Remote Sensing Analysis to Quantify Change in Woodland Canopy Cover on the San Carlos Apache Reservation, Arizona, USA (1935 vs. 2017)

Barry Middleton,Laura Norman

doi:10.3390/land10040393

Abstract

Since the late 1800s, pinyon–juniper woodland across the western U.S. has increased in density and areal extent and encroached into former grassland areas. The San Carlos Apache Tribe wants to gain qualitative and quantitative information on the historical conditions of their tribal woodlands to use as a baseline for restoration efforts. At the San Carlos Apache Reservation, in east-central Arizona, large swaths of woodlands containing varying mixtures of juniper (Juniperus spp.), pinyon (Pinus spp.) and evergreen oak (Quercus spp.) are culturally important to the Tribe and are a focus for restoration. To determine changes in canopy cover, we developed image analysis techniques to monitor tree and large shrub cover using 1935 and 2017 aerial imagery and compared results over the 82-year interval. Results showed a substantial increase in the canopy cover of the former savannas, and encroachment (mostly juniper) into the former grasslands of Big Prairie. The Tribe is currently engaged in converting juniper woodland back into an open savanna, more characteristic of assumed pre-reservation conditions for that area. Our analysis shows areas on Bee Flat that, under the Tribe’s active restoration efforts, have returned woodland canopy cover to levels roughly analogous to that measured in 1935.

Highlights

Among the most widespread and dramatic vegetation changes to occur since the late 1800s in the western United States is the expansion and increase in density of juniper (Juniperus spp.) and pinyon (Pinus spp.) trees [1,2,3,4,5,6,7,8,9,10]
Our results depict a substantial increase in the woodland tree canopy cover between 1935 and 2017 (Figures 6a–h and 7c,d), with encroachment into the Big Prairie grasslands (Figure 7a,b)
Grassland of Big Prairie shows some areas of encroachment by woodland trees over this time frame

Summary

Introduction

Among the most widespread and dramatic vegetation changes to occur since the late 1800s in the western United States is the expansion and increase in density of juniper (Juniperus spp.) and pinyon (Pinus spp.) trees [1,2,3,4,5,6,7,8,9,10]. Former savanna areas dominated by native grasses and forbs were replaced with higher-density pinyon–juniper woodlands, with the degraded soils and herbaceous understories that are often associated with them [15]. Periodic fire in these historical savanna environments had resulted in the low density of trees. Higher-density woodlands were confined to rocky ridges, hillslopes, and flats where grasses were discontinuous and fine fuels were insufficient to carry fires [5,6,7,13,15,16,17]

Methods

Results

Discussion

Conclusion