Forests and Erosion: Insights from a Study of Suspended-Sediment Dynamics in an Overland Flow-Prone Rainforest Catchment. Zimmermann, A., Francke, T., & Elsenbeer, H. Journal of Hydrology, 428-429:170–181, March, 2012.
doi  abstract   bibtex   
[Abstract] Forests seem to represent low-erosion systems, according to most, but not all, studies of suspended-sediment yield. We surmised that this impression reflects an accidental bias in the selection of monitoring sites towards those with prevailing vertical hydrological flowpaths, rather than a tight causal link between vegetation cover and erosion alone. To evaluate this conjecture, we monitored, over a 2-year period, a 3.3~ha old-growth rainforest catchment prone to frequent and widespread overland flow. We sampled stream flow at two and overland flow at three sites in a nested arrangement on a within-event basis, and monitored the spatial and temporal frequency of overland flow. Suspended-sediment concentrations were modeled with Random Forest and Quantile Regression Forest to be able to estimate the annual yields for the 2~years, which amounted to 1~t~ha-1 and 2~t~ha-1 in a year with below-average and with average precipitation, respectively. These estimates place our monitoring site near the high end of reported suspended-sediment yields and lend credence to the notion that low yields reflect primarily the dominance of vertical flowpaths and not necessarily and exclusively the kind of vegetative cover. Undisturbed forest and surface erosion are certainly no contradiction in terms even in the absence of mass movements. [Highlights] [::] We estimated suspended sediment yields (SSYs) of an undisturbed rainforest catchment. [::] SSY were 1~t/ha in a dry and 2~t/ha in an average-precipitation year. [::] These estimates rank high in the spectrum of SSY from undisturbed forests. [::] High SSY in undisturbed forests primarily reflect the frequency of near-surface flow. [::] The impression of low SSY from undisturbed forests reflects a bias in site selection. [Excerpt: Conclusions] Forest cover and erosion rates as high as 2 t ha-1 yr-1 are no contradiction in terms where distinct soil anisotropy in shallow depth coincides with high rainfall intensities and amounts. Overland flow, which is frequently activated in the latter environment, acts as an important driver of erosion processes. Our observations indicate that during single, large rainfall events, overland flow-prone hillslopes lose more than 0.2 t ha-1 of suspended-sediment within hours, and the erosion of the stream channel further increases the catchment's event-based suspended-sediment yield to 0.3 t ha-1. These findings do not only add to the wealth of knowledge that overland flow occurs in undisturbed forests but they also challenge the wide-spread belief that erosion rates are generally low under undisturbed forest cover. [\n] Based on our results and using available background data from our study site, we hypothesize that undisturbed forest ecosystems are particularly susceptible to erosion where a trinity of [::] active near-surface flowpaths, [::] relatively nutrient rich soils (that coincide with sparse root mats), and [::] a low light availability at the forest floor (preventing the growth of ground vegetation) [\n]exists. These conditions are probably more widespread than expected. In Panama, at least, they occur in extensive areas of the Panama Canal watershed. [\n] Given that our monitoring period covered a dry (2008, annual rainfall: 1975 mm) and an average year (2009, annual rainfall: 2544 mm), it remains to be seen how extremely wet years with totals up to 4500 mm increase sediment yields.
@article{zimmermannForestsErosionInsights2012,
  title = {Forests and Erosion: Insights from a Study of Suspended-Sediment Dynamics in an Overland Flow-Prone Rainforest Catchment},
  author = {Zimmermann, Alexander and Francke, Till and Elsenbeer, Helmut},
  year = {2012},
  month = mar,
  volume = {428-429},
  pages = {170--181},
  issn = {0022-1694},
  doi = {10.1016/j.jhydrol.2012.01.039},
  abstract = {[Abstract]
Forests seem to represent low-erosion systems, according to most, but not all, studies of suspended-sediment yield. We surmised that this impression reflects an accidental bias in the selection of monitoring sites towards those with prevailing vertical hydrological flowpaths, rather than a tight causal link between vegetation cover and erosion alone. To evaluate this conjecture, we monitored, over a 2-year period, a 3.3~ha old-growth rainforest catchment prone to frequent and widespread overland flow. We sampled stream flow at two and overland flow at three sites in a nested arrangement on a within-event basis, and monitored the spatial and temporal frequency of overland flow. Suspended-sediment concentrations were modeled with Random Forest and Quantile Regression Forest to be able to estimate the annual yields for the 2~years, which amounted to 1~t~ha-1 and 2~t~ha-1 in a year with below-average and with average precipitation, respectively. These estimates place our monitoring site near the high end of reported suspended-sediment yields and lend credence to the notion that low yields reflect primarily the dominance of vertical flowpaths and not necessarily and exclusively the kind of vegetative cover. Undisturbed forest and surface erosion are certainly no contradiction in terms even in the absence of mass movements. 

[Highlights]
[::] We estimated suspended sediment yields (SSYs) of an undisturbed rainforest catchment. 
[::] SSY were 1~t/ha in a dry and 2~t/ha in an average-precipitation year. 
[::] These estimates rank high in the spectrum of SSY from undisturbed forests. 
[::] High SSY in undisturbed forests primarily reflect the frequency of near-surface flow. 
[::] The impression of low SSY from undisturbed forests reflects a bias in site selection.

[Excerpt: Conclusions] Forest cover and erosion rates as high as 2 t ha-1 yr-1 are no contradiction in terms where distinct soil anisotropy in shallow depth coincides with high rainfall intensities and amounts. Overland flow, which is frequently activated in the latter environment, acts as an important driver of erosion processes. Our observations indicate that during single, large rainfall events, overland flow-prone hillslopes lose more than 0.2 t ha-1 of suspended-sediment within hours, and the erosion of the stream channel further increases the catchment's event-based suspended-sediment yield to 0.3 t ha-1. These findings do not only add to the wealth of knowledge that overland flow occurs in undisturbed forests but they also challenge the wide-spread belief that erosion rates are generally low under undisturbed forest cover.

[\textbackslash n] Based on our results and using available background data from our study site, we hypothesize that undisturbed forest ecosystems are particularly susceptible to erosion where a trinity of [::] active near-surface flowpaths, [::] relatively nutrient rich soils (that coincide with sparse root mats), and [::] a low light availability at the forest floor (preventing the growth of ground vegetation) [\textbackslash n]exists. These conditions are probably more widespread than expected. In Panama, at least, they occur in extensive areas of the Panama Canal watershed.

[\textbackslash n] Given that our monitoring period covered a dry (2008, annual rainfall: 1975 mm) and an average year (2009, annual rainfall: 2544 mm), it remains to be seen how extremely wet years with totals up to 4500 mm increase sediment yields.},
  journal = {Journal of Hydrology},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-10396996,forest-floor-low-light-availability,forest-resources,ground-vegetation,near-surface-flowpaths,nutrient-rich-soil,precipitation,rainforest,sediment-transport,sediment-yield,shade-tolerance,soil-erosion,soil-resources},
  lccn = {INRMM-MiD:c-10396996}
}
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