genetic change associated with mesquite invasion

The invasion of desert grassland environments by shrubs is an active area of concern for both resource managers and researchers. Previous research has emphasized the ecological basis for the invasion and its long-term consequences. In southern Arizona, velvet mesquite [Prosopis velutina (Fabaceae)] began to invade upland grassland environments around 1880. This invasion is generally associated with a combination of factors caused by very high livestock densities, severe drought, and reduced fire frequencies. Before this, mesquite existed primarily only along regular watercourses where saturated soil layers are relatively shallow. The invasion involved colonization of upland environments where saturated soil layers may be relatively deep.

It is generally assumed that this invasion is driven principally by ecological changes that permitted mesquite seedlings to establish and persist in upland environments. Nothing is known about the extent to which natural selection may have acted to modify mesquite populations to affect their establishment and persistence in upland environments. If such natural selection has occurred, it is reasonable to assume that it continues to occur and affects the ongoing invasion of upland environments.

In this research we are addressing the following questions:

Are there heritable differences in seedling growth or development that might be associated with upland establishment between trees in the source (watercourse) and neighboring upland population?

We are utilizing populations of mesquite from a large watercourse with many large trees and an adjoining upland on the Santa Rita Experimental Range south of Tucson. Repeat photography here permits visualization of the mesquite invasion of upland environments since 1900.

Our long-term goal is to use findings from this research to begin characterize how mesquite populations change genetically in response to a variety of environmental changes.