Canopy temperature variability in a tropical rainforest, subtropical evergreen forest, and savanna forest in Southwest China

Abstract

(1-2) Canopy temperature (Tc) measurements with infrared thermometry have been widely used to assess plant water status. Here, we evaluated Tc and its controlling factors in a primary tropical rainforest (TRF), subtropical evergreen broad-leaved forest (STF) and valley savanna forest (SAF) in southwestern China. We found differences between Tc and air temperature (Ta) of as much as 2.2 °C between the dry and wet seasons in the TRF. However, the canopyto-air temperature difference (Tc-Ta) was only 0.3 °C between the dry and wet seasons in the STF. Solar radiation (SR) was the dominant factor in Tc-Ta variations during the dry and wet seasons at the three sites. The increased heating in the canopy leaves was likely the result of low stomatal conductance leading to low transpiration cooling. Changes in Tc-Ta in the TRF were highly sensitive to the degree of stomatal closure. The change in Tc-Ta was controlled by the climate, but inherent plant traits, such as stomatal conductance, also played an important controlling role.

Contrasting Temperature and Precipitation Patterns of Trees in Different Seasons and Responses of Infrared Canopy Temperature in Two Asian Subtropical Forests

“…Our analysis showed that the Tc of the two sites exhibited clear diurnal and seasonal variation above Ta throughout the day and the year, indicating that the canopy of the studied subtropical forests is typically warmer than ambient air. These warming effects are consistent with previous findings in tropical, subtropical, and temperate natural forests, as well as urban forests [4,6,7,23,25]. By contrast, Asian subtropical forests (2.42-4.58 °C) have a comparable canopy heating effect to ∆T also showed opposite patterns with increasing g c between the two sites (Figure 5b,d).…”

“…Despite the fact that the two sites shared similar climate zones, different temperature and precipitation patterns impacted ∆T and its responses to environmental change. Our results showed that ∆T increased linearly with Rg and decreased nonlinearly with Ws (Figure 5a1-a2,c1-c2), which confirmed the warming effect of radiation and cooling effect of wind on Tc [6,7,23,27]. However, we found that the ∆T at the two sites responded differently to the temperature-influenced variables, such as VPD and g c .…”

“…Comparative results further showed that the mean Rg during the dry season at the QYZ site was 5.8 MJ m −2 d −1 higher than that during the wet season, and it was 5.69 MJ m −2 d −1 higher than that of the DHS site (Table 2). The intensive Rg input on the canopy during the dry season most likely drives ∆T [6,7,27].…”

“…Meier and Scherer [5] quantified the spatio-temporal patterns of Tc in urban forest, and found that a high fraction of impervious surfaces enhanced the increase of plant Tc. Tropical rainforests, subtropical evergreen forests, and savanna forests were found to show different Tc responses to climate change, with a higher sensitivity occurring in tropical rainforests [6,7]. Forest Tc measurements provide rapid and direct information on the plant water status over a large area [4,6].…”

“…This infers that water stress coupled with a high temperature drives plants to reduce stomatal conductance and plant transpiration for holding water, thereby warming leaves of the canopy upwards [11,12]. However, studies of a subtropical montane forest where the temperature was relative cool also pointed out that the dry season only caused weak canopy heating compared to the wet season [6,7]. This implies that the impacts of the temperature and precipitation condition on Tc are variable among stands, and the responses of Tc to temperature and precipitation factors may also be highly variable; however, the roles of temperature and precipitation factors (especially their pattern) have been less discussed.…”