Effects of exotic eucalypt plantations on organic matter processing in Iberian streams

Abstract

Eucalypt leaves are an organic matter source of poor quality, and therefore, extensive eucalypt plantations can affect stream ecology. Nevertheless, it is difficult to discern the effects of altered inputs from other impacts of plantations, such as increased erosion from periodic clearcuttings. To assess the effects of eucalypt inputs on organic matter decomposition in streams, we manipulated litter inputs in two headwater streams in the north of the Iberian Peninsula. Three contiguous 50‐m long reaches were selected per stream: the upstream reference reach was left unaltered, while the others were covered and separated by mesh to prevent inputs from the surrounding vegetation, and via downstream transport. Regularly, we provided the covered sites with either native deciduous (middle reach) or eucalypt (lower reach) leaf litter, mimicking the amount and seasonality of inputs from the respective forest types. Two decomposition experiments with alder and eucalypt leaves were performed half a year and 1 year after the beginning of the exclusion of natural inputs. Decomposition for both species was lower in sites receiving eucalypt leaf litter, corresponding with the lower density of shredders in these sites. Moreover, higher alder:eucalypt macroinvertebrate density and biomass ratios in sites receiving eucalypt inputs suggest that invertebrates concentrated on the higher quality alder leaves in those sites. Our study shows that eucalypt inputs can impair stream ecosystem processes in a short time span and suggests that maintaining native leaf litter inputs into streams by intact riparian corridors could help to mitigate the effect of eucalypt plantations on streams.

A meta-analysis on the effects of changes in the composition of native forests on litter decomposition in streams

“…In the latter study, however, the native forest was mostly evergreen and there was no major change in the phenology of litter inputs to streams when it was replaced by eucalyptus (Lac ´an et al, 2010). The replacement of native deciduous litter inputs by eucalyptus litter inputs in a manipulative experiment at the stream reach level (Larrañaga et al, 2014) also resulted in a strong inhibition of litter decomposition (À46%), which supports the suggestion that effects of euca-Fig. 5.…”

Litter Decomposition as an Indicator of Stream Ecosystem Functioning at Local-to-Continental Scales

“…Total oak leaf litter decomposition was not significantly affected by forest change (Ferreira et al, 2015b*), likely due to the lower contribution of macroinvertebrates to the decomposition of nutrient-poor litter (Hieber and Gessner, 2002). Microbially driven alder and oak leaf litter decomposition were generally not affected by forest change (Ferreira et The effects of eucalyptus plantations on litter decomposition vary within and among studies (Abelho and Grac ¸a, 1996;B€ arlocher and Grac ¸a, 2002;Dı ´ez et al, 2002;Ferreira et al, 2006aFerreira et al, *, 2015bLa can et al, 2010;Larran ˜aga et al, 2014;Molinero et al, 1996;Pozo, 1993;Pozo et al, 1998; Table 4), but a recent meta-analysis has found an overall 20% inhibition of litter decomposition in streams flowing through eucalyptus plantations compared with reference streams (Ferreira et al, 2016a). Eucalyptus plantations cover >20 million ha worldwide (Iglesias-Trabado et al, 2009), but their impacts on litter decomposition in streams have been addressed mostly in the Iberian Peninsula and thus generalisation to other regions is limited due to differences in the type of native forest, eucalyptus species used in plantations, climate, etc.…”

Efectos dos eucaliptos sobre os ecosistemas ibéricos. Unha revisión

“…Os cursos de auga en plantacións de eucaliptos teñen menor riqueza de taxóns, densidade e biomasa total de invertebrados (particularmente detritívoros de maior tamaño), comparados cos que flúen a traverso de bosques autóctonos (Larrañaga et al, 2006(Larrañaga et al, , 2009a. Os macro-invertebrados detrítivoros tenden a concentrarse nas follas de amieiros e non nas de eucalipto (Larrañaga et al, 2014).…”

“…Para mitigar os efectos negativos dos monocultivos intensivos de eucaliptos sobre os invertebrados acuáticos e protexer o funcionamento ecolóxico dos pequenos cursos fluviais en áreas forestais, en todas as publicacións científicas recentes recoméndase conservar e/ou restaurar as ripisilvas (Abelho & Graça 1996;Graça et al, 2002;Molinero & Pozo 2004;Larrañaga et al, 2014;Ferreira et al, 2015Cordero-Rivera et al, 2017).…”