Forests are the major carbon storage of the world, as they absorb circa 2.4 billion tonnes of carbon each year, which is one third of the gross primary production (GPP) (Pan et al., 2011). Net productivity of ecosystems is determined by the balance between carbon (C) assimilation through photosynthesis (also known as carbon sequestration) and carbon loss through plant respiration and heterotrophic soil respiration (De Deyn, Cornelissen and Bardgett, 2008). Recent anthropogenic emissions and deforestation have increased the concentration of atmospheric carbon (CO2). This has led to an inevitable change in climate including global warming, ice diminishing, the consequential sea level rise and the increase of extreme events (IPCC, 2014), all increasing the risk on human welfare. As to prevent further warming, a better understanding of the global system is needed. Of all carbon present in vegetation, 77% is stored in forests and 39% of the total carbon present in soils is stored in those of forests (Eliasch, 2008). Houghton et al. (2012) found that tropical rainforests alone can contain four times more carbon per hectare than cropland, which makes them the most productive systems in the world.
In the process of taking up carbon, 16 other nutrients are essential (Stevenson and Cole, 1999), with N and P being the most important. As CO2 enters the leaves through photosynthesis, carbon can be directly replenished from the atmosphere. Atmospheric nitrogen (N2) can be fixed by N-fixing heterotrophic bacteria and converted into amino acids and ammonium (NH4+) (Vitousek et al., 1997; Van Der Heijden, Bardgett and Van Straalen, 2008). These compounds are then available for plants to take up. Plant material consists of quite large quantities of N (10-50 g kg-1 of dry matter) (Larcher, 2003). N has a major influence on many aspects of plant physiology and photosynthesis, including production of ATP and NADPH (Marschner, 1995). As P is not present in the atmosphere, plants take it up from the soil, where it is available only in small quantities. P is released into the soil from bedrock and decomposed material. Plants take up only a small amount of P (≤ 10 g kg-1 dry matter) (Larcher, 2003), but it plays a role in many processes (Vance, Uhde-Stone and Allan, 2003), including photosynthesis, respiration and N fixation.
The cycles of the key nutrient elements N and P have been immensely altered by human activities. This makes it essential to understand how photosynthetic production across diverse ecosystems is limited by N and P (Elser et al., 2007). Abundant data indicate that large-scale ecosystem primary production is frequently limited by supplies of N or P in terrestrial environments (Aerts and Chapin, 1999; Elser et al., 2007; Vitousek and Howarth, 2007). Elevated inputs of these nutrients have been implicated worldwide in massive changes in biodiversity and ecosystem services (Smith, Tilman and Nekola, 1999), reflecting the fact that global cycles of N and P have been enlarged by circa 100% and 400% respectively, by post-industrial human activities (Falkowski et al., 2000). Predicting and mitigating the effects of altered nutrient loading requires an understanding of if and by how much these key nutrients limit production (Elser et al., 2007).
P becomes increasingly sequestered because of mineral transformations over large time scales (103 – 105 years) (Vitousek, 2004). Tropical ecosystems are thought to be more frequently P-limited because of their great soil age since they were not disturbed by glaciation (Elser et al., 2007). With their high temperatures and wet climate, tropical forests are likely to have high rates of N fixation (Cleveland et al., 1999). Tropical forests have not been affected by ice ages for millions of years, which means they have accumulated very large N stocks. This combined with the optimal conditions for N fixation, decomposition and mineralization explain why NO3- availability is high in tropical forests (Vitousek and Sanford, 1986). In tropical forests, generally, C:N ratios are low while N:P ratios are very high comparing to other biomes, meaning N availability is not a limiting factor of growth (Vitousek and Sanford, 1986). On average, leaf N:P ratios increase from the poles towards the equator, presumably related to latitudinal trends in temperature and biogeographical gradients in soil substrate age (McGroddy, Daufresne and Hedin, 2004; Reich and Oleksyn, 2004).
With rising CO2 concentrations and N deposition, but only small P inputs in natural ecosystems, a shift in the C:N:P balance towards a strong P limitation is seen (Peñuelas et al., 2012). Limitation by P and other elements can occur, sometimes together with N limitation (Vitousek and Howarth, 2007). Little is known about the gas exchange and photosynthetic capacity of tropical rainforests and their nutrient limitations (Carswell et al., 2000; Vitousek and Howarth, 2007).
Primary production often reflects variability in N and P availability across multiple scales of measurement (i.e., individual to landscape) as well as the tight relationship between nutrient uptake and photosynthesis (Walker et al., 2014; Koller et al., 2016). The general association of net photosynthetic capacity (Amax) with leaf N levels within and among wild species has become well accepted (Reich et al., 1991; Reich, Walters and Ellsworth, 1991, 1992, 1997). Amax has been defined in these studies as light-saturated net photosynthetic rate under near optimal environmental conditions, including ambient concentrations of CO2. The physiological basis for the Amax-leaf N relationship involves the central role of N associated with the amount of Rubisco (Ribulose – 1,5 – bisphosphate carboxylase/oxygenase), which is a photosynthetic enzyme involved in the first major step of C3 carbon fixation, as well as the role of N in other photosynthetically important leaf constituents (Mooney and Ehleringer, 2009). Variation among C3 species in Amax appears to be largely due to differences in biochemistry rather than to differences in CO2 supply (Reich, Ellsworth and Walters, 1998). Thus, the so-called photosynthesis-N relationship has important implications, ranging from our understanding of the physiology of leaf function to our ability to predict global carbon balance patterns based on remote sensing of canopy chemistry and many other areas in between (Reich, Ellsworth and Walters, 1998).
Selection has led species to differ intrinsically in both responses to soil nutrition and in their impacts on soil nutrient supply (Aerts and Chapin, 1999). These patterns of plant responses and feedbacks on nutrient supply have been conceptualized in terms of a general trade-off between growth rate and nutrient conservation (Aerts, 1999) itself integrated into well-known plant strategy models that characterize plant functioning according to axes of specialization (Westoby et al., 2002). Although there is great diversity among plant species in growth form, leaf size, leaf shape and canopy arrangement, there are also some general relationships, occurring across a wide range of species, in leaf traits central to the ‘carbon fixation strategy’ of plants (Wright et al., 2004). Much variation between plant species can be understood as a single spectrum of correlated traits.
The ‘leaf economics spectrum’ (Wright et al., 2004) runs from species with the potential for quick returns on investments of nutrients and dry mass in leaves to those with a slower potential rate of return. At the quick-return end are species with high leaf nutrient concentrations, high rates of photosynthesis and respiration and low dry-mass investment per leaf area. At the slow-return end are species with low nutrient concentrations, and low rates of photosynthesis and respiration. In organisms lacking major mineral storage of P (as in vacuoles or bones), the potential for rapid growth tends to be correlated with low biomass C:P and N:P ratios. This is thought to reflect increased allocation to P-rich ribosomal RNA, as rapid protein synthesis by ribosomes is required to support fast growth (Elser et al., 2000; Sterner and Elser, 2002).
Photosynthetic capacity is not the only plant trait heavily affected by nutrient availability in leaves, the leaf nutrient composition also has a large effect on plant herbivory. Herbivory rates are higher in tropical forests than in temperate ones, and in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding (Coley and Barone, 1996). Insect herbivores, which typically have a narrow host range in the tropics, cause most of the damage. Leaves in tropical forests are defended by having low nutritional quality and greater toughness (Coley and Barone, 1996). For insect herbivores, the N concentration of the host plants, and in particular the N concentration in their leaves, strongly controls processes such as growth, survivorship, development and reproductive rates (White, 1993; De Bruyn, Scheirs and Verhagen, 2002; Richardson et al., 2002). Changes to these processes result from both the direct effects of N on host plant quality and its influences on plant defensive chemistry (Throop and Lerdau, 2004). Low nitrogen contents have been repeatedly associated with reduced preference and performance of insects (Thorp, 1993). Young leaves are almost uniformly higher in nitrogen than mature leaves, an unavoidable consequence of cell growth. Although N limitation is well documented in insect herbivores, P limitation is poorly studied (Huberty and Denno, 2006). The few studies that have investigated P limitation indicate that it can affect survivorship (Ayres et al., 2000), fecundity, growth rate (Perkins et al., 2004) and population density (Schade et al., 2003) in insect herbivores.
All the above combined following hypotheses were formulated: If leaf N and/or P increases, photosynthetic capacity (Amax) and total herbivory will increase within or between species, meaning both are strongly correlated with leaf nutrient availability. As our first hypothesis states that both photosynthetic capacity and total herbivory will be affected in a similar way by nutrient availability, our second hypothesis states that photosynthetic capacity and total herbivory will be strongly correlated to each other.