**2.1. Chlorophyll meter SPAD-502**

condensation in tropical forests play a pivotal role in the regional and global atmospheric circulation [4], and the rivers' system produces about 20% of the world's fresh water discharge [5]. Photosynthesis and respiration process are more than twice the carbon of the annual rate of anthropogenic fossil fuel emissions [6]. Tropical forests store large amounts of carbon in high diversity ecosystems and play an important role in the global carbon cycle due to its net primary productivity (NPP). According to the estimates of Ref. [7], Amazon forests contain 70–80 billion metric tons (Pg or 1015 g) of carbon in plant biomass and assimilate 4–6 Pg of carbon each year in NPP. Despite its importance, a better understanding is needed of the interactions between the tropical forest and the global processes, such as climate change. During the last decades, the Amazon forest has been threatened by deforestation, selective

Tropical forest deforestation and degradation have raised international concerns since they contribute approximately 20% to the global greenhouse gases (GHGs) emissions [8]. Reducing emissions from deforestation and forest degradation (REDD) is a United Nations Framework Convention on Climate Change (UNFCCC) initiative that developed a financial framework and mechanisms to reduce forest losses and the associated GHGs emissions aiming to prevent

Deforestation is defined as the "permanent" conversion of a forest type to another land cover. "Forest degradation" is a reduction in biomass density within a forest cover. The relative contribution of deforestation and degradation to the net emissions of carbon is not readily distinguished [9]. Research has aimed to quantify global deforestation from satellite and census data, but there is an ongoing debate on the uncertainties of the estimates [10]. On the other hand, forest degradation has been more difficult to measure with remote sensing and there are no estimates for the entire tropics [9]. Therefore, accurate estimations of photosynthetic activity of forested areas are needed to quantify forest degradation and evaluate envi-

Photosynthesis is probably the most important biochemical process on earth. It allows plants to absorb certain wavelengths of the incoming radiation from the sun and transform its energy into organic compounds. Photosynthetically active radiation (PAR) is the amount of sunlight in the 400–700 nm wavelength range that is available for photosynthesis. Its agents are the photosynthetic pigments in the chloroplasts of which chlorophyll is the most important.

The leaf chlorophyll content is closely related to the plant's health and physiology. This characteristic has been considered to assess vegetation stress in agricultural areas and forest plantations [11–14], but studies of chlorophyll content in tropical rainforest environments, and specifically in the Amazon rainforest, are rare [15, 16]. A better knowledge of leaf chlorophyll content in the tropical forest is required to contribute to detecting and modeling vegetation stress during drought or pollution events by using satellite data and in this way better understand the potential of photosynthetic capacity and its implications in regional and global

Traditional methods for estimating pigment content in vegetation need to be performed in a well-equipped laboratory. They require the extraction of plant pigments from the leaves by

logging, hunting, fire, and global and regional climate changes [4, 5].

52 Tropical Forests - The Challenges of Maintaining Ecosystem Services while Managing the Landscape

further deforestation and consequently mitigate climate change.

ronmental services provided by flora in the tropical forest.

carbon cycle and climate models.

Chlorophyll meters based on transmittance have been produced and are available commercially. They offer an inexpensive, easy, rapid, and portable approach for an indirect estimation of chlorophyll content. One of these is the SPAD-502 chlorophyll meter (SPAD-502, Konica-Minolta, Osaka, Japan) which bases its measurements on the light that is transmitted by the leaf in two wavelength regions: the first is located in the red region at 650 nm, which corresponds to the chlorophyll absorption peak unaffected by carotene, and the second is located in the infrared region at 940 nm where chlorophyll absorption is extremely low. The light emitted by the instrument and transmitted by the leaf is measured by the receptor and converted into electrical signals. Finally, a chlorophyll index is calculated by using the ratio of the intensity of the transmitted light [26]. Chlorophyll meters have been used extensively in agriculture to estimate chlorophyll and nitrogen in different species [27–31] and also in forest studies [15, 32–36]. Furthermore, chlorophyll meters have been used in the indirect assessment of foliar nitrogen [29, 30, 37], and carotenoid content [29, 38].

Chlorophyll content estimates in the tropical rainforest are rare. A published generalized homographic model for trees of the Amazon region [15] has been used as standard model to estimate chlorophyll content for more than 700 Amazonian tree species. A comparison of chlorophyll estimation between the homographic model and the second-order polynomial model proposed in this study illustrates good agreement for a wide range of SPAD-502 reading (15–95 units).

The accuracy of the SPAD-502 decreases at high chlorophyll index readings. When applying the proposed second-order polynomial model, caution should be taken for readings higher than 80 where estimation increases markedly compared to other optical methods (reflectance indices and PROSPECT) assessed in this study. Moreover, SPAD index has shown to be a valuable indicator to detect main impacts of land use changes in the tropical forest.
