Impact of a eucalyptus (Eucalyptus globulus Labill.) plantation on the nutrient content and dynamics of coarse particulate organic matter (CPOM) in a small stream

Litterfall inputs, benthic storage and the transport of coarse particulate organic matter (CPOM) were studied in two headwater streams, one flowing through a mixed deciduous forest and one through a plantation of Eucalyptus globulus. Vertical and lateral traps, transported CPOM and benthic CPOM were sampled monthly to biweekly and sorted into four categories: leaves, twigs and bark, fruits and flowers and debris. The litterfall inputs were about 20% lower at the eucalyptus site but this reduction was unevenly distributed among the litter categories. The reduction of the nitrogen and phosphorus inputs was larger (50%) than that of CPOM because of the low nutrient concentration of the CPOM at the eucalyptus site. Transported CPOM was also lower at the eucalyptus site. Although total CPOM inputs to the stream were reduced in the eucalyptus plantation, benthic storage of CPOM was 50% higher due to (1) high inputs of CPOM and low discharge during summer, (2) more twig and bark inputs, (3) eucalyptus leaves being retained more efficiently in the stream than deciduous leaves (4) a lower discharge, which may in part be attributable to eucalyptus-induced changes in the hydrological cycle. Increased retention balanced lower nitrogen and phosphorus content of CPOM, so benthic storage of nitrogen and phosphorus was similar at both sites. This work demonstrates that the timing, quality and quantity of inputs and benthic storage of CPOM in streams changes substantially because of the substitution of natural deciduous forest with eucalyptus plantation. Maintenance of buffer strips of natural vegetation may be the best way to protect ecological functioning of small, forested streams.