I Will Tell You a Story … Houston Area Forests

Houston Area Forests, The Scene is Set

We are fortunate that the Houston Area has ecological conditions that allow for the growth of many kinds of forests. Many of these ecological conditions are created by the geology of the area and the diverse array of soils that have been formed by that geology. Although topography (elevation) in the Houston Area is low there are very diverse micro-topographies and micro-climates that make Houston Area Forests diverse. This presentation will only cover a few of the many different forests that exist near Houston.

The Houston Area marks the end or transition zone (what ecologists call an ecotone) of the great eastern, southern, and northern forests which abut up against other great ecosystems (Pineywoods Ecosytem). Ecosystems are large areas with all of their habitats. These other great ecosystems include the Coastal Tallgrass Prairie Ecosystem, the Blackland Prairie Ecosystem, and the Post Oak Savannah Ecosystem. An ecotone is where two or more ecosystems inter-grade into one another, making a more diverse landscape. Sort of like a “shifting mosaic” (patches of different habitats that intermingle with each other) on a landscape-scale.

Photo 1, shows the Houston Wilderness Map of ecosystems in the Houston Area

Trees can grow over a wide range of elevations. We can generally classify them by the topographic position where they are usually found. In the Houston Area, due to the long growing season, high rainfall, and low elevation, even a few feet or inches can determine what trees are found in a particular forest. Topographic elevation varies from 0 to about 300 feet, from Coastal Texas to Polk County, on the north and south, and the Louisiana Border and Fayette County on the east and west, or about 100 miles on both sides of Houston. The terms uplands, slopes, and bottomlands are used to denote where trees live in terms of topography.

Soil moisture is a characteristic that determines where forests exist and trees live. A hydric soil in a floodplain, is wet or saturated; a xeric soil on a deep, sandy ridge, is dry; and a mesic soil on a terrace or slope, is intermediate between wet and dry. Trees are often classified according to soil moisture differences. For example, Southern Magnolia is a mesic tree species, Bald Cypress is a hydric tree species, and Shortleaf Pine is a xeric tree species.

Houston Area Forests, Ecology

Trees do not exist by themselves. Each tree is part of a forest, a community, an association of a variety of trees, herbaceous plants (wildflowers), grasses, shrubs, soils, animals, waters, and terrains that all fit together in a landscape. Each of these community parts interact and has a relationship with the others. Thus ecology, is the relationship of organisms, with themselves, other organisms (biotic), and the non-living environment (abiotic).

Forests exist in many sizes, from 20 acre woodlots to 11 million acres of forest in East Texas. Landscapes are usually tens of thousands to millions of acres and are defined by the lay of the land and the geology, hills, rocks, soils, valleys, rivers, and streams.

In forest ecology, the most important concept is the role disturbance plays in perpetuation of forests. A disturbance event results in the killing of trees and the alteration of ecological succession on a site. Ecological succession is the change in vegetation at a site as one vegetative community changes the site and another vegetative community takes over that site. At one time it was thought that forests evolved to a climax or self-perpetuating state, where a few dominant (tallest, largest, most numerous) trees successfully reproduced, decade after decade, and century after century.

Photo 2, shows a forest blowdown in East Texas

Today we know that a “climax forest” is too simple of an explanation for why and where we find forests. Scientists now believe that due to hurricanes, tornadoes, droughts, windstorms, ice storms, hail storms, floods, fires, soil characteristics, insects, diseases, lightning, and other disturbance events, an ever-changing collection of trees is created to form forests.

A constantly changing collection of trees in a forest is called a shifting mosaic. There are different trees living in different places; different ages of each tree species (seedling, sapling, pole, mature, and old growth); and different layers or structure of plants (grass, herbaceous, shrub, tree understory, tree midstory, and tree overstory or canopy); that create conditions so that other plants and animals have a place to live (habitat).

Each tree species has its own job to do (niche) which makes the forest a collection of interrelationships and not simply a collection of individuals. For instance, Flowering Dogwood brings calcium to the surface by its roots (storing the calcium in its leaves which fall each autumn) and provides this essential nutrient to other trees or plants that need calcium for their growth, like Loblolly Pine.

Dead or sick trees play a role in these interrelationships. When a large tree loses a limb or is blown over this creates a hole or gap in the canopy of the forest (where the tree tops or crowns come together) which allows tree seedlings, grass, herbaceous plants, and shrubs to compete and grow into the space in a race for sunlight.

A downed tree (coarse woody debris) provides a seedbed for seedlings (nurse tree); nutrient and organic enrichment for the soil; hiding places for many animals; places for fungi to grow and turn the cellulose of the tree into rich fertilizer; natural check dams to prevent soil erosion; and ensures that moisture is retained for longer periods of time during our long, hot, summers.

A standing dead tree (snag) provides a home for woodpeckers, bats, squirrels, and other animals. A sick tree with fungal rot that leaves a hollow base or a space in a trunk or branch provides a den or cavity for raccoons, opossums, birds, snakes, frogs, other animals, and in the case of large trees like Black Gum, Water Tupelo, and Bald Cypress, a shelter for black bear. Both downed trees and snags are “ecological legacies” that ensure increase the quality of wildlife habitats.

Finally, trees must deal with tolerance. Tolerance refers to how much shade a tree can withstand. This differs depending on how old a tree is. When a tree has slowed or stopped growing due to a reduction in sunlight it is said to be suppressed or to have been over-topped by other trees that shade it.

Some trees, like Southern Magnolia, can exist in heavy shade for decades. Other trees, like southern pines (Longleaf, Shortleaf, and Loblolly Pines) need a lot of sunlight or they will die. When shaded, many trees develop larger leaves to catch as much of the filtered sunlight as possible to increase their chances for survival.

Photo 3, shows a Southern Magnolia in Big Creek Scenic Area

Without the forest there would be few trees. When trees grow together as a forest they protect each other better than when growing alone. Forests withstand wind blow-downs and forest drying better than individual trees. Individual trees need a forest for the mutual protection it provides and the forest needs different kinds of trees to make it a diverse forest.

Houston Area Forests, The Stories

This presentation focuses on four Houston Area Forests and tells you what they are, how they were protected, and how you can protect them in the future. The four forests are, Sam Houston National Forest, Big Thicket National Preserve, the Columbia Bottomlands, and the Trinity River National Wildlife Refuge.

Sam Houston National Forest (SHNF)

Photo 4, shows SHNF in 1938

Photo 5, shows Upland Pine Forest in SHNF

What is it?

SHNF is the largest unit of public land near Houston, Texas. This national forest, about 50 miles north of Houston, covers over 163,000 acres. SHNF is unique because it not only includes the original vegetation of the “Big Thicket” but is near the westernmost boundary of the Great Eastern Deciduous Forest and the Southeastern Evergreen Forest. Because of its' location, Western Riparian Forest, Post Oak Savannahs, and Blackland Prairies exist in SHNF which increase vegetative, ecologic, and biologic diversity.

In Del Weniger’s, “The Explorers’ Texas, The Lands and Waters,” De Cordova is quoted in 1858 about the area that is SHNF, “What is extensively known as Big Thicket lies on the eastern border of the country, between the forks of the San Jacinto. A great deal of this land is high, sandy and very productive, covered with a dense growth of large timber, post-oak, white oak, also black walnut, hickory, ash, and in some places magnolia and wild peach.”

Photo 6, shows an old-growth pine with man

An Ecological Classification System (ECS) was prepared by Stephen F. Austin State University and The Nature Conservancy for the U.S. Forest Service (FS) in 1999 and was revised in 2007. The ECS lists and describes vegetation in landtype associations and their landtype phases. The ECS uses topography, soils, and existing vegetation as criteria to determine what likely vegetation existed before extensive European settlement occurred. The ECS uses these three criteria to name the vegetation landtype associations and their landtype phases. For SHNF the landtype associations and their landtype phases include:

Raven Hills Landtype Association

Photo 7, shows an Upland Pine Forest in SHNF

The Raven Hills Landtype Association occurs on gently rolling to undulating landscapes of the Fleming and Willis formations. The Fleming formation is older and consists of calcareous clays, silt, and sandstone while the younger Willis formation occurs topographically higher and is mostly loam to loamy sand over clay subsoils. The Fleming formation has Mixed Hardwood – Shortleaf – Loblolly Pine Forests and small blackland prairie openings with Little Bluestem – Indiangrass communities while the Willis sand-caps have Shortleaf Pine – Post Oak Forests. Raven Hills marks the western edge of the West Gulf Coastal Plain and the southeastern U.S., with Blackland Prairies and Post Oak Savannahs immediately west. The Raven Hills Landtype Association occurs in most of the northern and western parts of SHNF.

a. Shortleaf Pine – Blackjack Oak/Schizachyrium (Little Bluestem) Arenic Dry Uplands Landtype Phase, historic fire return interval was frequent (high, every 3 to 5 years)

b. Loblolly Pine – Post Oak/Callicarpa (American Beautyberry) – Chasmanthium (Inland Seaoats) Loamy Dry – Mesic Slopes Landtype Phase, historic fire return interval was moderately frequent (moderate, every 5 to 10 years)

c. Dalea (Purple Prairie Clover) Herbaceous Dry Clayey Prairie Landtype Phase (blackland prairies), historic fire return interval was moderately frequent (moderate, every 5 to 10 years)

d. Loblolly Pine – Southern Red Oak/Callicarpa (American Beautyberry) Loamy Mesic Lower Slopes and Terraces Landtype Phase, historic fire return interval was infrequent (low, every 10 to 20 years)

e. Water Oak – Loblolly Pine/Bignonia (Crossvine) – Arisaema (Green Dragon) Loamy Mesic Stream Bottoms Landtype Phase, historic fire return interval was infrequent (low, every 10 to 20 years)

f. Cedar Elm – Hackberry/Justicia (Water Willow) Loamy Wet – Mesic Stream Bottoms Landtype Phase, historic fire return interval was very infrequent (very low, greater than 20 years)

Big Thicket Landtype Association

Photo 8, shows a Loblolly Pine – Southern Magnolia – American Beech Forest

The Big Thicket Landtype Association is found on broad, gentle slopes with well-defined drainages of the Willis formation. Soils are usually sandy loam to loamy sand over sandy clay loam subsoil. Vegetation is Shortleaf Pine – Oak Forest and Loblolly Pine – Oak Forest on uplands, Hardwood Forest on lower slopes and smaller streams, and Sweetbay Magnolia Forest Seep Communities in groundwater seepage areas. The Big Thicket Landtype Association occurs in most of the southeastern part of SHNF.

a. Loblolly Pine – White Oak/Callicarpa (American Beaityberry) – Chasmanthium (Inland Seaoats) Sandy/Loamy Dry – Mesic Slopes and Uplands Landtype Phase, historic fire return interval was moderately frequent (moderate, every 5 to 10 years)

b. Laurel Oak – Loblolly Pine/Callicarpa (American Beautyberry) Loamy Mesic Lower Slopes and Terraces Landtype Phase, historic fire return interval was infrequent (low, every 10 to 20 years)

c. Water Oak – Loblolly Pine/Bignonia (Crossvine) – Arisaema (Green Dragon) Loamy Mesic Stream Bottoms Landtype Phase, historic fire return interval was infrequent (low, every 10 to 20 years)

d. Cedar Elm – Hackberry/Justicia (Water Willow) Loamy Wet – Mesic Stream Bottoms Landtype Phase, historic fire return interval was very infrequent (very low, greater than 20 years)

e. Sweetbay – Swamp Tupleo/Osmunda (Cinnamon Fern) Loamy Wet Forested Seeps Landtype Phase, historic fire return interval was very infrequent (very low, greater than 20 years)

San Jacinto Flatwoods Landtype Association

Photo 9, shows a Longleaf Pine Wetland Savannah Forest

The San Jacinto Flatwoods Landtype Association is found on broad, newly featureless plains, shallow depressions, pimple mounds, and poorly defined drainages of the Lissie formation. Soils are variable and silty, loamy, or sandy and moderately well-drained to poorly drained and usually have a seasonally high water table. Vegetation is Water Oak – Willow Oak – Black Gum Forests on flat plains and Loblolly Pine – Oak Forests on slightly higher positions. Wetland Longleaf Pine Savannahs may have occurred here. The San Jacinto Flatwoods Landtype Association is found on the southeastern edge of SHNF.