IF A TREE FALLS: PLANT REGENERATION AND SOIL TRENDS IN A PENNSYLVANIA TREE TIP-UP CHRONOSEQUENCE
Open Access
- Author:
- Dillner, Benjamin Mcgowan
- Graduate Program:
- Ecology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 09, 2019
- Committee Members:
- Jason Philip Kaye, Thesis Advisor/Co-Advisor
Margot Wilkinson Kaye, Committee Member
Timothy Stapler White, Committee Member - Keywords:
- forest disturbance
ecology
tree tip-up
botany
critical zone
tree regeneration
soil formation - Abstract:
- Tree tip-up is a dominant form of natural disturbance in Pennsylvania’s forests. Tree tip-up occurs when high winds topple trees, ripping out the root system and creating a pit and mound. These pit and mound microsites may have fundamentally different vegetative and soil characteristics than the surrounding intact forest. In ecology, understanding these differences is key to predicting forest species change and diversity. For soil science, tip-ups may help explain spatial variability in soil horizon development and nutrient cycling rates. While research has largely focused on tip-ups at a single time point, knowledge of changes in tip-up plant and soil characteristics over multiple decades is limited. The goals of this thesis project were threefold: 1) compare plant communities of different aged tip-ups and undisturbed forest floor; 2) determine whether soil and light trends underlie vegetation patterns; 3) examine soil formation on pits & mounds and implications for the shale bedrock study forest. To answer questions about tip-ups over time, we established a chronosequence of three trunk decay classes with ages of 1 year (new), 5 to 25 years (mid-age), and 30 to 50 years (old). We conducted surveys of tree seedlings, tree saplings and low stature (non-overstory) plants on tip-ups and control sites during the summer of 2018. We also evaluated abiotic characteristics such as sunlight, nutrient, and water availability as well as soil profile cores. Distinct diversity trends emerged for tree seedlings and low stature plants. Tree seedling communities increased in diversity (Shannon index) over time on the tip-ups, while low stature plants showed declining diversity. Certain understory forest plants, particularly those adapted to full sun, grew preferentially on new tip-up microsites (which had significantly higher sun exposure than older tip-ups). Ten low stature taxa (out of fifty-four total) were only observed on tip-up microsites. Moss reached a mean cover of 50% on older mounds, which was much higher than on control sites. Unlike findings from other studies, tip-up pits and mounds do not appear to facilitate regeneration of canopy trees; tree seedling and sapling counts were not significantly higher on tip-up sites than the controls. In recent tip-ups, plant nutrients such as nitrogen and phosphorus showed increased availability, but this may not necessarily be a boon for plants since water availability was low compared to the control soils. We observed that pits accumulated thick organic horizons from leaf litter over time, while mounds remained dry and organic matter impoverished. Even after 30 to 50 years, mounds had soil profiles that were distinct from controls; mounds of this age lacked high soil organic matter enrichment in the top 5cm and had remnant O horizon at ~30cm depth. Despite only turning over 0.02% of the soil surface each year, we propose that tip-ups should be incorporated into models of the study forest because they have a disproportionate effect on plant communities, and mounds persist for hundreds of years.