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Disturbance and recovery in tropical dry forests

ResearchBlogging.orgWhen 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 a forthcoming paper7 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 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.
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9 Responses

  1. […] to focus on rainforests. Tropical dry forests tend to get overlooked.” Trinidadian blogger Further Thoughts explains. Share […]

  2. […] 「熱帯林の破壊や荒廃について考えるとき、熱帯雨林を重視する傾向がある。熱帯乾燥林は見逃されがちだ」と、トリニダードのブロガーFurther Thoughtsは説明している。 […]

  3. Great synopsis!

    It’s especially intriguing to me that dry forests are dominated by species that sprout when cut and, by implication, rain forests are not dominated by such species but rather by species that depend on seed germination for reproduction. I had come to think that tropical plants in general were apt to sprout when cut, so this observation was an eye-opener to me.

    Mary

  4. There’s some interesting literature on that question – hopefully I will get to it one of these days. It has been proposed that there’s a trade-off between “seeders” and “sprouters” – that you can either specialise in one or the other, not both.

    Dry forest trees put much more of their biomass below ground – they need more roots to forage for water. I suppose just doing that gives them an advantage when it comes to sprouting – the stem may represent as little as 50% of the biomass of a dry forest tree, while it’s likely to represent 70 or even 90% of the biomass of a rainforest tree (pulling the RF figures off the top of my head, so they may be wrong). That alone is likely to give dry forest trees a leg up when it comes to sprouting. Also many dry forest trees are also savanna trees (or their close relatives) – in savannas, of course, sprouting is an adaptation to fire.

    That isn’ t to say that rainforest trees don’t sprout when cut. They’re just less good at it. (There’s an interesting series on sprouting in wetter forests after Hurricane Joan, I think, in Nicaragua. John Vandermeer was one of the authors on that set of papers. Something else worth blogging about.)

  5. Thanks, Ian. Your comment is as interesting as your post. The depth of the roots in dry forest species (the shallow roots of rain forests are famous, but who ever hears of the deep roots in dry forests?) – the adaptation to fire – great stuff. Thanks!

    Mary

  6. Rooting depths – that’s another fascinating story, although one that I’m not up to date on. And it’s even more complicated than just a dry forest/wet forest distinction. There’s some fascinating stuff from the eastern Amazon, where the forests are taller and more evergreen than rainfall alone would predict. Turns out, if I remember correctly, that the trees are dependent on ground water from several metres down in the dry season. (8 m is the number that comes to mind, but I’d really have to look it up.) The suggestion was that this may also be a factor slowing recovery on abandoned pastures in the area.

    I believe this is the paper (that’s a story that would be well worth blogging on as well)

    Jipp, P. H., D. C. Nepstad, D. K. Cassel, and C. R. Carvalho. 1998. Deep soil moisture storage and transpiration in forests and pastures of seasonally-dry Amazonia. Climatic Change 39:395-412.

  7. And thanks for reminding me how much cool stuff there is out there to talk about. I tend to focus on stuff that’s new to me, forgetting that there’s so much cool stuff that’s new to everyone else…and just really fun to write about.

  8. […] [Repost from 2008] 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. […]

  9. […] from my old blog] When people think about the destruction and degradation of tropical forests, they tend to focus on […]

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