Ecosystem Ecology ch 4-7

Surface conductance

_______________________ determines the flux of water vapor from inside soil and leaves to the near-surface air, and limits ecosystem water losses when soils are dry.

Aerodynamic conductance

____________________________________ determines the flux of water vapor from the near-surface air to the bulk air, and controls the rate of water loss when soils are moist, particular in rough, well-coupled canopies.

What similarities do the controls on primary productivity in lakes share (a.) with controls on productivity of the marine coastal boundary zone biome, and (b.) with controls on productivity in the open ocean marine biomes?

(a.) Eutrophic lakes are similar to the marine coastal boundary zone biome because both receive high nutrient inputs from land sources. Eutrophic lakes receive nutrients via groundwater inputs and surface water runoff. Marine coastal areas primarily receive nutrients through runoff from freshwater rivers and adjacent land. In having high nutrient availability, they are both instead limited by light. Small lakes and the coastal boundary zone biome both have high levels of benthic productivity, as the bottom sediments are within the euphotic zone of both types of systems. Mixing from the benthos supplies nutrients to the water column in both lakes and the marine coastal boundary zone biome. (b.) Oligotrophic lakes experience nutrient limitation, similar to open oceans that do not have any nearby land to supply nutrients. These two types of ecosystems are likely to be nutrient instead of light limited. However, both lake pelagic zones and open oceans can have varying degrees of stratification and vertical mixing, and in some cases mixing may be deep enough to supply nutrients from depth to surface waters (e.g., during coolseason destratification in lakes and at higher latitudes where ocean waters are cooler and less strongly stratified). Deep mixing can also carry phytoplankton below the euphotic zone, thereby reducing primary productivity, in both the open ocean and deeper lakes.

Contrast the effect of high primary productivity on fish between (a.) lakes with strong "bottom-up" influences on secondary productivity, and (b.) lakes experiencing severe eutrophication.

(a.) High rates of primary production will support large fish populations in lakes with strong bottom-up influences on secondary productivity. That is, if phytoplankton densities are greater, then there is more food for the zooplankton to eat, and therefore more food for the fish to eat. (b.) In eutrophic lakes, conversely, high levels of primary productivity may reduce fish biomass by causing anoxia, possibly toxicity, and ultimately fish kills. The large phytoplankton biomass produced in eutrophic lakes dies and sinks, generating large inputs of organic matter and heterotrophic respiration in deeper water. This heterotrophic respiration consumes deeper-water oxygen, which cannot be resupplied from the atmosphere and surface waters because of strong stratification and a shallow mixing depth (the shallow mixing depth results from poor light and thus poor heat penetration, which are outcomes of the turbidity [murky water] created by all the phytoplankton). Additionally, many of the types of algae that are abundant in eutrophic lakes produce toxins.

(a.) When irradiance is low, which chemicals in photosynthesis don't get produced in adequate supply (i.e., biochemically, why is light needed in photosynthesis)? (b.) When nitrogen availability is low, which enzyme critical to photosynthesis doesn't get produced in adequate supply (i.e., biochemically, why is nitrogen needed in photosynthesis)?

(a.) When irradiance is low, ATP and NADPH do not get produced in adequate supply. Some additional comments: Recall that ATP and NADPH are produced in the light reactions in the thylakoids—structures within chloroplasts, which are themselves organelles within plant mesophyll cells and eukaryotic algae cells. Some prokaryotic bacteria like cyanobacteria ("blue-green" algae) also photosynthesize, and these simpler cells essentially are chloroplasts. After being synthesized in the thylakoids, ATP and NADPH are then transported to the stroma, the watery medium inside chloroplasts where they help fuel carbon fixation and biosynthesis of organic molecules like glucose. (b.) When nitrogen supply is low, the enzyme Rubisco does not get produced in adequate supply. Some additional comments: Recall that enzymes are not consumed in biochemical reactions. Instead, enzymes are the "stage" on which biochemical reactions take place. For many life-essential chemical reactions within cells, the reactions would take place v e r y s l o o o w l y (or not at all) without an enzyme to "host" the reaction and speed it along (to catalyze the reaction). The Rubisco enzyme hosts the reaction between RuBP (a 5-carbon compound) and CO2, to yield two 3-C compounds, one of which eventually plays a role in synthesizing glucose and the other of which helps regenerate new RuBP. Even if a lot of light generates a lot of ATP and NADPH (part (a.), above), photosynthesis slows down if there isn't enough Rubisco to make use of the outputs from the light reactions. Synthesizing Rubisco requires a lot of nitrogen, so a scarcity of nitrogen limits photosynthesis by, in part, constraining the capacity of plants and algae to build copies of Rubisco.

Examine figure 6.9. The thick line shows the "universal" relationship between terrestrial aboveground NPP (ANPP) and precipitation, while the thin lines show this relationship for individual sites. (i.) As precipitation increases from 600 to 1300 mm y-1 , what is the range in ANPP according to the universal relationship? Over this same precipitation range, what is the range in ANPP at the JSP site? (ii.) Why is ANPP at JSP less sensitive to precipitation than is the universal ANPP relationship (i.e., why does JSP show a smaller ANPP increase, or slope, for a given increase in precipitation)?

(i) According to the universal relationship, aboveground NPP increases from approximately 350 to 575 g m-2 y-1 as precipitation increases from 600 to 1300 mm y-1. Meanwhile, aboveground NPP for the JSP site remains practically the same with very little change (perhaps an increase from 475 to 500 g m-2 y-1) over this precipitation range. (ii) Aboveground NPP at the JSP site is less sensitive to temperature because it hosts fewer plant functional types than the high diversity of plant types represented by the universal curve, which spans all biome types. JSP is just a single site, so contains a limited number of plant functional types. With just one or a few functional types, the plant community at JSP (or at any single site) has a limited capacity to increase productivity in response to precipitation. A given functional type of plant has an upper limit on its productive potential even if resources (e.g., water, nutrients) are abundant. In the case of JSP, this site is a grassland, with less leaf area than a forest has. Accordingly, even though JSP occasionally gets as much precipitation as forests get (e.g., as much precipitation as received at HFR or HBR, which are forests) JSP could never have as much ANPP as those sites because of its smaller leaf area. Conversely, the universal relationship reflects a change in average NPP as plant functional type changes in accordance with the different climates to which those functional types are adapted. This "all-sites" relationship shows more sensitivity to increased precipitation because, unlike the "JSP-only" relationship, it includes plant functional types with a high productive capacity at high precipitation levels. To summarize, the line for JSP is relatively flat because it shows how productivity of a given plant functional type changes in response to changes in precipitation, whereas the universal relationships is steeper because it shows how productivity changes in response to changes in both precipitation regime and plant functional type adapted to each precipitation regime.

(i.) Describe hydraulic lift, and explain why it occurs primarily at night. (ii.) How can the presence of trees help meet the water and nutrient demands of shallow-rooted herbaceous plants?

(i.) Hydraulic lift occurs when plant roots transmit water from deep in the soil to dry surface soils. It primarily occurs at night because plant stomata are closed, and plant water potential is in equilibrium with the water potential of deeper soil. Accordingly, the surface soil has a lower water potential than either deep soil or leaves. Remember that water tends to move toward the lowest water potential. So, water drawn up by roots from deeper soil will tend to exit the roots into surface soil rather than continue upward into the aboveground plant tissues. (ii.) Hydraulic lift directly provides shallow-rooted herbaceous plants with water that they would not otherwise be able to access. Additionally, hydraulically lifted water stimulates the decomposition of organic matter in surface soils (it relieves water limitation of the microorganisms that perform decomposition). Recall that decomposition releases nutrients into forms that plants can use. So, by stimulating decomposition, hydraulic lift of water also provides plants with a source of nutrients that would not otherwise be available to them.

Use the property of rooting depth to explain (i.) why deeper soils might retain water even during a drought, and (ii.) why high transpiration rates can be maintained in arid ecosystems.

(i.—short answer) Deeper soils might retain water even during a drought if plant roots are not deep enough to access that water and bring it to the surface, where the water could then be lost through evapotranspiration. (i.—extra detail) Dry soil has low hydraulic conductivity (i.e., doesn't transmit water well). Even though surface soil is very dry during a drought, and thus according to the principles of water potential gradients, deep-soil water should flow toward dry surface soils via the physical process of diffusion, the pathway for that flow is essentially broken. Remember that water normally moves through soil by flowing from particle to particle along a continuous film of water. During a drought, this film is broken and there is a lot of air gaps between soil particles, so the deeper water can't flow up very well. Accordingly, plant roots are the only way for deep water to be brought up to the surface during drought. But this mechanism only works if the roots are deep enough to access this water. (ii.) If plant roots are deep enough to access the water of deep soils, then plants can continue to photosynthesize, and thus maintain high transpiration rates, during droughts. (Remember that transpiration is the loss of water out the open stomata in leaves, which plants keep open when water is sufficient so they can obtain CO2 from the air). Plants capable of growing long tap roots to access deep water are referred to as phreatophytes, which are common in arid ecosystems.

Consider amplifying feedbacks between GPP and NPP. It makes sense that high GPP results in high NPP, as more photosynthetic fixation of carbon (GPP) will result (after accounting for plant respiratory carbon loss) in more organic carbon available for growth and maintenance (NPP). However, for an amplifying feedback to occur, the reverse must be true as well—that is, high NPP must promote high GPP. This is only possible if a plant uses its organic carbon (its NPP) to build photosynthetic tissue (the machinery for GPP). Compare figure 6.7 with table 6.2 (or with the first paragraph of the "terrestrial NPP" section on pg 168) to explain why amplifying feedbacks between GPP and NPP are more likely for stream ecosystems than for forests.

A lot of the biomass produced by terrestrial plants consists of roots and support structures. These are not photosynthetic tissues. While they are important for supporting and supplying resources to leaves, the roots and support structures (stems, branches) do not directly enhance the photosynthetic capacity (GPP) of the plant. Conversely, stream primary producers mainly build biomass that is photosynthetic, so stream NPP directly contributes to GPP.

Describe the mechanisms, with regard to stratification and nutrient cycling, that give rise to algal blooms in lakes. In your description, identify the season when algal blooms occur. At what latitudes do you most expect to observe lakes that show seasonal algal blooms?

Algal blooms in lakes occur when light and nutrients are both plentiful. These conditions typically occur in spring in the euphotic zone of temperate-zone (mid-latitude) lakes [recall that the euphotic zone is the surface layer of water, down to the depth at which light is sufficient to drive photosynthesis, typically about 1% of full sunlight]. In addition to supplying light, the increased solar radiation in spring also warms (and thus reduces the density of) lake surface water, leading to stratification, with a layer of lower-density water on the surface, and higher-density water below. This stratification prevents surface water from mixing too deeply, thereby concentrating phytoplankton within the warmth and light of the euphotic zone. These favorable light and temperature conditions in surface waters allow phytoplankton to exploit nutrients that had mixed into surface waters before stratification. Prior to the springtime warming of surface waters, the full water column had a similar density, so the nutrient-rich deep waters could mix to the surface [recall that deep waters tend to accumulate nutrients because of all the dead stuff that sinks]. The algal bloom ends when phytoplankton have depleted surface nutrients, and when grazers (zooplankton) have reduced phytoplankton biomass. The seasonal algal blooms are most common at middle latitudes because of seasonality in temperature and light. That is, middle latitudes are where you find a cool early spring with weak stratification that allows mixing to bring nutrients to the surface water, followed by late-spring stratification and high light availability that allows phytoplankton to use those nutrients.

Imagine a grassland covered with vegetation that is the exact same color as the vegetation of a nearby forest. The forest might have greater rates of energy gain and energy loss than the grassland. (i.) Why would the forest have a lower albedo (i.e., absorb more incoming solar radiation) than the grassland, despite having vegetation of the same color? (ii.) Why would the forest have greater heat loss through mechanical turbulence than does the grassland?

Architectural complexity and canopy roughness account for both (i) a lower albedo in a forest than in a similarly colored grassland, and (ii) greater heat loss through mechanical turbulence in a forest than in a grassland. (i.) Forests present canopies with plant structures at a wide variety of heights and orientations. Accordingly, light that is initially reflected may eventually be absorbed by other features (e.g., other leaves, stems, etc.). In plant communities with smooth, structurally simpler canopies, such as grasslands, reflected light will more likely escape to space than be directed toward another leaf that might absorb it. Thus, even if forests and grassland have similar color, forests provide more opportunity for incoming light to be absorbed before being reflected back out of the ecosystem. (ii.) When air flows through the canopy of a forest, all those branches, stems, and leaves of different sizes and at different heights cause the airflow to slow down unevenly. This uneven slowing creates mechanical turbulence in the form of eddies of air (small whirling currents of air). These eddies transport bulk air (air from the atmosphere above the canopy) into the forest, and transport canopy air and energy back out, resulting in heat loss from the forest. This creation of turbulence, mixing of canopy and bulk air, and the resulting exchange of heat is much less pronounced in grasslands because grasslands have smoother canopies.

Why are decomposer bacteria most abundant in the rhizosphere, dead animal carcasses, and macropores, and less abundant in bulk soil further from roots?

As single-celled, sessile organisms, bacterial decomposers generally cannot explore the soil environment for resources. Rather, resources typically need to be brought to them, or they need to exist in a resource-rich site. These needs are best met in macropores, where the easy flow of water through soil brings resources to bacteria, and in the rhizosphere and dead animal carcasses, where multiple resources are abundant. Additionally, bacteria tend to produce only the types of enzymes that break down labile organic compounds (sugars, proteins), which will be particularly abundant in the rhizosphere and animal carcasses. By contrast, these needs are poorly met in bulk soil further from roots and macropores—in such places, organic compounds will not be labile (instead, they will be recalcitrant), and resources will neither be concentrated in single "hotspots" nor delivered to those locations by water flow. Instead, fungi predominate in such locations because their ability to grow hyphae (functionally similar to plant roots) allows them to explore the soil environment and take different resources from different microsites, and because their ability to produce lignin-degrading enzymes allows them to consume more recalcitrant (non-labile) organic materials.

Some of the CO2 that humans have emitted to the atmosphere through fossil fuel combustion has been removed from the atmosphere by oceans. That is, the increased atmospheric concentration of CO2 since the Industrial Revolution has accelerated the air-to-ocean flux of CO2 above what it was before the Industrial Revolution. However, the oceans now appear to be declining in their capacity to absorb the excess CO2 that we emit. Why?

As the CO2 concentration in ocean water has increased, it has approached saturation capacity. With less capacity to absorb additional inputs of CO2, the air-to-water flux of CO2 is slowing down. Ocean water can absorb and dissolve only so much of a gas (such as CO2) until it reaches saturation capacity with respect to that gas compound. The ocean has many mechanisms for capturing and storing CO2, including biological processes, downwelling, and equilibrium dynamics based on the atmosphere's ever-increasing "push" of CO2. But, at some point, the ocean has an upper limit to the amount of CO2 (or any gas compound) it can absorb. Accordingly, new CO2 we continually add to the atmosphere is increasingly going to remain in the atmosphere rather than be incorporated into ocean water.

Suppose that the bulk air, i.e., the air well above a plant canopy, is very dry. Explain why dry bulk air results in water loss from leaves in forests more so than in grasslands. Consider the decoupling coefficient in your answer.

Because of their rough canopy architecture, forests interact substantially with bulk air through mechanical turbulence (see question 1, part ii). This tight interaction between the forest canopy and bulk air is represented by a low decoupling coefficient for forests (i.e., forests are coupled with bulk air). Accordingly, forest leaves come in contact with bulk air. If that air is dry, there will be a steep gradient in water potential (Ψ) between the air and the moisture-laden leaves, so forest leaves will lose water quickly. On the other hand, grasslands have smooth canopies and are therefore highly decoupled from the bulk air. Accordingly, water loss from grassland leaves is little influenced by how dry the bulk air is.

Photosynthesis in freshwater ecosystems is rarely limited by CO2 because these ecosystems have a lot more CO2 than is needed by primary producers. Identify the source of CO2 in freshwater ecosystems. That is, where is all this CO2 coming from?

CO2 in aquatic systems comes from three sources. i. Soil CO2 that is delivered to freshwater bodies (lakes, streams) through groundwater flow. When organic matter decomposes in soil, and CO2 is generated, a bunch of that CO2 does not get emitted from the soil to the atmosphere. Instead, it gets entrained ("caught") in soil water and groundwater that moves laterally into a stream or lake, thereby delivering this CO2 to that body of fresh water. [As a gas, CO2 can dissolve in water—something you're reminded of every time you open a soda or seltzer water.] ii. Terrestrial organic matter that is delivered to freshwater bodies, and then decomposes in the water. Leaves fall or are blown into lakes and streams. Likewise, groundwater that flows laterally into streams can carry a high concentration of dissolved organic compounds. This organic matter is then further decomposed in the water body by aquatic heterotrophs (bacteria, fungi, and animals), and the carbon in the organic matter is mineralized to CO2. Note that process i. and ii. differ. In both cases, the organic matter is terrestrial in origin (i.e., it is allochthonous to the water body). But in process i., the organic matter is mineralized on land, and the CO2 is delivered to the stream or lake instead of being emitted from land to the atmosphere. Conversely, in process ii., the organic matter is first delivered to water, and then gets mineralized in the water by aquatic organisms. iii. Autochthonous (locally produced) organic matter is decomposed and mineralized in water bodies. Streams and lakes have their own photosynthesizers (algae, rooted aquatic vascular plants) that create organic matter within the water body itself. This organic matter then dies and get decomposed and mineralized by aquatic organisms, thereby producing CO2.

Why is denitrification unimportant in a large volume of uniformly anaerobic soil, but important in soil with a patchy mix of aerobic and anaerobic conditions?

Denitrification, a process performed by specialized bacteria, is the use of nitrate (NO3 - ) as a terminal electron acceptor in cellular respiration. It is less energetically favorable than aerobic respiration, which is the use of oxygen (O2) as the terminal electron acceptor in cellular respiration (see Table 3.4 on page 86). Accordingly, denitrification will generally only proceed in anaerobic (O2-free) conditions. While denitrification typically requires anaerobic conditions, it also requires a resource (nitrate) produced through the aerobic process of nitrification. Accordingly, if aerobic and anaerobic microsites in the soil are right next to each other, the nitrate produced in aerobic microsites can be available to denitrifying bacteria living in neighboring anaerobic microsites. On the other hand, a very large volume of uniformly anaerobic soil will not exhibit high denitrification rates. Even though the lack of oxygen would favor denitrification, there would be no nitrate available for denitrifiers to act on. Additional thoughts: This question asked about soil, but don't confine your thinking strictly to terrestrial environments. Patch mosaics with adjacent aerobic and anaerobic conditions are often found around terrestrial/aquatic interfaces, such as riparian zones and wetland/upland boundaries. In these places, you have oxygenated environments (e.g., surface water or unsaturated soil) and anaerobic environments (e.g., saturated soil or subaqueous sediments) side-by-side, where nitrification can produce nitrate that, via water flow, is then delivered into zones of denitrification. This process is critical for preventing nitrate pollution of surface waters, and highlights the importance of intact riparian zones and wetlands in watersheds.

Consider a stream/river drainage network. Distinguish headwater streams from major rivers in terms of (a.) the size of organic matter, and (b.) the source of organic matter (i.e., where the OM comes from).

Headwater streams — In headwater streams, the major source of organic matter is coarse particulate organic matter (CPOM) that comes from terrestrial detritus. Since headwater streams are narrow, they are often completely spanned by the overhead canopy of streamside (riparian) trees. These trees drop a lot of leaves and other detritus into streams (i.e., they drop a lot of CPOM). Generally, small streams do not generate their own autochthonous (internally synthesized) organic matter because shading by streamside trees limits photosynthesis by instream plants and algae. So in headwater streams, (a.) the size of organic matter is large, and (b). the source is riparian plants. Desert streams can be an exception to this generality, because they may have little or no streamside riparian vegetation. Since desert streams aren't shaded by streamside trees, they often experience high sunlight and therefore high in-stream photosynthetic rates by benthic algae. Desert streams can thus produce their own autochthonous organic matter that consists of algal tissue instead of leaves and woody tissue that have fallen in. Major rivers - In major rivers, the organic matter is mainly fine particulate organic matter (FPOM), which comes from the upstream breakdown of CPOM (so (a.) the size is small, and (b.) the source is from upstream flow). That is, CPOM falls into small headwater streams and gets fragmented by shredders into FPOM, and the FPOM is then carried down the drainage network into major rivers. Because major rivers are wide, they are not shaded by trees. Accordingly, another potential organic matter source for major rivers is the autochthonous production of organic matter through photosynthesis by in-stream plants and algae. However, in-stream photosynthesis is major rivers is still often light limited because the water column is frequently turbid (i.e., all the FPOM makes the water cloudy with poor light penetration). ̶ Note that streams and rivers of any size can obtain substantial amounts of terrestrially generated organic matter in the form of dissolved organic matter (DOM) molecules (even smaller than FPOM). This DOM leaches from terrestrial detritus and soil organic matter, moves through the soil, and flows into the stream or river (see chapter 5 question set, question 5, response ii).

Soil heterotrophs use soil organic matter (SOM) as a food source, so it may seem like the abundance of SOM should correlate with rates of heterotrophic respiration (i.e., that the abundance of food would correlate with the rate at which food is consumed). But it doesn't. Why?

Heterotrophic soil respiration (the rate at which SOM is consumed) shows little relationship with the total quantity of organic matter in soil because a large amount of SOM is inaccessible or unusable. In particular, large quantities of organic matter are adsorbed on to mineral surfaces, which protects organic molecules from being consumed. The extent of organic matter sorption to soil minerals is influenced by the composition of mineral particles, and is pronounced in clayrich soils. Another reason that large pools of SOM may not translate into fast rates of heterotrophic soil respiration is that a large amount of SOM is chemically recalcitrant—that is, much of the organic matter has some molecular structure that makes it difficult for soil microbes and their enzymes to break down. Finally, much SOM may be unavailable because it is in a soil environment that is unfavorable for microbial activity; such environments include places with low temperature or low oxygen availability. Bonus material: Soil aggregates are soil physical structures that make SOM unavailable to heterotrophic soil respiration, helping to cause this decoupling between the amount of SOM and the rate at which it is consumed. Soil aggregates are essentially miniature "vaults" that lock SOM away and prevent heterotrophic decomposer microbes from accessing it. They are "crumbs" of soil particles that generally form when organic "glues" excreted by microorganisms hold fine mineral (clay) particles together. These aggregates act as capsules that contain organic matter, thereby shielding it from microbes. The interiors of aggregates, moreover, are often anoxic, further worsening conditions for decomposition. Soil disturbances that break up aggregates can expose organic matter to aerobic heterotrophic decomposer microbes, thereby elevating soil respiration rates.

At the base of a food web, there are actually two sources of energy: live green plants and detrital organic matter. These two energy sources give rise to "green" and "brown" trophic systems. Why are the plant-based ("green") and detritus-based ("brown") trophic systems more intermingled in lakes that in terrestrial ecosystems?

In the "green" and "brown" food webs of terrestrial ecosystems, the consumer trophic levels above the live plants and detritus consist of specialized herbivores and detritivores, respectively. Terrestrial herbivores and detritivores select different food types based on food quality. In lakes, conversely, primary producers (algal cells) and detritus (dead algal cells) are both consumed by a general grazer functional group that chooses food based on size more than on whether it is alive or dead. Additionally, these grazers create fast-sinking fecal pellets that supply organic matter to decomposers in the benthic sediments. That is, the benthic decomposers do not consume just dead algae matter; they consume fecal pellets from the pelagic herbivores grazing on both live and dead algae. Thus, aquatic systems do not have separate food chains arising from distinct "green" and "brown" energy bases.

If a grassland is ineffective at shedding heat via turbulence, then what other process exports heat from grassland ecosystems?

Longwave radiation emitted by ecosystems to the atmosphere is the main non-turbulent process that allows grasslands to export heat. (With their high albedo, grasslands also reflect a large amount on incoming shortwave solar radiation during the day, but some might not consider that mechanism an "export" because that radiation was never absorbed in the first place—it was just directly reflected.)

How do macrofauna affect decomposition indirectly, i.e., in ways other than directly consuming organic materials?

Macrofauna change the properties of litter and the soil environment in ways that influence the capacity of microorganisms such as bacteria and fungi to decompose litter. Specifically, macrofauna fragment litter, which pierces the protective coating that is often found on plant tissues, thereby allowing microorganisms to penetrate into the interior of litter. Fragmenting litter also increases the amount of litter surface area onto which microorganisms can colonize. Moreover, macrofauna burrow into and digest soil, reduce soil bulk density, break up soil aggregates, and increase soil aeration and water filtration, all of which stimulate decomposer microorganisms. Macroorganisms may create tunnels (macropores) that water and roots can readily pass through. Animals are important in moving new soil to the surface, thereby changing its environmental properties (e.g., temperature, density) in ways that affect the activity of microbial decomposers. Soil animals excrete nitrogen and phosphorus, supplying nutrients for decomposers, but also graze on bacteria and fungus in the soil, thereby reducing decomposer population densities.

Examine panels e and f in figure 7.21. Note that across a broad gradient in environmental conditions (water availability [IWA] or temperature), there is little change NEP (the data points don't show much of an increasing trend, and the regression model line is nearly flat). Why do you suppose this is? That is, explain why NEP is not sensitive to changes in climate conditions.

NEP is not very sensitive to changes in water availability or temperature because both gross primary production (GPP) and ecosystem respiration (Recosyst) increase with increasing moisture and temperature. The two fluxes have similar changes that essentially cancel each other out. That is, if GPP and Recosyst each increase similarly with increasing temperature or with increasing moisture, then the quantity "GPP minus Recosyst" will show no change with increases in these environmental factors.

Pelagic GPP can be limited by light and nutrients (among other factors). Distinguish the roles of light and nutrients by identifying which resource limits the total amount of algal biomass and which limits the rate at which individual algal cells photosynthesize.

Nutrient supply regulates the total amount of algae biomass in pelagic zones, while light regulates the rate at which algae photosynthesize. That is, more nitrogen (or whatever the limiting chemical element is) yields more phytoplankton cells, while more solar energy drives those cells to photosynthesize at a faster rate.

Why would a plant increase the production of biochemical components (e.g., enzymes) involved in photosynthesis when CO2 levels are high, but not do so when CO2 levels are low?

Plants match photosynthetic capacity to resource supply. Maintaining photosynthetic capacity is energetically costly, so plants will only invest in the expensive biochemical components (e.g., enzymes) that underly photosynthetic capacity to the extent that they can fix carbon. When the potential for fixing carbon is low (because CO2 levels are low), the energetic costs of producing and maintaining a large photosynthetic capacity exceeds the energy that could be gained through carbon fixation.

Discuss how the linkage between precipitation and stream flow is governed by properties of the soil in the landscape around a stream. In your answer, consider the soil properties that influence the capacity of soil to store water, and the role of soil water storage in sustaining base flow.

Precipitation and stream flow are linked, in the sense that precipitation results in increased stream flow. (This seems intuitive—more water falling out of the sky leads to more water in streams.) But, the story is more complicated. When it rains (or when snow melts), water does not immediately rush into the nearest stream. Instead, water first infiltrates soil until soil moisture reaches field capacity, and then after the soil "bucket" is full, additional water either drains downward to groundwater or flows laterally across the landscape above or just below the ground surface into streams. Accordingly, a lot of the water that falls as precipitation either never reaches a stream (something else, such as evapotranspiration, happens to that water first), or that water reaches a stream gradually days to weeks after rainfall. This gradual, delayed release of stored water into streams maintains baseflow (the background "non-flood" water level in a stream) during periods with no precipitation. Precipitation and stream flow will be more disconnected in landscapes where soil can store more water (e.g., places with deep organic-rich soil with finer particle texture). Conversely, the link between precipitation and stream flow will be more direct (rainfall will more immediately reach a stream channel, potentially causing flash floods) where there is minimal soil water storage, which includes places with paved surfaces, already-saturated soils, or impenetrable soil horizons (see the second "□" bullet in my response to question 6). So, the short answer is that the link between precipitation and stream flow is buffered by the storage of water in soil.

What would happen to nutrient cycles and biological processes if there was no decomposition? What is the link between decomposition rates about 320 million years ago and the abundance of fossil fuels today?

Recall that decomposition is the breakdown of no-longer-living organic matter, or detritus. During decomposition, the carbon and nutrients that the decomposing material (e.g., leaf, animal carcass) had accumulated while it was alive are recycled back into the environment in forms that can be used by living organisms. If there was no decomposition, this recycling would not happen. Instead, ecosystems would quickly accumulate large quantities of detritus that sequesters (traps) carbon and nutrients in forms that are unavailable to plants, and atmospheric carbon dioxide (CO2) would be depleted. This trapping of biologically essential chemical elements in unusable pools would cause biological processes to come to a halt. Moreover, atmospheric CO2 levels would plummet, causing temperatures to become very cold, likewise slowing biological processes. A total shut-down of decomposition has never actually happened. However, the Carboniferous Period (320 mya) gives us a taste of slow decomposition. During the Carboniferous Period, decomposition did not keep up with primary productivity, which led to vast accumulations of carbon and nitrogen in coals and oils. That is, the fossil fuels we burn today are the buried accumulations of incompletely decomposed organic matter from hundreds of millions of years ago.

Explain why (a.) water losses and (b.) photorespiration are lower from C4 plants than from C3 plants.

Refresher note: Before providing a response to this prompt, first a refresher. Recall that in C4 photosynthesis, the enzyme PEP carboxylase first captures CO2 and adds it to a 3-carbon compound to make a 4-carbon compound (thus the name "C4 photosynthesis"). This initial carbon capture happens in mesophyll cells. Next, the 4-carbon compound is delivered to a bundle sheath cell (deeper in the leaf tissue), where it releases one of the carbon atoms as CO2, which then enters a "normal" C3 pathway with Rubisco and RuBP. (a.) Short answer: Water losses are lower from C4 plants than from C3 plants because C4 plants can keep their stomata more tightly closed and still obtain sufficient CO2 from the atmosphere. More detail: PEP carboxylase makes this possible. PEP carboxylase has a higher affinity for CO2 than Rubisco does. Therefore, in the mesophyll cells, PEP carboxylase scours up nearly all the CO2, which drives the internal (within the cell) concentration of CO2 down to very low levels. As a consequence, there is a very steep gradient between a high CO2 concentration in the air outside the leaf and a low CO2 concentration within the leaf. This concentration gradient drives a diffusive flow CO2 into the leaf, even if the stomatal openings are reduced. (b.)Photorespiration is lower in C4 plants than in C3 plants because C4 plants build up high concentrations of CO2 around Rubisco. This favors Rubisco catalyzing the RuBP + CO2 reaction (carbon fixation) instead of catalyzing the RuBP + O2 reaction (photorespiration). Additionally, when any photorespiration (which releases CO2) does happen to occur in a C4 plant, the released CO2 will likely get captured by PEP carboxylase before it can escape the leaf (because, again, PEP carboxylase is an excellent "CO2 catcher"). Reminder: Recall that Rubisco either can catalyze the reaction of RuBP with CO2, which is called carbon fixation and leads to the biosynthesis of energy-storing molecules like glucose and starch, or can catalyze the reaction of RuBP with O2, which is called photorespiration and is energetically unfavorable for the plant. The favorable RuBP + CO2 reaction will be promoted over the unfavorable RuBP + O2 reaction if CO2 is highly concentrated around Rubisco (if Rubisco is "bathed" in CO2). CO2 can be highly concentrated around Rubisco in C4 and CAM photosynthesis because the PEP carboxylase enzyme is so good at "catching" CO2 and ensuring that it gets delivered to the neighborhood around Rubisco.

The two right-hand panels of figure 5.8 show that rivers and streams generally have negative net ecosystem production. Explain why, using the GPP and ecosystem respiration panels in this figure. (Pay attention to axis values.)

Rivers and streams generally have negative net ecosystem production (NEP) because gross primary production (GPP) is less than total ecosystem respiration (Re). That is, GPP < Re, so GPP - Re = NEP = a negative value. Additional comments: Why is GPP less than Re in rivers and streams? GPP is generally low because several factors constrain photosynthesis within rivers and streams, including the flow and scouring action of water, and light limitation. Ecosystem respiration, on the other hand, is often quite high because streams and rivers get a lot of organic matter from land (some fresh detritus like newly fallen leaves, and some dissolved organic compounds in runoff) that is decomposed and respired by heterotrophic organisms in the stream water and stream sediments. (See answer 5.ii., below.)

Explain why the annual increase in plant biomass in a forest ecosystem is not an accurate estimate of that ecosystem's annual NPP.

The annual increase in plant biomass is not a good estimate of NPP because NPP has many other fates besides contributing to plant biomass. Recall that NPP is all the organic carbon that a plant synthesizes and does not use in its own respiration. This organic carbon (i.e., this NPP) can go many places besides into the plant's biomass increment. Specifically, plants can lose substantial amounts of their organic matter through the death/senescence of plant parts, to herbivores, to heterotrophic root symbionts (e.g., mycorrhizal fungi and N-fixing bacteria), through secreting or exuding organic compounds into the soil (which are often used as food by microbes), and through volatilization of organic compounds to the atmosphere. Moreover, the growth (biomass increment) early in any given year could be fueled by carbohydrates produced from previous year's NPP.

Consider two terrestrial ecosystems that both receive the same high inputs of solar radiation, but one ecosystem has plentiful water while the other has very little water. Will the two ecosystems have the same local temperatures? Explain why or why not.

The ecosystem with more water will be cooler because a large fraction of the energy in incoming solar radiation will be carried away from the ecosystem in latent heat flux. Water will allow the wetter ecosystem to avoid heating up, because it can transfer much of the incoming solar energy to water vapor that is then carried into the overlying atmosphere. The drier ecosystem, lacking water, does not have the capacity to transfer energy back to the atmosphere in latent heat flux, so the drier ecosystem will heat up as it absorbs incoming solar radiation.

Liebig's law of the minimum states that only one resource will limit productivity at any one time. However, consider an experiment showing that the productivity of a diverse grassland community increased if either nitrogen or phosphorus fertilizer is added. Explain why it is possible that the productivity of this grassland could be limited by both N and P.

The most likely reason that plant productivity in a diverse plant community could be limited by both N and P (i.e., by multiple resources) is that different species can be limited by different resources, so productivity by the entire plant community, in the aggregate, is limited by multiple resources. There are a couple additional reasons why a plant community could be limited by multiple resources. For instance, plants have several mechanisms for making the most-limiting nutrient not quite so limiting. Accordingly, there is not a severe imbalance among the many resources a plant needs, and the plant is therefore capable of responding positively to the addition of many different resources. These mechanisms include investing in the tissues (e.g., roots at the right depth) and biochemistry (e.g., particular enzymes) needed to acquire the most limiting nutrient, increasing the rate at which the most limiting nutrient flows to the plant (e.g., by investing in root symbiotic organisms or the release of "phosphorus mining" chemicals into the rhizosphere), and storing nutrients for later use when they are unavailable in the environment. Another reason that productivity could be limited by multiple resources is that different resources could be scarce or abundant at different times; in this case, multiple resources would be limiting over some time span (e.g., over a year) even if only one was limiting at any specific time.

Atmospheric CO2 concentration has increased 35% (from ~280 to >400 ppm) since the start of the industrial revolution, largely driving climate change. One proposal is that this increase in atmospheric CO2 will stimulate plant growth, ultimately drawing CO2 levels back down, thereby neutralizing the greenhouse gas/climate change problem. Consider the principle of down-regulation to evaluate whether this proposal has merit.

The proposal has questionable merit. Initially, with an increase in CO2, there may be an increase in photosynthesis, drawing CO2 levels down a bit. This increased rate of photosynthesis will not persist, however. According to the principle of down-regulation, plants will acclimatize to elevated levels of CO2 by reducing stomatal conductance. By reducing stomatal conductance when the atmospheric CO2 concentration is high, plants still uptake the amount of CO2 they need to maintain growth rate, but also reduce transpiration rates and water loss. By maintaining (rather than increasing) growth rates, plants will not draw down the excess CO2 from the atmosphere nor halt climate change—they will simply keep acquiring the same quantity of CO2 they've always gotten, but lose less water in the process. Additionally, plants will only increase growth in response to increased CO2 supply rate if CO2 is the limiting resource. If plants are limited by other resources such as water or nitrogen, then additional supplies of CO2 will not result in increased plant growth and, accordingly, not result in drawing atmospheric CO2 levels back down to pre-industrial levels.

Discuss why the timing and amount of water available to a terrestrial ecosystem is not directly linked with the timing and amount of precipitation falling on that ecosystem.

The timing of water availability to a terrestrial ecosystem may not be directly linked to the timing of precipitation due to several processes. For instance, in some terrestrial ecosystems, winter precipitation falls as snow, so this precipitation is not available to those ecosystems until it melts and infiltrates soil in the spring. Additionally, fog (a form of precipitation) condensing as droplets that later fall to the ground, or canopy interception of rainfall that later falls to the ground (throughfall), will both result in a short time lag between when precipitation occurs and when that water is available. Lags between the occurrence of precipitation and when that water is available to terrestrial ecosystems can also be very long. For instance, ecosystems may access stored deep-soil water and groundwater that fell as rain months to years earlier. The amount of precipitation is not directly linked to the amount of water available to terrestrial ecosystems because of processes (i.) that prevent precipitation water from infiltrating soil, (ii.) that remove water from soil before an ecosystem can use it, or (iii.) that add water which fell as precipitation elsewhere to soil. Some of these processes include: 1. canopy interception, in which precipitation is caught in plant canopies and then evaporates back to the atmosphere before ever getting to the ground; 2. runoff, in which water from precipitation moves laterally towards aquatic ecosystems like streams and lakes instead of being stored in the soil (this is particularly important where there is a lot of pavement, already-saturated soils that can't soak up any more rain, and soil with impermeable layers like compacted clay, calcic horizons, and permafrost); 3. sublimation, in which snow and ice do not melt, but instead directly vaporize back to the atmosphere; 4. direct evaporation of water droplets that formed from fog condensation, before those droplets could fall to the ground and infiltrate soil; and 5. lateral drainage of water that removes it from where it fell as precipitation and adds it to other locations (e.g., water drainage down catenas from hillslopes to valley bottoms).

Why does heterotrophic respiration in soil remain high late in the growing season, when conditions become poor for plants?

There are many reasons why heterotrophic respiration in soils remains high following the plant growing season. First, microbial respiration can occur over a broader range of temperature and soil moisture than plant growth can, meaning that the plant growing season is not the only productive time for microbes. Second, because they are below ground, soil microorganisms that perform heterotrophic respiration will experience less extreme temperatures than will plants. Third, soil temperatures will lag behind air temperatures. Taking points 2 and 3 together, soil temperatures will remain warm for some time after the seasonal cooling of air temperatures and will not ultimately get as cold as air temperature. These warmer thermal conditions in the soil, combined with the broad thermal tolerance range of the microbial community, permit microorganisms to remain active later into the winter. Finally, the senescence of large quantities of plant material late in the growing season provides a pulse of substrate for decomposers, further decoupling the timing of plant growth and heterotrophic respiration.

Parts of a plant may become no longer profitable. For example, a cluster of branches may no longer capture light because it is shaded by overhanging foliage, and a section of the root mass may no longer acquire soil nutrients because that area of soil has been fully depleted. A large fraction of plant respiration (i.e., a huge expenditure of GPP) is maintenance respiration, so maintaining these unprofitable "body parts" is very costly. With this consideration in mind, describe how plants adjust in order to maintain positive NPP when the spatial availability of resources in their nearby environment is constantly changing.

To balance resource requirements with resource supply from the environment, plants can lose tissues in a process called senescence, which is the programmed breakdown of tissues that are unprofitable. The energy a plant saves by no longer paying maintenance costs for unprofitable tissues can instead be invested in the growth of new tissues that grow into new microsites and explore for resources. These microsites might be gaps in the leaf canopy through which abundant sunlight flows, or undepleted pockets of soil that have abundant resources. Senescence is the only mechanism by which plants can reduce biomass and maintenance costs when resources decline in availability.

As stomatal conductance increases, the loss of water is accelerated more than is the gain in CO2. Explain why.

To reach a chloroplast, CO2 must pass through three barriers: (i.) the barrier created by the boundary layer of still air immediately adjacent to the leaf, (ii.) the barrier associated with the stomatal opening, and (iii.) the barrier associated with diffusing through the cell membrane, cytoplasm, and into the chloroplast. To exit a leaf, conversely, a water molecule only faces the first two of those barriers. Accordingly, any increase in stomatal conductance will disproportionately affect the rate of water efflux relative to the rate of CO2 influx. Moreover, water has a steeper concentration gradient (difference in water potential between air and within-leaf spaces) and a smaller molecular size than does CO2, which are other reasons that increasing stomatal openings will accelerate water loss more than it will accelerate CO2 gain.

The energy an ecosystem gains from net radiation can be transferred vertically downward, below the ecosystem surface, through a process called ground heat flux. In terrestrial ecosystems, ground heat flux usually has a negligible influence on energy budgets, because the heat transferred down during the day is transferred back up to the soil surface during the night. In lakes, by contrast, "ground" heat flux (i.e., downward flux of the heat produced when radiation is absorbed at the surface) can result in longer-term storage of heat energy. Explain why.

Water has a high heat capacity, so once it is warmed by "ground" heat flux, it tends to hold that heat. Moreover, the vertical mixing of lake water carries heat to deeper water where it is protected from being released back to the overlying atmosphere. Also keep in mind that downward ground heat flux is driven by thermal gradient, and the thermal gradient is steeper in lake water than in soil (i.e., the temperature difference between the surface and deeper positions is greater in lake water than in soil, thereby driving more downward heat flux). Finally, water is transparent while soil is not, so sunlight (and thus heat) penetrates deeper in lake water than in soil.