Category Archives: Tropical biology

Disturbance and recovery in tropical dry forests

[Repost from my old blog]
When people think about the destruction and degradation of tropical forests, they tend to focus on rainforests. Tropical dry forests tend to get overlooked. They aren’t as striking – no cathedral-like understorey, no mind-boggling biodiversity. But more importantly, they often just aren’t there. Over much of their potential range they have simply been erased from the landscape. They may have covered as much as 42% of the land area in the tropics1, but have been reduced to less than 27% of their former range in Mexico2, and as little as 2% in Central America3 and New Caledonia4.

Despite the fact this, tropical dry forests are often seen as being quite well-adapted to human disturbance. Being less species-rich than wetter forests, they tend to support fewer rare species, and may be less extinction-prone. In addition, dry forests are dominated by trees that sprout after being cut. This means that if you cut down a patch of dry forest, most of the stumps will re-sprout. This type of recovery is much quicker than you would get if the trees had to germinate from seeds – not only does it take much longer for seedlings to grow large (stump sprouts can draw on resources stored in the roots of the tree), but there’s likely to be a time lag as seeds disperse into the area from surviving trees (tropical forests tend to lack long-lived seedbanks).

Much of our understanding of succession in tropical dry forests comes from Jack Ewel’s dissertation work. Ewel looked at the effect of cutting and herbicide application on succession in a series of plots across the Neotropics. One of his important findings was the dry forests were quicker to recover their stature that wetter forests. Since most of the recovery comes from stump sprouts, the recovering forest is also close to the original forest in terms of species composition.

While lightly used dry forest sites recover rapidly, recovery is slower in more intensively used sites. Seedling survival rates are very low in dry forests – while seedlings establish in the wet season, most (often all) of them die in the subsequence dry season. So while intensively used sites in Guánica Forest recovered well in terms of structure, biomass and leaf fall in 50 years after abandonment, the recovery of species composition was very slow6.

Resilience is the rate of recovery of disturbed sites to their pre-disturbed state. Ewel’s work helped to establish the idea that dry forests are more resilient than wetter forests. But there is no single rate – or pathway – of recovery. Measures of “recovery” depend on the parameter measured – canopy height, biomass, species richness, nutrient cycling… It also depends on the baseline against which recovery is measured: if the same site is measured before and after disturbance, you need to know if the site represented “mature” forest before disturbance. If another site is used, you need to wonder if it is really representative of initial conditions in your experimental plot.

In an article7 published in the journal Biotropica, Edwin Lebrija-Trejos and coauthors looked at what it really means to say that tropical dry forests are more resilient than wetter forests. They looked at a sequence of 15 sites in Oaxaca, Mexico, which had been cultivated and then abandoned for 0-40 years, and compared them with nearby mature forest. All of the sites had been cultivated for a short period (1-2 years) and then abandoned without being converted to pasture8. They considered a variety of different ways to measure resilience – they looked at forest height, plant density, basal area (the area occupied by tree stems), crown cover, species richness, species density (number of species per 100 m2), Shannon evenness and Shannon diversity. Not surprisingly, they found that certain features (canopy height, plant density, crown cover) recovered rapidly (in less than 20 years) while others (including basal area and species richness) had not recovered after 40 years.

When compared their sites with other comparable studies, they found that their sites were among the quickest to recover canopy cover and height. On the other hand, they found that their sites were among the slowest to recover species diversity and average in terms of the recovery of species richness. Overall, in terms of the structural measures that Ewel focussed on, it’s reasonable to conclude that dry forests are more resilient that wetter forests. On the other hand, with regards to things like basal area and species richness, the assertion of resilience for dry forests isn’t well supported.

  1. Brown, S., and A. E. Lugo. 1982. The storage and production of organic matter in tropical forests and their role in the global carbon cycle. Biotropica 14:161-187.
  2. Trejo, I., and R. Dirzo. 2002. Floristic diversity of Mexican seasonally dry tropical forests. Biodiversity and Conservation 11:2063–2084
  3. Janzen, D. H. 1988. Tropical dry forests: The most endangered major ecosystem. In E. O. Wilson (Ed.). Biodiversity, pp. 130–137. National Academy Press, Washington, DC
  4. Gillespie, T. W., and T. Jaffré. 2003. Tropical dry forests in New Caledonia. Biodiversity and Conservation 12:1687–1697.
  5. Ewel, J. J. 1971. Experiments in arresting succession with cutting and herbicide in five tropical environments. Ph.D. University of North Carolina, Chapel Hill.
  6. Molina Colón, S., and A. E. Lugo. 2006. Recovery of a subtropical dry forest after abandonment of different land uses. Biotropica 38:354–364.
  7. Lebrija-Trejos, E., Bongers, F., Pérez-García, E.A., Meave, J.A. (2008). Successional Change and Resilience of a Very Dry Tropical Deciduous Forest Following Shifting Agriculture. Biotropica 40(4):422-431 DOI: 10.1111/j.1744-7429.2008.00398.x
  8. Conversion to pasture tends to slow recovery significantly; not only does the prolonged period eliminate almost all root stocks, it also establishes a grassy layer that makes it more difficult for tree seedlings to establish.

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.

Names…

I suppose it’s time to stop calling this by the default My Blog that WordPress gave it. To be honest though, it’s a pretty descriptive name – it’s a blog, it’s mine…what more do you want?

I suppose some sense of the place might help. But what do I really want to do here? What’s my vision for this site? And how do you encapsulate it in a single phrase? OK, well, I can’t. Well…what about something clever? Something catchy? Something profound? Sounds great – now all I need is skill in coming up with clever, catchy names. If I could do that, wouldn’t I be making lots of money in marketing?

OK then – how about a reference to something that’s kinda cool and somewhat descriptive? Something with a distinctly Trini flavour. But what? I’m sure there are dozens of ‘river lime’ blogs and ‘rum shop’ blogs. “A Mayaro state of mind” would be nice. But it’s probably been done already. And, more importantly, I’m not shooting the breeze in Mayaro, and I’m not in that sort of a state. I don’t want peace with the universe, I want intellectual engagement. With a sense of what is Trini and botanically inclined, nature inclined.

And then I realised what I was looking for. The samaan tree – Samanea saman. For starters, it’s a really attractive tree with its open, spreading crown. Its long, almost horizontal branches are excellent hosts for epiphytes. The campus of the University of the West Indies in St. Augustine (Trinidad) was dotted with them when I was an undergrad, and (weather permitting) the grass under a samaan tree was a good place to sit and talk, sit and contemplate the world. With the added bonus of random small insects falling out of the tree above you.

The first meeting of the group that was to become the Association for Tropical Biology (and Conservation, in recent years) had their initial meeting in Trinidad, and it was the samaan tree outside the Sir Frank Stockdale Building on the UWI campus that has graced the cover of their journal, Biotropica, ever since.

Although not a native species, there is something very Trinidadian about the samaan tree. In many ways it speaks to an older, less industrialised time when people understood that trees mattered. It has a personal connection, in memories of my undergrad days at UWI. And it is connected with one of the pillars of Neotropical biology – the journal Biotropica.

So it is with that in mind that I have renamed this blog. Too pompous and self-indulgent? Maybe. But so be it.

Bob Ricklefs: My Life as a Naturalist

Robert Ricklefs gave the American Society of Naturalists‘ Presidential Address today at Evolution 2011. For me, this was one of the high points of the entire conference. In a meeting where ecologists and tropical biologists were fairly thin on the ground, it was great to see someone speak who had such standing, not only as an ecologist, but also someone who had done some of his best work on the biogeography of the insular Caribbean.

Ricklefs talked about what it meant to be a “naturalist”. Naturalists and natural history have gone out of fashion in the last half century. Naturalists were Victorian field biologists, especially the amateur biologists and explorers who played such an important role in the early development of fields like ecology, evolution and taxonomy. But as growing academic disciplines in the second half of the twentieth century, these fields felt the need to divorce themselves from their antecedents in “natural history” and claim their place as rigorous academic disciplines. In his memoir Naturalist, E.O. Wilson spoke of some of the challenges that organismal biologists faced in the early years of the age of DNA, when they were labelled ‘stamp collectors’ and portrayed as a field being superseded by molecular biologists. It was against that backdrop that the fields of ecology, evolution and systematics transformed themselves into hard, quantitative scientific disciplines in the second half of the twentieth century. Modern systematics is about building consensus trees based on genetic data, and evolutionary biologists talk about SNPs and haplotypes. You can be an organismal biologist who has never seen your organism in the wild.

It was against a backdrop such as this that Ricklefs presented his talk. Direct observations of nature – direct experience of nature – is important for the progress of ecology and evolutionary biology. Most people realise, at least on some level, that observations of nature are an important means by which hypotheses can be generated. Broad theories and models of how the world work also have their roots in observations of the natural world. But, as Ricklefs pointed out, we also need to test our theories and models against nature, ensure that our explanations aren’t only internally coherent and logical, but that they also work when presented with the real world.

In addition to his own work on taxon cycles in Caribbean bird species, Ricklefs also discussed Steve Hubbell’s Unified Neutral Theory of Biodiversity and Biogeography. Hubbell’s neutral theory is a rather interesting idea. in essence, it suggests that tree species are competitively neutral, and that species diversity in tropical forests is maintained by a a mixture of random drift and speciation. Ricklefs expressed the opinion that Hubbell’s neutral theory, which has been around for a decade, probably has another decade before it fades into obscurity, as these things tend to do in ecology. The flaws in the neutral theory, he said, are seen when the theory is confronted with data.

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