• Energy Research

Featured paper: See Editorial p553

Abstract Cool materials are a suitable way to mitigate urban heat islands and help dimmish CO2 emissions and thermal discomfort in cities. However, the deposition of particulate matter and microbial growth reduces the reflectance of cool materials over time. There is little literature on this, especially in tropical climates. This research aimed to investigate the effects of biofilm formation over a white cool paint exposed in different Brazilian environments with Koppen’s climate classifications Cfb, Cfa, Am and Af. The paint was applied over fiber cement panels, without or after accelerated carbonation, to mimic new and aged tiles, respectively. Pre-carbonation of the fiber cement favored intense colonization by Scytonema that influenced the loss of reflectance in the panels aged in Belem, northern Brazil. The panels exposed in Pirassununga in South-East Brazil, on the other hand, presented an intense growth of phototrophs and fungi, that, together with deposition of particulate soil rich in iron oxide, caused a reduction of over 0.35 reflectance for the carbonated tiles. The loss of reflectance varied from 19% in the samples exposed in Sao Paulo to 36% in the samples exposed in Belem. This study has shown that the reduction in reflectance caused by biofilm development is not uniform in different Brazilian environments even when the substrate and coating are the same.

Summary Plant traits are increasingly being used to improve prediction of plant function, including plant demography. However, the capability of plant traits to predict demographic rates remains uncertain, particularly in the context of trees experiencing a changing climate. Here we present data combining 17 plant traits associated with plant structure, metabolism and hydraulic status, with measurements of long‐term mean, maximum and relative growth rates for 176 trees from the world’s longest running tropical forest drought experiment. We demonstrate that plant traits can predict mean annual tree growth rates with moderate explanatory power. However, only combinations of traits associated more directly with plant functional processes, rather than more commonly employed traits like wood density or leaf mass per area, yield the power to predict growth. Critically, we observe a shift from growth being controlled by traits related to carbon cycling (assimilation and respiration) in well‐watered trees, to traits relating to plant hydraulic stress in drought‐stressed trees. We also demonstrate that even with a very comprehensive set of plant traits and growth data on large numbers of tropical trees, considerable uncertainty remains in directly interpreting the mechanisms through which traits influence performance in tropical forests.

Summary 1. The Amazon region may experience increasing moisture limitation over this century. Leaf dark respiration (R) is a key component of the Amazon rain forest carbon (C) cycle, but relatively little is known about its sensitivity to drought. 2. Here, we present measurements of R standardized to 25 °C and leaf morphology from different canopy heights over 5 years at a rain forest subject to a large‐scale through‐fall reduction (TFR) experiment, and nearby, unmodified Control forest, at the Caxiuanã reserve in the eastern Amazon. 3. In all five post‐treatment measurement campaigns, mean R at 25 °C was elevated in the TFR forest compared to the Control forest experiencing normal rainfall. After 5 years of the TFR treatment, R per unit leaf area and mass had increased by 65% and 42%, respectively, relative to pre‐treatment means. In contrast, leaf area index (L) in the TFR forest was consistently lower than the Control, falling by 23% compared to the pre‐treatment mean, largely because of a decline in specific leaf area (S). 4. The consistent and significant effects of the TFR treatment on R, L and S suggest that severe drought events in the Amazon, of the kind that may occur more frequently in future, could cause a substantial increase in canopy carbon dioxide emissions from this ecosystem to the atmosphere.

Background: There is much interest in how the Amazon rainforest may respond to future rainfall reduction. However, there are relatively few ecosystem-scale studies to inform this debate. Aims: We described the carbon cycle in a 1 ha rainforest plot subjected to 8–10 consecutive years of ca. 50% through-fall reduction (TFR) and compare these results with those from a nearby, unmodified control plot in eastern Amazonia. Methods: We quantified the components of net primary productivity (NPP), autotrophic (Ra) and heterotrophic respiration, and estimate gross primary productivity (GPP, the sum of NPP and Ra) and carbon-use efficiency (CUE, the ratio of NPP/GPP). Results: The TFR forest exhibited slightly lower NPP but slightly higher Ra, such that forest CUE was 0.29 ± 0.04 on the control plot but 0.25 ± 0.03 on the TFR plot. Compared with four years earlier, TFR plot leaf area index and small tree growth recovered and soil heterotrophic respiration had risen. Conclusions: This analysis tested and extended the key findings of a similar analysis 4 years earlier in the TFR treatment. The results indicated that, while the forest recovered from extended drought in some respects, it maintained higher overall Ra relative to the undroughted control, potentially causing the droughted forest to act as a net source of CO2.

Abstract Solar reflectance and thermal emittance are fundamental parameters in the evaluation of a building's thermal and energetic performance, particularly important for “cool” surfaces. However, there is little information on typical values of these parameters in Brazil. We determined values of reflectance, thermal emittance, surface roughness and color difference, for a white high-reflectance paint exposed for two years in 4 cities in the States of Sao Paulo (SP) and Para (PA), south-eastern and north Brazil, respectively. After 2 years, painted panels exposed in Pirassununga, SP, showed significantly lower reflectance and increased color difference compared with the original paint and the panels exposed at other sites. This appeared to be related to the presence of a biofilm, in addition to particles of the red clay characteristic of this area. This was likely also to be the explanation of the increased surface roughness determined in Pirassununga samples. Emissivity of the paint changed very little at any of the exposure sites over 2 years, but Solar Reflectance Index, calculated from the average values of reflectance and emittance in the four cities, was reduced from between 105 and 107 to between 94 (in Sao Sebastiao, SP) and 77 (in Pirassununga, SP) after two years. Macroscopically, panels from Pirassununga and Belem, were the most discolored and the greatest presence of biofilm was demonstrated by digital and scanning electron microscopy. Such natural aging studies are important for the production of standard tests and determination of control strategies to increase durability of cool surfaces in Brazil.

AbstractTranspiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function with climate change can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. We report the only study of forest transpiration following a long‐term (>10 year) experimental drought treatment in Amazonian forest. After 15 years of receiving half the normal rainfall, drought‐related tree mortality caused total forest transpiration to decrease by 30%. However, the surviving droughted trees maintained or increased transpiration because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent nondroughted forest. If these drought conditions were accompanied by a modest increase in temperature (e.g., 1.5°C), water demand would exceed supply, making the forest more prone to increased tree mortality.