Legacies on the landscape: Changing patterns with changing environments

Background/Question/Methods

In recent years, studies focusing on long-term anthropogenic legacies on the landscape have increased dramatically.  It is now well established that legacies of past events can alter landscape patterns and processes for centuries and millennia.  The recent work of many researchers clearly demonstrates the prevalence of legacies that alter both patterns and processes in soils, vegetation and other landscape elements.  An assumption inherent in many of these studies is that the strength of the legacies recorded represents a specific and persistent change to the local conditions.  In contrast, we are finding that legacies effects can vary with significant variation of climactic conditions.  In some cases legacie effects may not be apparent unless ecological conditions are stressed or otherwise significantly different from ‘normal’.  In this paper we provide evidence that a legacy of ancient farming on certain populations of woody species in the lower Sonoran Desert appears to be visible only during extreme drought conditions while other populations are less changed.  Under normal conditions, two drought tolerant woody species Larrea tridentata and Ambrosia deltoidea, dominate the desert bajadas in the northern Phoenix Basin of central Arizona. The area within our study site contains ancient agricultural terraces and other dry farming features that were abandoned ca. AD 1250.  Initial studies of the woody vegetation in this area showed that the population of L. tridentata on fields appeared to be comprised of smaller individuals spaced further apart, while the populations of A. deltoidea on fields did not have any significant differences between locations. 

Results/Conclusions

This study documents the effect of a severe drought that spanned several seasons in our study area beginning in 2000 and extending into 2003.  We found that under extreme stress conditions, A. deltoidea suffered significantly higher mortality on the ancient fields than in off field locations. We conclude that the high mortality in stress conditions is likely due to the removal of cobbles and other clasts by farmers over 700 years ago.  The removal of clasts reduces that available moisture in the upper soils of the fields causing severe stress for the shallow-rooted A. deltoidea, while the more deeply rooted L. tridentata, were able to survive the drought conditions.  When studying long-term effects of change on landscapes it is important to keep in mind that any given legacy measured is the legacy apparent within the constraints of the conditions occurring during the study period.