Gas exchange instrumentation
Gas exchange of trees and forest floor is being monitored continuously with automated chambers. Altogether the instrumentation can accommodate 12 chambers, each chamber is recording the momentary gas exchange 8–100 times a day. The measurements on trees are running throughout the year, forest floor gas exchange measurements are paused for midwinter.
The instrumentation consists of chambers, sample tubing, gas analysers and a control unit operating the system automatically. Different chamber designs are used for measuring tree shoots and leaves, stems and forest floor gas exchange. Dedicated chamber design also exists for measuring shoots inside tree crowns where the spatial variability of light is measured within each chamber.
The shoot/leaf and soil chambers are of the transient type, that is, they are open most of the time exposing the chamber interior to the ambient conditions. Air in the chamber is continuously mixed by a small fan. For measuring gas exchange, the chambers are closed intermittently for period of 1–15 minutes with a pneumatically operated lid mechanism. Before and during chamber closure, gas sample is drawn from the chamber through heated Teflon FEP tubes to gas analysers (CO2, H2O, O3, NO, NOx, VOC) located in the main instrument cottage. Gas exchange is calculated from the measured dynamics of gas concentrations inside the chamber during the closure. The stem chambers are operated at steady state, the studies gases are CO2 and VOC.
The sample air flow is compensated by letting ambient air flow into the chamber through small vents. There is also a supply air system that can feed replacement air with known gas concentration into the chamber. The mass flow rates of the compensation air and the sample air are regulated by two separate pumps and mass flow controllers.
Air temperature inside the chambers and photosynthetically active radiation (PAR) outside of the chambers is also measured.
Instrument configuration and data description
Description of different chamber types, and
Old cylinder type shoot chambers, nowadays used for VOC measurements
Box type shoot chambers
Slide type shoot chamber
Old stem chamber
New stem chamber
Old soil chamber