At Home in the Forest: How Landscapes Form

We often take for granted why certain trees may grow in a given region of the Unites States, but others do not. Yet, have you ever pondered the complexities of how landscapes form? Of which environmental conditions give rise to various tree species? Of how conditions like drought or flooding might affect which trees might thrive, and which might perish? In the following article from Landscaping with Native Trees, you’ll learn how a variety of variables like temperature, microclimate, soil conditions, competition, and allelopathy affect the health and well-being of trees across the landscape! We have found this book to be such a valuable research, that we wanted to share this information with our clients and blog readers. Read on to learn more…

A Delicate Balance

Nature is a system of overlapping parts, with edges that are constantly changing. Each piece or species follows certain strategies for success in a competitive world, where the occupation of every space is contested by myriad organisms. Over time, the competing flora sort themselves into a predictable assemblage for specific soil and climatic conditions.

Superior trees and other organism equally suited to a particular set of soil, temperature, moisture, light and air quality conditions form a natural community that can be found wherever those conditions prevail. The balance of species can be so finely tuned that subtle changes in growing conditions result in different species dominating in different areas or in some species being entirely replaced by others.

Climate & Temperature Conditions

The minimum winter temperature in your area will dictate which trees are hardy there. Some southern species drop out of the picture at the first hard frost. Others are eliminated progressively as one travels northward, in a pattern reflected by the zones shown on the USDA Hardiness Zone Map. Many species are able to protect their sap from freezing at temperatures well below those that turn water to ice. This process, and the trees that use it, will fail precisely at 40 degrees below zero, which is the northern limit of USDA zone 3. Other trees tolerate even greater cold, but many will wither in the heat of a southern summer or be unprepared for a late killing frost in spring.

Local conditions, or the microclimate, can explain a tree’s ability to survive in certain portions of your site but not in others. This selective survival is increased by factors such as protection from drying winds, soil-temperature moderation on north-facing slopes (or under mulch) and shade from the hot afternoon sun. It can be adversely affected by summer heat that builds up in urban areas with their expanses of pavement and concrete and by the cumulative impact of our vehicles, our pets and our daily activities. The genetic pattern of an individual tree also plays a part, with trees of local origin frequently having an edge over trees of the same species from a distant area.

Some species that are native to one area may succeed in adjacent areas where only some of the conditions are the same, as long as their most prominent limiting factors – drought, perhaps, or deep shade – remain unchanged. Such species are often geological generalists, capable of surviving reasonably well under varied conditions. Other species will specialize, becoming perfectly adapted to a narrow habitat at the expense of their adaptability to other niches. These specialists are indicator species. Their occurrence on your site is strong evidence of certain specific growing conditions. Such specialists are often found on marginal sites where their competitors cannot thrive, but some will do fine under cultivation on more productive land, if competition is controlled.

Soil Conditions & Competition

Many trees prefer acidic soil; others grow in limestone areas. Some require ample moisture, while others can compete under drier conditions. Either type might be tolerant, or intolerant, of flooding or of the poor drainage that accompanies tight, compact soils. Some trees resist damage from fire, wind, ice, insects or ocean salt spray better than others. Some will take charge on ideal growing site that offer everything in moderation but will fail under less than perfect conditions. And some, called climax species, will succeed in displacing others over the long term as local conditions are changed by the growth of the pioneer trees.

Such changes can be caused by the shade or leaf litter cast by the pioneers, which can suppress weedy herbaceous plants or reduce evaporation from the soil. Other changes occur with the establishment of forest-inhabiting mycorrhizal fungi, important to the survival of the trees that follow the pioneers. There also might be changes in wildlife influence, because of habitat development, that affect seed dispersal. Many plants are allelopathic, releasing chemicals that selectively retard certain competitors. Usually, changes affecting germination and seedling establishment are the most important, because a tree is most vulnerable at the seedling stage.

By learning what types of trees are growing on your land and what types are not, you can make some reasonable judgements about your overall growing conditions. Some of these can be changed. Pulling weeds or providing irrigation, even for a short time, might allow a desired species to become established. On the other hand, some conditions, such as soil acidity or summer heat, can be difficult or impossible to alter.

Planning Out Your Landscape

As you read about the needs of various trees, you will notice certain patterns beginning to emerge. You can look at the trees in your neighborhood and assume that you can plant those same species and have them flourish. As you come more familiar with forest guilds, or groups of trees, you can begin to make reasonable assessments of which other uncultivated native plants will also succeed.

Why aren’t those other species already growing on your property? The trees may have been cut for timber or killed by livestock grazing. There might be increased road salt leaching into thee ground or more exhaust fumes in the air. Fire control could have eliminated the necessary conditions for some seeds to germinate.

Development brings changes to the environment, and the consequences can be felt by trees as much as by more delicate-looking flora.

If these changes persist, select trees that are adapted to some of these new or stressful conditions. Many swamp trees, for instance, tolerate compacted urban soil. Trees from floodplains are resistant to the soggy state of heavily irrigated lawns. Prairie species are more tolerant than forest types to the competition and allelopathic effects of turf roots.

Additional Considerations

Trees that are the first to establish in a sunny field, so-called early successional species, may be able to handle the heat near a south-facing brick wall or a city sidewalk. Those accustomed to hardpan might best adapt where the natural soil profile has been destroyed by a careless contractor. If you are lucky enough to have great soil and are careful to limit the encroachment of turf or pavement in your planting area, by all means put in some of the magnificent classic tree species that require optimum conditions.

When you begin to draw up a list of trees for your landscape, you may want to include a favorite native or an exotic species that is not ideal for your site. That would be understandable; this is what turns landscapes into gardens. However, remember that the farther you reach for special trees to achieve a certain effect, the more you will be burdened with site preparation and maintenance. This stands in contrast to the premise of native-plant landscaping. Pick adaptable natives and you will be on a path toward a beautiful and varied landscape that makes comparatively few demands.

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