Ecological Stress (Plant Physiology, Ecosystem Ecology, Field Ecology, Entomology, Mammalogy, Stable Isotope Techniques). Students could incorporate more sophisticated and accurate measures of plant growth to better understand net primary productivity along the gradient (e.g., measuring annual stem and branch growth and root ingrowth). Using stable isotope techniques on tree rings, students might reconstruct historical patterns of how water availability (?D), water-sources (?18O), and atmospheric nitrogen deposition (?15N) influence productivity. Students could also examine biotic constraints on productivity, such as competition and herbivory.

Dynamic Ecotones (Field Ecology, Microbial Ecology, Ecosystem Ecology). Students might quantify the importance of nurse plants - which are critical in arid environments (Holmgren et al. 1997) - for survival of piñon trees in response to the 100-year record drought described in Module 2. Data derived from georeferenced aerial photographs and GPS receivers could be incorporated into GIS layers using ArcView and S-SpatialStats for Windows to map and Meadow and quantitatively analyze patterns of piñon and juniper survival. Students could determine how nurse-plant mortality and the associated sudden decrease in nutrient demand and increase in organic matter input to soil affects soil nutrient dynamics.

Dominant and Keystone Species (Entomology, Field Ecology, Ecosystem Ecology, Mammalogy). Students might determine the ecosystem roles of dominant and keystone species (e.g., prairie dogs, elk) by studying effects of their removal on ecosystems. For example, the piñon scale causes needles to drop prematurely, reducing foliar biomass and leaf area index substantially (Cobb and Whitham 1993, Gehring et al. 1997), likely leading to changes in the microenvironment (e.g., increased soil temperatures) with important ecosystem-level consequences (e.g., increased decomposition rates).

Climate Change and Carbon Balance (Plant Physiology, Field Ecology, Ecosystem Ecology, Stable Isotope Techniques). Students could integrate litterfall with stem and branch increment, root growth, and litter decomposition for a more complete picture of carbon balance. Also, students might transplant soil monoliths along the gradient to simulate predicted future climate change, and monitor soil carbon processing to understand soil feedbacks to atmospheric CO2. In Stable Isotope Techniques, soil transplants could include a natural 13C tracer (due to differences in C3 and C4 plant inputs to soils) that exists along the CHM Gradient allowing one to separate the losses of old carbon from inputs of new production (e.g. Cheng 1986).

Biodiversity and Ecosystem Function (Plant Physiology, Ecosystem Ecology, Field Ecology). Students may actively seek to integrate data from other modules in developing more robust interpretations of ecosystem function. For example, litterfall and soil respiration (Module 1), soil respiration (Module 4), and dendrochronology (Module 2) may supplement the measurements outlined for this module. Reconsideration of data from Module 3 on Keystone Species may alter students' thinking in the sense that species richness alone may be insufficient to understand the importance of biodiversity. Students may also seek out data at spatial scales other than those originally used in this module; for example, might NDVI data obtained at scales less than 1km be more appropriate, or lend different interpretations to, the analysis of NPP based on remote sensing?