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 À1 . 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. #

Historical and current landscape‐scale ponderosa pine and mixed conifer forest structure in the Southern Sierra Nevada

“…However, a paired t-test on the two canopy cover estimates indicated no significant difference (P ¼ 0.57) suggesting that our FVS estimates of canopy cover are accurate. It is worth noting that both the plot-based and FVSprojected canopy cover estimates agree with a previously reported canopy cover estimate (25.3%) from a larger sampling area in the same forest of northern Baja California, Mexico, which was based on ten 100 m canopy line intercept transects (Stephens and Gill 2005).…”

“…FVS then calculates the percentage of ground area directly covered with tree crowns, correcting for canopy overlap (Dixon 2002). To investigate how well FVS canopy cover estimates approximate field-based observations in old-growth forests, we compared field-measured canopy cover from an intact Jeffrey pine-mixed forest in the Sierra San Pedro Mártir, Mexico (Stephens and Gill 2005), to FVSestimated canopy cover for the same plots. This was done for 25 0.1-ha circular plots, which were established on a regular 125-m grid.…”

Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests

“…The first is that over our 4000 ha study area historical forest structure and composition varied considerably (Fig. 3), suggesting that average conditions alone are very much an oversimplification of historical forest conditions (Stephens and Gill 2005). As such, the historical data we present do not support the idea of basing management targets for restoration and forest resilience treatments on mean values (North et al 2009b).…”

Landscape‐scale modeling of reference period forest conditions and fire behavior on heavily logged lands

“…Standard surface fuel models were used, rather than custom models, because the standard models have been calibrated with observed fire behavior under conditions similar to those simulated in the model runs Rinehart 1983, Burgan andRothermel 1984). To more accurately evaluate potential crown fire behavior for each subtype, we added seedlings and saplings to each subtype tree list based on estimates from previous research (Parker 1989, Taylor 2004, Stephens and Gill 2005, Scholl and Taylor 2006, Beaty and Taylor 2007.…”