Structure and Composition of a Dry Mixed-Conifer Forest in Absence of Contemporary Treatments, Southwest, USA

Douglas Cram,Terrell Baker,Pradip Saud

doi:10.3390/f8090349

Abstract

Dry mixed-conifer forests in the Southwest occupy an important ecological and hydrological role in upper watersheds. In the absence of reoccurring fire and silvicultural treatments over the last 50 years, we quantified forest structure and composition on prevailing north and south aspects of a dry mixed-conifer forest in southcentral New Mexico using mixed models and ordination analysis in preparation for an experiment in ecological restoration. Results indicated overstory and midstory were dominated by Douglas-fir (Pseudotsuga menziesii) and shade tolerant/fire intolerant white fir (Abies concolor) with interspersed mature aspen on north aspects, and Douglas-fir and Southwestern white pine (Pinus strobiformis) on south aspects. Ponderosa pine (Pinus ponderosa), which was historically co-dominant with Douglas-fir on north and south aspects, was subdominant on south aspects and almost entirely absent on north aspects. Regeneration was dominated by white fir saplings and seedlings on north aspects while ponderosa pine was completely absent. South aspect saplings and seedlings were characterized by Douglas-fir and Southwestern white pine, but almost no ponderosa pine. Ordination analysis characterized the effect of aspect on species composition. Understanding contemporary forest structure and composition is important when planning for desired future conditions that are to be achieved through ecological restoration using silvicultural techniques designed to foster resilience.

Highlights

Quantification of forest stand attributes and composition are important metrics in forest management
The alpine tundra and subalpine coniferous forest lie the two dominant cover types of the Sacramento Mountains: (1) upper montane coniferous forests composed of Douglas-fir, white fir, and ponderosa pine; and (2) lower montane coniferous forests composed of ponderosa pine, piñon pine (Pinus edulis), juniper (Juniperus spp.) and oak (Quercus spp.)
Forest structure and composition of mixed-conifer stands were primarily contrasted between north and south aspects

Summary

Introduction

Quantification of forest stand attributes and composition are important metrics in forest management. Compositional complexity and mixed severity fire regimes have been cited as to why dry mixed-conifer forests in the Southwest are less understood [1,2]. A number of factors combined to change forest structure and composition, fuel conditions, and the historic natural fire regime in Southwestern forests over the last 130 years. Similar to lower elevation pine forests, frequent fire historically shaped stand development, structure, and vegetation composition in Southwestern mixed-conifer forests [3,4,5,6,7,8,9]. Depending on weather and fuel conditions, torching and crowing (i.e., mixed-severity fire regime) on a stand scale influenced forest development [10]. In the absence of frequent fire, a shift in overstory species dominance from shade intolerant ponderosa pine to shade tolerant white fir (Abies concolor) and intermediate shade tolerant Douglas-fir (Pseudotsuga menziesii) has been reported [11,12,13,14]

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Conclusion