What is land management?
John Tirpak, previous Mianus River Gorge (MRG) land manager and current US Fish and Wildlife scientist, said, “Land management is to ecology as engineering is to physics.” Where physics attempts to understand the very nature of matter and energy, the engineer uses this understanding to build, create, and innovate, designing new devices or constructing better materials. Ecology is the study of the interaction of living creatures with one another and the physical world they inhabit. The land manager takes what ecologists have discovered and uses this knowledge to bring about some desired result, such as protecting biodiversity, endangered species, and maintaining ecosystem services.
Land management is a tricky thing. First, it relies on ecology, a young science (relative to chemistry and physics) with a lot of unanswered questions. Ecology is also a science of immensely complicated interactions operating on many spatial (from microscopic to global) and temporal scales (from hourly to evolutionary). You can’t look at an ecological phenomenon (or problem) in a vacuum: ecology is interaction!
Why manage the Gorge?
One strategy for land management is a “hands-off-policy,” e.g., letting nature do what it will do, free of human interference. For the first 50 years of our history, this has been the predominant policy of the MRG (and many other institutions). We protected our forests through gifts of land, direct purchase, and conservation easements, and limited human use of the Preserve to hiking in order to better protect the old-growth forest, and its many denizens. This philosophy was one of seeing humans as external from the natural world and that the best way to protect “nature” was to remove man from the equation. We prohibited hunting, extraction of forest resources, did not intervene as exotic plants became established or as native plants declined in abundance.
In reality, removing man from the equation is not possible. Even if we limit use of the Gorge to hiking, the ecology of the Mianus River Gorge is shaped by a multitude of human-related activities we can’t control. The last 50 years have seen an explosion in urban development around NYC (more pressure on the MRG watershed), the rise of globalization (more exotic species introduced), and global climate change (changes in temperature and altered water and nutrient cycles). Colonists and early Americans cleared much of our forests for pasture which altered soil characteristics. Even hundreds of years later these changes play a role in how the forest is currently growing back and functioning. We are learning more about sophisticated land management strategies of the Native Americans, such as prescribed burns which encouraged oak trees to produce more acorns. The land management strategy of doing nothing simply allows everything else humans do to shape the Gorge in an uncontrolled manner. Humans are part of nature – for better or worse.
Forests of the Mianus River Gorge Preserve
There are two major types of forest at the Preserve, the old-growth hemlock forest and the young, post-agricultural hardwood forests. Old-growth forests are unique elements of the ecosystem. These “virgin” forests have well-developed soils that take hundreds and thousands of years to fully develop. Such soils support unique fungal communities which in turn support rare wildflowers like orchids. We have lost nearly 90% of old-growth forest in the US since colonization making every remnant stand very special and worthy of the highest protection.
When you walk along the Preserve’s blue trail, you enter a young hardwood forest on our hemlock edge. It’s a pleasant woodland crisscrossed with colonial stonewalls and well-shaded with deciduous trees. What may not be apparent is that fact that it is a forest highly disturbed by human activity. These woodlands – similar to most forests in Westchester County – began growing back as farms were abandoned 70-100 years ago. They are dominated by maples, black locust, and cherry trees of even age. There is little ground cover and poor regeneration owing to heavy deer browse. Soils have often been tilled or have their nutrient cycles disturbed by centuries of livestock use or other agricultural activities. Earthworms (which are all non-native!) are in high abundance as are non-native flora such as barberry, stilt grass, and garlic mustard. Communities of fungi – many which are needed by trees such as oaks, hemlocks, and birch – have been damaged by human tilling and earthworms.
Mast trees such as oaks do not seem to do as well in these forests. Seed banks for many native wildflowers are largely depleted. Left on their own, these forests appear to be on a trajectory dominated by non-native species. Deer exclosures erected in these forests do not show the rapid recovery documented in old-growth forest.
Despite their impacted state, these young hardwood forests buffer our old-growth core from housing developments, roads, and other urban stressors. Left unmanaged, these buffer forests will continue to be a staging ground for non-native flora establishment and threaten the long-term health of our core forests. Through our research-based education programs, the Gorge is working to restore these forests and increase the diversity of the hardwood buffer through several management strategies.
To learn more about the threats facing our hemlock forest and our efforts to mitigate these stresses click on the following links:
Hemlock Woolly Adelgid
The biggest threat to the Preserve old-growth forest is the dual interaction of deer over-browsing (see Deer Management) and forest pathogens. The primary pest of hemlock is the hemlock wooly adelgid (HWA, Adelges tsugae), a Japanese insect that lives off the sap of new growth. HWA is currently found from northern Georgia to southern Maine and can take as little as 10 years to kill an entire hemlock forest. The Preserve has had HWA since 1986 and in 2007 we found that over 60% of our trees were infected. Interestingly and fortunately, we still have many adult, old hemlocks despite this pervasive and persistent infestation. Our trees have been weakened but have, uniquely, survived for decades.
In 2000, to protect our mature trees, the Gorge introduced, a Japanese lady beetle, Sasajiscymnus tsugae, as a biocontrol agent , to feed on HWA. The beetle is notoriously hard to monitor as it immediately disperses high into the canopy to upon release. Through our high school Wildlife Technician Program (WTP) and with the help of the Connecticut Agricultural Experimental Station, we initiated a study in 2007 to find the beetle using canopy nets, but have yet to find any.
We still do not know why we still have large hemlocks despite over 25 years of infestation. S. tsugae may be active high in the canopy keeping HWA in check. It is also possible that winters in the Gorge are cold enough to kill a substantial number of dormant HWA. It is also possible that the discovery of a still unidentified “black” fungus found growing on another hemlock pest, hemlock elongate scale, is protecting our trees from the 1-2 punch that have killed so many others.
Too many deer is a common problem to many suburban areas of the northeast. And like most of our environmental problems, it is ultimately a human-caused problem. Deer were nearly exterminated in NYS and CT by the end of the 19th century. Active reintroduction programs, new hunting regulations, the reforestation of NYS in the 20th century, and the extirpation of large of wolves and cougars led to the rapid recovery of white-tailed deer. These same forces have allowed the pendulum to swing the other way with deer densities now exceeding what is sustainable for forest regeneration in many areas.
Suburbanization was a major contribution to this phenomenon. Hunting is much less a cultural pass-time among suburbanites than among rural folk. Second, with every shrub they plant and every dose of fertilizer they add, humans subsidize the food that deer would normally only find in forests and fields. As deer densities rise and food becomes sparse in the forest, deer rely on suburban lawns for a meal.
Deer are traditionally browsers – they feed on woody shoots and buds. But, as their populations have grown, the true breadth of the deer’s diet has become known. Deer will consume many, many different species of trees and wildflowers. And with such a broad diet, deer become ecosystem engineers: They can reshape their environment with very broad impacts on other creatures living the forest.
The Gorge was lucky to have pre-eminent scientists inventory their flora and fauna in early in our history. One of these early studies was by Dr. Gile Bard of CUNY. She conducted an inventory of our old-growth forest and found no immediate threat to the sustainability of our hemlocks. In 2004, we recreated her study and found that the seedling density declined by 85% and saplings of 12 tree species could not be found in the understory. Exclosure (fenced areas that keep out deer) studies confirmed that deer were a major factor in declines in forest flora. We documented a dramatic increase in wildflower abundance in as little as 3 years.
Because of the threat that a deer herd unchecked by predation poises for the regeneration of our woodlands, the MRG initiated the first deer management plan in Westchester County. Our program was not done hastily, but was tied to good research and a commitment to scientifically evaluating our deer management program. To that end, our Director of Research and Land Management, Mark Weckel, is completing his doctorate investigating the efficacy of suburban deer management. In addition, the Gorge is committed to investigating and advocating for our newest predator, the eastern coyote and the role it may play in controlling deer abundance.
Non-native flora can also be called alien, invasive, or exotic, each having a slightly different meaning. In effect, non-native plants are those that have established themselves in area from outside its native range. They move around with people as a result of immigration, migration, and trade. In their new homes, they can have a multitude of impacts from very little to very big (e.g. invasive). They can put tremendous pressure on native biodiversity and out-competing natives for habitat. They can have wide-ranging impacts in the ecosystem as they form distinct plant communities, represent new food sources, and alter nutrient cycles.
Dealing with non-native flora must be done on a case-by-case basis. Strategies can be mechanical (using human or machine power to physically remove plants), chemical (using pesticide), biotic (using other species as control agents), or physical (using fire).
The Gorge’s goal for dealing with non-native plants is often not-eradication, but control. This is especially important in deer-ravaged environments where in some cases, non-native flora are less preferred deer browse. This has allowed them to grow and thrive over the past 2 decades. At the Preserve, we strive to keep exotics in check so as to not numerically overwhelm our weakened, native communities of flora especially as they begin to recover.