Ecosystem Function Measurement, Terrestrial Communities Table 75 Continued Parameter measured Usual units Techniques Emphasis Water use efficiency (C assimilated per unit water transpired, integrated over a long period, e.g., a whole season or longer) Evapotranspiration Water potential (the more negative the water potential, the more negative the balance between absorption and transpiration) mmol CO2 mmol H2OÀ1; d13C1/00 Ratio of stable isotopes 13C to 12C in plant material compared with standard (PeeDee Belemnite) P mg H2O mÀ2 minÀ1 Mpa H2O exchange measurements Pressure chamber measurements (pressure needed to expel a drop of sap out of the xylem) F F a dw, dry weight Source: More details can be found in Pearcy et al (1989), Ehleringer and Field (1993), Sala et al (2000), Berg and McClaugherty (2000), and Hoover et al (2008) Photosynthesis Growth Productivity (µmoI CO2 m−2 s−1) (mg dw gdw−1 day−1) (g dw m−2yr−1or t dw ha−1yr−1) Leaf level Gas exchange Plant level Harvest Paddock / patch level Site level Sequential Equations Remote sensing harvest involving rainfall and water holding capacity Regional/ global level Equations involving physiological models or rainfall evapotranspiration, radiation and/or water holding capacity Figure Carbon assimilation processes at different scales and some methods for measuring them Chapin et al (2005), NEP is the net biomass accumulation by a whole ecosystem and depends not only on NPP, but also on carbon losses due to the respiration of animals and microbes, leaching, erosion, exportation by animals, and in some cases volatilization due to fires At the regional scale, NPP can be largely accounted for by climatic factors, rainfall, and temperature For example, precipitation, potential evapotranspiration, and radiation are enough to account for the net aboveground primary production (NAPP) of North American forests, deserts, and grasslands In regions of the US with up to 1400 mm of annual rainfall, annual precipitation is enough to account for 90% of the variability in NAPP of grasslands (Figure 4(a)) At higher precipitation, NAPP depends more on other factors, and equations based on annual rainfall lose part of their predictive power At the site level, variability in production seems to be accounted for by annual precipitation and soil water-holding capacity (whc; Figure 4(b)) Soil whc can have a positive or negative effect depending on the precipitation value In dry regions, major losses of soil water occur via bare soil evaporation However, where sandy soils occur, bare soil evaporation is lower than in loamy soils because water penetrates deeper into the soil For the same reason, surface runoff is also lower in sandy soils than in loamy soils In more humid regions, substantial water losses occur via deep percolation, which is reduced in soils with high whc This is known as the inverse texture hypothesis, proposed by Noy-Meir in 1973 At finer scales of analysis (e.g., paddocks and vegetation patches), more variables are needed to account for NAPP Species composition and land-use regime become important factors, although drivers at a coarser scale are still in operation and constrain responses (e.g., irrespective of management or species composition, annual precipitation will set an upper boundary to NAPP) For instance, the NAPP of Argentine