Abstract
Reforestation following timber harvests and natural disturbances is an essential component of sustainable forest management. As disturbances such as drought-induced mortality and wildland fires spread across many forests of the western U.S., a better understanding of the influences of stand structure on seedling physiology can foster more effective reforestation efforts. Moreover, as climate throughout the West is projected to become hotter and drier, it is important to investigate regeneration under xeric conditions, particularly for species restricted to mesic habitats. To study the influences of stand structure and climate on regeneration success, we monitored physiology (water potential [Ψ] and stomatal conductance [gs]), growth (change in basal diameter and biomass accumulation), and mortality rate of planted Douglas-fir (Pseudotsuga menziesii) and coast redwood (Sequoia sempervirens) seedlings for two growing seasons after partial harvesting in inland northern California. Compared to seedlings in the no-cut and moderate-retention treatments, seedlings in the gap (100% cut) treatment had the highest Ψ and gs, greatest growth, and lowest mortality. We also found that compared to Douglas-fir, redwood had higher Ψ and gs, greater growth, and lower mortality. Overall, our study indicates that low-retention silvicultural treatments can minimize water stress and maximize gas exchange, growth, and survival in regenerating seedlings. Our results also demonstrate that redwood, a species generally restricted to mesic coastal habitats, can successfully establish in xeric inland sites when planted after partial harvesting, even during drought conditions.
Highlights
Global climate change is forecast to have widespread impacts on forested ecosystems (Jia et al, 2019)
Using, gs, basal diameter growth, biomass accumulation, and mortality to evaluate seedling success in interior northern California during drought, we investigated the following questions and corresponding hypotheses: (1) How does thinning treatment influence physiology, growth, and mortality? We hypothesized that seedlings in moderate treatments would be the most vigorous in these metrics due to an optimal increase in light availability buffered with partial shading from the residual overstory to minimize excessive evapotranspirational water losses
There was no difference in light availability between 2015 and 2016 in plot centers (X2 = 3.5, df = 1, p = 0.06); data from both years were pooled for a better estimate of monthly light levels
Summary
Global climate change is forecast to have widespread impacts on forested ecosystems (Jia et al, 2019). Forest management to foster productive, healthy, resistant, and resilient forests in the Seedling Physiology and Growth face of increasing drought frequency and severity is an important and widespread goal across many land ownerships (Keenan, 2015; Newton and Preest, 1988; Millar and Stephenson, 2015). Following drought-induced tree mortality in a climate predicted to continue warming and drying for decades (Allen et al, 2010, 2015), the successful establishment and regeneration of former forestlands is dubious, as water stress can threaten successful seedling establishment in Mediterranean climates with hot, dry summers (Plamboeck et al, 2008). Seedling growth rates during establishment can strongly influence later growth rates in older trees (Newton and Preest, 1988), such that the establishment phase of a seedling’s life can have lasting impacts on forest productivity
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