Net Primary Productivity: Plants Play An Important Role In E ✓ Solved

Net Primary Productivity: Plants play an important role in E

Net Primary Productivity: Plants play an important role in Earth’s carbon dioxide budget. Plants take in carbon dioxide via photosynthesis during the day and give off carbon dioxide via respiration at night. The difference between carbon uptake and release by plants is the net primary production. Answer the following questions: 1) What do negative net primary productivity values mean? 2) Would you expect net primary productivity in northern Africa to be high or low, and why? 3) Why does the northern half of South America have high positive net primary productivity year round? 4) Why is net primary productivity lower in the U.S. Southwest than the Northeast year round? 5) What is displayed on vegetation maps and what factors control it? 6) In which months are vegetation values high but net primary productivity near or below zero in the United States, and what season(s) is this in the Northern Hemisphere? What does this indicate about the relationship between net primary productivity and day length? 7) How do net primary productivity and temperature relate in Southern Hemisphere winters where day length is comparable to Northern Hemisphere winter, and what factor helps moderate temperatures in the Southern Hemisphere? 8) How do monthly mean CO2 concentrations change over one year at Mauna Loa Observatory, Hawai’i, and explain this pattern.

Paper For Above Instructions

Overview

Net primary productivity (NPP) is the net carbon gain by plants after accounting for autotrophic respiration. It is a central metric for understanding terrestrial carbon uptake and seasonal carbon dynamics (Field et al., 1998; Running et al., 2004).

1. Interpretation of Negative NPP Values

Negative NPP values indicate that autotrophic respiration exceeds gross primary production (GPP) over the time step used to compute NPP. In practical remote-sensing products this often occurs in periods or locations where plants are dormant, stressed, or where sensor/model artifacts or non-vegetated surfaces (e.g., bare soil, deserts, burned areas) are present (Running et al., 2004; Zhao & Running, 2010). A sustained negative NPP implies the ecosystem is releasing more carbon via plant respiration than it is assimilating through photosynthesis during that period (Chapin et al., 2002).

2. Expected NPP in Northern Africa

Northern Africa (the Sahara and adjacent arid regions) is expected to have low NPP because of severe water limitation, high temperatures, and sparse vegetation cover (Nemani et al., 2003). Even where sensor maps show missing data, the climatic context—low precipitation, low vegetation density, and extensive bare ground—implies low or near-zero NPP for much of the year (Field et al., 1998; Tucker et al., 1985).

3. High Year‑Round NPP in Northern South America

The northern half of South America (Amazon basin and adjacent humid tropics) maintains high positive NPP year round due to abundant solar radiation, warm temperatures, and, critically, high and relatively continuous precipitation that supports evergreen, high-leaf-area ecosystems (Bonan, 2008; Running et al., 2004). High leaf area index (LAI) and vigorous photosynthetic capacity lead to large, sustained GPP and therefore large NPP except during extreme droughts (Zhao & Running, 2010).

4. Lower NPP in U.S. Southwest vs. Northeast

The U.S. Southwest has lower NPP year round relative to the Northeast because it is limited by aridity and lower soil moisture, despite often similar or higher temperatures (Field et al., 1998). The Northeast benefits from higher precipitation, greater soil moisture, and extensive temperate forests with larger seasonal leaf areas, contributing to greater annual and seasonal NPP (Chapin et al., 2002; Nemani et al., 2003).

5. Vegetation Maps: What They Show and Controls

Vegetation maps derived from satellite indices (e.g., NDVI, EVI) display fractional vegetation cover, greenness, leaf area index, or similar metrics that reflect canopy density and photosynthetic potential (Tucker et al., 1985; Huete et al., 2002). Controls on these maps include climate (precipitation, temperature, seasonality), phenology (leaf-on/leaf-off cycles), land cover type (forest, grassland, cropland), disturbances (fire, logging), and human land use (agriculture, urbanization) (Bonan, 2008; Running et al., 2004).

6. Months with High Vegetation but Low or Negative NPP in the U.S.

In mid- to high-latitude regions of the United States, satellite greenness can remain relatively high in late autumn or early spring (depending on evergreen cover and snow masking) while NPP may be near or below zero during months with low temperatures and short photosynthetically active radiation (PAR) (Field et al., 1998). For example, late autumn and early spring months can show green vegetation indices for evergreens or residual greenness, but photosynthetic rates drop and respiration can exceed photosynthesis during cold spells or low-light periods. These months correspond to shoulder seasons around winter in the Northern Hemisphere (autumn and spring transitions), indicating that NPP depends not only on leaf presence but also strongly on day length and light quality/intensity (Chapin et al., 2002; Nemani et al., 2003).

7. NPP, Temperature, and Southern Hemisphere Winters

Although day length in Southern Hemisphere winter is comparable to Northern Hemisphere winter, mean temperatures in many southern midlatitudes (e.g., southern South America, southern Australia) are moderated by large oceanic influences and the greater proportion of ocean in the Southern Hemisphere. Oceans moderate seasonal temperature swings through heat capacity, leading to milder winters and thus somewhat higher wintertime metabolic activity and NPP relative to what would occur over continental interiors at the same latitude (Bonan, 2008; IPCC, 2013). Consequently, NPP is a function of both day length (light availability) and temperature/water availability, with the oceanic climate of the Southern Hemisphere often mitigating cold-temperature suppression of NPP.

8. Seasonal Cycle of CO2 at Mauna Loa and Its Explanation

Monthly mean CO2 concentrations at Mauna Loa Observatory exhibit a strong seasonal cycle: concentrations rise during the Northern Hemisphere autumn and winter, peak in late winter/early spring, then decline during the spring and summer, reaching a minimum in late summer before the cycle repeats (Keeling et al., 1995; NOAA ESRL, 2024). This pattern is primarily driven by the seasonal cycle of terrestrial photosynthesis in the Northern Hemisphere, which contains most of the Earth’s land mass and vegetation: photosynthetic uptake in spring and summer draws down atmospheric CO2, while respiration and reduced uptake in autumn and winter allow CO2 to accumulate (Field et al., 1998; Keeling et al., 1996). Interannual variations in the amplitude and timing are influenced by climate anomalies (e.g., El Niño/La Niña), droughts, and long-term trends from fossil fuel emissions (Zhao & Running, 2010; IPCC, 2013).

Conclusions

Remote-sensed NPP and vegetation indices provide complementary views: vegetation maps show canopy presence and greenness, while NPP quantifies net carbon uptake and responds to light, temperature, and water constraints. Negative NPP signals net carbon release by plant metabolism or non-vegetated conditions. Spatial patterns—low NPP in arid northern Africa, high NPP in Amazonia, and regional contrasts within the U.S.—are explained by precipitation, temperature moderation by oceans, day length, and vegetation type. The Mauna Loa CO2 seasonal cycle reflects the integrated terrestrial photosynthesis and respiration of the Northern Hemisphere land mass (Keeling et al., 1996; NOAA ESRL, 2024).

References

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