A novel in situ passive heating method for evaluating whole-tree responses to daytime warming in remote environments

Abstract

Background: Many significant ecosystems, including important non-forest woody ecosystems such as the Cerrado (Brazilian savannah), are under threat from climate change, yet our understanding of how increasing temperatures will impact native vegetation remains limited. Temperature manipulation experiments are important tools for investigating such impacts, but are often constrained by access to power supply and limited to low-stature species, juvenile individuals, or heating of target organs, perhaps not fully revealing how entire or mature individuals and ecosystems will react to higher temperatures.

Results: We present a novel, modified open top chamber design for in situ passive heating of whole individuals up to 2.5 m tall (but easily expandable) in remote field environments with strong solar irradiance. We built multiple whole-tree heating structures (WTHSs) in an area of Cerrado around native woody species Davilla elliptica and Erythroxylum suberosum to test the design and its effects on air temperature and humidity, while also studying the physiological responses of E. suberosum to short-term heating. The WTHSs raised internal air temperature by approximately 2.5 °C above ambient during the daytime. This increased to 3.4 °C between 09:00 and 17:00 local time when thermal impact was greatest, and during which time mean internal temperatures corresponded closely with maximum ambient temperatures. Heating was consistent over time and across WTHSs of variable size and shape, and they had minimal effect on humidity. E. suberosum showed no detectable response of photosynthesis or respiration to short-term experimental heating, but some indication of acclimation to natural temperature changes.

Conclusions: Our WTHSs produced a consistent and reproducible level of daytime heating in line with mid-range climate predictions for the Cerrado biome by the end of the century. The whole-tree in situ passive heating design is flexible, low-cost, simple to build using commonly available materials, and minimises negative impacts associated with passive chambers. It could be employed to investigate the high temperature responses of many understudied species in a range of complex non-forest environments with sufficient solar irradiance, providing new and important insights into the possible impacts of our changing climate.

Keywords: Cerrado; Climate change impacts; In situ heating; Open top chambers; Passive heating; Plant physiology; Remote environments.

Fig. 1

The novel, whole-tree in situ passive heating structure (WTHS). A Drone image of the prototype built in the Cerrado around an individual of D. elliptica. B Diagram depicting shape, size and materials

Fig. 2

Four-sided WTHSs in situ in the Cerrado. A Aerial image of WTHSs S2 and S3 in situ in the Cerrado, with the position of control individuals C2 and C3 marked with umbrellas. B Photograph of WTHS S2 partially opened for taking measurements for the analysis of plant responses to temperature

Fig. 3

Mean diurnal patterns of A temperature; B relative humidity (RH); and C vapour pressure deficit (VPD); calculated from data recorded at treatment individuals inside each of the four WTHSs (the prototype, S1, S2 and S3; red lines), and their relative controls (blue lines), with the differences calculated between the two (orange lines). In A temperature difference is given on the right-hand axis for greater definition, and the dashed line indicates the target of 3 °C

Fig. 4

Mean differences in temperature and RH between inside each WTHS (prototype, S1, S2, S3) and their relative controls during given time periods, including average values for all four WTHSs together. Mean differences were calculated for A 09:00 to 17:00 (period of strongest heating), B daytime (06:30 to 18:30), and C night-time (18:30 to 06:30). Red error bars are the average (over all days of measurement) of the daily standard deviation of the mean difference in temperature or RH. Black error bars are the standard deviation of the daily mean differences over all days of measurement

Fig. 5

Climatic conditions and example results from the heating experiment on E. suberosum. A Mean daytime (06:30–18:30) temperature and RH experienced by treatment and control groups in relation to the number of days before or after heating began. B1–B4 Results of the photosynthesis and respiration parameters B1 T_max, B2 T_opt, B3 A_opt, and B4 R₄₅, estimated from leaf measurements taken at 0, 7, 14 and 21 days after experiment initiation for each individual. Each point represents the results from one leaf on one day. Lines show average results for treatment and control groups