Constructed Climates

Figure 1.2: The top left graph shows plant growth (called net primary productivity) across 23 ecological areas with different levels of evapotranspiration (data from Cleveland et al. 1999). My desert photograph shows green areas living up to their high potential evapotranspiration through irrigation, whereas the surrounding brown area has a much lower actual evapotranspiration strongly limited by low precipitation. Interestingly, the top right plot shows that the number of tree species also increases with an area’s evapotranspiration (after Currie 1991).

Humans need to eat food, food ultimately requires plant growth, and plant growth requires water, warmth, light, and nutrients. It really comes down to evaporating water. Ecologists sweep all the biologically relevant ways of turning liquid water into vapor under the term evapotranspiration, but only two really important ways catch my interest for the questions at hand: simple evaporation of liquid water, which accelerates when water is heated, and transpiration, which takes place when plants use and lose water while growing.

Two main ingredients limit evapotranspiration: heat and water. I took the desert photograph shown in Figure 1.2 out of a plane flying over the western United States, and, in a wonderful way, it shows the difference between potential evapotranspiration and actual evapotranspiration. A location’s potential evapotranspiration measures how much liquid could change into vapor if only an unlimited water supply existed. Assuming infinite water means that heat ultimately limits potential evapotranspiration: How much water can a location’s heat evaporate and transpire? In the Arctic, for example, there’s plenty of (frozen) water, and if only there were more heat there’d be more evapotranspiration. Actual evapotranspiration, in contrast, tells how much evapotranspiration  actually takes place in a spot given both its heat and water. A large difference between potential evapotranspiration and actual evapotranspiration of the desert motivates the farmer irrigating the fields.[3] The upper left plot turns my photograph into numbers,[4] showing how the production of plant material increases as evapotranspiration increases[5] across 23 vastly different ecological areas ranging from deserts to rainforests.[6] My photo shows this graph’s most extreme points in just one spot: the dry extreme by nature, another extreme made wet by irrigation.

For ecologists, plant growth means the uptake of carbon from the atmosphere, while appreciating that plants both take up carbon through photosynthesis and release carbon through respiration. We call the balance–uptake minus release–net primary productivity, or NPP for short. This plot shows that the net amount of carbon absorbed by plants depends on how much warm water they have available. Annual NPP totaled across the continental United States amounts to 3.4 x 10¹² grams of carbon distributed over the United States’ 7.9 million km², giving an average plant growth of 430 gC/m²/year[7]. Biomass production excites people, historically for food and more recently for energy. However, packaging biomass into species[8] excites botanists and ecologists even more. Along with increased plant biomass production, the upper right plot of Figure 1.2 shows that increasing evapotranspiration also correlates with an increasing number of tree species.[9] Through some unclear mechanism, the process of natural selection yielded more species where greater plant growth occurs.

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[3] Besides capitalizing on the difference between potential and  actual evapotranspiration in deserts, farmers gain at least one further benefit when irrigating desert fields — nearby noncrop areas support few bugs and weeds that recruit to crop plants. Desert farmers’ crops need fewer pesticide and herbicide treatments.

[4] Many people have trouble reading graphs; scientists who teach nonscientists know that, but because graphs transfer so much information so efficiently, graphs just won’t go away. I’ve added an appendix to this book covering basic graph-reading skills. A number of websites run by scientific associations provide guidance into these troubles. For example, concerns for teachers are at tiee.ecoed.net/teach/essays/students_interpreting_graphs.html,  and from more of a student’s perspective, see tiee.ecoed.net/teach/essays/figs_tables.html. Here’s what you do: Break reading the graph into two steps, describe and interpret. Break  describing into four parts: (1) Examine the axes and understand the variables; (2) look at the units used to measure the variables; (3) determine the symbols or lines used for the variables; and (4) see the patterns made by the variables.  Some questions you ask yourself while looking at patterns: Is it steady or fluctuating? Are fluctuations random or systematic? Do the data trend upward or downward, and is the trend stronger than the fluctuations?  No important scientific discoveries are really accepted until other independent scientists double-check the information provided in these graphical descriptions. In the next step of graph-reading, you interpret what the description tells you, and you’re allowed to be skeptical and bring in other information that the graph-maker might not have known or hasn’t told you.

[5] Cleveland et al. (1999) provide the data showing how net primary productivity increases with evapotranspiration. It’s unclear whether their numbers refer to potential or actual evapotranspiration.

[6] The plant growth versus evapotranspiration data in 1.2 has N=23 points, and the fit has a correlation coefficient of 0.81 to the fitted function y=2.2121+5.7865 x.

[7] Potter et al. (2006) look at the United States’ carbon budgets, including net primary productivity (NPP). Hicke et al. (2002) examine trends and  average North American NPP.

[8] Typically, the term species refers to a genetically isolated, interbreeding set of organisms, but the concept becomes stressed because some sets are isolated only by space, others by behavior, and still others can reproduce offspring with much reduced fitness.

[9]Tree species richness increasing with evapotranspiration plot comes from Currie (1991). My fit for the tree species vs. evapotranspiration graph in 1.2  has N=258 points, a correlation coefficient of 0.813, and a fitted function of y=-13.776+0.1494  x.