Date of Award

Summer 8-17-2018

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Shawn Fraver

Second Committee Member

Aaron Weiskittel

Third Committee Member

David Hollinger

Abstract

Individual tree growth and mortality drive forest stand dynamics and are important, universal metrics of tree success. Studying the factors that affect growth and mortality is particularly challenging in mixed-species, uneven-aged systems due to their defining heterogeneity and strong temporal and spatial variability. However a better understanding of the factors driving growth and mortality in mixed-species, uneven-aged forest is crucial to managing and maintaining these valuable systems for the future.

The goal of this study was to determine the relative importance of individual tree attributes (e.g., species, size, neighborhood crowding, crown position) and environmental characteristics (e.g., soil moisture) in driving growth and survival. In particular we aimed to test if the factors regulating growth were the same as those regulating mortality, as is often assumed. Due to its large size and intensive sampling, the 3-ha, stem-mapped plot (established in 1989) at Howland Research Forest in central Maine allowed us to address additional relevant questions regarding the influence of sapling crowding, neighbor species identity, and past disturbance.

Growth and survival of over 3000 plot trees was assessed after 25 years and modeled using multiple linear regression (growth) and binary logistic regression (survival). We found that species, neighborhood crowding, and diameter, in that order, were the top predictors of both growth and survival. Growth, but not survival, was inhibited by soil moisture, especially in poorly-drained portions of the plot. Growth was also inhibited for individuals that grew in neighborhoods with more conspecific neighbors, which likely have more similar resource requirements when compared to those of heterospecific neighbors. In individual species growth analysis, we found that not all species are equivalent competitors, namely white pine (Pinus strobus) was more competitive, and red maple (Acer rubrum) was less competitive than would be expected if all species were equivalent competitors. Unexpectedly, we found that individuals with greater crowding from sapling neighbors were more likely to survive. For both growth and survival, we found a significant interaction between crowding and soil moisture, suggesting that within a single stand, individual success can be limited by both excess and insufficient water, depending on the crowding neighborhood. We also found that the growth of larger trees was enhanced when they were surrounded by more cut stumps, implying that the effects of a 100-year-old disturbance were surprisingly persistent.

These results demonstrate the broad range of variables driving growth and survival in uneven-aged mixed-species forests, as well as the benefit of differentiating between metrics of success when assessing stands and individuals. Given the importance of uneven-aged, mixed-species forests in storing and sequestering carbon, maintaining biodiversity, and providing resistance and resilience to an uncertain future, we suggest studies such as this that address a full range of interacting drivers of success are necessary to better manage and maintain these complex systems.

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