General radial pattern of J s

The general radial patterns in the (axial) sap flow of European beech – a decrease of Js with increasing distance from cambium – as shown in Chapter 5.3 and discussed above are indicative of the variation in hydraulic conductivity Kh and may be ex-plained by general variation in wood anatomical parameters due to tree growth and ageing (e.g. Gartner and Meinzer 2005, Chap. 2.3). Superimposed on these general anatomical trends are changes in wood properties caused by environmental factors (e.g. Bouriaud et al. 2004, Skomarkova et al. 2006) which in turn may be modulated by competition (Piutti and Cescatti 1997). The points discussed in the following are basically also valid for Q. petraea (cf. Cermák et al. 1992, Granier et al. 1994, Poyatos et al. 2007), only on different spatial scales since the sapwood radius of this ring-porous species is smaller than that of diffuse-porous F. sylvatica.

In this study, no significant radial trend in wood density ρwd was observed on 5 mm long segments of increment cores taken from three beech trees (Fig. 5.1.1a), as found in the majority of reports on this species in the literature (see Chap. 5.1.1).

Similarly Lang (1999), in concert with the findings of Sass and Eckstein (1995) and Bouriaud et al. (2004; see Chap. 2.3), observed no tendency of vessel density (mm^-2) in the earlywood of beech over the outermost 20–34 growth rings of trees aged 60–

68 years, and only a weak tendency of percentage vessel lumen area (conductive area) of the earlywood to decrease from the youngest to older growth rings. Based on the theoretical treatise of Roderick and Berry (2001), Barbour and Whitehead (2003) postulated that wood density and xylem sap velocity should not correlate in angiosperms while a relationship was observed in gymnosperms – at least from hydric sites – due to the fact that angiosperms generally have a few large, velocity-determining vessels embedded in a matrix of smaller tracheids.

Water content Wf (in g cm^-3) and relative water content RW (%) measured on the same increment core segments as ρwd decreased steadily with increasing radial depth from bark approximately halfway through to the pith and remained rather constant in innermost xylem (Fig. 5.1.1b, c). The depth where Wf and RW did not decline any further indicated the transition from moister sapwood to drier ripewood.

These observations are consistent with published patterns of Wf and RW for beech (see Chap. 5.1.1). Corresponding with changes in Wf and RW, absorption values α (in cm^-1), measured with a mobile computer tomograph (Chap. 4.3) on the same trees prior to core extraction, changed as well (Fig.5.1.3).

Direct measurements of radial variation of Kh on trunks are destructive and calcula-tion of theoretical Kh requires more detailed anatomical information, both approaches being beyond the scope of this study. Lang (1999) observed a barely significant ra-dial decline of theoretical specific conductivity Ks in the earlywood of European beech, in contrast to a prominent radial decrease in Js. This could be due to the fact that Lang (1999) measured vessel lumen areas only of part of the earlywood of a growth ring or due to the presence of embolisms or tyloses not accounted for on the microscopic sections prepared by Lang (1999). Differences between theoretical and actual Ks may also arise from other additional resistances to flow along the path not accounted for in theoretical Ks (see Chap. 2.3). In Pinus ponderosa, Domec et al.

(2005) measured lower (maximum) Ks in inner than in outer sapwood, but the decrease in Ks was lower than the typical radial decline of Js in this species (not measured concurrently in their study). Thus they considered that Js and Ks may not be linearly (but non-linearly) correlated, supported by the findings of James et al.

(2003), Domec et al. (2006) and Meinzer et al. (2006). This tendency of Js to saturate with increasing Ks indicated that factors other than Ks began to limit maximum Js

(James et al. 2003). The latter authors considered the most likely explanation to be that for a certain water potential gradient between leaf and soil (the driving force for Js), regions of sapwood with higher Ks would experience smaller axial tension gradi-ents, and this in turn would diminish the response of Js to increasing Ks (James et al.

2003). Meinzer et al. (2006) concluded from results for temperate conifers compara-ble to the findings of James et al. (2003) for tropical tree species that the similarity of the relationship between theoretical Ks and measured Js implies a comparable sto-matal regulation of xylem tension gradients among co-occurring species to produce similar rates of Js at a particular value of Ks. Clearly, more specific experiments are desirable to elucidate general patterns and differences in the relationships of Js and Ks, between species and anatomical-systematical groups of plants.

Radial trends in Js may also be brought about by patterns in the hydraulic connec-tivity of certain parts of the sapwood radius with specific regions in the crown. A sap flow gauge in other words could merely be sensing the supply to one particular branch. Waring and Roberts (1979) stated for Pinus sylvestris that “the older, inner sapwood possibly supplies water mainly to the older lower branches which generally have less foliage and are less well irradiated than limbs higher in the crown”, a view also taken by Nadezhdina et al. (2002) for the same species. As well, Dye et al.

(1991) suggested from measurements on P. patula that inner xylem originally sup-plied branches and leaves that are now shed and therefore inner xylem would not be as well-connected or functional as outer sapwood. And similarly, Jiménez et al.

(2000) concluded that sap flow in the inner xylem layers is connected with the lower, deeper parts of crowns and outer xylem connected to the well-illuminated upper can-opy of broad-leaved tree species (Laurus azorica and Persea indica). Nadezhdina et al. (2007) attributed different radial patterns in the trunks of Pinus sylvestris growing in shallow and deep soil to different rooting and water extraction patterns, with deep sinker roots, which they propose are connected to the inner xylem, contributing more to sap flow than sinker roots on the shallow soil.

Studies of patterns of vascular sectoriality to date have mostly been carried out on branches, shrubs and small trees or saplings (e.g. Waisel et al. 1972, Orians et al.

2004, Zanne et al. 2006), which are easier to handle experimentally. But the results from such studies cannot be assumed to be directly transferable to axial long-dis-tance transport in tall stems of large adult (forest) trees as studied in the present work. This is first of all because wood anatomy changes from juvenile to adult wood (see Chap. 2.3). Secondly, patterns of tangential and radial spreading of vessels along the stem axis as retrieved from dye injections have to be interpreted with cau-tion since pressure gradients and hence flow direccau-tions are altered upon applying dye at atmospheric pressure and because dyes do not necessarily behave like xylem sap (Cermák et al. 1992, Tyree and Zimmermann 2002, p 32, p 44, pp 255–257).

And thirdly, the probability of contact between conduits increases with an increasing axial length of the pathway, as detailed below:

Vité (1958) pointed out the significance of the spiral grain of xylem for water transport in conifers. He stated that spiral grain is a physiological necessity which enables the distribution of different water demands of different parts of the canopy across the whole water-conducting stem cross-sectional area. Braun (1959) showed for a deciduous broad-leaved tree species (Populus sect. Aigeiros) that vessels are connected within growth rings and across growth rings (growth ring bridges), forming a radial and tangential network of vessels. Tangential spreading of vessels was also shown for Fraxinus

excelsior (Burggraaf 1972) and F. lanuginosa (Kitin et al. 2004). Bosshard and Kučera (1973) quanti-fied the network of vessels in a mature Fagus sylvatica, where they found an average radial spread of vessels of 218 µm • 10 mm^-1 (maximum 420 µm • 10 mm^-1) and an almost twice as large average tan-gential spread of 402 µm • 10 mm^-1 (maximum 594 µm • 10 mm^-1). One vessel was on average in contact with 5.6 (range 3–8) vessels over a length of 10 mm, and the contact length ranged from 0.1 mm to 1.9 mm, with lengths between 0.1 mm and 0.2 mm being the most frequent class (22 %;

Bosshard and Kučera 1973).

The distance between the height of installation of the thermal dissipation probes and the height of the lowest living branch was ≥ 7 m (but about 3.9 m in two suppressed trees) for beech from Steinkreuz, ≥ 6 m for Großebene, and ≥ 7 m for Farrenleite,

≥ 13 m for oak from Steinkreuz and ≥ 12 m for oak from Großebene (Fig. 5.2.3).

Using the findings from Bosshard and Kučera (1973, see above), the tangential spread for an axial distance of 7 m would be on average 28.1 cm, at most 41.6 cm.

The calculation of a radial spread over 7 m axial distance is not reasonable because a vessel cannot leave its growth ring. Still, the meandering of vessels within a growth ring in combination with the existence of growth ring bridges allows the distribution of water across and between growth rings:

Kučera (1975) noted frequent vessel-to-ray contacts in beech, demonstrating that ray parenchyma is part of the hydraulic system of trees, particularly in the radial movement of water (Braun 1984, Hirose et al. 2005) and in water storage (Meinzer 2002). In Quercus robur and Fraxinus excelsior a smaller radial tissue displacement was observed than in beech (Bosshard et al. 1978, 1982), which was in part counterbalanced by vessel contacts across growth ring borders, and tangential spread was simi-lar in magnitude to radial spread (Bosshard 1976, Bosshard et al. 1982). Q. robur exhibited a pro-nounced vessel-to-ray network to compensate for more isolated vessels (with less vessel-to-vessel contacts) than in diffuse-porous F. sylvatica (Bosshard et al. 1978, 1982). Bosshard et al. (1982) hy-pothesised that in the wood of beech the tangential spreading of vessels has a similar function for the water transport as the ring of large vessels in ring-porous species (and the radial cell wall pores in earlywood tracheids of conifers). And Bosshard (1976) stressed that growth-ring boundaries are not functional boundaries, but imposed on the tree by external factors.

A sectorial connection of a certain root with a certain part of sapwood as observed in orchard-grown apple (Cabibel 1994), cherry (Cabibel and Isberie 1997) and mango trees (Lu et al. 2000), may introduce a circumferential (azimuthal) variation of Js on top of the radial variation since the sap flow gauges need to be spaced out from neighbouring gauges for thermal isolation. But the circumferential variation of sap flow is much less pronounced in tall, long-stemmed trees growing in closed stands (e.g. Schulze et al. 1985, Granier 1987b). Azimuthal variation has also been attrib-uted to soil texture (Cermák et al. 1995, Cienciala et al. 1999, Oliveras and Llorens 2001). The same general considerations as for the stem–leaf connectivity regarding tangential spreading (see above) apply here. Nevertheless, by installing thermal dis-sipation probes between 1.3 m and 2 m above the ground it was sought to circum-vent this additional source of uncertainty in the present study, as well as by installing probes in a fashion that allowed minimising the tangential distance between them (cf.

Chap. 4.1.3). Thus for the large forest trees studied here it seems that patterns in the hydraulic connectivity of roots or branches to the trunk would only have a minor influ-ence on the observed radial patterns of Js. This is also the reason why it seemed sta-tistically appropriate to confine the sampling of sap flow to one side of the trunk which was not randomly chosen but decided to be in the thermally least noisy northern ex-position on the stems (cf. Wullschleger and King 2000, Wilson et al. 2001, Ford et al.

2004b). This sampling strategy then allowed a larger number of individual trees to be monitored with the limited resources of such a study and balanced the aims of the investigation at the different spatial levels (within-tree, tree and stand level).

6.2.4. Effects of soil conditions, and seasonal trends in general, on radial