Modification of Fire Regimes Inferred from the Age Structure of Two Conifer Species in a Tropical Montane Forest, Mexico

Effect of Fire Severity on the Species Diversity and Structure of a Temperate Forest in Northern Mexico

Forests experience disturbances, such as fire, that affect their functioning, structure, and species composition. The objective of this work was to compare the species diversity and forest structure at sites affected by different degrees of fire severity, 9 years after a forest fire event. We used the differenced Normalized Burn Ratio (dNBR) index. Vegetation was sampled in three severity levels: high (HS), moderate (MS), and low (LS) and included the unburned (U) level as control. In addition, we calculated the species richness (S), the Shannon index (H), and the Evenness Index (E). The structural diversity of tree diameters and heights was measured applying the indices H, E, and the coefficient of variation (CV). The differences in the indices calculated across the fire severity levels were determined through analyses of variance (ANOVA) and Tukey’s multiple comparison tests. The results showed no significant differences (p ≤ 0.05) in the species diversity indices between fire severity levels. The structural diversity of tree diameters and heights was lower at the HS level. dNBR was negatively related to structural diversity; thus, it is concluded that HS tends to reduce structural variability in terms of diameter, height, and age. These results provide a baseline to understand how fire can modify forest structure and species diversity.

Tree-Ring Patterns and Growth Response of Abies jaliscana to Climate along Elevational Gradients in the Mountains of Western Jalisco, Mexico

Jalisco fir (Abies jaliscana) is an endemic tree with restricted distribution, and is considered a Pliocene relict inhabiting the cloud montane forests in the humid-temperate upper zone of western Jalisco Mountains. Despite its ecological importance, to date, there are no studies for this taxon aimed to understand its vulnerability to regional climate variability. We used dendrochronological techniques to investigate tree-ring patterns and the effects of climate variability on the radial growth of Jalisco fir through its elevational range in both southern and northern aspects. We developed tree-ring series that ranged in length from 79 to 152 years. High-elevation trees (2400 m.a.s.l.) showed higher annual radial growth compared to those present in low-elevation sites. Over the last 150 years, Abies jaliscana trees have had higher annual radial growth rates compared to those exhibited by other Abies species. Both winter and early spring precipitation are the main factors determining the growth rates at all the elevation sites for both aspects. In the cold season, mean maximum temperature negatively affected ring-width growth at most of the elevation zones except for two study sites. We conclude that the radial growth of A. jaliscana appears to be smaller during lower rainfall periods in the dry seasons (winter and early spring), and A. jaliscana annual radial growth responds differently to regional climate variability along the elevational gradient and mountain aspects.

Reciprocal Common Garden Altitudinal Transplants Reveal Potential Negative Impacts of Climate Change on Abies religiosa Populations in the Monarch Butterfly Biosphere Reserve Overwintering Sites

Research Highlights: Reciprocal altitudinal transplants of Abies religiosa seedlings within the Monarch Butterfly Biosphere Reserve (MBBR) allow prediction of the impacts of climatic change, because they grow in sites with a climate that differs from that of their origin. Background and Objectives: Climatic change is generating a mismatch between the sites currently occupied by forest populations and the climate to which they have adapted. This study determined the effect on the survival and growth of A. religiosa seedlings of transfer to sites that were warmer or colder than that of the origin of their seeds. Materials and Methods: Eleven provenances of A. religiosa, collected along an altitudinal gradient (3000 to 3550 m a.s.l.), were assayed in common gardens in three sites of contrasting altitude: 3400, 3000 and 2600 m a.s.l. The results were evaluated by fitting a response curve with a mixed model. Results: The climate transfer distance for the seasonal balance between the temperature conducive to growth (degree days above 5 °C) and the available precipitation (a ratio expressed as dryness index) dominated the shape of the response function curve. The rainy season (June–October) dryness index transfer distance was critical for survival, while that of the cold and dry season (November–February) was critical for aerial biomass, and the annual index was critical for the increase in basal diameter. The effect of climatic transfer distance is much more negative (triggering about 45% mortality) when transfer is toward warmer and dryer sites (at 400 m lower in altitude, +1.9 °C warmer and 16% less precipitation), than when shifting toward colder and wetter sites (400 m higher in altitude, resulting in 95% survival). Conclusions: The projected higher temperatures and lower precipitation due to climatic change will undoubtedly cause severe mortality in young A. religiosa seedlings. A 400 m shift upwards in altitude to compensate for climatic change (assisted migration) appears to be a feasible management action.