An Introduction to Tropical Rain Forests (II)

First an update on the issues I mentioned earlier this week: I had a guy come by and ‘fix the internet problem’ yesterday. Approximately an hour after he left I lost my connection, and it was gone for the rest of the day. I have internet now. If the problem is not solved by a second visit on Monday (they’ll send another guy over), the ISP just lost a customer – I’ll give them no more chances, I can’t live like this. The uncertainty is both incredibly stressful and frankly infuriating. I actually lost internet while writing this post. Down periods seem completely random and may last from 5 minutes to 12 hours. I’m much more dependent on the internet than are most people in part because most of my social interaction with others takes place online.

I’ve read four Christie novels within the last week and I finished The Gambler by Dostoyevsky earlier today – in case you were wondering why I’ve suddenly started reading a lot of fiction, the answer is simple: I’m awake for 16+ hours each day, and if I can’t go online to relax during my off hours I have to find some other way to distract-/enjoy-/whatever myself. Novels are one of the tools I’ve employed.

The internet issue is more important than the computer issue also in terms of the blogging context; the computer I’m using at the moment is unreliable, but seems to cause a limited amount of trouble when I’m doing simple stuff like blogging.

Okay, on to the book. I was rather harsh in my first post, but I did also mention that it had a lot of good stuff. I’ve included some of that stuff in this post below.

“Forests, because of their stature, have internal microclimates that differ from the general climate outside the canopy. […] In general terms, it is cool, humid, and dark near the floor of a mature patch of forest, progressively altering upwards to the canopy top. Different plants and animal species have specialized to the various forest interior microclimates […] Night is the winter of the tropics, because the diurnal range of mean daily temperature exceeds the annual range and is greater in drier months. […] Rain forests develop where every month is wet (with 100 mm rainfal or more), or there are only short dry periods which occur mainly as unpredictable spells lasting only a few days or weeks. Where there are several dry months (60 mm rainfal or less) of regular occurence, monsoon forests exist. Outside Asia these are usually called tropical seasonal forests. […] To the biologist […] there are major differences, and this book is about tropical rain forests, those which occur in the everwet (perhumid) climates, with only passing mention of monsoon forests.”

“Tropical rain forests occur in all three tropical land areas […]. Most extensive are the American or neotropical rain forests, about half the global total, 4 x 106 km2 in area, and one-sixth of the total broad-leaf forest of the world. […] The second largest block of tropical rain forest occurs in the Eastern tropics, and is estimated to cover 2.5 x 106 km2. It is centred on the Malay archipelago, the region known to botanists as Malesia. Indonesia[25] occupies most of the archipelago and is second to Brazil in the amount of rain forest it possesses. […] Africa has the smallest block of tropical rain forest, 1.8 x 106 km2. This is centred on the Congo basin, reaching from the high mountains at its eastern limit westwards to the Atlantic Ocean, with outliers in East Africa. […] Outside the Congo core the African rain forests have been extensively destroyed.”

“It is now believed that about half the world’s species occur in tropical rain forests although they only occupy about seven per cent of the land area. […] Just how many species the world’s rain forests contain is still […] only a matter of rough conjecture. For mammals, birds, and other larger animals there are roughly twice as many species in tropical regions as temperate ones […]. These groups are fairly well studied, insects and other invertebrates much less so […] The humid tropics are extremely rich in plant species. Of the total of approximately 250 000 species of flowering plants in the world, about two-thirds (170 000) occur in the tropics. Half of these are in the New World south of the Mexico/US frontier, 21 000 in tropical Africa (plus 10 000 in Madagascar) and 50 000 in tropical and subtropical Asia, with 36 000 in Malesia. […] There are similarities, especially at family level, between all three blocks of tropical rain forest, but there are fewer genera in common and not many species. […] In flora Africa has been called ‘the odd man out’;[52] there are fewer families, fewer genera, and fewer species in her rain forests than in either America or Asia. For example, there are 18 genera and 51 species of native palms on Singapore island,[53] as many as on the whole of mainland Africa (15 genera, 50 species) […] There are also differences within each rain forest region. […] meaningful discussions of species richness must specify scale.[60] For example, we may usefully compare richness within rain forests by counting tree species on plots of c. 1 ha. This within-community diversity has been called alpha diversity. At the other extreme we can record species richness of a whole landscape made up of several communities, and this has been called gamma diversity. The fynbos is very rich with 8500 species on 89 000 km2. It is made up of a mosaic of different floristic communities, each of which has rather few species. That is to say fynbos has low alpha and high gamma diversity. Within a single floristic community species replace each other from place to place. This gives a third component to richness, known as beta diversity. For example, within lowland rain forest there are differences in species within a single community between ridges, hillsides, and valleys.”

“Most rain forest trees […] exhibit intermittent shoot growth […] The intermittent growth of the shoot tips is seldom reflected by growth rings in the wood, and where it is these are not annual and often not annular either. Rain forest trees, unlike those of seasonal climates, cannot be aged by counting wood rings […] tree age cannot be measured directly. It has [also] been found that the fastest growing juvenile trees in a forest are the ones most likely to succeed, so growth rates averaged from a number of stems are misleading. […] we have very little reliable information on how long trees can live. […] Most of the root biomass is in the top 0.3 m or so of the soil and there is sometimes a concentration or root mat at the surface. […] Roots up to 2 mm in diameter form 20-50 per cent of the total root biomass[79] and their believed rapid turnover is probably a significant part of ecosystem nutrient cycles”

“Besides differences between the three tropical regions there are other differences within them. One major pattern is that within the African and American rain forests there are areas of especially high species richness, set like islands in a sea of relative poverty. […] No such patchiness has been detected in Asia, where the major pattern is set by Wallace’s Line, one of the sharpest zoogeographical boundaries in the world and which delimits the continental Asian faunas from the Australasian […]. These patterns are now realized to have explanations based on Earth[‘s] history […] Gondwanaland and Laurasia were [originally] separated by the great Tethys Ocean. Tethys was closed by the northwards movement of parts of Gondwanaland […]. First Africa and then India drifted north and collided with the southern margin of Laurasia. Further east the continental plate which comprised Antarctica/Australia/southern New Guinea moved northwards, broke in two leaving Antarctica behind, and, as a simplification, collided with the southeast extremity of Laurasia, at about 15 million years ago, the mid-Miocene; this created the Malay archipelago (Malesia) as it exists today. Both super-continents had their own sets of plants and animals. […] Western and eastern Malesia have very different animals, demarcated by a very sharp boundary, Wallace’s line. […] the evolution of the Malay archipelago was in fact more complex than a single collision.[145] Various shards progressively broke off Gondwana from the Jurassic onwards, drifted northwards, and became embedded in what is now continental Asia […]  The climate of the tropics has been continually changing. The old idea of fixity is quite wrong; climatic changes have had profound influences on species ranges.”

“Most knowledge about past climates is for the last 2 million years, the Quaternary period, during which there has been repeated alternation at high latitudes near the poles between Ice Ages or Glacial periods and Interglacials. During Glacial periods tropical climates were slightly cooler and drier, with lower and more seasonal rainfall. During these times rain forests became less extensive and seasonal forests expanded. Most of the Quaternary was like that; present-day climates are extreme and not typical of the period as a whole. Today we live at the height of an Interglacial. […] At the Glacial maxima sea levels were lower by as much as 180 m […] Sea surface temperature was cooler than today, by 5 ° C or more[147] at 18 000 BP in the tropics. […] Rain forests were more extensive than at any time in the Quaternary during the early Pliocene, parts of the Miocene, and especially the early Eocene; so these were all warm periods. Then, in the late Tertiary, fluctuations similar to those of the Quaternary occurred. […] Africa [as mentioned] has a much poorer flora than the other two rain forest regions.[152] This is believed to be because it was much more strongly dessicated during the Tertiary. […] Australia too suffered strong Tertiary dessication. At that time its mesic vegetation became mainly confined to the eastern seaboard. The strip of tropical rain forests found today in north Queensland is only 2-30 km wide and is of particular interest because it contains the relicts of the old mesic flora. This includes the ancestors from which many modern Australian species adapted to hot dry climates are believed to have evolved […] New Caledonia is a shard of Gondwanaland which drifted away eastwards from northeast Australia starting in the Upper Cretaceous 82 million BP. Because it is an island its vegetation has suffered less from the drier Glacial climates so more of the old flora has survived. The lands bordering the western Pacific have the greatest concentration of primitive flowering plants found anywhere […] It is most likely that they survived here as relicts.”

“rain forests have waxed and waned in extent during the Quaternary, and probably in the Tertiary too, and are not the ancient and immutable bastions where life originated which populist writings still sometimes suggest. In the present Interglacial they are as extensive as they have ever been, or nearly so. At glacial maxima lowland rain forests are believed to have contracted and only to have persisted in places where conditions remained favourable for them, as patches surrounded by tropical seasonal forests, like islands set in a sea. In subsequent Interglacials, as perhumid conditions returned, the rain forests expanded out of these patches, which have come to be called Pleistocene refugia. In the late 1960s it was shown that within Amazonia birds have areas of high species endemism and richness which are surrounded by relatively poorer areas. The same was soon demonstrated for lizards.[153] Subsequently many groups of animals have been shown to exhibit such patchiness […] The centres of concentration more or less coincide with each other […] These loci overlap with areas that geoscientific evidence suggests retained rain forest during Pleistocene glaciations […] In the African rain forests four groups of loci of species richness and endemism are now recognized […] Most parts of Malesia today are about as equally rich in species, including endemics, as the Pleistocene refugia of Africa and America. At the Glacial maxima the Sunda and Sahul continental shelves were exposed by falling sealevel. Rain forests were likely to have become confined to the more mountaineous places where there was more, orographic, rain. The main development of seasonal forests in this region is likely to have been on the newly exposed lowlands, and when sea-level rose again at the next Interglacial these and the physical signs of seasonal climates […] were drowned. The parts of Malesia that are above sea-level today probably remained, largely perhumid and covered by rain forest, which explains their extreme species richness and their lack of geoscientific evidence of seasonal past climates. […] Present-day lowland rain forest communities consist of plant and animal species that have survived past climatic vicissitudes or have immigrated since the climate ameliorated. Thus many species co-exist today as a result of historical chance, not because they co-evolved together. Their communities are neither immutable nor finely tuned. This point is of great importance to the ideas scientists have expressed concerning plant-animal interactions […] Those parts of the world’s tropical rain forests that are most rich in species are those that the evidence shows have been the most stable, where species have evolved and continued to accumulate with the passage of time without episodes of extinction caused by unfavourable climatic periods. This is similar to the pattern observed in other forest biomes”


September 20, 2014 Posted by | Biology, Books, Botany, climate, Ecology, Evolutionary biology, Geography, Geology | Leave a comment