All plants are subject to various stresses. Plants may respond to stress (such as resource limitations) in a number of ways, primarily through the amount of foliage that develops and physiological activity of leaves (photosynthesis and transpiration -- both involving opening/closing of stomatal openings or pores on leaves). These responses operate at different time scales. Stomatal control is a short-term response to factors such as temperature, moisture and light. Foliage display is, for the most part, a cumulative response to these same stress factors acting over a longer period of time. Plants better able to withstand stress or to use resources more efficiently than others may have an adaptive advantage with improved chances of survival and reproduction.
Photosynthesis operates within a bounding range of temperature from just above the freezing point of water (0°C/32°F) to about 50°C (~120°F), though few organisms function efficiently over this entire span. Plants are adapted to the climate in which they occur so that plants from various climate regimes have different optimum temperatures for photosynthesis. For example, boreal tree species may have a maximum photosynthetic rate at ~20°C, whereas tropical trees may show peak rates at ~30°C (other factors not limiting).
Surface temperature has been monitored using remotely sensed observations in the thermal portion of the spectrum. Air temperature can be statistically inferred using a combination of spectral vegetation indices (SVIs) and surface temperature. This technique assumes that areas with very high vegetation cover will have a canopy temperature that is close to the ambient air temperature.
If the atmosphere surrounding a leaf is dry, the leaf is always losing water to the atmosphere if its stomatal pores are open. Leaf moisture must be replenished constantly for a leaf to keep photosynthesizing. For plants to preserve essential moisture, stomata respond to the relative difference between the vapor pressure (a measure of the amount of water) inside a leaf and that of the outside air. This difference in vapor pressure inside and outside a leaf is termed the vapor pressure deficit (VPD). It is an index of the drying capacity of the air, and it varies with temperature and humidity conditions.
VPD strongly affects photosynthetic rates. In dry conditions, leaves may be unable to maintain adequate moisture and respond by closing stomatal pores. Stomatal closure restricts not only the diffusion of water out of the leaf but also carbon dioxide diffusion into the leaf, resulting in reduced photosynthesis. Plants respond differently to VPD depending on the environment in which they have adapted.
VPD can be estimated from remotely sensed observations using two thermal channels from the Advanced Very High Resolution Radiometer (AVHRR) that are affected differently by atmospheric water vapor. The difference in temperatures detected by the two channels can be related to the total water vapor amount between the satellite and Earth's surface. Atmospheric water vapor amount at the surface can then be extrapolated and a corresponding absolute humidity value inferred. The difference between the absolute humidity and the potential humidity the air could hold, based on its temperature, is the VPD.
Photosynthesis and stomatal response are controlled not only by VPD, but also by the amount of moisture available to the plant through soil moisture and the associated "leaf water potential." Leaf water potential is a measure of how tightly or strongly a leaf holds its moisture. To measure leaf water potential, a leaf is clipped off and inserted into a pressure chamber. Pressure in the chamber is increased just until water starts to ooze out of the cut end of the leaf. The greater the pressure needed to exude water from the leaf, the more tightly the leaf is holding on to its moisture, indicating that the leaf may have been experiencing water stress.
Soils also have water potentials that affect the rate at which soil moisture can be extracted by roots. Whenever soil water potential, leaf water potential and/or VPD reach certain levels, the stomata respond accordingly (resulting in greater or lesser diffusion of carbon dioxide and water).
Estimating the moisture content of foliage with remotely sensed observations has met with mixed results. Surface soil moisture can be estimated using a method similar to that described for air temperature inference. The slope of a spectral vegetation index (SVI) vs. temperature relationship is related to surface wetness. Soil moisture can also be assessed by radar remote sensing.
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