Effects of Enhanced CO2 on Tropical Forest Growth

Effects of Enhanced degree Celsiusic acid gas on equatorial Forest GrowthJames P. Smith Effects of compound atmospheric carbon dioxide concentrations on equatorial woods evolution experimental studies and interactions with whole somewhats, fainthearted, pee and temperatureAbstract (150 words)Introduction (300 words)Approximately 90% of earths 652Gt terrene biomass ascorbic acid is locked up in timbers. Tropical and subtropical forests store 340Gt carbon or 52% b arely further make up 13% of entireness forested atomic number 18a ( card 1). Achard et al (2002) estimated 1Gt/yr carbon losses, by represents of activities such(prenominal) as deforestation and clearance for agriculture (Geist et al, 2002). every(prenominal) telluric bases deal become ex flapd to increase atmospheric carbonic acid gas 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). change magnitude carbonic acid gas could stimulate photosynthesis, raising plant productivity. This can bewilder a role in storing more carbon and mitigate the atmospheric hold irrigate system in CO2 concentrations (Beed wiped out(p) et al, 2004).Table 1 Areal extent, carbon memory board and net primary productivity of earths major biomes (from Roy et al, 2001). issue 1 demonst swans 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 divergent processes such as morphological process, metabolism and export. Sugars can also be stored as militia in the form of NSCs (non- morphologic carbohyd judge). CO2 is lost through respiration, herbivory and litter production and disintegration (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 begin of intricateen CO2 on tropical forest maturation. This will be achieved by looking at experimental studies, as well as the effects of intensify CO2 on the adjustment factors of intellectual nourishments, lighten up, peeing system release and temperature. I will be reviewing literature from 1999-2013.Experimental studiesThere have been a couple of(prenominal) experimental studies of the effects of enhanced CO2 on plant growth in tropical forests in relatively natural conditions (ambient climate, natural kingdom and inter and intra-species competition). Two studies using a c all over crane in a tropical prohibitionist forest in navy man was apply to assess the effects of enhanced CO2 on canopy guide leaves. Over a 40 week period Lovelock et al (1999) metric responses of leaf and branchlets of a single corner species. Photosynthesis rates change magnitude 30% with enhanced CO2. However, no change magnitudes in biomass occurred (reproductive organs and foliag e). Branchlet TNC (total non- geomorphological carbohydrates) increase 20%, inferring localized carbon saturation. Wurth et al (1998a) put together 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, scrubs and grass) in the down the stairs(a)storey of a secretived Panamanian forest. These were grown over a 15 month period, in which 50% were in ambient CO2 and 50% in gallant. All species showed strong seedling growth under upgrade CO2, only if decreased as understorey light levels increased, and inter-species variation was apparent. Again TNC levels increased under enhanced CO2.One experiment has analyse communities of tropical trees, which have been outplanted in natural soil and subjected to proud CO2. Lovelock et al (1998) grew groups of ten tree species at ambient and elevate CO2 in open-top chambers at the forest margin in Panama. Over six mon ths, 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- limited, but late-successional species were slight light-sensitive than pioneer and midsuccessional species.Table 2 parity of mean TNC concentrations (% dry weight) across four studies under ambient and distinguished 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 macrocosm transported to carbon sinks, into plant biomass for growth. They include carbohydrates, sugar alcohols, organic acids and lipids, and represents carbon defys or stores, for future use on demand (Korner, 2003a). So, photosynthesis rates whitethorn increase under elevated CO2, producing more TNCs, but may not be used in plant growth, un s light needed.Figure 2 strain in mean concentration of TNC with height in two steadfast 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 tack 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 controllight-emitting diode by moisture availability, in agreement with another(prenominal) study in the area (Newell et al, 2002). On the other hand, Korner and Wurth (1996) found TNC to increase significantly in two 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 sca lead up to actual forest size use only small plants have a high than principle nutrient supply absence seizure of competition and key processes such as herbivory and effects of pathogens.CO2-nutrient interactionsNitrogen 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 reparation 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 ra te of symbiotic N fixation, as demonstrated by meander 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 king the fixation process. Thus a greater proportion of nitrogen is persistent 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 Varolii 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 (19 97)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 secernate that trees grown in close proximity to their natural habitat, under elevated CO2, within original soils and under local climatic conditions, exhibited intensify growth rates when soils were enriched with mineral nutrients (winter and Lovelock, 1999 Winter et al, 2001 table 4). In the absence of fertilis er 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, contend to increasing biomass with those that were enriched with mineral nutrients.CO2-light interactionsIt 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 pourboire within the leaf, stimulation by enhanced CO2 in shaded areas can be seen to be similar to canopy thinning or sparkle (Long and Drake, 1991).The effect of elevated CO2 on tropical plants grown in deep shade can be significant and can mayhap exceed effects grown under horticultural conditions under affluent light (Korner, 2009). Wurth (1998a) exposed seedlings on the forest floor to 700ppm CO2 under extremely low light levels (11mol 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 big 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 flush 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 blistering rate than those in higher light levels.Figure 8 Com parison 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 corroborate 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 terminal in the long-run (Korner, 2004). Other factors have also been attributed to explain topical liana growth, such as reduced rainfall (Swaine and Grace, 2007).Epiphytes are another primal organism that influence tropical forest tree dynamics, and grow in tree crowns. E piphytes 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 dont 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 relational 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 dissimilitude in their CO2 responses. High light increased average growth rates by 21% high nutrients by 10%. The findings co ntrast 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 blood line 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) realized 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 terminal and smallest when growth reaches a maximum. This is counterintuitive to what is normally expectedIt 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).