How quickly do tropical forests recover from disturbance?
Anyone reading this post probably does not need reminding of how important tropical forests are. In addition to their status as some of the most biodiverse ecosystems on earth, the rich collection of floral species they contain translates into a diversity of resources that benefit us all, from medicinal plants to timber and non-timber forest products, many of which are a focus of Kew’s research and conservation programmes. However, many tropical ecosystems continue to be deforested at rapid rates, due to the pressures of expanding human population, rising consumption, and the associated agricultural footprint that accompanies this growth (Geist & Lambin, 2002).
Disturbance and recovery in tropical forests
Despite widespread exploitation of tropical forests, some areas are being given a chance to recover from past disturbance, helped by incentives from recent international forest conservation initiatives and by rising rural-to-urban migration, among other factors (Mather, 1992). But how long does it take these damaged forests to recover? Research conducted in the last few decades has attempted to answer this fundamental question, along with other related questions about the pattern and process of forest regrowth, for example Chazdon et al. (2007). However, few studies have monitored change over timescales of more than 50 years, ie a fraction of the lifespan of a tropical tree. Through capturing only a part of the ecological process, results on forest recovery may be misleading and provide precarious recommendations for the development of sustainable forest management strategies. Thus in our recent study, we attempted to answer the question again, this time by looking into the past to gather data over longer timescales that could offer a more complete picture of forest recovery post disturbance.
The palaeoecological approach
Palaeoecology, otherwise known as long-term ecology, uses fossils to decipher how plants and animals interacted with their environment in the past. Fossil pollen grains come in all shapes and sizes, and their morphological characteristics can be used to identify the plant family, genus or even the species to which they belong. When a collection of these grains are identified and counted from a layer of sediment, we can reconstruct what the vegetation was like at that point in time when those grains were deposited. Although pollen of some tropical species is not preserved in the fossil record, such as that of Passiflora lobata (below), the dominant species are usually well represented and can therefore give an accurate picture of the past environment.
In this project, we were interested in studies that documented disturbance-induced changes in fossil pollen from forested communities across the tropics over the last 20,000 years (with a particular focus on the last 12,000 years - the Holocene). Types of disturbances ranged from climatic drying events and landslides, to shifting cultivation and human-induced biomass burning. We found 71 studies published on tropical forest palaeoecology that satisfied our selection criteria (e.g. within 23 degrees N/S of the equator, possessing a sufficient chronology), documenting 283 disturbance and associated recovery events. The rate at which recovery occurred across the different forests and disturbance events was the key variable of interest and calculated as the percentage increase in forest pollen abundance per year relative to the pre-disturbance level.
How far and how fast have tropical forests recovered in the past?
Our results demonstrate that in the past the majority of forests regrew to less than 100% of pre-disturbance levels, prior to declining again or reaching a new baseline forest abundance - the median recovery was to 95.5%. They also recovered at a large variety of different speeds post-disturbance, ranging from rates that would lead to 95.5% regrowth in less than 10 years to those taking nearly 7,000 years - the average was 503 years. This is significantly longer than the periods adopted by logging companies between extraction cycles!
What affects the rate of recovery?
We investigated a number of different factors to assess whether they had an effect on the recovery rate of tropical forests, and three seemed to be of particular importance: geographical location, disturbance type and frequency of disturbance events. Of the four key tropical regions, Central American forests recovered the fastest and those in Asia the slowest. This is somewhat concerning, given that forests in Southeast Asia are currently experiencing some of the greatest rates of deforestation of all tropical regions, primarily due to the economic profitability of oil palm agriculture.
The most common form of disturbance, and one from which forest regrowth happened relatively slowly, was anthropogenic impact, i.e. via logging, burning and/or for agriculture. The fastest rates of recovery followed large infrequent disturbances, such as hurricanes, landslides and natural fire, whereas the slowest rates occurred after longer term, more gradual changes in climate. The results are somewhat intuitive given that large infrequent disturbances are a natural part of all ecosystems, and thus the native plant communities have evolved to respond dynamically to such perturbations.
Insights into resilience
When we looked at the standardised rate of disturbance events (SRD), i.e. the number of disturbance events per 1,000 years, we found that the greater the frequency events occurred in the past, the more quickly the forest responded to each subsequent disturbance. This runs counter to contemporary theories on resilience that describe slowing rates and diminishing ability to recover with each subsequent perturbation (e.g. Veraart et al., 2012). Our results suggest that over ecologically meaningful timescales, i.e. over the life-span of entire forest communities rather than single trees, increased exposure results in adaptation to that disturbance over time, leading to a greater ability to recover quickly from the perturbation.
What does this all mean for tropical forests?
From looking back into the past, it seems that tropical forests can take a long time to recover from disturbances, and that different regions may require different management regimes to encourage more complete reforestation after natural or anthropogenic events, such as fire. Central American and African forests may bounce back from impacts more quickly than the other regions, with disturbances such as tropical hurricanes and climatic fluctuations being a more common component of these ecosystems than in the other tropical regions. However, all of the forests we looked at demonstrated a greater vulnerability to anthropogenic impacts and gradual climatic changes (the two major forms of disturbance occurring today) than to large infrequent disturbances.
Identifying and understanding the different ecological requirements of forests across the different geographical regions, and of the forest-types within those regions, is vital for developing more sustainable landscape management plans. With increasing international concern about deforestation rates and the associated loss of biodiversity,and elevated carbon dioxide emissions, the conservation and restoration of tropical forests is becoming more politically and economically feasible. Indonesia, for example, has introduced ‘ecosystem restoration concessions’ in the last decade, providing a legal means for forest protection from the further expansion of industrial agriculture. The potential of the UN Reducing Emissions from Deforestation and Forest Degradation programme (now REDD+) to save the world’s forests also continues to generate international debate.
Of importance to all of these programmes and initiatives, is the suggestion from our study that forests take time to recover and, if we give them that time, that they will persist and continue to provide their faunal inhabitants, including us, the greatest collection of biological riches on Earth.
- Lydia -
Chazdon, R.L., Letcher, S.G., van Breugel,M., Martínez-Ramos, M., Bongers, F. & Finegan, B. (2007). Rates of change in tree communities of secondary Neotropical forests following major disturbances. Philosophical Transactions of the Royal Society B Biological Sciences 362: 273-289. Available online
Cole, L.E.S., Bhagwat, S.A. & Willis, K.J. (2014). Recovery and resilience of tropical forests after disturbance. Nature Communications. DOI: 10.1038/ncomms4906. Available online
Geist, H.J. & Lambin, E.F. (2002). Proximate causes and underlying driving forces of tropical deforestation. Bioscience 52: 143-150. Available online
Mather, A.S. (1992). The forest transition. Area 4: 367-379. Available online
Veraart, A.J., Faassen, E.J., Dakos, V., van Hes, E.H., Lürling, M. & Scheffer, M. (2012). Recovery rates reflect distance to a tipping point in a living system. Nature 481: 357–359. Available online