0

0.01

0.02

0.03

0.04

0.05

0

0.01

0.02

0.03

0.04

0.05

1980 1985 1990 1995

0

0.01

0.02

0.03

0.04

0.05

B a s a l a r e a ( % )

Wet site:

Oak-Gum

Moist site:

BeechOak-Pine

Dry site:

Oak-Pine

Flood

Hurricane

Effects of Climate Change on Southeastern Forests

Scientists from Rice University

sponsored by the National Wetlands

Research Center are using a long-term

study of forest structure and change in

east Texas to investigate potential

effects of climate change on forests of

the coastal plain (Fig. 1). Study areas

include sites in the Big Thicket of east

Texas, which are representative of

important vegetation types on the

coastal plain, including bottomland

hardwoods, one of the two major types

of forested wetlands in the region.

Unique records of forest variation have

been created for three study sites. The

database includes records on more

than 42,000 individual trees.

Tree-ring Studies

In addition to the long-term study,

they use tree-ring measurements and

weather data to reconstruct past

relationships of tree growth to climate

and to investigate species behavior

near range limits. Annual tree-ring

chronologies have been developed

from 16 tree species growing on 38

sites distributed across the region.

Initial results of tree-ring measurements indicate that individual growth

is not substantially slower near the

range boundary of a species, which

implies that species range boundaries

in this region are not set by climatic

limitations on individual growth. Thus

factors influencing reproduction may

be more critical in setting range limits.

Also, analysis of the relationship of

tree-ring width to climate revealed

another surprising result: several

groups of related species (black oaks,

white oaks, and pines) respond

differently to climate. This fact

indicates that groups will respond

differently to climate change, and

therefore the relative importance of

these groups in forests may change.

This kind of information is presently

only possible to obtain through treering analysis.

Effects of Flooding and

Fire

Long-term studies reveal that Big

Thicket forests can show strong

variation in tree mortality over time

(Fig. 2). Flooding at the wet site in

1989-92 initiated a strong pulse of

mortality. At the moist upland site, a

hurricane caused a mortality peak in

Forests of the coastal plain region of the southeastern United States are among the most productive in North America.

Because they form the basis of a large timber and wood products industry, these forests are of considerable economic

importance. Also, the forests are rich in plant and animal species. Because they are diverse as well as productive, they have

considerable conservation importance. Therefore, understanding potential impacts of climate change on southern forests is

critical.

Fig. 2. Natural forest mortality.

1987 that was about three times the

long-term average. Variations in

abundance of saplings of some species

(Fig. 3) are partially explained by these

disturbances. For example, Chinese

tallowtree, an exotic species, took

advantage of the disturbance openings

to invade the wet and the moist upland

sites. Also, other species increased in

abundance following the disturbances.

Nevertheless, not all of the trends in

saplings were related to these disturbances. At the wet forest site, it

appears that changes in the river flow

related to construction of two upstream dams much earlier favored Fig. 1. Rice University researcher

measuring tree growth at Big Thicket

National Preserve.
0

200

400

600

800

1000

0

20

40

60

80

100

1980 1985 1990 1995

0

20

40

60

80

100

N u m b e r o f s t e m s p e r h e c t a r e

Wet site

Moist site

Dry site

Sweetgum

White oak

Chinese 

tallowtree

Chinese 

tallowtree

Red maple

American holly

change will affect individual growth

rates directly by way of overall

warming or change in regional moisture

balance. Climate shifts will also affect

death and recruitment rates indirectly

by altering the frequency and intensity

of stand disturbances. Results suggest

that disturbance effects are stronger

and more immediate than growth

effects. Because individual trees grow

quickly and species are not particularly

long-lived, responses to climatic

change may be rapid. In natural area

preserves, Chinese tallowtree and

other non-native woody species will

become more important if disturbances

increase. In commercially managed

forests, increasing disturbance rates

may result in higher timber losses.

The variety of spatial and temporal

influences on forest processes,

coupled with uncertainties associated

with climate prediction, makes it

difficult to assess the effects of

changes in climate on forest dynamics

at the ecosystem level. Nevertheless,

this research not only identifies

specific climatic effects on particular

life stages or processes, but it also

provides critical information for

improving our understanding of the

context within which these effects

occur.

For more information, contact

Paul A. Harcombe

Rice University

Department of Ecology and Evolutionary

Biology

6100 Main Street

Houston, TX 77005

713-527-4924

harcomb@rice.edu

or

National Wetlands Research Center

virginia_burkett@usgs.gov

http://www.nwrc.gov

sapling survival, resulting in an overall

eightfold increase in sapling abundance and strongly altering the open

understory that is characteristic of

many bottomland forests. Altered river

flows may also be predisposing this

system to uncharacteristic increases of

certain tree populations; these increases are then followed by catastrophic declines after unusual floods.

Changes in regional climate will further

increase this variability if they increase

the frequency and intensity of both

floods and droughts. Increasing

variability of this type could result in

altered ecosystem function in wetland

forests and could also influence

success of post-harvest regeneration

in commercial forestry operations.

At the dry study site, the changes

in sapling abundance were related to

the history of human activity (logging

and fire exclusion). Future effects of

climatic changes on sites such as this

will probably involve interactions

between climate and fire regimes.

Changes in climate influence both

within-season timing and intensity of

fires, even under managed conditions,

thus affecting forest composition.

Effects of Storm

Disturbances

At the moist site, researchers

directly observed the effects of a 1986

hurricane. Even though it caused only

a small rise in mortality of canopy

trees, this storm had an important

impact on the forest by creating

canopy openings which promoted

growth of a non-native woody

species, as well as several native

species. If large storms begin to occur

with increasing frequency, however,

small trees and shrubs may benefit

most, with detrimental consequences

for canopy tree regeneration. This

suppression of tree regeneration,

coupled with higher storm-related

mortality, could result in a long-term

decline in the carbon storage capacity

of southern upland forests.

Influence of Climatic

Fluctuations

Researchers tested for variability in

growth of trees and saplings over

short and long time periods and small

and large areas that might be related to

climatic fluctuation. The emerging

conclusion is that climate variation

does produce small but detectable

effects on tree and sapling growth in

natural forests. However, subtle

climatic variability does not seem to

affect recruitment or mortality, though

earlier work showed that drought

strongly increased seedling mortality

in the wet forest. Failure to detect

effects on tree mortality may be related

to the fact that trees die slowly and

that multiple causes are often involved.

Continued collection of these important data over the next 10-20 years will

more definitively show how climate

affects tree recruitment and mortality.

Climate Change Will Affect

Southeastern Forests

Overall, evidence from the longterm monitoring indicates that climate

Fig. 3. Small sapling abundance.

USGS FS 093-97

June 1997

U.S. Department of the Interior

U.S. Geological Survey