Hedgerows have a checkered reputation. From England comes the image of hedgerows and windbreaks as bastions of botanical diversity. Surely a truism once one has removed most of the forest. From the Midwest comes the demonizing of hedgerows as the eyries of raptors who fall upon hapless grassland birds. This is perhaps most true when hedgerows are the only trees in sight. The role of hedgerows, in our landscape, where forest abounds, is probably more subtle.
When we discuss “hedgerows”, we also mean windbreaks or fencerows – basically any stretch of woody vegetation bordered on either side by grass and/or brush. This can include riparian woods along the banks of a stream that winds through agricultural land. At what width a stretch goes from being a hedgerow to being a patch of woods is arbitrary and depends upon which wood-like attribute one chooses to focus on. For some small insects, a 6-foot wide strip of trees may feel sylvan indeed. For a large buck, such a slim portion would seem poor forest grounds.
As the Brit Oliver Rackham has outlined in his book The History of the Countryside, hedgerows can arise through several different routes:
1) as relicts – the last standing remains of what was once a forest blanket.
2) as spontaneous incidentals – the woody “weeds” that happen to grow up along walls and fences as those areas escape the repeated clearing occurring on the fields that they border.
3) as plantings – shrubs or trees intentionally planted as living fences or windbreaks.
Mixed origins are possible, but these scenarios help one think about what hedgerows can represent ecologically. Relict fencerows are probably the rarest in our part of the country. For example, judging by historical images, most of our Hawthorne Valley's hedgerows probably grew in of their own accord along fences and walls – certainly, 19th century landscape images are neat and trim (Fig. 1) and the 1948 aerial photo of Hawthorne Valley show thinner fencerows than today (Fig. 2). Hedgerow planting is probably most common as a form of domestic gardening rather than farming. There may have been sporadic agricultural attempts at ‘live fences’ (i.e., fences constructed by planting rows of certain spiny trees or bushes), but none seemed to be widely successful in the County. Multiflora Rose, initially introduced for live fencing, has taken up hedgerow building of its own accord. It is one of the first species to appear along new fences or field margins. However, it is generally too invasive for farmers to willfully want to encourage it; and it has recently been hit by Rose Rosette Virus.
Several years ago (2004), we spent late autumn and early winter mapping the woody vegetation of many Hawthorne Valley hedgerows. Our goal was to better understand which plants make their home in such areas and to gain a better idea of how these hedgerows may have arisen and evolved. While only one small case study, there's no reason to believe that our hedgerows are particularly unique, and so we describe their botanical patterns here.
The diversity of native woody plants was highest where hedgerows abutted forest and lowest in the center of the Farm (see Fig.3). Two collaborating factors probably resulted in this pattern. On the one hand, because these are mostly spontaneous hedgerows (“planted” by birds, squirrels and the whims of wind blown seeds), they are most diverse nearest the natural source seeds, i.e., the forests. At the same time, the more centrally-located stretches are probably the ones most heavily influenced by farming activity such as grazing cows, which may well partially control their growth.
We have divided the fencerow species into five different ecological groups based upon their distribution patterns and their means of seed dispersal. The most abundant fencerow species are the Super-Colonizers. These are species which have many small, bird-dispersed seeds; which are thorny (and thus deter browsing) and fast-growing; and which prosper in full sunlight. The archetypical species in this group are the Multiflora Rose and the various brambles (Blackberry, Raspberry and their ilk). These species were found in almost all hedgerows, although, because of their sun-loving nature, they probably become less common in those fencerows with taller, more forest-like woody vegetation. See Fig.4.
Next in apparent abundance are the Browse-sensitive Colonizers. These are species which share the love of sunlight and ease of seed dispersal characteristic of the first class, but who are unarmed. Their smooth stems do little to deter browsing. Exemplars of this group are the arrowwoods and the dogwoods, both native taxa. While they range widely in the hedgerows, they are largely absent from the most intensively-used central stretches. See Fig.4.
The Weedy Trees are also fairly widespread. They produce fleshy fruits and have bird-dispersed seeds, or they have light, wind-dispersed seeds. They are apparently dispersed widely, and they are eager to grow in sunlit spaces. Being slower growing, and perhaps more delectable to browsers, they are somewhat rarer than the earlier classes. Typical of this group are Black Cherry, hawthorns, apples, and American Elm. At least with Black Cherry and the Elm, one begins to see hints of greater abundance near forested areas. See Fig.5.
Bringing up the rear are the Adventurous Forest Trees. These species tend to have heavier seeds; some are still wind-dispersed, others distributed by mammals and gravity. They are likely browse-sensitive. Representatives of this group include Red Maple, White Ash, Red Oak, and the hickories. Here the pattern of greater abundance near forests is readily apparent. See Fig.6.
Finally, there are a few species whose distributions may be more affected by soil conditions; for example, the lovers of moist soil such as willows, Red-osier Dogwood, Speckled Alder, and Spiraea species.
That our hedgerow species can be rather neatly categorized into these groups provides, together with historical research, strong evidence that our hedgerows evolved spontaneously as fencerows went uncleared.
Our results also support an interesting supposition more strongly developed by other researchers: the idea that those woody plants growing along fences may differ from those growing along overgrown stonewalls. Picture for a moment a wire fence and a stone wall. Likely as not, a bird will fly into your image of the first and a chipmunk scurry into your vision of the latter. Think then about what these animals eat, and you will quickly realize how plants such as Black Cherry, Multiflora Rose, and Viburnum species may quickly arrive below fences, and how oaks and hickories may rapidly colonize stonewalls.
In our case, the pattern may be somewhat confused by the fact that most of our “stonewalls” are probably more accurately described as long stone heaps, piles that field-clearing farmers created as they threw stones beneath wooden fences that were subsequently replaced by wire. Thus, both bird and rodent have likely visited our stone wall “fence lines”. However, the hedgerows along simple barbed wire do show an abundance of bird-dispersed species such as cherries, brambles and roses.
So what? What role do hedgerows play in on-farm conservation? We have already alluded to their probable value as buffers to riparian areas. But what value do they have in their own right for native plants and animals? While few of the woody plants that we found in hedgerows were unusual, the growth of a few native woodland shrubs, such as beaked hazel and nannyberry, seemed particularly exuberant in certain hedgerows. Winterberry and flowering dogwood, two woody species of conservation interest, were found in hedgerows. Hedgerows also happened to be the only place where we found Carrion Flower, Smooth Sumac, and Common Elderberry at Hawthorne Valley, although these are not generally considered unusual species. Hawthorns are a particularly interesting edge/hedgerow farm species. This native group has surprising diversity - Claudia and a botanical colleague, David Werier, identified at least 10 hawthorn species at Hawthorne Valley Farm. We did not systematically survey the herbaceous plants of hedgerows.
Researchers in Quebec looked at the role of hedgerows as reservoirs of weeds and avian pests and as sources of native plant biodiversity. Studying both woody and herbaceous plants, these workers found that the lowest weed density was in natural, woody fencerows, as opposed to planted hedgerows or mainly herbaceous ones. This work implied that, from the perspective of avoiding a weed refuge, rather than periodically cutting back hedgerows, they should be allowed to develop into wooded margins. Such hedgerows also were home to a higher number of native plants of “conservation interest”. No evidence was found that hedgerows in their area were home to high numbers of crop-damaging birds (see notes below).
We did tally Groundhog holes along our fencerows. In slightly over 3 km of fencerows, we found the entrances to roughly 140 Groundhog holes, with the highest densities in the hedgerows around the vegetable gardens. Because Groundhogs may dig numerous burrow entrances and because probably not all of these holes were active, this is not an estimate of Groundhog numbers (thank goodness), but it does indicate patterns of occurrence. Some holes were found at a distance into the neighboring fields, and whether the Groundhogs were looking for hedgerows per se or simply for less-utilized land near the gardens was not clear. While removing hedgerows might make Groundhog control easier in and of itself, it might not reduce their populations.
Probably the strongest ecological reason for maintaining hedgerows, aside from their afore-mentioned buffering ability along streams, is as wildlife corridors and homes to shrubland birds. While we are still assembling our winter tracking data, we have followed mink, fisher and bobcat through (mainly riparian) hedgerows of Hawthorne Valley Farm. Such wooded links provide corridors by which forest animals can easily pass between woodland patches. Game camera work suggested that, while travel through hedgerows may be cumbersome for larger animals, such creatures may move parallel to hedgerows, using them as ready refuge. We did record around 50 bird nests in about 3 km of hedgerows (Fig.7). Although we were not able to positively identify which birds made which nests, our bird watching has told us which species frequented these habitats. Please see our 2005 Farm Biodiversity report for information on the birds.
Hedgerows are also sometimes considered in relation to agricultural productivity. Their effects on windspeed and evapotranspiration apparently result in a slight net positive influence on crops, at least in large Midwestern and Australian agricultural settings. Their importance for agriculture at the scale practiced in Columbia County has been less thoroughly researched. The swirling air associated with our small fields and hilly terrain, coupled with a climate that perhaps does not see the growing season extremes of more continental areas, may reduce their microclimatic benefits. Indeed, shading from tall hedgerows may reduce field productivity. Their agricultural influence through biotic effects is probably mixed. On the one hand, as mentioned, they do harbour weeds and certain invertebrate and vertebrate pests that can plague crops; on the other hand, they are also home to native pollinators and beneficial birds. Greater research is needed to clarify all these effects. However, given the general abundance of forest in the region and the often relatively small field sizes, it may be hard to document any strong effects on production.
The Quebec work mentioned has been carried out by Céline Boutin, Benoît Jobin and their colleagues. Three of their most relevant papers are the following: Jobin et al. (2001), “Bird use of three types of field margins in relation to intensive agriculture in Québec, Canada.” in Agriculture, Ecosystems and Environment 84: 131–143; Boutin et al. (2001) “Comparing weed composition in natural and planted hedgerows and in herbaceous field margins adjacent to crop fields” in Can. J. Plant Sci. 81: 313–324; and Boutin et al. (2002) “Plant diversity in three types of hedgerows adjacent to cropfields.” in Biodiversity and Conservation 11: 1–25, 2002.