So in today's lecture, I'll be talking to you about tropical forests and their climate history as well as soil nutrients. So why the ecology of tropical forest? What's so special about tropical forests? What makes them unique? Well, the first thing that usually comes to mind is diversity. Tropical forests are known for harboring an extremely high number of species worldwide. There is estimated to be over 40,000 different tree species across the tropics, and even within a single hectare, an area the size of a few football fields, you can get over 200 different tree species. And this is really high compared to temperate europe, for example, only has 125 tree species, and this trend for tropical trees is also the same for other organisms. For example, mammals and birds both have their peak species richness in the tropics. This high diversity also means that most species in the tropics are extremely rare. So besides diversity and rarity, tropical forests are known for their complexity. There both structurally complex and ecologically complex. So in terms of structure, tropical forests have many different vegetation layers and many different growth forms. So in addition to canopy trees, understory trees, shrubs, you also get things like liana, woody vines and epiphytes that are plants growing on top of other plants. In terms of ecological complexity, the tropics are well known for having many different plant-animal interactions. For example, many tropical plant species are pollinated or have their seeds dispersed by animals like this hummingbird pollinated species you see here. Tropical forests also provide numerous ecosystem services that benefit humans. I'm not going to talk about all of them today, but just to list a few, there are a large number of non-timber forest products that people use. They also harbor insects that serve as crop pollinators. Tropical forests improve air and water quality, provide erosion control, and they also provide opportunities for recreation and ecotourism. For many cultures, forests hold cultural and spiritual values, and as we'll talk about in a later lecture, tropical forests also harbor a large amount of carbon, so they are particularly important for carbon storage. So where are tropical forests? Well, Tropical forests are located in a band around the equator in the area between approximately 23° North and South latitude. But if you look at the map, you'll notice that not all areas in the tropics are forest. So what determines where you find forest? Well, in general, climate determines ecosystem type. In the tropics, you see areas that are warm throughout the year and in areas within the tropics that are wet that receive high annual rainfall, you get areas of forest. Now, in contrast to the temperate zone, variation at a site in the tropics is primarily in precipitation, not temperature. In fact, in the tropics, daily temperature fluctuations exceed average annual seasonal fluctuations in temperature. However, most tropical forest experience distinct wet and dry seasons, so some parts of the year where there is much lower rainfall compared to the rest of the year. So climate shapes ecosystems, but so does the past. There are several regions around the world where you get tropical forests, including in the Americas, Africa and Southeast Asia. And throughout all of these regions, the vegetation and the forest structure looks surprisingly similar across all of these regions, but in fact they have very little overlap in species composition. This has to do with the biogeographic history related to plate tectonics and evolution across these different regions. So, a biogeographic region is a large area with particular flora and fauna due to their isolation during continental drift. There are five major regions in the humid tropics that support tropical forests, tropical Americas, Africa, Southeast Asia, Madagascar and New Guinea. And these areas occurred on separate continental fragments from about 100 million years ago to 20 million years ago. And as a result, each region is a distinct ecological and biogeographical entity. And it's important to recognize this because when you go to identify conservation or restoration strategies, you need to make sure they are appropriate to the regional conditions where you're working. But even within a single biogeographic region, historical events have shaped the forest that we see today. I'm just going to give you one example of that, the Great American Biotic Interchange. So this began in the early Pleistocene with the emergence of the Isthmus of Panama approximately 2.5 million years ago, which connected North America and South America. Now prior to the formation of the Isthmus of Panama, no relatives of these animals were present in South America. There are also a number of South American migrants to North and Central America. This image shows what migration after the emergence of the Isthmus look like, with animals and green are ones that migrated from the south to the north and animals in blue from the north to the south. And interestingly northern families of land mammals did better in both North and South America. So in addition to climate in evolutionary history, soil nutrients shaped tropical forest communities. In terms of nutrient cycling in tropical forests, tropical force is the most productive of all terrestrial ecosystems. Yet large areas of tropical forests occur on very nutrient poor soils. And so how can this happen? How can we get such high diversity and high productivity on nutrient poor soils? Well, it turns out that a large proportion of the nutrients that are available to plants are tied up in the living biomass and are recycled with plant litter. And this recycling is highly efficient, so there's a highly efficient cycling of nutrients in organic matter and tropical forests. In addition, plant communities in the tropics are likely to be well adapted to nutrient shortages. Often they form mutualisms with mycorrhizal fungi or nitrogen fixing bacteria that help them gain nutrients. They often have well defended leaves that prevent those nutrients being lost when their leaves are attacked by herbivores. So, in tropical forests, nutrients are mainly found in living plant biomass and in the layer of decomposing litter. Little nutrient content is found in the deeper soils, unlike in temperate forests. And a longstanding paradigm is that phosphorus availability is what limits productivity of most lowland tropical forests. And this is in contrast to nitrogen limitation, which commonly occurs in temperate forests. Although phosphorus is is most often the limiting nutrient in tropical forests, and you also see very low nutrient availability in general, across the tropics, there is a wide range of soil fertility, and this wide variation and tropical soils is due to a number of factors. First, tropical forests vary widely in the soil age and weathering status, and this produces differences in the availability of multiple nutrients. In addition, there are differences among regions in erosion and tectonic uplift. And these processes rejuvenate soils by bringing weatherable parent material to the surface. And finally, variation and plant species composition can affect nutrients by altering rates of production, decomposition and nutrient cycling. So in summary, tropical forests occur in warm and wet places around the equator between 23 degrees north and south, most experience distinct dry and rainy seasons, and they're known for their high diversity rarity and complexity. Tropical forests provide a wealth of ecosystem services upon which humans depend. And across the tropics, we see differences in the biogeographic history that result in differences in ecological communities found throughout the tropics. We also see wide variation in tropical forest soils, but all tropical forest soils are generally characterized by nutrient storage, primarily in plant biomass and a highly efficient cycling of nutrients.
