Dave Gorchov studies plant population ecology and community ecology. One major focus of Dr. Gorchov and his graduate and undergraduate students is the interactive effects of abundant deer and invasive plants on forest plants. Dr. Gorchov’s current research projects are in Ohio; past projects were carried out in Florida, Maryland, Mexico, and Peru.
Interactions between deer and invasive shrubs
AoB PLANTS Special Issue, edited by D.L. Gorchov and B. Blossey, on Interactions between White-tailed Deer and Invasive Plants in North American Forests. 2017
In many places, wild ungulates occur at unnaturally high densities, with negative impacts on plant communities. White-tailed deer are overabundant in many parts of the United States, due to predator eradication and human modification of the landscape. High deer densities have been shown to reduce regeneration of tree seedlings and abundances of some other forest plants.
Many of the forests with overabundant deer are also invaded by non-native plant species. Invasive plants often reach high density, and in numerous cases have been shown to reduce native forest plant abundances and diversity. Collaborating with graduate and undergraduate students, I have assessed the impacts of the invasive shrub most abundant in our forests (as well as many other areas of the Midwest), Amur honeysuckle (Lonicera maackii). We also investigated the effects of the invasive biennial herb, garlic mustard (Alliaria petiolata).
AoB PLANTS Special Issue, edited by D.L. Gorchov and B. Blossey, on Interactions between White-tailed Deer and Invasive Plants in North American Forests. 2017
In many places, wild ungulates occur at unnaturally high densities, with negative impacts on plant communities. White-tailed deer are overabundant in many parts of the United States, due to predator eradication and human modification of the landscape. High deer densities have been shown to reduce regeneration of tree seedlings and abundances of some other forest plants.
Many of the forests with overabundant deer are also invaded by non-native plant species. Invasive plants often reach high density, and in numerous cases have been shown to reduce native forest plant abundances and diversity. Collaborating with graduate and undergraduate students, I have assessed the impacts of the invasive shrub most abundant in our forests (as well as many other areas of the Midwest), Amur honeysuckle (Lonicera maackii). We also investigated the effects of the invasive biennial herb, garlic mustard (Alliaria petiolata).
One of five deer exclosures installed in the Miami University Natural Areas in 2010 to study how deer and invasive shrubs impact forest plants and animals
We are interested in the interactions between invasive shrubs, particularly Amur honeysuckle, and white-tailed deer. We are investigating whether their combined effects on forest floor plants in southwest Ohio are additive or synergistic, using a set of five deer exclosures, each paired with an unfenced control plot, in the Miami University Natural Areas. While both deer and honeysuckle have negatively impacted native plants, their combined effect was not significantly greater than the effect of either alone (Peebles-Spencer et al. 2017 ). This means that restoring forests will require both removing the invasive shrubs and reducing the density of deer. Honeysuckle has broad effects on understory cover, deer primarily reduce tree seedlings and native shrubs, and actually promote annual plants by increasing bare ground cover (Haffey and Gorchov 2019).
Shrub removal should probably be done after reduction of deer density, as honeysuckle shrubs provide sugar maple seedlings with some protection from deer browse, an example of facilitation (Peebles-Spencer and Gorchov 2017). For his MS thesis, Kevin Lash investigated whether killed honeysuckle shrubs also facilitate native tree survival and growth, and whether this depends on whether the shrubs are cut or killed standing (basal bark herbicide application).
Shrub removal should probably be done after reduction of deer density, as honeysuckle shrubs provide sugar maple seedlings with some protection from deer browse, an example of facilitation (Peebles-Spencer and Gorchov 2017). For his MS thesis, Kevin Lash investigated whether killed honeysuckle shrubs also facilitate native tree survival and growth, and whether this depends on whether the shrubs are cut or killed standing (basal bark herbicide application).
We are also exploring another indirect interaction between deer and invasive shrubs, apparent competition (where two prey species reduce each other’s abundance by increasing the abundance of a shared enemy). We hypothesize that Amur honeysuckle increases deer populations, which in turn reduce preferred native plants. In the Miami University Natural Areas, where honeysuckle is abundant in the shrub layer, over 20% of its twigs are browsed by deer, comprising a large fraction of the deer diet (Martinod and Gorchov 2017). Deer browse on this shrub is particularly high in early spring, when native deciduous woody plants are leafless, but honeysuckle, with its ‘extended leaf phenology,’ has already expanded its leaves, providing higher protein content for deer (Martinod and Gorchov 2017).
We continued our investigation of whether deer seek out honeysuckle in spring by comparing browse rates in forests that vary in honeysuckle and deer density. MS student Elle Wright, along with undergrad advisee Ieva Juska, found that the proportion of honeysuckle twigs browsed by deer is high where this invasive shrub is sparse and low where it is common (Wright et al. 2019). We are interested in investigating whether high deer abundance can impede new invasions.
Roles of Disturbance vs. Propagule Pressure in Plant Invasions
An important question in invasion ecology involves the relative importance of community invasibility vs. propagule pressure (e.g. seed dispersal) in shaping where invasion occurs. Plant invasions are often associated with disturbances. We found that both tree-fall gaps and proximity to seed sources are important in the establishment of several invasive plant species in the Forest Dynamics Plot at the Smithsonian Environmental Research Center in Maryland (Driscoll et al. 2016, Emsweller et al. 2018). Brian Hoven’s PhD dissertation investigates how the death of ash trees due to Emerald Ash Borer affects growth and recruitment of Amur honeysuckle, as well as native tree seedlings, in Ohio (Hoven et al. 2017). We also investigate dispersal of seeds of invasive shrubs by birds (Bartuszevige and Gorchov 2006) and deer (Guiden et al. 2015)
Growth in basal area of Amur honeysuckle shrubs from 2012 to 2014 was greater in sites where ash trees were in poorer condition due to Emerald Ash Borer across central and southwest Ohio (Hoven et al. 2017).
Pattern and Process in Plant Invasions
Invasion ecologists are also interested in the relative importance of diffusion (expansion of existing patches) vs. long-distance dispersal (and new invasion foci). We used age structure analysis to show the importance of both in Amur honeysuckle’s invasion in southwest Ohio (Gorchov et al. 2014).
(Left) Edge of a Tradescantia fluminensis (small leaf spiderwort) invasion in the Apalachicola River floodplain, Florida. Right, at the edge of this invasion, iButton temperature sensors were placed inside fiberglass mesh and kerosene lantern mantles; the former allowed better heat loss at night, and was used to contain iButtons in documenting soil surface temperatures at the invasion front in winter 2016/17.
I also investigated abiotic factors that shape the diffusion of small-leaf spiderwort, Tradescantia fluminensis, on slopes adjacent to the floodplain of the Apalachicola River in the Florida Panhandle, a hotspot of biodiversity. (There is no seed production in the invasive populations, so long-distance dispersal is limited to fragments floating and re-rooting within the floodplain.) Temperatures below -4.2° C kill this perennial herb, but no points along the invasion front got that cold in the winter of 2016/17. Relating field temperatures on the soil surface to nearby weather stations indicates that sufficiently cold temperatures were historically common, but not after 2012. Invasion fronts further from the Apalachicola River had colder minima in 2016/17, and had more frost-induced necrosis after a cold event in January 2018 (Gorchov 2019). Hurricane Michael caused extensive treefall damage on these slopes in October 2018, providing an opportunity to investigate the effects of this disturbance on invasion.