Topography, Soil, and Trees : How the Land Affects the Forest Type*

Henry McNab, U.S. Forest Service Bent Creek Experimental Forest, Asheville, NC August 6, 2010 WOODSCAPING YOUR WOODLANDS & FIREWISE MANAGEMENT Woodland Steward Series

Table of Contents Introduction to the subject .............................................................. 3 Introduction to "cold-deciduous" forests ........................................ 4 Ecoregions of NC .......................................................................... 5 What do trees need? ..................................................................... 7 Grouping to simplify ....................................................................... 7 Rocks ............................................................................................. 8 Topography .................................................................................. 10 Soil ............................................................................................... 11 Site index and tree growth ........................................................... 13 Moisture Regime Index ................................................................ 14 Moisture regime index real example ....................................... 15 Shrubs - what do they tell us? ..................................................... 16 For more information on trees and forests .................................. 17 Soil Map-Holmes Educational Forest .......................................... 18

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Introduction to the subject Natural environments of the Southern Appalachian Mountains are among the most diverse in the eastern U.S. when described by temperature, water, and fertility. Because naturally occurring vegetation is an expression of the environment, these mountains are home to a large number of native trees, shrubs, and vines. In his 1964 book "Trees, Shrubs, and Trees = 81 Shrubs = 119 Woody Vines of the Great Smoky Mountains Vines = 21 National Park, Park Naturalist Arthur Stupka lists ?? = 1 222 species. Also, a small number of introduced Total = 222+ species have become well established and are beginning to call this area their home; often causing serious problems to the natives. We'll focus on trees to simplify our discussions. Many trees are common throughout the region and are well known in our back yards, such as the oaks, yellow-poplar and red maple. Others are less common, such as bitternut hickory, sugar maple and yellowwood. Regardless of their frequency of occurrence, however, each of these species is adapted to certain environmental conditions. Knowing those conditions allows us to predict where a species is likely to occur and how quickly it will grow from a young sprout to old age. This information could be useful when making decisions about how to manage your back yard, whether it's half an acre or half a mountain side. This booklet has several purposes: (1) provide an outline of topics covered in this class, (2) summarize on several scales (from global to local) what we know about environmental factors that are important for hardwood forests, (3) review site factors that influence the species composition of hardwood stands, (4) describe an alternative method to site index for estimating quality of a stand for timber production, and (5) provide a guide for a brief walk in the woods around our classroom.

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Introduction to "cold-deciduous" forests Cold-deciduous, broadleaf forests, where trees lose their leaves as a result of freezing temperature - not drought - occur across extensive areas in only three regions of the world: eastern Europe, western Asia, and eastern North America. Although not apparent on the vegetation map (left), these widely separated, large ecosystems of broadleaf forests have something in common: a temperate, humid climate: where winters are short and mild, and summers are long and hot. More important annual rainfall in these regions is well distributed resulting in only minor soil moisture deficits. In the eastern U.S., hardwood dominated forests gradually change to evergreen conifers (spruce, hemlock, white pine) in the colder north, pines and evergreen hardwoods (loblolly and longleaf pines, live oak and magnolias) in the hotter south, and grasses in the drier west. Composition of North Carolina forests varies in a similar manner in relation to climate, but on a smaller scale. North Carolina may be subdivided into three large ecological zones of similar climate and vegetation: (1) coastal plains (from the coast west to about Raleigh) with mostly pine forests, (2) piedmont (between Raleigh and Old Fort) where hardwoods become more common, and (3) mountains (west to GSMNP) dominated by deciduous hardwoods, as shown on the large center-fold map. In the mostly hardwood-dominated mountains, three landform related ecosystems are apparent, each with its characteristic climate: (1) warm and dry intermountain basins of hardwoods - mostly oaks (light green on map), (2) cool and moist mountains - oaks with other hardwoods (medium green) and (3) cold and wet high peaks northern hardwoods and conifers (dark green). The red box on the map on the next page, between Asheville and Greenville, indicates our current general location at Holmes Educational State Forest.

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Blue Ridge Mountains

Piedmont

Coastal Plain

A magnified view of the area in the red box on the map is shown below. Holmes Educational Forest, indicated by the white box on the map (and the nearby DuPont State Forest) are located just outside of the very large Asheville basin. Aspect has minor effect on site quality in the hilly terrain of the basin, but becomes much more important in the steeper terrain of the mountains. The dark green area on the map, northwest of Brevard, is the high-elevation (cold and wet) landscape of Devil's Courthouse, along the Parkway, in an ecosystem where conifers of spruce and fir dominate the landscape. We will use the trees to help us evaluate and make assumptions about the small parts of the landscape (sites) where they are growing, which is a lot easier than taking samples of the environment to figure out the amounts of environmental components that are present. On a state scale, for example, if I say "bald cypress" for many listeners that brings up a mental image of a shallow swamp along the Atlantic coastal plain, "southern red oak and loblolly pine" suggests (to me) the piedmont red clay hills from Raleigh to Marion, and "Frazer fir" takes us up to the cool heights of Richland Balsam, in the high mountains just west of here. These large "ecoregions," shown on the centerfold map developed by NC-DENR and the US Environmental Protection Agency to monitor water quality are defined by climate, landform and vegetation. Ecosystems in western NC at this scale are defined by elevations: (1) low basins 4500 ft, and (3) the land between.

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What do trees need? Trees, like most life on Earth, need four things to live: (1) Energy, as sunlight for photosynthesis by plants, (2) Heat, sufficient time during the year when the temperature is above freezing to allow for growth and reproduction, (3) Water as a raw material for photosynthesis and subsequent growth, and (4) Nutrients, such as C, O, N, P, K, S, Ca, Mg, and other elements, for production of carbohydrates and cells for growth. Although all of these things are important to trees, a priority of needs exists beginning with light. In places without light, such as a cave, it doesn't matter how much of the other things are available. Likewise for heat; even though the North Pole has plenty of water (and light during half of the year), a sufficiently long period is lacking when air temperature is above freezing to allow for a summer growing season. Fortunately, the mountains of western NC have adequate, but varying supplies of the four requirements.

Grouping to simplify Although each species of tree is an individual in its needs for light, temperature, moisture, and nutrients, we know enough about the needs of certain trees so that we can make assumptions about other species that occur 80 species with them. For example, many of the but only 24 commercial species, such as oaks, have environments, been much studied. But, we know little about the physiological requirements of or sites sourwood, for example, a nontimber species well known for it value for honey production, which occurs in the understory of oak forests. From formal studies, observations, and experience we can group the 80 species into fewer classes to simplify the job. First, let's begin the task by saying that light doesn't vary; it's about the same everywhere on the landscape. And, let's continue the process by grouping other components of the environment. Using elevation Energy as a substitute for actual measures of Heat temperature, we will group species into one or Water more heat zones ranging from warm (4500 ft). Most trees of the Appalachians grow in the warm and cool zones, but we'll concentrate on the warm zone, where Holmes Forest lies. Let's continue the process of simplifying by using four moisture classes (mesic, submesic, subxeric, and xeric) and two 7

nutrient classes (average and rich). By viewing the landscape as 24 different environments (3 temperatures, 4 moistures, and 2 fertilities = 24), we can place each species where it does best and usually occurs. Availability of water during the growing season is the factor most important in the distribution of trees within a uniform zone of temperature and nutrients, and that is what we will concentrate on today. An example of several typical tree species that would likely be present on four sites defined by temperature and moisture is shown in the table below. Environments

Xeric (ridge) site

Mesic (cove) site

2500 - 4500 ft cool site

Black oak, chestnut oak, red maple, mockernut hickory

Yellow-poplar, northern red oak, black locust, red maple

2000 - 2500 ft warm site

Post oak, scarlet oak, dogwood, red maple

Yellow-poplar, red maple, sweet birch

Rocks Any discussion of trees and environment in the southern Appalachians would be out of sequence and incomplete by not beginning with rocks. The rocks that formed these mountains are among the oldest in North America, dating back over a billion years to a Precambrian time when the mountains were much higher Rocks = topography, (some say similar to the Rocky soil and nutrients Mountains) and devoid of vegetation. Over millions of years the mountains were eroded and that material was buried and then pushed up into new mountains when continents collided, perhaps several times, then worn down again. For the most part, rocks in this area can be put into three groups for purposes of species composition and forest site quality:

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(1) Gneisses and schists - This group is the common "garden variety" of rocks, either light or dark in color, and often with streaks or layers. Soil formed from these rocks is generally highly acidic and low in fertility, and have a wide range of textures, from sandy loam to clay loam. (2) Amphibolites - This type of rock is uncommon in most areas, although large outcrops are present along NC-64 between Franklin and Murphy, and in Ashe County in the northwest corner of NC. The rock has a very dark appearance and because of its high mineral content of iron, magnesium, and calcium compounds it weathers to form soils that are less acidic and have higher fertility. Limestone also forms soils with high fertility, but it occurs only is very small areas, such as Cades Cove, in the Great Smoky Mtn. National Park. (If you have these rocks in your backyard then many possibilities for diversity of tree species is possible.) (3) Granites - These rocks are obvious as domes, for example Lookingglass Rock, and particularly in the Highlands area. Soils from granites generally have a coarse texture that is low in capacity to hold water and low in fertility. Many of the granitic formations are in areas of high rainfall, so fertility is more of a factor affecting site quality than is soil moisture. In summary, the two most important considerations that soils have for site quality are depth and texture. Both of these measures are associated with availability of moisture during the growing season. Organic matter, decomposition of parent material, and precipitation are the main sources of nutrients. Topography often reveals clues about the underlying rocks, or geology. High ridges are often underlain by hard rocks that have resisted weathering; granite domes, such as Looking Glass Rock, are a good example. Geologic maps are available for the Appalachian Mountains, but detail is often lacking because mapping bedrock formations covered by soil requires a certain amount of interpolation based on scattered outcrops. Rocks are important in affecting forest types only indirectly, through their influence on topography, soils, and nutrients.

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Topography As suggested by the picture on the cover of this booklet, topography is an obvious way to size up the landscape and make assumptions about how trees are distributed. As shown in the figure below, a moisture gradient exists from ridge tops to valley bottoms. Because water flows downhill, we know from many observations that ridges are usually dry with no flowing water. Small streams develop on hill sides and combine to form larger streams and rivers in the valleys. Soil moisture is difficult to sample because a rain the previous day could result in a wet ridge top.

The direction a mountain slope faces also affects the water available and species composition. As shown below, south facing slopes receive greater sunlight and are hotter and drier than north facing slopes that receive less direct sunlight. The influence of aspect is particularly apparent in winter when snow remains on the ground much longer on colder north and east slopes than on warmer south or west slopes.

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Other features of topography are also important in affecting the distribution of tree species, particularly elevation. Rainfall increases with higher elevation and the rate of moisture evaporation decreases because temperatures are cooler. Soil characteristics also change with elevation, becoming shallower in some places but higher in organic matter in other locations; soils will the topic of our next discussion. Using topography to predict forest types works adequately for three broad categories of trees: those that need little water (xerophytes: pines, Carolina hemlock), those that need considerable water (mesophytes: yellow-poplar, white ash), and an intermediate group (oaks, hickories). A fourth group of trees are generalists and are less demanding in their water needs and are widespread (red maple, sassafras).

Soil Soil is closely associated with geology and topography. Soil mapping units are usually restricted to certain rock formations, such as metamorphic or sedimentary bedrocks. Topography is important, too, because characteristics such as depth, texture, and stoniness are associated with landform (ridge, slope, cove), elevation, and aspect. Soils have been studied in more detail than rocks and recent mapping is available for almost all counties in NC. An example of soil maps available from NRCS on the internet (http://websoilsurvey.nrcs. usda.gov/app/ ) from the web soil survey mapping application is shown for the Holmes State Forest on the outside back cover of this booklet, where five series with several classes of slope steepness have been mapped. Soil maps and the taxonomic units displayed are primarily for agricultural application although other uses can be determined, such suitable for home septic systems. 11

In addition to providing information on soils, these maps show tree species that are well suited for management. Information most easily obtained is related to a limited number of commercial species and is shown as annual cubic volume production and site index. An example of information on trees obtained from a soil map is shown for about 10 acres around our picnic shelter that extends across three soil mapping units, as shown in the photo at left. The red polygon shows the area where one type of soil (Evard) was mapped. White oak is one of several tree species recommended for Evard. Another series, Cordorus, is mapped in the broad floodplain where the parking lot is located. A small area of a colluvial soil, Tusquitee is mapped in a cove in the lower left portion of the photo and is the only area recommended for growing black cherry. Several tree species are recommended for each of these three soil series, as shown by site index (tree height at 50 years) in the table below. Soil series

E. white pine

Black cherry

White oak

Yellowpoplar

Cordorus

85

-

-

95

Evard

91

-

75

95

Tusquitee

100

83

-

103

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Site index and tree growth Site index is the most common method used by foresters to measure land productivity for management and economic purposes. As described in the Woodland Owner Notes on Forest Soils and Site Index, in your participant's notebook, site index is defined as the height a tree of a certain species will attain at 50 years of age (Figure 1 in Woodland Owner Notes). Site index variation among species of the upper canopy (oaks, not dogwood) is mostly related to the amount of water in the soil that is available for growth during the summer; fertility (nutrients) is more important in affecting the species composition (white ash vs oak). In general, all of the oaks except for white oak will have about the same site index; white oak will be slightly lower because it grows slower. Where a site is dominated by oaks, white pine will be taller because it is better adapted to a wider range of conditions than the oaks, but other species, such as yellowpoplar will be shorter because it needs more water than the oaks and therefore is usually found on lower slopes and in coves, which have higher soil moisture content due to deeper soils and protection from wind. Site index is based on the principle that trees serve as a living measure of climate, topography, and soil and their height expresses the long-term availability of the site physical resources, mainly water. One (of many) limitations of site index is that it is a timber based measure; it tells you little about the ecological characteristics that might be important for restoration objectives, species diversification, or wildlife habitat.

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Moisture Regime Index Moisture regime index (MRI) is a quick way to evaluate the moisture characteristics (and site index) for land stocked with trees. Because topography can affect moisture regime of a site through variation in landform, aspect, slope gradient, and soil, the trees take this into account over many years and are an indicator of environmental conditions on the site. I assigned a weight value to each moisture class (xeric=1, subxeric=2, submesic=3, and mesic=4) that is equivalent to the relative moisture needs of each species. The numbers 1 - 4 can indicate any unit of measure, from a gallon, barrel, or swimming pool of water. Silverbell (a weight of 4), for example, needs 4 times as much water as post oak (weight of 1). MRI is determined by listing all tree species growing on a small area (say a tenth acre) and then calculating the average moisture weight. Below is a made-up example showing how the method works. Five species are present on the example plot; note that the count of trees for each species is not needed, only if it is there. MRI is 3.0. Species Post oak E. white pine White oak Hickory N. red oak Silverbell Basswood Total Avg. (15/5=)

Present 0 1 0 1 1 1 1 5

Weight 1 2 2 2 3 4 4

Product 0 2 0 2 3 4 4 15 3

The value of MRI may be converted to site index for oak using the chart shown below. On the X-axis (horizontal) locate the value of MRI obtained for a field plot; from there extend a line up to the diagonal line; then extend that line left, to the Y-axis, where the value of site index is obtained. This method utilizes all of the trees on the plot to estimate an average moisture regime, which can be translated to a measure of site quality (site index). For example, an MRI value of 3.0 (see table above) is about equivalent to a site index of 76 for northern red oak.

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Moisture regime index real example As explained above the moisture regime index is a quick, easy, and relatively accurate (at least in Bent Creek EF) method to evaluate the moisture characteristics for a site, and obtain an estimate of the site index. On the handout MRI field form I have checked all tree species present on the site surrounding the picnic shelter. Fourteen species were present, not counting red maple. Red maple, and perhaps other species, is not particular in its moisture requirements and is not counted because it provides little useful information about the soil moisture characteristics of a site. This is illustrated on the chart on the back of the field form. Converting MRI to a measure of site quality, using the graph below, we estimate site index for red oak as 76 feet (which might be a little low).

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Shrubs - what do they tell us? Last, but not least, what can non-arborescent vegetation, shrubs for example, tell us about site quality? Perhaps a lot. When used with trees in calculation of the moisture regime index we have found that two shrubs are helpful for estimation of site index: mountain laurel and spice bush. Mountain laurel (Kalmia latifolia) - if it is present, then subtract 4 feet from site index based on MRI for trees alone. Spice bush (Lindera benzoin) - if present, then add 6 feet to site index estimated from trees. Other common shrubs such as rhododendron and sweet shrub (both common here, outside the picnic shelter), we have not determined their value, if any, for estimating site quality.

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For more information on trees and forests Braun, E. Lucy. 1950. Decidous forests of eastern North America. The Blankeston.Company. 596 p. (She gives the big picture on trees in hardwood forests in the East) McNab, W.H.; Loftis, D.L. (In press) A preliminary test of estimating forest site quality using species composition in a southern Appalachian watershed. (A research publication that describes how to use species composition to estimate site index; soon to be printed by the Southern Research Station.) Stupka, Arthur. 1964. Trees, shrubs, and woody vines of Great Smoky Mountains National Park. The University of Tennessee Press. Knoxville. 186 p. (A good checklist of trees, shrubs, and vines of the Southern Appalachian Mountains and the environments where many of them occur.) Neelands, R.W.. 1999. Important Forest Trees of the eastern United States. FS-466. US Department of Agriculture, Forest Service, Southern Region, Atlanta, Ga. 111 p. (Out of print by but is available [as of Aug. 5, 2010] from internet at http://library.rawlingsforestry.com/fs/important_forest_trees/import ant_forest_trees.pdf) Information presented in this booklet is in draft format and was assembled specifically for a presentation at the Woodscaping Your Woodlands & Firewise Management workshop on August 6, 2010, at Holmes Educational State Forest.

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Soil Map-Holmes Educational Forest

Soil map of Holmes Educational State Forest, Henderson County, NC. (downloaded from Natural Resources Conservation Service web page) (http://websoilsurvey.nrcs.usda.gov/app/)

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