Environmental Gradientsand Forest Composition

(Trees on the landscape)

2007 Upland Hardwood Silviculture TrainingBent Creek Experimental Forest

Henry McNab

Research Forester

Lesson Plan (23 July 2007, 1300-1350)

• Objective - Review the influence of temperature, moisture, and nutrient environmental components on species composition of vegetative communities.

• Presentation – 10-12 min.

• Questions – 5 min.

Forest Species-Site Quotation

“Perhaps the single most important decision a forester can make is to manage for the right species on the right site.” (Anon.)

What do trees need?

• A combination of ingredients including carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, magnesium (CHOPKNSCaFeMg + Mo, B, Cu, Mn, Zn, and Cl) and water, needed for photosynthesis and growth with sunlight as the energy source (Botany 101).

• Where do the ingredients come from? (the site)

Vegetation patterns of central N.A.

Raven Fork watershed, GSMNP

S. App. arborescent vegetation• Eighty ± common forest trees (Frothingham 1931): white

ash, basswood, beech, blue beech, river birch, sweet birch, yellow birch, yellow buckeye, butternut, red cedar, black cherry, pin cherry, (chestnut), chinquapin, coffeetree, crab apple, dogwood, American elm, slippery elm, winged elm, balsam fir, black gum, red gum, hackberry, hawthorn, Carolina hemlock, eastern hemlock, bitternut hickory, mockernut hickory, pignut hickory, shagbark hickory holly, hophornbeam, black locust cucumber magnolia, Fraser magnolia, umbrella magnolia, red maple, sugar maple, red mulberry, black oak, blackjack oak, chestnut oak, pin oak, post oak, northern red oak, scarlet oak, shingle oak, southern red oak, swamp white oak, water oak, white oak, pitch pine, white pine, shortleaf pine, Virginia pine, persimmon, great rhododendron, yellow-poplar, redbud, sassafras, serviceberry, silverbell, sourwood, red spruce, sycamore, black walnut, black willow + a few others.

• Problem: How to group the species for managers.

Vegetation distribution• Botanists (and foresters) had long observed that

vegetation of the S. App. Mtns is distributed in relation to elevation (spruce/fir) and moisture classes associated with landforms (cove, moist slope, dry ridge).

• Whittaker, working in the Smoky Mtns. N.P. in the early 1950s, was the first to quantify the relationship of species with environmental “gradients” in support of Gleason who proposed in 1926 that species were distributed individually, not as an organic group, as advocated by Clements.

• Gleason stated that ".... every species of plant is a law unto itself, the distribution of which in space depends upon its individual peculiarities of migration and environmental requirements"

Raven Fork watershed, GSMNP

How plants regulate water use

• Within an elevation zone, moisture availability is the primary factor affecting the distribution of species.

• With a few exceptions most species adjust their water needs with availability using a similar mechanism – by opening and closing stoma (plural: stomata) on leaf surfaces, which allows CO2 in for photosynthesis and water vapor out through transpiration.

• Stomata vary by species and range from ~1,000 to >1,000,000 / sq.cm. of leaf surface.

Leaf stoma and water relations

• Hydrophytes have few or no stoma and modified leaves and stems (water lily).

• Mesophytes have many stoma.

• Xerophytes have few stoma and modified leaves (cacti, pines).

• Stoma response to:– Increased light: Open– Less water: Close– More CO2: Close– Higher temp: Open

• Speed of response varies by mesophytes vs xerophytes.

Vegetation types: nutrient regimes

• Nutrient regimes– Very poor < ----------- > very rich– Oligotrophic, mesotrophic, eutrophic

3-D matrix of temperature-moisture-fertility gradients

Tem

pera

ture

Moisture

low >low <high high

high

med

.lo

w

Fertility

low

med.

high

2-D matrix of temperature-moisture classes

Tem

pera

ture

Moisture

low >low <high high

high

med

.lo

w

S. Appal. temp-moisture classes

Tem

pera

ture

Moisturelow >low <high high

high

med

.lo

w cold

xeric

coolxeric

warmxeric sub-

xericsub-mesic

mesicwarm warm warm

coldmesic

S. App. topographic relationships

Tem

pera

ture

Moisture

low >low <high high

high

med

.lo

w >4500

<4500>2200

<2200

ridge sw-slope

ne-slope

cove

ridge

ridge

<2200 <2200 <2200

S.A. species by temp and moisture

Tem

pera

ture

Moisture

low >low <high high

high

med

.lo

w N. redoak

pitchpine

Va.pine

S. redoak

bass-wood

beechredspruce

sweetbirch

yellow-poplar

chest-nut o.

pignuthick.

bitter-nut hic.

Site nutrient availability

• Site fertility is largely a function of soil pH.

• Higher pH results in greater availability of some elements.

• Amphibolite rocks form soils with higher pH forming eutrophic sites.

• Highly acidic soils (<5 pH) can contribute to oligotrophic nutrient conditions.

S. Appal. geology and fertility

Nut

rient

s

Moisture

low >low <high high

low

med

.hi

gh

amphi-bolite

gneissschist

granite

ridgesw-slope

ne-slope

cove

Species vs nutrients and moisture

Nut

rient

s

Moisture

low >low <high high

low

med

.hi

gh

pitchpine

Va.pine

yellowwood

yellow-poplar

chest-nut o.

bitter-nut hic.

whiteash

? ?

? ?E. wh.pine

Environmental building blocks

• Landscape may be viewed as a matrix of blocks described by temperature, moisture, and fertility.

• Topographic-geologic variables correlate well with environmental variation; soil series not as good.

Equivalent sites

• Upper NE slope = lower SW slope • Lower NE slope = upper SW cove• Vegetation integrates physical components of

the site and provides a means of identifying equivalent sites.

• Some plant species indicate similar conditions - - (black oak, chestnut oak, pignut hickory, sourwood, sassafras, shortleaf pine)

Whittaker’s species distribution in Smoky Mtns. Nat. Park

Red m

aple V

a. p

ine

Silverbell

Chestnut oak

Dogwood Pitch

pin

e

Per

cent

of

stan

d

Per

cent

of

stan

d

Hemlock

Y-P Sourwood

Summary

• Vegetation is a biological integrator of environment.

• Moisture availability during the growing season is the primary factor affecting distribution of species.

• Refer to sites in terms of a moisture regime: mesic, submesic, subxeric, xeric.

• How to classify sites by moisture regimes and productivity is the topic of my next presentation.