Forest structure and mortality in an old-growth Jeffrey pine-mixed conifer forest in north-western Mexico

Abstract

Over 10 million hectares of forests in the western United States are in need of restoration. Restoration targets benefit from quantitative descriptions but many old-growth definitions are qualitative. Quantification of live forest structure and mortality in Jeffrey pine-mixed conifer forests in the Sierra San Pedro Martir (SSPM), Mexico, was done to assist in the development of restoration goals and to increase our understanding of old-growth forests. Conifer forests in the SSPM have not experienced systematic fire suppression or harvesting making them unusual in western North America. Tree and soil data were collected from a systematic design of plots. High variability characterized all structural attributes measured in this forest. This high variation is probably the result of the relatively intact frequent surface fire regime and no history of harvesting in the sampled area. Cumulative tree mortality was 2.7–3.6%; the annual rate of tree mortality was 0.162% yr −1. Hierarchical cluster analysis determined that 33% of sampled plots included a relatively small number of large trees, 24% of plots had bi-modal diameter distributions, and 43% of plots had inverse-J diameter distributions. Separating these categories into seral stages is difficult since all plots included relatively large trees. Stand structure classes include old forest single-stratum, young multi-strata, and old forest, spatially distinct multi-strata. The forests of the SSPM have a great deal of variation and western United States forests with similar species, soils, topography, and disturbance regimes would be expected to have similar variation prior to fire exclusion. Restoration of similar western United States forests should not use uniform restoration targets. Methods must be developed to incorporate more variation in stand-level prescriptions. Conservation of the forests in the SSPM is critical because it is one of the last landscape-scale, old-growth-mixed conifer forest with a relatively intact frequent surface fire regime in western North America.