Trajectory of forest vegetation under contrasting stressors over a 26-year period, at Morristown National Historical Park: Focused condition assessment report

Epiphan, Jean; Handel, Steven

doi:10.36967/2297281

The Jockey Hollow section and the New Jersey Brigade Area of Morristown National Historical Park (MORR) are predominantly comprised of upland oak-hickory forests that have regrown over the past 200 years from previous land uses. The forest is being damaged by two major stressors, a large population of white-tailed deer and an abundance of non-native, invasive shrubs and herbaceous species. This study explores changes to the forest over 26 years and suggests management techniques to avoid future degradation. The forest is typical of many upland stands in the region, and studies here would be applicable to many lands controlled by the National Park Service and to many public and private land owners. In 1995, 18 vegetation experimental plots were established in the forest, each 20 x 20 m. Ten plots were in areas that had no non-native, invasive plants. The other eight plots had invasive species. All trees, shrubs and a sampling of herbs were recorded in each of the 18 plots. At that time, no GPS technology was available and handwritten maps were used to record locations. The plots were revisited and resurveyed in 2001; however, only 13 plots were found. This 2021 study is a new survey of the plot conditions. The investigators were able to relocate 17 of the original plots. New GPS locations were recorded for these 17 plots to facilitate future studies. The goal of the study was to test if changes over 26 years in forest conditions differed between the original invaded plots as compared to the uninvaded plots. Also, these data will allow us to measure the progress of invasion into previously uninvaded areas. Together, these results will allow the forest managers to focus attention on the most aggressive plant invaders and to understand the fate of this forest type that is being challenged by deer and non-native plants. Over the last 26 years there has been no hunting for deer here. Also, the plots were not within the few deer exclosures at MORR; deer were able to enter the landscape from surrounding heavily wooded areas. Data were collected in four layers of vegetation – mature trees, saplings, shrubs, and herbaceous groundcovers. The mature trees in the invaded forest plots demonstrated declining trends. The species richness declined by 6%, the average number of trees declined by 30%, white ash and flowering dogwood had the most losses, and basal area did not increase over time because very few new saplings grew into mature tree sizes. The uninvaded plots’ mature trees also revealed a 20% declined in richness, number of trees declined by 18%, the greatest losses occurred for red maple and black birch, but basal area increased slightly due to growth of large persisting trees. Saplings in the invaded forest experienced declines over the 26 years. Species richness declined by 38%, number of native saplings decreased by 44%, and number of invasive saplings increased by 600%. In the uninvaded forest, the conditions and trends were variable. Richness decreased by 21%, no invasive saplings found, number of native saplings increased by 37% (due to increases in American beech). With American beech excluded, the number of native saplings decreased by 60%. In both forest types, the declining number of native saplings was primarily caused by excessive deer damage. For the shrub layer in invaded plots, Japanese barberry stems increased by 122% by 2001 and 276% by 2021. Barberry became the dominant species. Similarly, wineberry stands increased 486% in 2001 and 157% for 2021. It is now the second most common species. However, in the uninvaded plots there was no significant increase in the number of barberry stems and wineberry was not present in 1995 or 2001, and only averaged 1.5 stems per plot in 2021. Neither species has a significant presence now and eradication is possible. A major finding is that the process of invasion of these shrubs over 26 years is very slow. For the herbaceous plants, in the invaded plots there was a sharp decrease in cover by 2021 due to the competitive impact by the abundant invasive shrubs. The invasive Japanese stiltgrass declined 86% in cover and native Carex (sedge) species declined by 78%. In the uninvaded forest plots, stiltgrass was present in very low amounts and did not increase significantly over 26 years. The number of quadrats with any stiltgrass only increased from 3 to 5 over the 26 years. These data show that stiltgrass invades slowly in the uninvaded plots, but in the invaded plots it was greatly replaced by invasive shrubs. Overall, the rate of change of the native herb cover was slow. Together, these data suggest that currently uninvaded forest areas can be protected by monitoring and rigorously eliminating any initial invasions by non-native shrubs and herbs. Once these species reach a significant presence in number of stems or cover in a plot they explode in number. Early intervention when invasive plants first appear is the most practical management tool. Reduction of deer density will also increase the reproductive potential and sustainability of our main forest tree species. Planting of new young canopy tree species, with protection from deer, can retard the ability of invasive plants to begin new populations.

Trajectory of forest vegetation under contrasting stressors over a 26-year period, at Morristown National Historical Park: Focused condition assessment report

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