CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration 

Woody plant encroachment into semiarid savanna parkland - a case study from southern Texas, USA

 Zou Chris¹  Archer Steve²

(¹ Department of Rangeland Ecology and Management, Texas A&M University, College Station, Texas 77843, USA; ² School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721-0043, USA)

Abstract: Woody plant encroachment into grassland and savanna has occurred worldwide in recent history. This phenomenon jeopardizes grassland biodiversity and threatens the sustainability of commercial and pastoral livestock production, alters wildlife habitat, and changes land surface-atmosphere interactions. Quantitative and historical assessments indicate that woody plant abundance has increased substantially in grassland during the last 50 –100 years in the subtropical savanna parklands in southern Texas, USA. However little is known of tree-tree interactions and how these might influence ecosystem function and dynamics. In this study we examine how facilitation and competition affect interactions between an overstory tree, honey mesquite (Prosopis glandulosa), and associated understory shrubs, so that we may mimic natural processes of facilitation and competition to reduce the amount of inputs needed to restore ecosystem function. 

Results indicated that the process of woody plant encroachment was initiated by the establishment of the phreatophytic mesquite tree. Initial seedling establishment of mesquite tree is attributable to its fast growing taproot which enables it to minimize resource competition with grasses; and to grazing-induced elimination of grass fire. We have found that mesquite tree roots can redistribute moisture from deep subsoils to drier upper soil profiles using the root system as a conduit in a process that has been termed ‘hydraulic lift.’ Established mesquite trees then serve as a nucleus to facilitate the ingress and expansion of shallow-rooted understory shrubs through shading, providing perch structure for birds disseminating seeds of other woody plants, and hydraulic lift. The present two-phase pattern is moving toward mono-phasic woodland as new clusters are initiated and existing clusters expend and coalesce.

In order to remain the grassland function and avoid the occurrence of encroachment, it is essential to prevent the facilitative species mesquite tree from establishment in the landscape, which can be realized by reducing grazing pressure and inducing grass fire at a certain frequency or even mechanical or chemical brush clearing at the initial downgrading phase of grassland. Once the facultative mechanism is removed from the system, the late stage ingress of unpalatable shrubs will be restricted or even eliminated. In restoration projects targeting increasing woody component in semiarid landscape, particularly in sites with favorable deep soil moisture, it is therefore ecologically meaningful and economically efficient by focusing on establishing deep-rooted plants into the landscape and let the natural facilitation mechanism to take place. 

The woody plant encroachment in southern Texas includes a grazing-induced soil carbon degradation phase followed by an aggradations phase that begins when unpalatable woody plants establish, grow, modify microclimate, and enrich soil nutrients. Current plant and soil C and N mass is substantially greater than that under “pristine” conditions. The recently developed shrub-dominated landscapes are highly resilient following disturbance and provide habitat for numerous wildlife species. In this case, the system is now ecologically diverse, productive and functional. It is likely degraded or dysfunctional only with respect to its socioeconomic value for domestic animal grazing. Due to the unclear influence of global climate change on herbaceous and woody plant growth forms, strategies for conservation of grasslands and savannas threatened by woody plant encroachment are evolving and are challenged to realize a balance between socioeconomic and ecological concerns.

Key words: vegetation dynamics; semiarid savanna parkland; woody plant encroachment; hydraulic lift; facilitation; restoration 

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