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    Disturbance And Recovery In Tropical Dry Forests
    By Ian Ramjohn | March 25th 2008 11:17 AM | 2 comments | Print | E-mail | Track Comments
    About Ian

    A Trinidadian in Oklahoma, I am an ecologist interested in tropical dry forests and island ecology. I also have a blog called View Ian's Profile

    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 this fact, 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.

    Comments

    I'm wondering if "time" comes into resilience as a major player, or as, merely, an artifact. I, for example, believe that resilience is more a measure of how "far" a system is from some threshold; i.e resilience is "how much change a system can absorb" before it begins behaving in another way and cannot recover easily (or changes to a new system altogether). Note that in this definition, time plays a peripheral role (if any).

    iramjohn
    True, but that sort of a definition assumes the existence of multiple stable (or quasi-stable) states. That may be true (I suspect as much for Puerto Rican dry forests), and it may be useful in a formalised mathematical model, but it's far less useful in a practical, field-oriented approach.