitute a sustained press that limits opportunities for natural and artificial regeneration.
Highlights
Introduction published maps and institutional affilThe ability of ponderosa pine forests to regenerate in undisturbed and post-disturbance landscapes will increasingly dictate their persistence in the western United States
Increasing temperatures and water limitation associated with climate change are expected to increase juvenile tree mortality [1,2], and climate change is expected to intensify disturbances including drought and wildfire [3,4,5]
Our study shows that even short duration declines in soil moisture availability will quickly initiate hydraulic stress in nursery-grown and planted conifer seedlings, even when microclimate conditions are much less severe than those found in post-wildfire landscapes
Summary
The ability of ponderosa pine forests to regenerate in undisturbed and post-disturbance landscapes will increasingly dictate their persistence in the western United States. Regeneration declines have been observed throughout the western US [4,6], and both natural regeneration and human-assisted artificial regeneration are often unsuccessful [5,7]. Severe climate and disturbance events impose environmental conditions that seedlings (
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