How will tropical forests respond to increasing heat extremes?

Tropical forests play a vital role in the Earth System, housing >50% of global plant biodiversity and regulating climate by virtue of their exchanges of carbon, water and energy with the atmosphere. The continued functioning of tropical forests is, however, critically dependent on the maintenance of favorable climatic conditions. There is growing concern that predicted higher temperatures in tropical regions may push tropical forests beyond their high temperature threshold. Temperatures are rapidly rising pan-tropically - e.g. the recent heat waves in India have substantially exceeded usual temperature levels, affecting on the order of 300 mio. people. The effect of high temperatures on forest vegetation is further exacerbated by drought and there is also a need to better understand the interactive effects of heat and drought on forest vegetation.
One approach to gain insight into heat effects on tropical forest vegetation is to use on-going warming and extreme events as a natural laboratory and harbinger of the consequences of future conditions. Thus key ecosystem state variables related to water/heat stress and key metrics of ecosystem functioning are continuously being recorded and the effects of heat peaks and waves can be observed in situ. A complimentary approach is to measure plant heat response under controlled warming conditions although it is unclear to what extent responses under controlled conditions compare with responses under natural conditions.
We are currently establishing two continuously observed forest sites in tropical South America, Nova Xavantina (Southern border of Amazon forests), and Tambopata (Western Amazon, wet but seasonal) based on a range of diagnostics measured partially from a tower overlooking tree canopies. If funding permits we will also equip a third site, Jennaro Herrera (Western Amazon, wettest region of the Amazon). Together these three sites bracket the Amazon precipitation amount and seasonality gradient. The Nova Xavantina site is particularly well suited to study heat effects because maximum daily temperatures are high and peak temperatures have increased rapidly over the past years. Between 2005 and 2014 (data from INMET, www.inmet.gov.br) they exceeded 40 C a total of 53 times. Xavantina is amongst the dry end of Amazonian forests, and generally experiences significant seasonal shortages in rainfall. These forests are characterized by Amazon species at their current thermal extreme and thus are a very good analogue for future conditions.

The main focus of this PhD project will be on observations of tree canopy status using cameras of phenology, heat radiation and fluorescence installed on purpose-built towers. A particular focus of the project is on establishing the capability to measure canopy - and to some extent leaf level - fluorescence with other key parameters like soil hydraulic status, sap flow and tree diameter growth, also being measured. Fluorescence is a measure of leaf level stress of the canopy leaf photosystems as well as a diagnostic of photosynthetic activity and thus is related to tree productivity (e.g. Guanter et al. 2014). The instrument used for this purpose is a highly resolving spectrometer, which will also permit to measure canopy chlorophyll content and various vegetation indices which will help understand tree responses on comparably short time-scales.