Monday, March 31, 2008


ReForLan (Restoration of Forest Landscapes for Biodiversity Conservation and Rural Development in the Drylands of Latin America) is a new project focusing on
the restoration of dryland forest landscapes for biodiversity conservation and rural development in Latin America.
The project has received three years of funding (2007-2010) and are working in seven sites around Latin America:
  1. Chiapas, Mexico
  2. Central Veracuz, Mexico
  3. Oaxaca, Mexico
  4. Central Valley, Central Chile
  5. Coastal range, Central Chile
  6. Northwestern Argentina
  7. Southwestern Argentina.
There's a write-up about the project in the March 2008 issue of Ecological Restoration.

Other definitions

Last year, Jeremy Bruno posted a description of dry forest at The Voltage Gate. (He has since moved to ScienceBlogs.) It's a pretty good overview - covers the main points, though I'm a tad uncertain about the assertion that erosion (post-deforestation) is higher than in rainforests.

So what is tropical dry forest?

So what is tropical dry forest? In trying to compile a list of recent dry forest-related literature that question comes to the fore. It's a very familiar question - I spent a lot of time wondering about that early in my grad school career when I thought that I should get a handle on all the literature related to tropical dry forest. (What can I say, I was ambitious naive.) Part of the problem comes from relying on Holdridge's definition - if you define dry forest in relation to precipitation and potential evapotranspiration (which basically boils down to precipitation and temperature), you end up including riparian forests and some forests dependent on ground water - forests that which resemble wetter forests in terms of physiognomy and species composition. When it boils down to it, regardless of rainfall, they don't function as dry forests. In addition, there's the issue of savannas - many savannas fall within the dry forest life zone, but if you look at them as "former" or "potential dry forest" you ignore the fact that they have been grassland for thousands and years. (Other grasslands though, are recent human creations.) Then there are areas that function as dry forest, but get too much rainfall.

In selecting literature I am trying to use broad criteria. I'm including deciduous and semi-deciduous forest even if they seem to be a bit on the wet side. I'm also including savannas. In a case like this, it's probably more useful to use a broad definition.

Recent literature

In my continuing attempt to catch up on recent literature on tropical dry forests and savannas

Oswalt, Sonja N., Thomas J. Brandeis, Christopher W. Woodall. 2008. Contribution of Dead Wood to Biomass and Carbon Stocks in the Caribbean: St. John, U.S. Virgin Islands. Biotropica 40(1):20–27 doi:10.1111/j.1744-7429.2007.00343.x

Bellairs, Sean M., Donald C. Franklin, Nicholas J. Hogarth. 2008. A Tropical, Gregariously Semelparous Bamboo Shows No Seed Dormancy. Biotropica 40(1) :28–31 doi:10.1111/j.1744-7429.2007.00336.x

Keuroghlian, Alexine, Donald P. Eaton. 2008. Fruit Availability and Peccary Frugivory in an Isolated Atlantic Forest Fragment: Effects on Peccary Ranging Behavior and Habitat Use. Biotropica 40(1) :62–70 doi:10.1111/j.1744-7429.2007.00351.x

Maffei, Leonardo, Andrew J. Noss. 2008. How Small is too Small? Camera Trap Survey Areas and Density Estimates for Ocelots in the Bolivian Chaco . Biotropica 40(1) :71–75 doi:10.1111/j.1744-7429.2007.00341.x

Martins, Eduardo Guimarães, Márcio Silva Araújo, Vinícius Bonato, Sérgio Furtado dos Reis. 2008. Sex and Season Affect Individual-Level Diet Variation in the Neotropical Marsupial Gracilinanus microtarsus (Didelphidae). Biotropica 40(1) :132–135 doi:10.1111/j.1744-7429.2007.00319.x

Casandra Reyes-García, Howard Griffiths, Emmanuel Rincón, Pilar Huante. 2008. Niche Differentiation in Tank and Atmospheric Epiphytic Bromeliads of a Seasonally Dry Forest
Biotropica 40(2) :168–175 doi:10.1111/j.1744-7429.2007.00359.x

Philip D. Round, George A. Gale (2008) Changes in the Status of Lophura Pheasants in Khao Yai National Park, Thailand: A Response to Warming Climate?
Biotropica 40(2) :225–230 doi:10.1111/j.1744-7429.2007.00363.x

Reuber A. Brandão, Alexandre F. B. Araújo (2008) Changes in Anuran Species Richness and Abundance Resulting from Hydroelectric Dam Flooding in Central Brazil
Biotropica 40(2) :263–266 doi:10.1111/j.1744-7429.2007.00356.x

Saturday, March 29, 2008

Recent literature

Some recent literature published on tropical dry forests and savannas.

Poorter, Lourens, William Hawthorne, Frans Bongers, Douglas Sheil. 2008. Maximum size distributions in tropical forest communities: relationships with rainfall and disturbance. Journal of Ecology doi:10.1111/j.1365-2745.2008.01366.x (early online)

Klop, Erik and Janneke van Goethem. 2008. Savanna fires govern community structure of ungulates in Bénoué National Park, Cameroon. Journal of Tropical Ecology 24(1):39-47 doi: 10.1017/S0266467407004609

de Lima, Renato A. Ferreira, Adriana M. Zanforlin Martini, Sérgius Gandolfi and Ricardo Ribeiro Rodrigues. 2008. Repeated disturbances and canopy disturbance regime in a tropical semi-deciduous forest. Journal of Tropical Ecology 24(1):85-93 doi: 10.1017/S0266467407004658

González-Ruiz, Teresa, Víctor J. Jaramillo, Juan José Peña Cabriales and Arturo Flores. 2008. Nodulation dynamics and nodule activity in leguminous tree species of a Mexican tropical dry forest. Journal of Tropical Ecology 24(1):107-110 doi: 10.1017/S0266467407004634

Castanho, Camila de Toledo and Alexandre Adalardo de Oliveira. 2008. Relative effect of litter quality, forest type and their interaction on leaf decomposition in south-east Brazilian forests. Journal of Tropical Ecology 24(2):149-156 doi: 10.1017/S0266467407004749

Janos, David P., John Scott and David M. J. S. Bowman. 2008. Temporal and spatial variation of fine roots in a northern Australian Eucalyptus tetrodonta savanna. Journal of Tropical Ecology 24(2):177-188 doi: 10.1017/S0266467408004860

Tuesday, March 25, 2008

Disturbance and recovery in tropical dry forests

[Crosspost from Further Thoughts]
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 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.