Eladio H. Cornejo-Oviedo

El Niño-Southern Oscillation effect on a fire regime in northeastern Mexico has changed over time

The El Niño Southern Oscillation (ENSO) is a climate-forcing mechanism that has been shown to affect precipitation and the occurrence of wildfires in many parts of the world. In the southern United States and northern Mexico, warm events (El Niño) are associated with moist winter conditions and fewer fires, while cool events (La Niñia) tend to favor dry winters and more fires. We tested this relationship in a region of northeastern Mexico by characterizing the historical fire regime and climatic influences: Fire regimes were reconstructed from fire-scar samples collected from 100 trees in three high-elevation sites on Peña Nevada in southern Nuevo Le6n. The sites were approximately 25 ha each, and the site centers were approximately 1 km apart. The earliest recorded fire occurred in 1521 and the time period we used for analysis was 1645-1929. The sites were characterized by frequent surface fires before the 1920s. In the three sites, mean fire intervals ranged from 8.6 to 9.6 years (all fires) and 11.9 to 18.6 years (fires that scarred > or = 25% of recording trees). The per-tree mean fire return interval was 17 years, and all three sites burned in the same year seven times between 1774 and 1929. After 1929, fires were nearly eliminated in all sites, likely due to human causes. We found a temporal change in the association between ENSO events and fires; before the 1830s La Niña events were significantly associated with fire years, while after the 1830s this association was not significant. In 1998, when the most severe El Niño event of the past century occurred, the three sites experienced severe, stand-replacing fires that killed many trees that had survived multiple surface fires in the past. Prior to the 1830s, fires tended to occur during dry La Niña years, but since then both La Niña and El Niño have been associated with dry years in this region, especially during the last three decades. This result suggests that ENSO effects have changed over time in this location and that phases of ENSO are not consistent indicators of precipitation, fire occurrence, or fire behavior in this area of northeastern Mexico.

Thinning dense sapling stands of mesquite (Prosopis glandulosa var. glandulosa) to optimize timber production and pasture improvement

Abstract Our objective was to identify mechanical, chemical or a combination of treatments that would convert dense stands (over 7000 stems ha−1) of small trees into widely spaced, large trees to maximize lumber production and pasture improvement. A randomized complete block design was utilized with four replicates and six treatments. Strips of 8 m width were cleared in two perpendicular directions leaving 2 m × 2 m squares on 10 m rectangular spacings. Pruned or unpruned crop trees were permitted to grow in the 2 m × 2 m squares. To prevent reinvasion of new mesquites until the crop trees became dominant, discing, discing plus seeding with winter rye, and spot spraying with herbicides were applied to the interlying spaces. After 2.5 years, significant treatment differences were found for growth in basal diameter (P = 0.0014), height (P = 0.0261), basal area (P = 0.0063), and dry weight (P = 0.0082). The greatest crop tree growth occurred with annual discing. The greatest reduction in resprouts was obtained with annual spot spraying with herbicides. The basal diameter growth of 1.25 cm year−1 for the disced and pruned treatment is comparable with growth of other high-value hardwoods (i.e. cherry) in northeastern USA.

Fine-scale factors influence fire regimes in mixed-conifer forests on three high mountains in Mexico

We investigated the influence of broad- v. fine-scale factors on fire in an unusual landscape suitable for distinguishing the drivers of fire synchrony. Our study was conducted in the Sierra Madre Oriental mountain range, in north-eastern Mexico. We worked in nine sites on three parallel mountains that receive nearly identical broad-scale climatic influence, but between which fires are unlikely to spread. We collected and cross dated samples from 357 fire- scarred trees in nine sites in high-elevation mixed-conifer forests and identified fire dates. We used Jaccard similarity analysis to evaluate synchrony among sites and quantified relationships between climate and fire occurrence. Fires were historically frequent(meanfire intervalrangedfrom 8to16yearsinallsites)anddates offireexclusionrangedfrom1887 to1962.Wefoundlowfiresynchronyamongthethreemountains,indicatingastronginfluenceoffine-scalefactorsonfire occurrence. Fire regime attributes were similar across mountains despite the independence of fire dates. La Nina events were associated with fire over time, although not significantly since the 1830s. Our results highlight the importance of scale in describing fire regimes and suggest that we can use fire history to understand controls on complex ecosystem processes and patterns. Additionalkeywords: climate,dendrochronology,firehistory,firesynchrony,SierradeArteaga,SierraMadreOriental.