## 8 Challenges of Working with Sensors in a Humid Tropical Rainforest and Lessons Learned  
As with any technology, there are limitations and challenges to working at La Selva with a warm and humid environment that is characteristic of tropical rainforest ecosystems. In our experience, we had to learn to anticipate practical problems when working with limited access to spare parts by developing procedures to maximize the life of the sensors and sensor arrays. Moreover, numerous modifications were necessary to successfully operate our sensor arrays under the extreme conditions of high temperatures and humidity, torrential periods of rainfall, and flooding. For example, rust and microbial growth in the humid environment requires high-quality stainless metal parts and care in the use of plastic tubing, which can be readily and quickly overgrown with fungal hyphae. The need to \begin{table}
\begin{tabular}{l l l} \hline \hline
**Category** & **Example** & **Comments** \\ \hline \multirow{2}{*}{Physical} & Temperature (e.g., thermocouples, thermistors, IR sensors) & Inexpensive to intermediate cost, reliable, low powerequirements \\  & Relative humidity & Intermediate, reliable, low power \\  & Leaf wetness & Intermediate, reliable, low power \\  & Soil moisture & Inexpensive to moderate, issues with calibration, low power, \\  & PFD, total irradiance & Intermediate, reliable with calibration issues, low power \\  & Wind speed and direction & \\  & cup anemometer & Inexpensive to intermediate, reliable, fail at low wind speed, low \\  & hot wire anemometer & Intermediate, less reliable, high power \\  & 2-D/3D sonic anemometer & Intermediate to expensive, very reliable, moderate power \\ \hline \multirow{2}{*}{Chemical} & Atmospheric CO\({}_{2}\) & Expensive, moderate power, requires careful calibration \\  & Soil CO\({}_{2}\) & Intermediate, reliable low power, requires careful calibration \\  & Soil CO\({}_{2}\) efflux & Expensive, reliable, moderate power. Requires careful calibration \\  & Nitrate sensor & Expensive, under development for field use \\  & Phosphorus sensor & Not available for field use \\ \hline \multirow{2}{*}{Biological} & Digital imagers & Moderately expensive, reliable, moderate power, high band width, \\  & Minirhizotron camera & software issues \\  & Sap flow sensors & Expensive, variable power requirements \\  & Sap flow sensors & Moderate cost, reliable, calibration issues \\  & Acoustic sensors & Moderate, reliable, high band width, software issues \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of major sensor modalities with comments on cost, reliability and power requirements. thoroughly seal all openings, holes, or gaps in sensor housings and instrumentation was also critically important to prevent insect colonization. We quickly learned that almost any exposed metal surface or opening on the sensors and instruments will rust, promote fungal growth, and/or be colonized by ants or other insects. Consistent AC power was generally not a problem, but power surges and brief outages are to be expected in remote field stations. AC-powered elements must be outfitted with surge protectors. Our only experience with an extended power loss at the towers occurred when thieves pulled and stole the 1 km of copper wire that linked the towers to the La Selva laboratory. An additional challenge of working in tropical rainforest ecosystems like La Selva is the presence of ferocious stinging ants and venomous fer-de-lance snakes. A focus on these biological hazards was a concern taken very seriously by the engineers in our group, which limited their field activities in the forest. However, we noted that the biologists in our group were much less affected. The three original towers forming the La Selva Canopy Tower Network were erected in 2008. Since the La Selva environment had a history of strong storms with high winds and lightning, we took care to take all manner of precautions to protect the towers. The towers were carefully grounded with heavy copper cable to protect them against lightning.