Mickie and I are interested in how plants respond to heat in urban environments. Heat responses are intricately linked to water and so we are interested in the interaction. To study this topic we've created:

The Sacramento leaf characteristic survey    

For Mickie's thesis we wanted to determine how urban street trees respond to heat. We hypothesized that leaf characteristics would exhibit phenotypic plasticity in response to differences in surface temperature. Specifically, in hotter areas we expected leaves to be smaller and more dissected for more efficient leaf convective cooling. We measured leaf characteristics of trees growing in relatively hot and cool areas of Sacramento, California. We studied two common street tree species, London plane (Platanus acerifolia, a non-native hybrid) and valley oak (Quercus lobata, a planted native species).

The Rad Weather Station

The full Radiation component Weather Station measures the standard weather station variables and all four up and down radiation components (i.e. reflection, emitted long wave radiation etc), has aspirated air temperature sensors at four heights, surface temperature (thermocouples and IR sensor) and NDVI. 

A model of reference leaf temperature in urban environments

In collaboration with KT Paw U, we developed a reference model that takes the weather station data and predicts a reference leaf water use and temperature. The general form takes into account all the radiation components that a plant might get in an urban environment i.e.:

absorbed SWR down + absorbed SWR up (reflection) + absorbed LWR down + absorbed LWR up 

=

sensible heat + latent heat  or evaporation + LWR out

The main idea is to evaluate the temperature and water use of a new sapling planted in the middle of a surface such as a asphalt concrete parking lot, lawn, cement concrete, soil, gravel, wood chips etc. The model is finalized but awaits publication.

The model and weather station results

The reference leaf model and the weather station data can be combined (see below). The first graph is for temperatures above a grassy lawn, with the reference leaf temperature for a leaf with closed stomata (solid red line) and open stomata (dashed red line). The second is the difference between air temperature (at 150cm height) and other temperature measurements. Notice the cooling effect of open stomata during the day, and the fact that most ground surfaces are much cooler at night than air, when no clouds are present. The third graph shows the radiation terms.