Stem flow

Apart from the economic significance of such loss there are potentially adverse effects on the environment arising from acidification of rain and soil. Ammonia may react with hydroxyl radicals in the atmosphere to produce NOx contributing to the acidification of rain (4). Wet and dry deposition of NH3/NH4+ inevitably contributes to soil acidification through their subsequent nitrification. This effect can be accentuated in woodland by absorption of aerosols containing NH4+ within the canopy followed by transport to the soil in stem flow (5). In more extreme cases, NH3 emission from feedlots, pig and poultry... 

The main sources of elements in soils are the parent materials from which they are derived (Thornton, 1983). Nutrients may be naturally deficient in soils, and elements may be removed in crop and livestock products. Geochemical maps are helpful in the location of areas with deficiencies or excesses of particular elements (Webb et al., 1978 Potter, 1992). Atmospheric metals, whether present as precipitation, or in an enriched form of throughfall or stem flow, represent a more dilute source, available on a less regular basis. 

Nutrient demands by plantation trees vary from season to season and with the developmental age of the stand (Drechsel and Zech 1993). During the life of the plantation, large quantities of nutrients are returned to the soil by above- and belowground litter, harvest residues, stem flow, and throughfall. These elements are mineralized in the soil and may be used by the trees or by associated crops. Deposition of certain mineral elements from the atmosphere can also be important, sometimes even compensating from losses in stemwood removal at harvest (Bmijnzeel 1989). Retranslocation of absorbed nutrients... 

A more detailed model is shown in this Fig. 5b. The throughfall Fa of Fig. 5a is replaced by the partial fluxes stem flow (Fu) and canopy drip (Fu). There are also included additional fluxes as Utter fall F e, dry deposition to soil Fse and internal fluxes of the system as uptake by roots and flow to the crown Fee, leaching of leaves Fse and flow from roots to soil F9e. 

Because natural weathering of minerals in the upper soil horizons is not sufficient to neutralize that amount of acid, a gradual acidification takes place so that even calcareous soils can be affected, especially at the stem base of beech and red oak, both possessing high stem flow rates. In silicate soils, clay minerals can be destroyed, releasing AP " and Fe, by replacing... 

More recent work has not confirmed the large proportion of stem flow. Work by Franken et al. (1982), carried out in typical Amazon high forest near Manaus, used a system of rain gauges in cleared areas and in the forest canopy, with an extensive series of stem flow collectors. They found that 77.7% of average precipitation reached the soil surface as throughfall, 22% of the rain was intercepted by the canopy, while stem flow only represented 0.3% of the total. 

Clearly, local conditions, such as tree species, the number of trees per unit area and the average intensity of the precipitation will affect the proportions. Thus in the Amazon forest in the region of San Carlos, Venezuela, Jordan and Heuveldop (1981) found 87% throughfall and 8% stem flow, while only 5% of precipitation was intercepted. 

In Malaysian tropical forest Sim (1972) found that interception varied between 25 and 80% as a function of precipitation class. Near Manaus, interception of rainfall. Table 3, was as great as 84.6% for the rainfall class of > 5 mm, with a throughfall of only 15.4%, while for the rainfall class 60-70 mm interception was reduced to 25.5% and throughfall increased to 74.1%. Stem flow was negligible regardless of rainfall class (Franken et al., 1982). 

Stem flow (SF), i.e., rain flowing down trunks to the forest floor. 

Stem flow is important for deciduous stands and young coniferous stands. Total stand precipitation might be underestimated by up to 30% in such sites when stem flow is not taken into account. In beech stands, the stem flow makes up a considerable part of the total deposition. 

The soil material at the base of the stems and that surrounding the roots of the 2C horizons were chemically and chromatically different from the bulk. The relatively high precipitation of the site and the fast drainage, due to the coarse texture, favoured the bleaching of the soil in contact with the stems and roots, as stem-flow and soil solutions passing through the soil used the coarse roots as... 

Several methods have been proposed for predicting values of Hy and Hi, as functions of stem, flow conditions, and packing type. These include the methods of Cornell et al., Onda et al., Bravo and Fair, and Bolles and Fair. The last-named has had the broadest validation and will be discussed here. 

Variation in the pH of the bark of the same tree species or even the same tree makes pH-lichen distribution correlations difficult to interpret. For example, there are variations in the pH of stem flow of Quercus petraea in different seasons (Carlisle et al., 1967) and significant differences with tree age in Populus (Margot, 1965). Young (1938) reported very small but consistently higher pH values on the north side of the trees she examined as compared with the south side, and Hale (1967) reported that bark acidity is greater at the base than at upper levels of a tree trunk. 

Carlisle, A., Brown, A. H. F., and White, E. J. (1967). The nutrient content of tree stem flow and ground flora litter and leachates in a sessile oak Quercus petraea) woodland. J. EcoL 55, 615-627. 

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