Date of Award

Summer 8-20-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Forest Resources

Advisor

Jay Wason

Second Committee Member

Yongjiang Zhang

Third Committee Member

Nicholas Fisichelli

Abstract

Climate change is expected to lead to novel drought conditions in the Northeastern United States. Therefore, experimental studies that mimic these conditions are crucial to understand the potential impact on forests. Further, recent large scale dendrochronological studies suggest that spring and summer droughts may immediately impact tree growth while fall droughts may cause delayed impacts on growth the following growing season. Therefore, in this study, we investigated the impacts of six-week-long spring, summer, and fall droughts on the physiology and intra-annual growth on 288 saplings of six tree species native to the Northeastern United States. These species (deciduous broadleaf angiosperms, hereafter “broadleaf”: Acer rubrum L., Betula papyrifera Marsh., Prunus serotina Ehrh.; and coniferous evergreen gymnosperms, hereafter “conifer”: Juniperus virginiana L., Pinus strobus L., and Thuja occidentalis L.) represent different anticipated drought tolerances and projected abundances with climate change according to previous studies. Additionally, we used experimental dry-downs of seventy-one leafy shoots and seventeen xylem segments to assess how structural and physiological adaptations of each species relate to water use during an extreme drought.

We observed marked differences in how the growth patterns of these six species responded to seasonal droughts. Spring and summer droughts generally caused height growth rate reductions for all species. Negative impacts on height growth were stronger for trees that had higher water-use and therefore experienced drought sooner. Importantly, some species such as A. rubrum, Pr. serotina, and T. occidentalis were able to compensate for these height growth reductions during spring and summer droughts with more rapid post-drought height growth. We also found that spring and summer droughts for Pr. serotina, Pi. strobus, and T. occidentalis resulted in reductions in diameter growth rates but only post-drought. Interestingly, these three species were not able to compensate for this decrease in diameter growth, which remained low throughout the rest of the growing season. These high-resolution data on intra-annual growth rates of trees in response to seasonal droughts reveal details about the growth phenology that supports and extends our understanding of annual resolution tree ring studies at larger scales.

In the benchtop dry-down experiment that simulated an extreme drought, we found that leafy shoots of conifer species dried more slowly than leafy shoots of broadleaf species. In general, conifer species lost water at equal rates between leaves and stems. In contrast, deciduous species lost water very quickly and experienced larger reductions in leaf water content compared to stem water content. We saw evidence of drought-deciduousness in our greenhouse experiment where B. papyrifera was the fastest to dry-down, and in two instances, its cambia remained hydrated enough to re-flush an additional cohort of leaves post-drought. On the other hand, conifers were slow to dry-down in the greenhouse experiment, only experiencing moderate drought by the end of each drought period. The clear division in response between fast-drying broadleaved deciduous angiosperm species and slow-drying needle-leaved evergreen conifers may be partly driven by lower leaf area per shoot of conifer species, which we observed in the simulated extreme drought experiment.

In the mixed wood forests common in the northeastern United States, stands may respond to drought in different ways depending on the species present. For example, we observed very different responses to growth and to some extent, recovery, in our species, and in a stand with species showing different responses, competitive dynamics may be altered among these species as the climate continues to change. Acknowledging that tree responses to drought as individuals and as communities may not align will be important moving forward from studies like these, which define drought responses of individual species, to studies which observe drought responses of entire forests.

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