Growing here or there? Changes in red maple growth performance along complex environmental and topographic gradients

Background/Question/Methods

The realized niche of a species can be defined as the set of conditions, both biotic and abiotic, that a species actually occurs in. However, within the realized niche space conditions are not homogenous and thus the species may have differential performance across this n-dimensional space. Fine-scale differences in factors such as nutrient availability, moisture availability, below canopy temperature, and topography may have significant influence on a species performance within its range limits. It is important to understand how these fine-scale factors affect species performance especially in the context of predicted range shifts due to climate change.

To evaluate performance we measured annual growth increment in red maple (Acer rubrum), using dendroecological techniques, along six transects in Great Smoky Mountains National Park. Transects spanned the full elevational range of red maple within the park (500-1500 m). Within the overall elevational gradient transects were positioned to capture differences in distance from streams, aspect, and slope, thus covering all major topographic aspects of the gradients that exist within the park. At eight points along each transect we collected increment cores from five red maple individuals. We also measured plant available soil nutrients and moisture, and below canopy temperature at each point. 

Results/Conclusions

Data were analyzed using mixed effect models to relate annual growth to nutrient availability, moisture availability, below canopy temperature, and topographic position. Results showed complex relationships between these factors and growth response. Annual growth increment was not related to elevation. There was a generally positive relationship between nutrient concentrations (Ca, Mg, K, and P) and growth increment. However, there was a negative relationship between nitrogen concentration and growth increment. Further modeling will work towards elucidating these relationships and the interactions between them.

Differential performance due to fine-scale heterogeneity may have implications for species success in colonizing new territory, or persisting within current range limits, as large-scale climate forcing’s change overarching conditions. While territory may be suitable based on large-scale climatic conditions, low competitive ability due to adverse fine-scale factors may limit a species ability to colonize a site and displace other species. Conversely, a species’ persistence within its current range-limits may depend on fine-scale suitability. Legacy effects may result in individual species ranges and communities that are not in sync with optimum climate conditions.