The purpose of this repository is to share the preliminary results of a project examining change in vegetation between the time a set of field plots (n = 285) was first established in the late 1990's and when the plots were revisited in 2014-2015. The field plots were located on five mountain ranges in the Madrean sky islands in Arizona, USA: Huachucas, Pinalenos, Santa Catalinas, Chiricahuas, and Santa Ritas.
Vegetation was measured in the field for six broad cover types: "Broadleaf Evergreen Woodland", "mixed conifer Forest", "Conifer Woodland", "Ponderosa pine forest", "Transition Forest" and "Grassland" but these analyses will focus primarily on mixed conifer, ponderosa pine, pine oak and Madrean oak vegetation types.
Initial goals of the analysis are:
- Comparison stats and graphs of the distributions of basal area (BA) and trees per hectare (TPH) by cover type
- Simple regression analysis using derived index of change for BA and TPH as response. Predictors will include fire history metrics, topography and climate
Broader goals and questions:
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Develop a conceptual model for a longitudinal study that will help inform the more recent field efforts and future projections.
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We are interested in evaluating change in vegetation communities over this time period, and will address some of the following questions with statistical models:
a) What are the differences with trees per hectare and basal area between the 90s and 2014/2015 within and between vegetation cover types, accounting for fire severity and time since fire?
b) What are the differences with shrubs and canopy cover between the 90s and 2014/2015 within and between vegetation cover types, accounting for fire severity and time since fire?
c) Are there locations that can be classified as type conversion (e.g., is there evidence of regeneration)?
During the modeling process, we aim to experiment with:
- developing indices that represent change in field measurements between the two time periods and quantify variability in climate and
- incorporate variability in climate and mountain range (latitudinal gradient) into the above questions.