Produce A Tree And Show The Water Movement

Fig. 8.24 (A) Transverse section of sector of a 1-year-old alder stem: red autofluorescence under blue light is due to chloroplasts in secondary cortex, secondary phloem, secondary xylem and medulla. Scale bar — 200 mm. (B) As (A) but at higher magnification showing individual chloroplasts fluorescing red in secondary phloem and in secondary xylem ray cells. Scale bar — 25 mm. (C) Sector of a 3-year-old alder stem showing red autofluorescence from chloroplasts in medulla and the inner two annual rings. Scale bar — 200 mm. (D) Transverse section of a 3-year-old alder stem through a lenticel. Some algal cells are fluorescing red on the outer surface of the flaking lenticel. Some red fluorescence is seen from chloroplasts in secondary cortex and secondary phloem. The pink colour in banded tissues of lenticel is not fluorescence but is due to anthocyanins. Scale bar — 100 mm. (E) Transverse section of a 1-year-old alder stem with transmitted white light. Chloroplasts (green) in secondary xylem rays and medulla. Scale bar — 200 mm. (Reproduced with permission from Armstrong & Armstrong, 2005.)

Fig. 8.24 (A) Transverse section of sector of a 1-year-old alder stem: red autofluorescence under blue light is due to chloroplasts in secondary cortex, secondary phloem, secondary xylem and medulla. Scale bar — 200 mm. (B) As (A) but at higher magnification showing individual chloroplasts fluorescing red in secondary phloem and in secondary xylem ray cells. Scale bar — 25 mm. (C) Sector of a 3-year-old alder stem showing red autofluorescence from chloroplasts in medulla and the inner two annual rings. Scale bar — 200 mm. (D) Transverse section of a 3-year-old alder stem through a lenticel. Some algal cells are fluorescing red on the outer surface of the flaking lenticel. Some red fluorescence is seen from chloroplasts in secondary cortex and secondary phloem. The pink colour in banded tissues of lenticel is not fluorescence but is due to anthocyanins. Scale bar — 100 mm. (E) Transverse section of a 1-year-old alder stem with transmitted white light. Chloroplasts (green) in secondary xylem rays and medulla. Scale bar — 200 mm. (Reproduced with permission from Armstrong & Armstrong, 2005.)

(Pfanz, 1999) net photosynthetic uptake of CO2 is rarely found. Instead, internal re-fixation of CO2 in young twigs and branches may compensate for 60-90% of the potential respiratory carbon loss. Corticular photosynthesis is thus thought to be an effective mechanism for recapturing respiratory carbon dioxide before it diffuses out of the stem. Furthermore, chloroplasts of the proper wood or pith fraction also take part in stem internal photosynthesis. Although there has been no strong experimental evidence until now, it has been suggested that the oxygen evolved during wood or pith photosynthesis may play a decisive role in reducing internal stem anaerobiosis. Whether or not this internally generated oxygen reaches the deeper roots in mature trees is still an open question. In some deciduous tree species (e.g. Betulapubescens, Alnusglutinosa and Populus tremula) the bark retains some photosynthetic activity in winter which, like the overwintering leaves in Juncus spp., will provide a source of oxygen.

Alders are particularly remarkable for the chloro-plast content of their woody tissues (Fig. 8.24). A combination of anatomical and physiological studies of alder has shown that a significant oxygen flux occurs from stems in high light periods, indicating a net carbon gain by stem photosynthesis (Armstrong & Armstrong, 2005). Chloroplasts are abundant in the secondary cortex and secondary phloem, and occur throughout the secondary xylem rays and medulla of 3-year-old stems. Marked diurnal patterns of radial oxygen loss have been observed when light reaches submerged portions of the stem, and it has been suggested that this internally produced oxygen may improve root aeration, especially when temperatures are low (Fig. 8.25).

Investigation of the xylem flow in birch using microsensors has shown that the oxygen status of the sapwood was related to the mass flow of xylem sap but was reduced by oxygen depletion in the root space

Fig. 8.25 Alder sapling showing the effects of light on ROL from a 13.5 cm long intact root. (a) ROL was unaffected by north light (50 mmol m-2 s-1) or supplementary light (550 mmol m-2 s-1) on an emergent shoot only but rose rapidly when emergent and submerged parts were exposed to a PAR of 550 mmol m-2 s-1 followed by a steady decline. ROL returned rapidly to background (approximately zero) when the light was switched off. (b) Continuation from (a) showing a similar pattern but lower ROL peak after exposure of emergent and submerged parts to 550 mmol m-2 s-1 PAR. (c) Continuation from (b) showing two further cycles of light application and a further diminished ROL peak despite some CO2 enrichment of the bathing medium. (Reproduced with permission from Armstrong & Armstrong, 2005.)

Fig. 8.25 Alder sapling showing the effects of light on ROL from a 13.5 cm long intact root. (a) ROL was unaffected by north light (50 mmol m-2 s-1) or supplementary light (550 mmol m-2 s-1) on an emergent shoot only but rose rapidly when emergent and submerged parts were exposed to a PAR of 550 mmol m-2 s-1 followed by a steady decline. ROL returned rapidly to background (approximately zero) when the light was switched off. (b) Continuation from (a) showing a similar pattern but lower ROL peak after exposure of emergent and submerged parts to 550 mmol m-2 s-1 PAR. (c) Continuation from (b) showing two further cycles of light application and a further diminished ROL peak despite some CO2 enrichment of the bathing medium. (Reproduced with permission from Armstrong & Armstrong, 2005.)

(Gansert, 2003). Such a finding makes it problematical as to whether or not tree roots will receive any substantial benefit to their oxygen from winter stem photosynthesis.

Tree species that live in wetlands often show a polycormic growth form. In dry habitats, both willow and alder will grow as pole trees, but when flooded, the basal buds are stimulated to develop and the bush form predominates. Alnus species appear particularly successful in exploiting the polycormic bush growth form for surviving in wetlands. The polycormic growth form is so frequently a characteristic of wetland alder carr that the woods can appear as if they have been coppiced. The greater area of stem surface that the poly-cormic form produces close to the ground may facilitate aeration, and also maximize the potential benefits of stem photosynthesis in winter. It is possible therefore that the combined effects of a greater bark surface together with the presence of adventitious roots in willow, and negatively geotropic roots in nitrogen-fixing alders, could provide a supply of oxygen to roots growing in waterlogged soils.

Trees appear to be most at risk from flooding in oceanic climates where winters are long, wet and not particularly cold. In the more continental regions of the boreal forests of Canada and Siberia tree growth is not prevented by prolonged cold winters. In the bogs of Latvia and Estonia it is possible to find large stands of pine growing on raised bogs. Further north, in some of the coldest regions of the boreal forest Picea mariana and Larix dahurica can grow on bog surfaces even when permafrost is not far below the surface. The most northerly location of the boreal forest (Section 5.2.1) at 72° 34' just to the west of the mouth of the Khatanga River in Siberia coincides with the greatest degree of continentality in the Siberian climate. However, in all these situations the soils are frozen for most of the winter, the roots are relatively shallow and oxygen demand is lower and supply less impeded (Crawford, 1992).

An entirely different winter situation is found in northern oceanic regions such as the British Isles and parts of western Norway where roots can be subjected to

Fig. 8.26 Diagrammatic representation of the seasonal cycles of the movement of carbohydrate in relation to the ability ofbirch trees to produce a strong upward flow of sap in spring even when soils are fully saturated or flooded and aeration is impeded (Braendle & Crawford, 1999).

winter flooding before they become dormant. In Sitka spruce (Picea sitchensis) flooding at temperatures above 6 °C leads to extensive death of root tips (Coutts & Philipson, 1987). Experimental studies of overwintering trees of Sitka spruce in which the flooding is carried out both at ambient temperature and ambient temperature plus 5 °C have shown that flooding under milder winter conditions causes a marked decrease in root carbohydrate reserves. This does not lead to the immediate death of the root, but when carbohydrate-depleted roots are exposed once again to air when the water table is lowered, there is extensive dieback of the root system, presumably due to post-anoxic injury (Crawford, 1996a). Death of the whole tree will not be immediate, but reduced root development will make the larger trees unstable and therefore prone to windblow.

The long winter, and the need to conserve carbohydrate for the resumption of sap flow and bud burst in the spring, creates a need for adequate overwintering supplies of carbohydrate. In birches, the active upward movement of xylem sap in spring carries with it substantial quantities of soluble carbohydrates, ethanol, organic and amino acids. This flow has been suggested as the means whereby roots of some woody plants compensate for the lack of oxygen in the soil-atmosphere by exporting hydrogen to the shoot and thus maintaining the redox balance of the inundated root system (Crawford, 1996a). The upward movement of hydrogen in reduced compounds as a constituent of the xylem flow is a more efficient means of repaying the oxygen debt of submerged organs than the slow diffusion of gaseous oxygen through more than a metre of woody tissue (Fig. 8.26). This replacement of oxygen import by hydrogen export does, however, have a metabolic cost, namely the provision of a high carbohydrate store in the overwintering root. The flow of sap in birch trees should not be confused with that of the sugar maple (Acer saccharin). In the latter the sap flow is dependent on diurnal freeze-thaw cycles affecting the trunk and stem and is not a function of root pressure. In birch, however, root pressure drives the spring ascent of sap (Kozlowski & Pallardy, 1997). It is relevant to note that in the bottomland trees of the USA it has been shown that high pre-flood root-starch concentrations are an important characteristic allowing flood-tolerant species to survive inundation. The bottomland Fraxinus pennsylvanica (green ash) and flood-tolerant Nyssa aquatica (water tupelo) are able to store more carbohydrate in their roots and retain less in their leaves than the less flood-tolerant Quercus alba.

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