Climate is critically linked to vegetation dynamics at many different spatial and temporal scales across the desert Southwest. Small-scale, short duration monsoon season thunderstorms can bring much needed precipitation to small patches of vegetation or can initiate widespread flooding. Long-term variations in climate related to ocean circulation patterns can create multi-decade wet or dry periods that can promote large-scale, episodes of recruitment of certain species (wet periods) or large-scale mortality (dry periods) (Allen and Breashears 1998).
Species native to the desert Southwest have evolved with this complex variability and have special adaptations to take advantage of ephemeral moisture sources and to endure long drought periods (Dimmit 2000). Competition for resources (moisture, light, nutrients) is fierce in the often resource-limited environments of the Southwest and the delicate balance between native species can be quickly disrupted by more competitive exotic species. Other arid places in the world besides the southwestern United States support similar vegetation communities, but the climate of Arizona has unique features important to the interaction between native species and non-native invasive species.
The US Global Change Research Program (Carter, 2003) described the mounting issue of climate variability and invasive plants as follows: “Some native species are unlikely to be able to adapt fast enough to the changing climate regimes, resulting in a lowered competitive edge and weakened resistance of ecosystems to infestations by invasive plants and animals. Potential impacts include shifts in the relative abundance and distribution of native species, significant changes in species richness and communities, and local extinctions of native species. Subtle changes in the diurnal (day/night) or seasonal patterns of temperature have also been shown to affect plant community composition.”