Calibration of LAI-2000 to estimate leaf area index (LAI) and assessment of its relationship with stand productivity in six native and introduced tree species in Costa Rica
Introduction
Physiologists and ecologists have long recognized the importance of both leaf surface area and foliar weight as factors affecting many tree and stand-level processes and functions such as photosynthesis, gas exchange, growth, stand productivity and canopy dynamics. Silviculturists had been also incorporating estimates of leaf area and foliar weight into their studies on stand manipulation and subsequent response (Gholz et al., 1979, Morataya et al., 1999, Samuelson et al., 2004, Waring, 1983, Waring and Running, 1998). Foliage is the element of the forest structure that adapts the most to small temporal changes in the environment and hence it has become a central and basic descriptor of vegetation condition in a wide variety of physiological, climatological and biochemical studies (Asner et al., 2003). A simple measure of the amount of foliage is the leaf area index (LAI), the projected area (one side) of foliage per unit of ground surface area (Asner et al., 2003, Fournier et al., 2003). Leaf area is the exchange surface between the photosynthetically active component of the vegetation and the atmosphere (Cohen et al., 2003, Fernandes et al., 2003, Fournier et al., 2003, Meadows and Hodges, 2002, Turner et al., 1999) and controls the light, thermal, and hydric conditions within the canopy (Fournier et al., 2003). Leaf area index (LAI) is one of the most frequently used parameters for the analysis of canopy structure (Beadle, 1997) and it has also been used as a predictor to explain processes in forest ecology, forest management and remote sensing studies (Asner et al., 2003, Battaglia et al., 1998, Bolstad et al., 2001, Cannell, 1989, Cannell et al., 1988, Kalácska et al., 2004, Kalácska et al., 2005a, Kalácska et al., 2005b, Sampson et al., 1998, Samuelson et al., 2004, Stickan et al., 1994, Sullivan et al., 1996). Although most of the studies on LAI and its applications to tree plantations and forest management have taken place in temperate and subtropical regions, a promising study on Tectona grandis and Gmelina arborea in Costa Rica by Morataya et al. (1999) demonstrated that silvicultural management of these tropical hardwood species can also be evaluated by monitoring LAI.
Leaf area index therefore, is a valuable tool in forest management, forest ecology and remote sensing and consequently it is important to develop appropriate methods to evaluate LAI in tropical forest plantations. Direct estimates of LAI are difficult in forest stands as one looks to relate leaf area to ground surface unit area (m2 m−2). Under such conditions, in order to obtain the LAI of a stand, one has to do a destructive sampling by harvesting the total leaf biomass of the trees to generate the total dry weight of the foliage. To complete the conversion of the weight of the foliage into leaf area values for the species of interest, it is necessary to know the specific leaf area (SLA), which relates the area of the leaf blade with its dry weight in cm2 g−1 (Arias, 2002). In practice, it is not always possible to make these direct measurements. Alternatively two indirect methods can be used: litter traps and allometric models to predict the leaf area of individual trees (Asner et al., 2003). Allometric equations and litter traps are labor intensive (Asner et al., 2003, Coops et al., 2004, Dufrêne and Breda, 1995, Leblanc and Chen, 2001, Maass et al., 1995). Therefore, optical methods are often preferred because LAI estimates can be completed relatively rapidly and accurately (Chen et al., 1997; Battaglia, 1998; Bolstad and Gower, 1990, Dufrêne and Breda, 1995, Leblanc and Chen, 2001). The LAI-2000 plant canopy analyzer is one of the most commonly used optical instruments to estimate LAI by measuring the amount of diffuse radiation that infiltrates the canopy. More precisely, optical instruments such as LAI-2000, estimate “plant area index” (PAI), which includes projected stem and branch area (wood-area-index; WAI) as well as leaves (Battaglia et al., 1998, Bolstad and Gower, 1990, Kalácska et al., 2005a, Smolander and Stenberg, 1996; Stenger et al., 1994).
Therefore, in the preceding context, the main objective of this study is to calibrate optical estimates of PAI from the LAI-2000 using leaf area index derived from allometric models for two introduced and four native fast growing tree species. A second objective is to investigate the corresponding relationship of LAI with stand productivity indices and environmental factors along an environmental gradient in the southern region of Costa Rica.
Section snippets
Experimental design and selected species
In June 1994, six fast growing native and introduced species were planted in the southern region of Costa Rica under a complete randomized block design nested in four Eco-regions. In this study an Eco-region considered as the combination of soil (USDA, Soil Taxonomic Classification System, Pérez et al., 1978) and climate (Holdridge Life Zones, Bolaños and Watson, 1993). In summary Eco-regions 1 and 2 represent the most challenging environmental conditions with their ustic soil moisture regimes
Specific leaf area (SLA)
The average values of specific leaf area (SLA) are summarized in Table 1. For all of the broadleaf species, a close linear relationship was found between an individual leaf's area (cm2) and its weight (g) and therefore this relationship can be used as a species specific constant. SLA values ranges between 81 cm2 g−1 (V. ferruginea) and 106 cm2 g−1 (G. arborea). Major differences between species were noted when comparing average values of leaf size. The smallest leaves, with areas of less than 40 cm2
Conclusions
This study demonstrates that it is possible to calibrate LAI-2000 to estimate LAI of fast growing tropical tree species planted in monoculture conditions with species specific allometric LAI estimates. According to the results, the LAI-2000 values appear to show very good results for four of the study species. For two species, there is a tendency to either under- or overestimate the LAI. In this study we concluded that under- or overestimation of LAI by the LAI-2000 depends on species crown
Acknowledgments
This project was supported by the Instituto Tecnológico de Costa Rica, University of Göttingen, Duke University, Ston Forestal Company, PINDECO Company, Programa Forestal Campesino/Ministry of Natural Resources and Energy, Costa Rica. Funding was provided by the Organization for Tropical Studies; through a grant from the USA-AID Project No. 515-0262-G-IR-2004. Special thanks to all collaborators: Dr. Donald Stone and Dr. Charles Schnell from OTS, Donald Zeaser M.Sc. from Ston Forestal, Dr.
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