Effects of Enhanced CO2 on Tropical Forest Growth

Effects of Enhanced CO2 on Tropical Forest Growth James P. Smith Effects of enhanced atmospheric CO2 concentrations on tropical forest growth: experimental studies and interactions with nutrients, light, water and temperature Abstract (150 words) Introduction (300 words) Approximately 90% of earth’s 652Gt terrestrial biomass carbon is locked up in forests. Tropical and subtropical forests store 340Gt carbon; or ~52%; but only make up 13% of total forested area (table 1). Achard et al (2002) estimated 1Gt/yr carbon losses, through activities such as deforestation and clearance for agriculture (Geist et al, 2002). All terrestrial plants have become exposed to increasing atmospheric CO2 concentrations, as part of global change. This has changed from 180ppm 18ka (Petit et al, 1999) to 390ppm today, by degassing from oceans and fossil C burning (Crowley et al, 2001). Increased CO2 could stimulate photosynthesis, raising plant productivity. This can have a role in storing more carbon and mitigate the atmospheric rise in CO2 concentrations (Beedlow et al, 2004). Table 1: Areal extent, carbon storage and net primary productivity of earth’s major biomes (from Roy et al, 2001). Figure 1 demonstrates CO2 enters plants at the source (leaf); where it becomes photoassimilated to produce carbon sugars; which are transported around the plant to carbon sinks; for different processes such as structural growth, metabolism and export. Sugars can also be stored as reserves in the form of NSCs (non-structural carbohydrates). CO2 is lost through respiration, herbivory and litter production and decomposition (Korner, 2003a). Figure 1: CO2 pools and fluxes in plants, as well as source-sink interactions (modified from Korner, 2003a). The aim of the review is to evaluate research on the effects of enhanced CO2 on tropical forest growth. This will be achieved by looking at experimental studies, as well as the effects of enhanced CO2 on the limiting factors of nutrients, light, water supply and temperature. I will be reviewing literature from 1999-2013. Experimental studies There have been few experimental studies of the effects of enhanced CO2 on plant growth in tropical forests in relatively natural conditions (ambient climate, natural soil and inter and intra-species competition). Two studies using a canopy crane in a tropical dry forest in Panama was used to assess the effects of enhanced CO2 on canopy tree leaves. Over a 40 week period Lovelock et al (1999) measured responses of leaf and branchlets of a single tree species. Photosynthesis rates increased 30% with enhanced CO2. However, no increases in biomass occurred (reproductive organs and foliage). Branchlet TNC (total non-structural carbohydrates) increased 20%, inferring localized carbon saturation. Wurth et al (1998a) found stronger TNC increases (41-61%), upon exposing canopy leaves of four tree species to enhanced CO2, in situ. Wurth et al (1998b) planted seedlings of five local species (tree, shrubs and grass) in the understorey of a closed Panamanian forest. These were grown over a 15 mo nth period, in which 50% were in ambient CO2 and 50% in elevated. All species showed significant seedling growth under elevated CO2, but decreased as understorey light levels increased, and inter-species variation was apparent. Again TNC levels increased under enhanced CO2. One experiment has studied communities of tropical trees, which have been outplanted in natural soil and subjected to elevated CO2. Lovelock et al (1998) grew groups of ten tree species at ambient and elevated CO2 in open-top chambers at the forest margin in Panama. Over six months, there was no enhancement in biomass accumulation. There were also reductions in leaf area index, increased photosynthesis rates and increased nitrogen: carbon ratios. Response was species-specific, but late-successional species were less sensitive than pioneer and midsuccessional species. Table 2: Comparison of mean TNC concentrations (% dry weight) across four studies under ambient and elevated CO2 concentrations. From table 2, it is clear that all four studies mentioned showed increased mean TNC concentrations when exposed to elevated CO2. Despite the increases, this does not necessarily mean TNCs from carbon sources are being transported to carbon sinks, into plant biomass for growth. They include carbohydrates, sugar alcohols, organic acids and lipids, and represents carbon reserves or stores, for future use on demand (Korner, 2003a). So, photosynthesis rates may increase under elevated CO2, producing more TNCs, but may not be used in plant growth, unless needed. Figure 2: Variation in mean concentration of TNC with height in two wet and dry seasons (from Wurth et al, 1998a). Wurth et al (1998a) also compared TNC concentrations, exposed to elevated CO2, with height from canopy height to roots, between wet and dry seasons (figure 2). They found TNC to increase in all plant compartments during the dry season. The TNC again not incorporated into structural growth, because growth was directly limited by dry conditions, and not photosynthesis. More TNC was being stored in reserves. In the wet season, TNC pools reduced, coinciding with resumed tree growth and new leaf production. They inferred TNC concentrations were controlled by moisture availability, in agreement with another study in the area (Newell et al, 2002). On the other hand, Korner and Wurth (1996) found TNC to increase significantly in both dry and wet seasons. This infers plants have a store of carbon, and can mobilize it when needed for growth. To further the understanding of increasing CO2 on tropical forest growth, more and longer-term experiments are needed. Arnone (1996) and Korner (1998) criticize these experiments, as they cannot be scaled up to actual forest size; use only small plants; have a higher than normal nutrient supply; absence of competition; and key processes; such as herbivory and effects of pathogens. CO2-nutrient interactions Nitrogen is commonly seen as the main limiting nutrient of tree CO2 responses (Finzi et al, 2006). However, although this is theoretically an unlimited resource (atmospheric), provided N fixation balances N losses through processes such as N20 losses or leaching (Korner, 2009). Litter mineralization is the predominate source of N in forests. All other nutrients are in limited supply in a given area, with older, more weathered (humid tropics) soils making these nutrients much more limiting to plant growth (Bergametti et al, 1998). Enhanced CO2 can accelerate the rate of symbiotic N fixation, as demonstrated by Tissue et al (1997). Seeds of fast-growing woody legumes from a seasonal tropical forest in Costa Rica were inoculated with N2 fixing Rhizobium bacteria and grown in greenhouses for ~70 days, exposed to ambient (35Pa) and elevated (70Pa) CO2 levels. Seedlings were watered adequately with N-free water solution. Under elevated CO2, photosynthesis rates increased by 49%, compared to those exposed to ambient CO2. As a result growth in elevated CO2 increased 36%. Figure 3 illustrates this, with total plant biomass growing 84% under elevated CO2. Greater rates of photosynthesis mean greater quantities of carbon are transported to the nodules. More carbon supplied to nodules means specific nitrogenase activity (SNA); that is N-fixing enzyme activity; is increased; more energy is available to power the fixation process. Thus a greater proportion of nitrogen is fixed by the legumes and incorporated into the plant for biomass accumulation and growth. Figure 4 shows this clearly, with increases in N content across all parts of the plant. Figures 3 4: Dry weight biomass (gDW) of whole plant, as well as different areas of the plant (left). N content (mg) of whole plant, and different sections of plant (right). (From Tissue et al, 1997). Although there is a high abundance of nitrogen, and fixing increases under CO2 levels, Pons et al (2007) inferred N-fixation is also strongly limited by phosphorus availability, and is absorbed by trees much more efficiently than N (Medina and Cuevas, 1994; Herbert and Fownes, 1995). Pons et al (2007) measured N and P concentration changes in leaves of leguminous plants, in different soil types, in a tropical forest in Guyana. From table 3, general increases in N and P led to positive accumulations of N in leaves. They inferred increases in phosphorus were the main cause for increasing N-fixation, with increasing N concentrations having negligible effect. Contrary to Tissue et al (1997)’s findings, Houlton et al (2008) found N fixation to be less prominent in tropical forests. Pons et al (2007) approximated 6% of total N uptake by trees in Guyana was by N-fixation, and only ~50% legumes used the symbiotic pathway. Nardoto et al (2008) found near negligible N-fixation levels in legumes in Amazonia. Thus, nitrogen is unlikely to majorly constrain C-fixation in tropical forests, but phosphorus is more likely to (Martinelli et al, 1999). Table 3: Phosphorus and nitrogen concentrations in five different soil types, and their affect on N-fixation rates by N contents in leaves (Modified from Pons et al, 2007). Studies in tropical forests in Panama provided clear evidence that trees grown in close proximity to their natural habitat, under elevated CO2, within original soils and under local climatic conditions, exhibited accelerated growth rates when soils were enriched with mineral nutrients (Winter and Lovelock, 1999; Winter et al, 2001; table 4). In the absence of fertilizer there was no significant change in growth rate under elevated CO2 (Lovelock et al, 1998; Winter et al, 2000). No major changes in growth rates were found again were found by Korner and Arnone (1992) and Arnone and Korner (1995). Table 4: The effect of fertilizer/absence of fertilizer application on biomass accumulation for tropical plants under elevated CO2. Clearly the effects of elevated CO2 on have caused mixed responses from different studies. In some studies, greater photosynthesis rates led to increased carbon supply to allow accelerated N-fixation for biomass growth. Other studies highlighted the greater importance of phosphorus in regulating N-fixation and biomass accumulation. Plants grown in the absence of nutrients consistently showed minimal to no change in growth rates, opposed to increasing biomass with those that were enriched with mineral nutrients. CO2-light interactions It is known that shaded plant growth rates are limited by light and CO2. Illuminating plants will lead to accelerated growth, by forest canopy thinning or removal. As enhanced CO2 increases light use efficiency and decreases the light compensation point within the leaf, stimulation by enhanced CO2 in shaded areas can be seen to be similar to canopy thinning or illumination (Long and Drake, 1991). The effect of elevated CO2 on tropical plants grown in deep shade can be significant and can possibly exceed effects grown under horticultural conditions under full light (Korner, 2009). Wurth (1998a) exposed seedlings on the forest floor to ~700ppm CO2 under extremely low light levels (~11ÃŽ ¼mol photons m-2s-1). Tree seedlings grew 25-44% and shrub seedlings grew 59-76%. Lovelock et al (1996) observed similar results of mycorrhizal growth of tree seedlings, although P supply may have had an influence. Thus elevated CO2 promotes expansion into shaded areas. As expressed, as most tree seedlings wait to exploit an opening in the canopy, lianas employ a different strategy. Lianas are situated in deep shade and aim to occupy maximal space, but with minimal structural investment (Korner, 2009). Elevated CO2 increases the probability of lianas reaching the upper canopy. Granados and Korner (2002) studied biomass and growth rates for three liana species; simulated in a tropical understorey environment with seed and soil from Yucatan; under high and low light levels; and under ambient and elevated CO2 levels. From figures 5-7 it is apparent that liana biomass increases at higher light levels for all three species. However, liana growth rate is much larger at lower light levels (up to +249%), opposed to higher light levels (up to +52%). These higher growth rates are at moderately elevated CO2 levels of ~420ppm. At ~700ppm, growth rates reduced or even reversed. Thus, individuals within the understorey with low light levels (under moderately elevated CO2 levels) have the potential to grow upwards towards the canopy at a faster rate than those in higher light levels. Figure 8: Comparison of biomass change and growth rates; under ambient and elevated CO2 concentrations; between temperate and tropical liana species (from Korner, 2009) This consistent trend in increased growth rates under low light levels has also been confirmed for temperate liana species (figure 8). Hattenschweiler and Korner (2003) found growth rates between 64-80% under low light opposed to 23-40% under high light. These results could support reasoning for the enhanced vigour and reproduction of lianas observed in recent decades in Panama (Wright et al, 2004) and Amazonia (Phillips et al, 2002). Elevated CO2 may cause lianas to behave more aggressively, thereby inducing faster forest turnover, and reducing tree carbon storage in the long-run (Korner, 2004). Other factors have also been attributed to explain current liana growth, such as reduced rainfall (Swaine and Grace, 2007). Epiphytes are another important organism that influence tropical forest tree dynamics, and grow in tree crowns. Epiphytes derive from succulents, and may utilize CAM (Crassulacean acid metabolism) photosynthetic pathways, although some can use C3 pathways also (Korner, 2009). Contrary to lianas, evidence suggests epiphytes don’t benefit from elevated CO2 (Monterio et al, 2009). They tested the effect of doubling CO2 concentration; as well as increasing light and nutrient levels; on growth of six epiphyte species from the Neotropics. Figure 9: Relative growth rate (mgg-1d-1) of six epiphyte species under increasing CO2, light and nutrient levels for six different species. C3 pathways (V=Vriesea; C=Catopsis; O=Oncidium). CAM pathways (T=Tillandsia; B=Bulbophyllum; A=Aechmea). From Monteiro et al (2009). From figure 9; across the six species; elevated CO2 increased relative growth rates by only 6%. Although C3 species grew 60% faster than CAM, the two groups showed no significant difference in their CO2 responses. High light increased average growth rates by 21%; high nutrients by 10%. The findings contrast with those noted by Granados and Korner (2002) and Wurth et al (1998a), who found significant positive responses of lianas to elevated CO2 and deep shade, opposed to high light intensities. Thus, epiphytes will pose a lower risk to forest turnover and carbon stock losses. CO2-water interactions CO2-water interactions have two sides: the CO2-driven stomatal response; and the interactions with weather; such as drought. Under elevated CO2 conditions, plants will always absorb more CO2 per unit of water lost; regardless of stomata respond. However, experimental evidence confirms stomata may not be as sensitive to CO2 as previously thought (Korner and Wurth, 1996; Lovelock et al, 1999). The increase in atmospheric CO2 over the last century has highlighted the dynamic relationships between CO2 gain and water loss. The evidence for this is within tree rings, in the form of stable carbon isotope signals. Hietz et al (2005) observed these changes in Amazonian trees, where a change in ∂3C over the past two centuries infers increased intrinsic water use efficiency. Traditionally, when water acts as a limiting factor, scientists have drawn upon an array of responses; such as stomatal closure; reduced photosynthesis and growth. However, it has been understood for decades that photosynthesis is less sensitive to reduced water potential than biomass growth. Most of the evidence is derived from non-woody plants (Korner, 2003a). Less water uptake reduces turgidity, which reduces tissue formation, eventually limiting CO2 uptake. Wurth et al (2005) completed an extensive inventory for 17 tropical tree species in both the dry and wet seasons in Panama. They found NSC pools to be largest when growth was lowest and smallest when growth reaches a maximum. This is counterintuitive to what is normally expected! It had been suggested that high NSC levels found in trees under growth limitations by environmental factors, such as drought, does not reflect source saturation by C, but a precaution strategy by which NSCs are stored in a reserve (Lewis et al, 2004a).