Conifer forests in northwestern Mexico have not experienced systematic fire suppression or logging, making them unique in western North America. Fire regimes of Pinus jeffreyi Grev. & Balf. mixed conifer forests in the Sierra San Pedro Martir, Baja California, Mexico, were determined by identifying 105 fire dates from 1034 fire scars in 105 specimens. Fires were recorded between 1521 and 1980 and median fire return intervals were less than 15 years at all compositing scales. Significant differences in mean fire return intervals were detected from 1700 to 1800, 1800 to 1900, and 1900 to 1997, most often at intermediate spatial compositing scales, and the proportion of trees scarred in the fires of the 1700s was significantly different from the fires of either the 1800s, the 1900s, or the combined post-1800 period. Superposed epoch analysis determined that moderate and large spatial scale fires occurred on significantly dry years during the length of the record, but before 1800, these fires were preceded by significantly higher precipitation 1 year before the fire. The dominance of earlywood fires in the Sierra San Pedro Martir is similar to the seasonality found in the southwest United States and is different from the western slope of the Sierra Nevada and Klamath Mountains of California.

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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. #

In Mediterranean environments in western North America, historic fire regimes in frequent-fire conifer forests are highly variable both temporally and spatially. This complexity influenced forest structure and spatial patterns, but some of this diversity has been lost due to anthropogenic disruption of ecosystem processes, including fire. Information from reference forest sites can help management efforts to restore forests conditions that may be more resilient to future changes in disturbance regimes and climate. In this study, we characterize tree spatial patterns using four-ha stem maps from four old-growth, Jeffrey pine-mixed conifer forests, two with active-fire regimes in northwestern Mexico and two that experienced fire exclusion in the southern Sierra Nevada. Most of the trees were in patches, averaging six to 11 trees per patch at 0.007 to 0.014 ha−1, and occupied 27–46% of the study areas. Average canopy gap sizes (0.04 ha) covering 11–20% of the area were not significantly different among sites. The putative main effects of fire exclusion were higher densities of single trees in smaller size classes, larger proportion of trees (≥56%) in large patches (≥10 trees), and decreases in spatial complexity. While a homogenization of forest structure has been a typical result from fire exclusion, some similarities in patch, single tree, and gap attributes were maintained at these sites. These within-stand descriptions provide spatially relevant benchmarks from which to manage for structural heterogeneity in frequent-fire forest types.

A time-series autoregressive moving average (ARMA) approach was used to develop stochastic models of tree crown profiles for five conifer species of the Sierran mixed conifer habitat type. Models consisted of three components: (1) a polynomial trend; (2) an ARMA model; and (3) random error. A Bayesian information criterion was used to evaluate alternative models. It was found that 70% of the crown profiles could be modeled using first-order ARMA [AR(l) or MA(l)] models, and that an additional 25% could be modeled using a white noise model [(AR(O)]. When the coefficients of the ARMA models were statistically significant, the models proved to be both visually and statistically an improvement over the polynomial trend (a Euclidean model). A binary classification system was used to determine if model type was related to tree or stand characteristics. Using this classification we found that it was possible to relate the appropriate model type to forest tree size and forest stand density with acceptable accuracy.