Category Archives: Land-use history

Survival and rebound of Antillean dry forests: Role of forest fragments

Antillean dry forests have experienced high levels of human impact for almost five centuries. Economic changes in the second half of the 20th century have facilitated forest recovery in Puerto Rico. We quantified the extent of forest cover and the community composition of representative forest fragments in the subtropical dry forest life zone (sensu Holdridge, 1967) in southwestern Puerto Rico. Forest cover, which was largely eliminated by the 1940s, stood at 48% in the western dry forest life zone in 1993. Fragments varied in land-use history and supported from 1% to 86% of the reference species sampled in Guánica Forest, a 4000-ha protected area. Reference species were well represented in forest fragments, even those smaller than 1 ha, if they had never been completely cleared, but were uncommon in forests regenerating on previously cleared sites. The studied fragments are novel ecosystems which combine native and introduced elements; Leucaena leucocephala (Lam.) De Wit, an introduced legume, was the most common species, regardless of land-use history.

I.A., Murphy, P.G., Burton, T.M., Lugo, A.E.  2012. Survival and rebound of Antillean dry forests: role of forest fragments. Forest Ecology and Management 284:124-132.

Full article: ScienceDirect (subscription) or International Institute of Tropical Forestry.

Human impacts on pre-Columbian tropical forests

When European naturalists first visited the New World Tropics they saw vast forests that seemed untouched by humans. While indigenous people often lived in these forests, their populations were small. This led to a perception of tropical forests as primeval, “virgin” forests. In the last few decades, this perception has changed – large areas now covered by mature forests have a history of cultivation. In many cases, “primeval” forests are less than 500 years old.

La Selva biological station in Costa Rica is one of the premier research stations for Neotropical biology. Prior to archaeological study of the site, much of it was assumed to be free of human influence. However, the discovery of pre-Columbian artefacts led to the discovery that the site had been occupied at least 3000 years ago. Charcoal was more abundant in alluvial terraces (flatter areas with deeper, more fertile soil) and less abundant in the less fertile upland soils. A chronology, established by Sol (2000)*, divided the La Selva into four archaeological phases: La Cabaña 1000 – 1550 CE; La Selva 500 – 1000 CE; El Bosque 300 BCE– 500 CE; La Montaña 1500 –300 BCE.

To better understand the history of the site, Lisa Kennedy of Virginia Tech and Sally Horn of the University of Tennessee, Knoxville, undertook a study of sediment cores extracted from the Cantarra swamp*, a 0.5 ha wetland dominated by perennial herbs. They used pollen, charcoal and macrofossils to reconstruct the environmental history of the site. Wetlands are frequently used to reconstruct vegetation histories. As sediments accumulate in bodies of water, plant pollen, fern spores and charcoal fragments are trapped. Pollen coats are extremely tough, and decay takes place very slowly in waterlogged soils. If the vegetation surrounding the site changes, different types of pollen will be deposited into the site. Someone with the patience to sort through these cores can observe thousands of years of history in a few metres of sediment.

The most obvious evidence of human activity is the presence of corn (Zea mays subsp. mays) pollen. Corn is a cultivated species which does very poorly without human intervention. Thus, the presence of corn pollen in the wetland sediments is direct evidence of agriculture. Corn pollen shows up from 880 CE to somewhere between the mid-1600s and mid-1800s. Pollen of other species like Amaranths, Asteraceae (the sunflower family), and other grasses and herbs also peak during and before the “corn zone”, often at the same time that charcoal density peaks. This may also reflect cultivation, although it could represent weedy species establishing after fires. Corn pollen was found in sediments about 1300 years older at another lake about 2 kn distance from this one. The authors suggested that disturbance in this time period at Cantarra swamp may have represented the cultivation of root crops (which don’t leave the kind of pollen signature that corn does.

As a forest ecologist, I find some of the “other evidence of disturbance” to be the most interesting. There are several peaks of Cecropia pollen, and to a lesser extent Trema pollen. These are fast-growing species that are usually associated with large gaps in the forest – specifically the type that human agricultural activities may have suggested. Other peaks of pollen belonging to forest species suggests that periods of forest recovery were interspersed with the cultivated times.

This is very interesting stuff. We are too inclined to interpret forests as “primeval”. In many cases, what our eyes see as ancient is only a few centuries old. It is important to understand that if we want to construct realistic models of forest dynamics.

  1. Sol, C., R. F. 2000. Asentamientos prehispánicos en la Reserva Biológica La
    Selva, Sarapiquí, Costa Rica: Sistemas de explotación de recursos naturales
    en un bosque tropical lluvioso. Licenciatura thesis, School of Anthropology and Sociology, University of Costa Rica.
  2. Lisa M. Kennedy, Sally P. Horn. A Late Holocene Pollen and Charcoal Record from La Selva Biological Station, Costa Rica. Biotropica (OnlineEarly Articles). doi:10.1111/j.1744-7429.2007.00334.x

What is natural? Reinterpreting rivers in the eastern US

[Repost from my old blog]
One of the fundamental questions faced in ecological restoration is that of trying to find a “natural” baseline to work towards. This is especially true in Europe and eastern North America – these are the areas where so much fundamental ecological work was done, but by the time this work was done, they were modified to the point where it was often impossible to distinguish “natural” from anthropogenic.

I’ve always been a little confused by streams. As a terrestrial ecologist, steams often seem to be these alien things that intrude into the landscape. It isn’t that bodies of water are themselves incomprehensible, but rather, the way that they interact with the terrestrial landscape. Too often, they just don’t seem to add up – it always feels like current processes couldn’t produce what I see. Turns out that, at least in eastern North America, I’m at least partly correct to feel that way.

In a paper published in the January 18, 2008 issue of Science Robert Walter and Dorothy Merrits of Franklin and Marshall College examined the established ideas about the characteristic form of rivers in the Mid-Atlantic and Western parts of the United States. The norm is seen to be a single meandering channel with a gravel bottom and alternating riffles and pools, often with a channel that is deeply incised into the surrounding streambanks. These streams flow through “valley flats” – broad deposits that fill the valley. The stream cuts into these relatively coarse deposits. These deposits were interpreted to have been deposited when the river floods, while the valley was thought to have been produced by the stream as it meandered across the valley. (That’s what I was taught in high school geography). However, many observations seemed inconsistent with these explanations. Modern flood patterns don’t deposit the type or amount of sediments that are observed in these valleys. In addition, Walter and Merritts observed that the tops of historic milldams (which were typically built 2.5 to 3.7 m tall) were at the level of the valley flats which suggested that the valley flats had been deposited in recent times, after the dams were built.

Dam construction began with European settlement. Until they were replaced by steam engines in the twentieth century forges, furnaces, mining operations and mills were driven by water from millponds. Walter and Merritts estimated that there are 16,000 – 18,000 millponds in Pennsylvania, covering a period of over 200 years. Since millponds were built early in the settlement period, they rapidly silted up as the surrounding landscape was deforested. In the late 1800s and early 1900s newer dams were built along streams that had breached older dams. Many of these dams have also breached, leading to an even more complex pattern.

When the dug into the valley flats, Walter and Merritts found several metres of sediment on top of soils characteristic of wetlands. These soils are rich in organic material and sometimes include tree stumps and the remains of corduroy roads – roads made of logs or planks on swampy ground. By digging trenches across the entire valley, they were able to get a sense of the entire river, and were able to conclude that:

The characteristics of the presettlement sediments and organicmaterial suggest that valley bottoms were broad, forested wetlands(alder shrub-scrub) with small, shallow (<1-m) anabranchingand chain-of-pool streams that experienced frequent overbankflow, which is consistent with accounts by early explorers ofubiquitous swampy meadows and marshes fed by springs at thebase of valley side-slopes

Walter and Merritts conclude by saying

These conclusions change the interpretation of hydraulic geometryin eastern U.S. streams that is based on the archetype of an”ideal meandering river form” and imply the need to reconsidercurrent procedures for stream restoration that rely on referencereach conditions and the assumption that eroding channel banksare natural and replenishable. The current condition of singlegravel-bedded channels with high, fine-grained banks and relativelydry valley-flat surfaces disconnected from groundwater is instark contrast to the presettlement condition of swampy meadows(shrub-scrub) and shallow anabranching streams described here.

These observations also have important implications for European rivers, were millpond construction dates back to the 1100s. It also makes me rethink Trinidadian streams and rivers – but that needs to be a separate blog post.

Walter, R.C., Merritts, D.J. (2008). Natural Streams and the Legacy of Water-Powered Mills. Science, 319(5861), 299-304. DOI: 10.1126/science.1151716