QUESTION SET FOUR
Forests, Fall 2018
DUE  by end of classes Friday 14 Dec.


ANSWER THESE TWO of these THREE QUESTIONS ABOUT ECOSYSTEM FUNCTION:

1.  Ecosystems come in all scales.  A compost pile is a decomposer-driven ecosystem, where the energy input comes through the break-down of organic matter from outside the ecosystem (small streams are often primarily 'decomposer-dominated' systems as well). The compost pile has internal trophic dynamics, food webs, nutrient cycles, etc.  A gardener might want their compost pile-ecosystem to break down organic wastes (thus liberating nutrients tied up in organic material to return to the garden) as quickly as possible; thus, they would want the pile to support large populations of decomposers with high rates of metabolism.  There are two main groups of decomposers -- fungi and bacteria.  Bacterial decay tends to be faster, while a fungus-dominated compost pile works more slowly.  In practice, people have long noticed that compost piles too full of some kinds of material become fungus-dominated and very slow to break down fully; these types of materials include, for example, wood-chips and dry tree leaves of some types.  Fast, bacterial decay can be sustained when there is a high proportion of green plant material (vegetable scraps, grass clippings, etc.) or animal wastes.  Think about this is in terms of ecosystem dynamics (READ THE HINTS below) and
a) come up with a hypothesis for what might drive a 'switch' between fungal and bacterial dominance in the compost-pile ecosystem.
b) Sometimes a compost pile can get TOO active, leading to breakdown of materials so fast that nutrients are lost before the compost is added back to the garden (or even getting so hot it catches fire!).  Given your hypothesis offer a suggestion for how you might cool/slow down a compost pile (perhaps by limiting bacterial dominance) -- and why it should work.  
    HINTS: The energy source for all decomposers is the break-down of chemical bonds between carbon atoms in organic molecules (especially carbohydrates -- which are 'pure' caron/hydrogen/oxygen).  But, of course, other nutrients/minerals are required to build organisms and support their function.  Wood and dry leaves have a very high carbon concentration (they're made up almost entirely of combinations of carbohydrates).  Green plant materials and animal wastes have much higher concentrations of PROTEINS (remember that proteins are made of nitrogen-containing amino acids).  CONSIDER that there is a parallel here with the Lake Washington story.

2. Many New England forests have been harvested at intervals of 70-80 years for over 200 years. The following graph shows general living biomass trends for such a forest over this time period, with four logging episodes.  You can think of this curve as showing the accumulation of NPP -- the excess of GPP over respiration and decay, accumulating as biomass (or as carbon). The sharply descending parts of the curve show removal of biomass (wood) in logging. Describe any other patterns or trends you see over the several cycles of logging and regrowth. Offer a hypothesis explaining the dominant patterns in terms of ecosystem processes; use terms and concepts from ecosystem ecology (e.g., you might need to refer to gross and/or net production, respiration, limiting resources,...). There's an appearance of unsustainability here; offer two possibilities (derived from your hypothesis) for ‘improving’ the situation; how might you change things to keep the biomass available for harvest from becoming less each time?

forest_growth














3. Human burning of fossil fuels injects large amounts of carbon dioxide into the atmosphere.  CO2 is, of course, the source of carbon for photosynthesis and so an essential resource for primary production (by photosynthetic autotrophs).  It has been suggested that added CO2 should, therefore, act as a fertilizer, increasing plant growth and NPP.  If this were the case, there is the potential that ecosystems would become 'carbon sinks', removing (some of) the excess CO2 from the atmosphere and sequestering it in added biomass, thus reducing the rate at which this most important 'greenhouse gas' builds up in the atmosphere.  This would be a desirable thing. However, as we have seen, the regulation of ecosystem productivity is complex, and this outcome depends on several other things. Describe at least one assumption --  in terms of ecosystem process and properties -- of this hypothesis; that is, what would have to be true before this  CO2 fertilization effect (increased NPP resulting in increased sequestration of carbon) could take place?  Imagine you were in charge of things; think of some practices you could implement to INCREASE the likelihood that vegetation would take up more  CO2 as it became available through increased rates of NPP.

AND TWO of these THREE ABOUT SUCCESSION

4. We have discussed some of the trends one might expect in successional dynamics, particularly in terms of changes in importance of 'life-history' types (for example, 'r-selected' opportunists are particularly well adapted for dispersal and for rapid exploitation of abundant resources; 'K-selected' competitors may grow and disperse more slowly, but have high competitive ability).. Taking all of this into account, discuss how you would expect SPECIES DIVERSITY (considered simply as the number of different species present in a community -- also called 'species richness') to change over ‘successional time’ – that is, over the years following a major disturbance?  TO MAKE THIS EASIER (maybe), assume that this happens in a habitat where late-successional communities are closed FORESTS (like most of our local landscape). Sketch a graph showing, generally, how you think species diversity would be related to time since disturbance (don't worry about actual numbers; focus on shape -- trends and slopes), and explain why you predict this pattern. Use appropriate concepts and terminology about competition and niche dynamics.

5. Review the case study of the Boundary Waters Canoe Area, from which Heinselman developed the 'shifting mosaic' model of landscape dynamics.  Suppose climate change caused a decrease in the frequency of fires such that return time increased from around 100 years to, perhaps, 150-200 years, with other factors staying much the same. Predict how the properties of the 'landscape mosaic' might change.   What changes would you predict if  return time DECREASED significantly?  Comment specifically on how these changes might affect overall diversity.

6. Increased movement of people and materials around the world has, in recent decades, led to the introduction and establishment of many non-indigenous species in our area (for example, of the approximately 2100 plant species growing 'wild' in Vermont, one-third are the result of human introduction).  Introduced species include plants, animals, fungi, and microorganisms (including some that are pests or pathogens).  Many conservationists view such introduced organisms as potential environmental threats, but views on this issue differ.  
     A. Using what you have learned about species interactions (competition and niche theory) and community dynamics (disturbance and succession), suggest two or three ways in which introduced PLANTS might affect regional plant communities and diversity.  Under what what circumstances would you regard introduction of a non-indigenous species as an environmental 'threat' ? Explain your reasoning (you may need to introduce some subjective valuations here; that's okay). 
    B. All of the species that live in our area have dispersed here from further south over the last 10,000 years as their ranges expanded following the retreat of the last ice sheet.  Consequently,  ecological communities have gone through long series of transformations as species entered and left any particular region.  How are these changes similar to or different from the  consequences of introductions of species from other parts of the world by human agency?