rainforests the rainforest is incredibly complex the rainforest is incredibly productive the...
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Rainforests The rainforest is incredibly complex The rainforest is incredibly productive The rainforest cycles nutrients incredibly quickly The rainforest soils are incredibly fragile Slide 2 Rainforests Productivity: the amount of solar radiation, sunlight, converted by plants into complex molecules such as sugars This is accomplished through photosynthesis CO 2 + H 2 O + sunlightsugar + O 2 Slide 3 Rainforests Slide 4 Oxygen production very important both in ecological and evolutionary time E.g. oxygen-free to 21% O 2 E.g. rainforest (2x) temperate forest (4-5x) grassland Slide 5 Rainforests NPP vs. GPP GPP very hard to estimate, typically NPP Despite high rates of respiration (50- 60% in maintenance), highest NPP of all terrestrial systems So what Slide 6 Rainforests Conversion to less productive systems As rainforests are replaced by other systems (predominately agriculture), all are much less productive! E.g. 40% of worlds NPP co-opted Slide 7 Rainforests Leaf-area Index (LAI) is the leaf area above a square meter of forest floor. In NH, LAI is almost 6, BCI almost 8 In Caatinga, 5.1, and in rainforest, a range of 10.6-22.4 (soil conditions) The relationship between LAI and understory cover is inversely related Slide 8 Rainforests Tropical growing season is year-round compared to temperate areas Easy to suggest comparable on a per unit time metric Answer is unclear But, some studies suggest tropical growth is significantly greater than temperate growth (an order or magnitude greater) Slide 9 Rainforests Growth: what influences it?? Water Light CO 2 Minerals Tropics do well in first 3, and plants have done relatively well in adapting to poor soil Why?? Slide 10 Nutrient Cycling Earth is a closed system Consequently, atoms present in dead tissue must be reacquired, recycled back into the living world (or tissues) The recycling is in the form of decomposition Slide 11 Nutrient Cycling Consider a unit of rainforest unit of energy fixed during net primary productivity can move in one of two major directions: either it can be consumed as part of living tissue (caterpillar), or it can remain as part of the leaf until it drops and enters the decomposer food web Slide 12 Nutrient Cycling Have you been to the tropics? Which route do you think the majority of energy takes? Why? Slide 13 Nutrient Cycling Who are the players in the decomposer food web? Fungi and bacteria But also slime molds, actinomycetes, algae, arthropods, earthworms, protozoans Having said that, our understanding of how the decomposer (microbial) food web works is relatively poor Slide 14 Nutrient Cycling It may be the biodiversity in soil community rivals that of the canopy, but there are relatively few studies on the pathways and extent of this community Slide 15 Nutrient Cycling Organisms facilitate a process called humification, in which complex soil organic matter is maintained at the interface between the tree roots and soil. Humus is important in forming colloids that cement soil particles, in helping aerate the soil, in possessing a negative charge, and important aid in retaining critical minerals, and in overall conservation of nutrients Slide 16 Nutrient Cycling The soil is an important storage bank for essential minerals E.g. nitrogen, calcium, magnesium, phosphorus, and potassium Nutrient cycling is often termed biogeochemical cycling because of the process moving things in between the living (bios) and non-living (geos) Slide 17 Nutrient Cycling The rate of biogeochemical cycling is strongly influenced by temperature and rainfall Since minerals are always taken up through roots via water, the uptake of water is essential the uptake of minerals as well. But evaporation can be a mixed blessing. Plants can lose too much water when subjected to constant high temperature. Slide 18 Nutrient Cycling Plants can lose too much water when subjected to constant high temperature. Many tropical plants retard evaporative water loss both by closing their stomata and by producing waxy leaves. Slide 19 Leaching Water can wash essential minerals and other chemicals from leaves, a process called leaching. Leaching can be especially sever in areas subject o frequent heavy rains. A defense against this is the thick waxy coating that aid in retarding water loss as well as deterring herbivores and fungi Slide 20 Leaching Rainfall also leaches minerals from the soil, washing them down into the deeper soil layers. Clay particles and humus have negative electrostatic charges tat attract mineral with positive charges such as calcium and potassium. Because H 2 O adds a positive H+ to the soil, these elements get washed deep into the soil or may wash out of the soil completely Slide 21 Leaching Consequently in the tropics, the combination of high temperature and heavy rainfall can often result in much leaching and strongly acidic soils. Typical Amazon soils (up to 75%) are frequently mineral-poor, high in clay, acidic, and low in available phosphorus Slide 22 Leaching As a result, in tropical forests most of the rapidly cycling minerals are in the living plants, the biomass. In the temperate zone, minerals are more equally distributed between the vegetation and soil bank. Slide 23 Mycorrhizae Most plants in the world exhibit an intimate, mutualistic association between tree roots and a diverse group of fungi collectively termed mycorrhizae Although located in tree roots and appearing parasitic, they are essential in mineral uptake from the forest litter. Slide 24 Rapid Recycling Walk through a rainforest and youll be surprised how little litter there is (think deciduous or more, coniferous) Rainforest have the same processes, they just occur much more rapidly E.g. One study estimates 80% of total leaf matter in an Amazon rainforest is annually returned to the soil. Slide 25 Rainforest Soils Despite previous statements, generalizations of tropical soils should be made cautiously Eastern and central Amazon Basin, soils are very old and mineral-poor (oligotrophic), while in other regions, such as volcanic areas of Costa Rica or much of the Andes, soils are young and mineral-rich (eutrophic) Slide 26 Rainforest Soils Soil characteristics vary regionally (and quite a bit) because soil is the product of several factors: climate, vegetation, topographic position, parent material, and soil age Much of the humid tropics falls into one of three categories: ultiols, oxisols, or alfisols, all of which are reddish to yellowish in color Slide 27 Rainforest Soils Ultisols: well-weathered (leached) Oxisols: deeply weathered, old, acidic, and found on well-drained soils (generally occurring on old geologic formations). Widespread and world wide Alfisols: common in the subhumid and semiarid tropics and are closer to a neutral pH (though still acidic), which less overall leaching than oxisols Slide 28 Rainforest Soils However, not all tropical soils are old or heavily weathered or infertile Perhaps 15% of moist tropical soils are situated on soils of at least moderate fertility. Soils generated from deposits during the flood cycle or from recent volcanic activity typify these categories Slide 29 Rainforest Soils A general pattern in the tropics is that heat and heavy moisture input cause the formation of oxides of iron and aluminum (neither of which is useful to plants), yielding a reddish color Clay content is high be leery of wet trails and mountain roads Slide 30 Rainforest Soils Slide 31 Slide 32 At the extreme, one can encounter laterization, the combined effects of intensive erosion and heat acting on soil If vegetation is removed and bare soil is exposed to extreme downpours and heat, it can bake into a bricklike substance, ruining it for future productivity (also utilized for bricks) In Amazonia, only 4% are at risk Slide 33 Rainforest Soils However, even without this extreme, the attempt to farm the Amazon and other tropical forests have failed due to the quick loss of soil fertility Limited agriculture could be supported (soil similar to SE US) more on this later Slide 34 Rainforest Soils white soils In some parts of the Amazon Basin, white and sandy soils predominate, from the Brazilian and Guianan Shields These are extremely old (100s of millions of years) and have lost their fertility However, relatively lush broadleaf rainforests grow on these soils Slide 35 Rainforest Soils Oligotrophic soils support less lush and smaller (in height) forests Up to 26% of roots can be on the surface and root mats can be very thick Certain minerals (e.g. calcium and phosphorus) are extremely efficiently taken up by root mat and mycorrhizae (99.9%) Slide 36 Rainforest Soils How might buttresses impact the nutrient cycling? Roots can spread widely at the surface Do you understand the value and need for a thin layer of forest humus with its mycorrhizal fungi Generalizations should be avoided as there are many variants as to what is truly productive and what is tight Slide 37 Nutrient Adaptations Some tropical plants have root systems that grow vertically upward, from the soil onto the stems of neighboring trees. These apogeotropic roots grow as fast as (5.6cm) in 72 hrs). Why? Slide 38 Nutrient Adaptations Arrested Litter: many plants (i.e. palms, epiphytes) catch litter before it reaches the forest floor. This is one way in which plants in the upper canopy can obtain the entire suite of nutrients needed to grown and reproduce Slide 39 Nitrogen Fixation Some plants, such as legumes, can take up gaseous nitrogen directly from the atmosphere and convert it to nitrate, a chemical form in which it can be used by the plant. How effective? One estimate 20kg of nitrogen fixed per ha per yr in the Amazon Basin (>3x from precip) Slide 40 Tropical Rivers White sandy soils are usually drained by blackwater rivers Color derived by tannins, phenolics, and related compounds (humic matter) Not confined to tropics (e.g. bogs) Poor soils contribute to such water Leaf production expensive, hence concentrate defenses Slide 41 Rainforest Gaps Slide 42 Tropical Rivers In contrast to white, sandy soils, soils in places such as Puerto Rico, much of Costa Rica, and much of the Andes Mountains are not mineral-poor but mineral-rich. They are much younger, mostly volcanic in origin, high rainfall and temperatures. Much sediment leaches into the river, they are cloudy and termed whitewater Slide 43 Tropical Rivers Slide 44 A dramatic confluence of such water is the wedding of the waters between the Rio Negro and Rio Brancho It lasts from 15-25km until the mixing is complete Slide 45 Tropical Rivers Slide 46