SAMUEL D. FUHLENDORF, Department of Agronomy, Oklahoma State University, Stillwater, Oklahoma 74074-6028.
Abstract: Over the past 100-150 years many of our rangelands have become dominated by several brush species that were previously limited to discrete portions of the landscape. These changes can be attributed to adaptations by the brush species involved, and changes in the environment that favor their growth and establishment. The species that have increased on these rangelands frequently have similar characteristics that may include (1) high levels of seed production, (2) persistent seed or seed bank, (3) effective seed dispersal, (4) tolerance to water or nutrient stress, (5) chemical or physical deterrents to browsing, (6) ability to regenerate vegetatively after top removal, and (7) extended longevity. Changes in the environment that are associated with increased dominance of these woody plants include altered fire regimes, introduction of grazing by domestic livestock, increases in atmospheric trace gases, and long- and short-term changes in climate patterns. An interaction between the adaptations of the brush species and the environmental changes that have occurred over the past century have allowed these species to gain dominance in areas where they were historically limited.
Early descriptions of our rangelands were highly variable with woody plants or brush frequently restricted to discrete portions of the landscape. Since settlement, woody plants have increased on many rangelands that were once dominated by grasses. Several factors have been associated with the increases in woody plants including introduction of livestock, altered fire regimes, short- and long-term fluctuations in climate, and increased atmospheric trace gases.A review of literature suggests that increases in woody plants have been (1) rapid occurring over 100-150 years, (2) accentuated by drought with variable rates over time, (3) influenced by soil and topographic features, and (4) non-reversible over time frames relevant to land management (Archer 1994). As a result, brush on Texas rangelands has increased dramatically over the past 100-200 years, frequently converting open grasslands and savannas to closed-canopy shrublands and woodlands.
Why does brush dominate our rangelands? What traits have allowed woody plants to gain dominance over grassland vegetation on many rangelands? What environmental factors have contributed to the increased dominance of woody plants? What limits brush dominance on some rangelands? My objective is to attempt to answer these questions by presenting a general overview of the life history traits of woody plants and how they relate to the environmental conditions on rangelands.
Seed dispersal.Woody plants cannot gain dominance in a grassland or savanna until seeds are brought into an area and distributed across the landscape. Seed dispersal refers to the spreading of individuals away from each other (Begon et al. 1990) and can occur through wind, water, humans and animals. Method of seed dispersal influences the rate and distance of spread of a species, as well as the type of community where the species is most likely to grow.
The woody plants that have increased in dominance on our rangelands frequently utilize several methods but most are partially dependent upon animal dispersal of seeds. Species with fleshy fruits around the seed, such as juniper (cedar) are frequently consumed by all types of livestock, as well as American robins, cedar waxwings, raccoons, ringtail cats, gray foxes, coyotes, jackrabbits and deer (Chavez-Ramirez and Slack 1993, 1994, Smeins and Fuhlendorf 1997). Bird dispersal appears to be the primary factor associated with distribution of these berry producing brush species. Clusters of bird dispersed woody plant seedlings are frequently found around perch sites that include fences, telephone poles/wires, and large mature trees.
Other species, such as mesquite, are much more effectively dispersed by livestock. Seeds consumed by livestock are usually less likely to be destroyed by mastication and are frequently more viable because of scarification of the seed coat. Seed dispersal by livestock is a major influence of grazing on rangelands that is frequently overlooked in developing management strategies. Introduction of new herds that have consumed fruit from other areas can result in introduction and establishment of many woody plant seedlings.
Seed bank and seed production. Seed bank is the number of viable seeds in the soil at any given time. Most invasive woody plants either produce many seeds per year, or their seeds remain viable in the soil for many years. Brush species usually have highly variable annual seed production that is dependent upon weather patterns. Ashe juniper can produce 230,000 seeds/tree in years with high rainfall (Smeins and Fuhlendorf 1997), but these seeds do not appear to be able to germinate after about 2 years in the soil. Other species, such as mesquite, produce fewer seeds per tree, however, these seeds are capable of remaining viable in the soil for many years (Archer 1995).
Long-term viability and high potential seed production allow brush species to exist in environments where seed germination and seedling establishment may be low most years and favorable conditions are rare or episodic. On good years where sufficient moisture is available for germination, many seeds are either produced or available in the soil to capitalize on the normally unavailable resources. Also, the presence of viable seeds results in immediate response following disturbance of mature canopies allowing brush to quickly regain dominance when periodic events kill mature trees.
Seedling establishment. Woody plants are capable of germinating and establishing under most conditions observed on North American rangelands. Germination rates are highly variable, species dependent and can be enhanced through seed coat scarification and exposure to cold temperatures (cold stratification). Seed coat scarification occurs naturally in the digestive tract of wildlife and livestock. Many plant species, especially those with fleshy berries that are dispersed by birds, have highest germination and establishment rates under existing tree or shrub canopies. Others are considered pioneer species (e.g. mesquite) because they are capable of establishing out in open grasslands (Archer 1989). Some species, such as juniper, can germinate under other canopies or in extremely open areas, such as old fields released from farming. Requirements of individual species can result in a succession of woody plants that begins with the pioneer species, and eventually be replaced by species that are more tolerant of low light conditions associated with dense, overlapping canopies (Archer 1989). Frequently, seedling establishment requires fairly specific conditions and in most years may be minimal. Establishment of many plants can occur in years when resources are available, forming stands of brush that are even in age and size.
Growth rates. Following germination and establishment woody plants continue to develop to a mature age, which can take 5-70 years depending upon the species, location, and grazing pressure. Woody plants are frequently associated with exponential growth rates which suggests that growth of individual trees is initially slow but can become very rapid as the plant increases in root and shoot biomass (Fuhlendorf et al. 1996). Exponential growth of individual trees and the development of a seed source results in exponential development of an entire stand of brush. A critical point or threshold is reached once the rate of change begins to increase. Beyond the threshold, changes are difficult to reverse and an area that was once an open grassland or savanna can eventually be converted to a closed canopy shrubland.
Community influences. As woody plants increase, many other changes occur within the community. Some brush species, such as juniper, have the ability to exclude other vegetation under the canopy which can lower the forage production and species diversity of a landscape (Fuhlendorf et al. 1997). In other situations the presence of some woody plants may actually increase herbaceous productivity by improving nutrient availability and translocating deep water to the surface where it can be utilized by herbaceous plants (hydraulic lift). Increased nutrient availability can occur from processes such as nitrogen fixation and the creation of islands of fertility.
In addition to altered productivity, the presence of woody plants can alter the composition of the herbaceous community. The presence of canopies alters the micro-environment, allowing species, particularly cool-season grasses and forbs, to become more important than when no woody plants occurred in the community. Cool-season species require shade in the summer and are capable of growing earlier in the spring than many other species so they can potentially be a benefit to livestock and wildlife production.
Plant longevity. Differences in longevity for grasses and woody plants may be particularly important in accounting for observed patterns of change from grass to woody dominance on our rangelands since settlement (Archer 1994). Longevity of grasses ranges from 1 – 50 years with a mean of about 8 years, while longevity of woody plants is correlated with degree of “woodiness” and ranges from 22 to >400 years (Canfield 1957, West et al. 1979, Archer 1994). Dominance of longer-lived plants results in less frequent openings within the plant community which reduces the propensity for change. These differences result in an asymmetry where changes from grass to woody dominance can occur much more quickly than changes from woody to grass dominance. For woody plants with potentially long life-spans and low mortality of mature individuals, the seedling recruitment phase is the most critical to allow succession toward woody plant dominance (Archer 1994). In addition, longer lived plants can exist through severe episodic events such as drought and encounter rare or infrequent windows of opportunity for successful seed production, germination and establishment.
Stress tolerance. Most of the woody plants that are increasing on rangelands have adaptation that allows them to survive stressful environments. Many increasing shrubs are evergreen (e.g. juniper) which can be an advantage in nutrient-poor environments (Aerts 1995). Evergreens have adaptations that reduce nutrient losses, such as reducing litter production, slow decomposition rates, efficient resorption from mature parts, and determent of grazers through production of secondary chemicals. There are trade-offs however, traits that enhance conservation of nutrients also limit the maximum growth rates that evergreen species can attain.
An alternative strategy to evergreenness is nitrogen fixation, which occurs in leguminous shrubs that have the ability, through a symbiotic relationship with micro-organisms, to use nitrogen from the atmosphere. Nitrogen fixing species, such as mesquite, have the ability to establish in nitrogen-poor environments and eventually they can increase nitrogen availability for understory species.
In addition to adaptations for nutrient poor sites, most shrubs that are increasing on rangelands today are able to gain dominance on sites where low water availability is normal or severe droughts are common. These adaptations can be divided into water savers and water spenders. Water-saving adaptations conserve water by limiting excessive loss through evapotranspiration and include leaf size, leaf shape, leaf texture, leaf angle, and specialized photosynthetic pathways. Water spenders typically have adaptations that allow the plant to acquire greater water resources than herbaceous plants, such as the extended tap root system of mesquite trees. This allows resource partitioning where soon after germination, shrubs are not competing with herbaceous plants for shallow resources.
Resistance to disturbance. Rangeland species are differentially exposed to disturbance by many factors, such as grazing, trampling, fire and episodic weather events. Many species that have increased in abundance on rangelands possess traits that contribute to their relative persistence in communities that experience specific types of disturbances. Resistance mechanisms can be divided into either avoidance or tolerance mechanisms depending upon their function. Avoidance includes mechanisms that minimize the probability and severity of disturbance while tolerance mechanisms facilitate growth following disturbance.
Avoidance mechanisms are frequently discussed in reference to grazing. Grazing animals select species based upon their abundance, availability, palatability and digestibility. Many brush species produce secondary chemicals that limit palatability and digestion. Other species have thorns and spines that limit access to highly palatable young leaves. Each of these traits reduce the probability that these species will be grazed because animals usually prefer species without these deterrents.
Some brush species can resprout after the above-ground portion of the plant has been killed. This tolerance mechanism allows these plants to re-establish following severe disturbances without following the progression of dispersal, establishment and development. Traits that infer tolerance to disturbance limit the success of many management practices by allowing plants to maintain or regain dominance rapidly and suggest that control should focus on dispersal or early in the establishment phase.
Environmental changes contributing to woody plant dominance
Altered fire regimes. Prior to European settlement, periodic fires burned across prairies, maintaining fairly open grasslands with woody plants limited to discrete landscape units where fires were rare. After settlement, fires were feared and attempts to control wildfires were given high priority.
Studies have demonstrated that fairly infrequent fires can maintain an open grassland and when fire is removed from the system, many grass-dominated landscapes will eventually become closed canopy woodlands (Fuhlendorf et al. 1996). With a long enough interval between fires, tree and shrub species can reach sufficient size or age to tolerate fires, but the required interval varies among species (Archer 1994). Western juniper species that are 50 years old are susceptible to fires (Burkhardt and Tisdale 1969) while survival of 2-3 year old mesquite seedlings when exposed to flames equivalent to a hot grass fire exceeded 60-90%, respectively (Wright et al. 1976, Archer 1994).
Mortality of woody plants following fire can provide a competitive advantage to grass species. However, reductions in grass production, basal area, and survival following very hot fires may generate windows of opportunity to invasive shrubs (Archer 1994). Reintroduction of fires to control woody plants is limited by landowner inexperience and fear of fire, removal of fuel by livestock and sub-division of many rangelands.
Introduction of domestic livestock. Early settlers described many of our rangelands as open grasslands suitable for livestock production. Initial stocking rates were severe, resulting in livestock mortality as high as 85% during droughts prior to 1900 (Lehmann 1969), which suggests that the resource had been over-utilized soon after settlement. These severe stocking rates likely resulted in complete removal of vegetation cover that eventually led to significant erosion. These influences should be considered when evaluating vegetation changes over the past 100-200 years, particularly when we base our discussions on plant community descriptions of pre-settlement conditions. The environment associated with the early descriptions may no longer exist.
Since the introduction of livestock, there has been a major increase in the dominance of woody plants. It has been suggested that grazing reduced the competitive ability of grasses to limit the establishment of brush species. Recent studies suggest that the presence of thick grass cover, as in ungrazed conditions, does not restrict the establishment of brush, but instead it may actually improve conditions for seedling survival and development (Archer 1995, Smeins and Fuhlendorf 1997). This has been experimentally demonstrated for mesquite, juniper, and acacia species and indicates that grazing may not directly increase rates of woody plant increase. This could be due to the creation of harsher micro-environmental conditions in a heavily grazed community which may limit seedling establishment. Woody plant seedlings are frequently more susceptible to mortality from grazing than mature plants because they can be more palatable with less defense mechanisms, such as secondary chemicals or thorns. Regardless of the importance of direct influences through seedling consumption and altered competitive relationships, grazing has had a major contribution in the increase of woody plants through the removal of fuel required to initiate and carry a fire (Fuhlendorf and Smeins 1997).
Increase in atmospheric trace gases. Recently, some scientist have suggested that increases in atmospheric trace gases such as CO2, alter the relationship between woody and herbaceous plants. Their arguments are based on observations that indicate: (1) woody plants typically posses the C3 photosynthetic pathway, whereas most grasses in Texas are primarily C4; 2) increased CO2 may confer a significant advantage to C3 species relative to C4 species with respect to physiological activity, growth and competitive ability; (3) C4 grasslands appear to have evolved at CO2 concentrations below 200 ppm and thus at low CO2/O2 ratios; and (4) invasion of woody plants into C4 grasslands has been accompanied by a 30% increase in atmospheric CO2 over the past 200 years (Archer 1994).
The relationship between increases in CO2 and increases in woody plants has yet to be determined. Experimental data has suggested that woody plant growth and establishment was improved under CO2 fertilization. Other studies have suggested that the C4 grasses also responded positively to these treatments and suggest that CO2 enrichment is probably not a major factor contributing to the increases in woody plants. It is difficult to determine the actual influences of changes in atmospheric gases because of the difficulties in developing experiments that are not confounded by other factors.
Climatic influences. The dominant vegetation on a landscape is largely dependent upon the climate. The importance of short-term and long-term weather patterns on the increase in woody plants over the past 100-200 years has been debated in the ecological literature (see Conley et al. 1992, Archer 1994). A series of studies in the desert Southwest suggested a variety of climate-related causal factors associated with the increase in woody plants, including short-term droughts, long-term downward trends in precipitation, and long-term warming and/or cooling trends (see Conley et al. 1992). Others have suggested that grazing was more important than climatic patterns as a causal factor of woody plant increase (Conley et al. 1992).
Fossil pollen and pack-rat midden data suggest that woody plants have increased and decreased with fluctuations in climate over the past 20,000 years. Mean annual temperature, rainfall and evapotranspiration of savannas and grasslands are precariously situated between shrublands, woodlands, and forest (Holdridge 1964, Archer 1994). This suggests that changes or fluctuations in any of these variables could result in a shift in the balance between grass and woody plant dominance on rangelands where they both occur. In addition to changes in the mean of these variables, changes in distribution and extremes have major influences on plant communities. The regular occurrence of periodic droughts may be as important as mean precipitation.
Climate-driven vegetation changes often lag behind the changes in climate, where vegetation established under one climate regime may persist for hundreds of years until disturbed. The pristine grassland described by many settlers may have been established under very different climatic conditions than those that exist today. Changes in climate over the past 1000 years, particularly when considered with previously discussed factors (i.e. altered grazing and fire regimes), suggest that the increase in woody plants could have potentially been initiated prior to settlement (Smeins 1984). Changes in climate, to conditions more appropriate for woody plants, may have occurred prior to settlement and human disturbances could have acted as the catalyst that facilitated the directional increase in woody plants.
At the time of settlement, woody plants (brush) were limited to discrete portions of the landscape with many of our rangelands dominated by grassland and savanna plant communities. Over the past, 50-150 years trees and shrubs have gained dominance because of many factors that include specialized plant adaptations, as well as natural and man-induced environmental changes. The increased dominance of woody plants has been fairly rapid (50-100 years), exponential and frequently associated with a threshold where return to grassland dominance requires major inputs.
The species that have increased on these rangelands frequently have similar characteristics that may include: (1) high levels of seed production; (2) persistent seed or seed bank; (3) effective seed dispersal; (4) tolerance to water or nutrient stress; (5) chemical or physical deterrents to browsing; (6) ability to regenerate vegetatively after top removal; and (7) extended longevity (Archer 1995). A combination of several of these traits can enable brush species to invade into a relatively open environment and limit the success of control practices. Knowing the life history traits of brush species allows management practices to focus on critical aspects of vegetation change.
In addition to plant adaptation, environmental changes on rangelands have contributed to the increased dominance of woody plants. Introduced grazing, reduction in naturally occurring fires, periodic droughts, and increases in atmospheric trace gases have a occurred simultaneously on rangelands, making it difficult to isolate the importance of any single variable. In addition to these human-induced factors, some scientists believe that climatic fluctuations over the past several thousand years may have played an important role in vegetation change. These multiple factors interact to create a complex environment that changes continuously and inhibits efficient management of our resources. It is essential for management of livestock and wildlife production that we understand specific responses of species to each of these changing environmental factors.
Historically, brush species have been considered a nuisance because our primary focus has been maximizing livestock production. Even with the most rigorous efforts to control these species, they continue to dominate many of our rangelands. Recently, our focus as natural resource managers has shifted to include factors such as wildlife, water quality and yield, and biodiversity, which has changed our appreciation of woody plants. It is difficult to place a general value on the occurrence of these species on rangelands because they have many beneficial and detrimental impacts. Understanding the ecology of theses species will allow a landscape level approach to be applied that will minimize negative impacts on the many pressures experienced by our rangelands today.
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Updated: Mar. 18, 1997