Adecuación

The woody species assayed for NR activity at the three sites between October 1987 and October 1988 (section 3.3.1), were divided into climax and pioneer species. Tables 3.2 and 3.3 show the annual mean leaf NR activities for climax and pioneer species. All species assayed possessed readily detectable activities in the field. These tables and Figure 3.2 clearly show that the highest rates of leaf nitrate reduction are present in the pioneers - the characteristic species of forest margins, gaps and early stages of forest succession. Previous studies of herbaceous and woody species (Stewart at al., 1987) and rain forest species (Chandler, 1981; Stewart et al., 1988) also support the idea that shoot nitrate reduction is a characteristic of pioneer species of both temperate and tropical habitats. High percentages of species in the top three activity classes (5 to 7) were pioneers: 73.8%, 96% and 100% of the species in NR activity classes 5, 6 and 7

respectively were pioneers. This contrasts with 63.1% of the species in class 3, 72% of class 2, and 100% of class 1 consisting of climax species.

Table 3-2 Mean annual foliar NR activity of climax species from three woodland sites.

Mean NR No. of

Species Activity Range samples Site

Aceraceae Acer pseudoplatanus 75.8129.1 2.3-215.0 22 T Aquifoliceae Ilex aquifolium 173.2142.7 17.2-577.2 75 B,H,T Corylaceae Carpinus betulus 85.1163.9 16.1-215.8 37 H Fagaceae Fagus sylvatica 77.3116.6 18.1-259.4 64 B,H,T Quercus petraea 58.2120.9 4 8 . 9 - 6 5 . 3 3 H Quercus robur 78.9115.5 20.0-249.2 74 B,H,T Hippocastanaceae Aesculus hippocastanum 36.5 30.7- 42.2 2 T Pinaceae Picea abies 78.51 9.4 36.1-160.0 44 H Pinas sylvestris 234.6139.4 17.8-553.8 80 B,H Pseudotsuga menziesii 37.41 8 . 8 1 6 . 4 - 7 3 . 9 17 B Rosaceae Prunus sp. 93.2 64.2-122.2 2 B Taxaceae Taxas baccata 59.6 5 2 . 5 - 6 6 . 7 2 B

Sampling sites were as follows: B= Boxhill, H= Bancroft Wood, Hertfordshire, T= Tooting Graveney Common.

Where three or more samples were analysed, 195% confidence limits are given.

Table 3.3 Mean annual foliar NR activity of pioneer species from three woodland sites.

Species

Mean NR Activity

No. of

Range samples Site Betulaceae Betula pendula Betula pubescens Caprifoliaceae Sambucus nigra Oleaceae Fraxinus excelsior 946.8±182.8 272.1± 78.5 642.4±157.3 197.8 868.1-1013.9 3 28.6- 950.0 68 75.0-1601.9 69 180.8- 214.7 B B,H,T B,H,T H Rosaceae Crataegus monogyna Prunus sp. Rubus fruticosus Salicaceae Populus deltoides Populus tremula Salix caprea Salix sp. 92.3±62.6 385.5±33.1 128.4±47.2 980.4±890.8 280.7±73.2 66.2 107.7 28.3- 158.6 8 372.8- 399.4 3 26.4- 462.8 68 676.4-1375.8 3 168.6- 569.2 9 65.6— 6 6 .7 2 100.3-115.0 2 B, T H B,H,T

H

Sampling sites were as follows: B= Boxhill, H= Bancroft Wood, Hertfordshire, T= Tooting Graveney Common.

Where three or more samples were analysed, ±95% confidence limits are given.

1600 pkat g"^ f.wt., and 49.3% of all pioneer species sampled had activities in excess of 250 pkat f.wt. Only 9.4% of climax species showed NR activities greater than 250 pkat g“^ f.wt. No pioneer species were found in the lowest NR activity class (<25 pkat g"^ f.wt.). In contrast, an average of 7% of climax species at each site had activities less than 25 pkat g“^ f.wt.

Climax species generally exhibited low foliar NR activities. An activity of 16.1 pkat g“^ f.wt. was recorded in leaves of

Carpinus betulus (Hornbeam) at Bencroft Wood. At Boxhill,

Bencroft Wood and Tooting Graveney Common respectively, 49.3%, 55.5% and 69.8% of climax species had foliar NR activities less than 100 pkat g*'^ f.wt. This contrasts with only 27.2%, 23.8% and 29.3% of pioneer species in these respective locations, with NR activities less than 100 pkat g'^ f.wt.

Members of the Caprifoliaceae, Betulaceae and Salicaceae (see Table 3.3), contained particularly high NR activities in their leaves, while members of the Fagaceae and the Corylaceae (see Table 3.2) contained low activities. These findings agree with the results of Al Gharbi and Hipkin (1984) who found similar species differences. Species with a very limited capacity to assimilate nitrate in their leaves were among the dominant (climax) species at the sites, where interaction of factors such as low pH, waterlogging and low temperature may have combined to inhibit nitrification (Stewart et al., 1974). There were exceptions to this trend, however. For example in other families - notably Pinaceae (Table 3.2), leaf NR

Pioneer species Climax species (a) Bencroft Wood

% Relative Frequency 100% 76% 5 0 % 25% - 0% (b) Boxhill 2 3 4 5 6

Nitrate reductase activity class

100%

75%

50%

% Relative Frequency

25%

Nitrate reductase activity class

(c) Tooting Graveney Common

100%

75%

5 0%

% Relative Frequency

25%

Nitrate reductase activity class

Figure 3.2 Percent relative frequency distribution of NR activity in the leaves of climax and pioneer species at three woodland sites.

activity was high in one genus (Plnus) but low in others

(Picea, Pseudotsuga). The significance of this finding is

discussed in Chapters 4 and 8 .

Some species, (e.g. Sambucus nigra, Populus deltoides, Betula

spp.) resemble herbaceous ruderals, not only with respect to the high NR activity in their leaves but in their relatively rapid growth rate and ability to colonize open habitats. The high NR activities in these species suggest that high levels of nitrate were available to them at the sites which they were occupying. Further confirmation of this was obtained by Havill et al. (1974) and Lee and Stewart (1978) when they showed that NR activity in the leaves of ruderals was not increased appreciably after plants had been fed with nitrate. This suggested that the supply of nitrate at these sites was already sufficient to maintain the enzyme level close to the fully induced state (Stewart et al., 1974).

Table 3.4 gives mean foliar NR activities for climax and pioneer species at the sites studied. Mean nitrate reduction rates for pioneer species differ significantly from those for climax species (P<0.001) and show that NR activity is approximately threefold greater in leaves of pioneers than in those of climax species at these sites.

It has been argued that in different serai stages of forests the form of available nitrogen varies (Chandler, 1981). In pioneer communities nitrate is the predominant form of available nitrogen, whereas in climax communities ammonium can

predominate (see Chapter 1). Rice and Pancholy (1972) observed that quantities of nitrate and nitrifying micro-organisms decreased with successional stage, whereas, amounts of ammonium increased. These workers (see also Keeney, 1980; Vitousek and Matson, 1985) suggested that climax ecosystems suppress nitrification. This results in conservation of nitrogen in these communities since nitrate is the form in which nitrogen is most easily lost. It is suggested that many temperate perennial species assimilate nitrate in their roots when external concentrations of nitrate are low (< 1 mol m"^)

and as nitrate concentration is increased, shoot nitrate reduction becomes more important (Ingestad, 1971; 1973; 1979a; 1979b; Andrews, 1986a).

Table 3.4 Mean(±SE) foliar NR activities for climax and pioneer species assayed between October 1987 and October 1988.

Mean Nitrate Reductase Activity (pkat g"^ f.wt. ).

Site_____________________ Climax Species_______ Pioneer Species Bencroft Wood 125.43 ± 15.21 312.83 ± 57.55

Boxhill 129.15 ± 16.74 332.88 ± 67.98

Tooting Graveney Cmn. 90.73 ± 12.64 344.98 ± 88.64 _____

Overall mean 115.10 + 14.23 330.23 ± 67.10

In undisturbed forests, the NH4/NO3 ratio is approximately 10:1

and root nitrate assimilators commonly predominate (Carlyle, 1986). However, tree species in other forest ecosystems and on different sites may be exposed to wide variations in the

NH4/NO3 ratio (Nadelhoffer et al., 1985). Bencroft Wood,

particularly has a reported history of having a very varied soil in terms of pH and nitrification rates (Bird, 1957). This woodland can support calcicolous species in some areas, but has other areas, especially beneath the high forest, where base rich soil has deteriorated to below pH 3.5. Such variations in the nitrogen status of the soil have led to the woody species supported in such communities developing different strategies for efficient nitrogen utilization, such as leaf nitrate reduction in excess of root nitrate assimilation. In the present study, this inhibition of nitrification in climax communities is not universal since appreciable nitrate utilization was observed in some climax species examined (Figure 3.2), but reduced leaf NR activities measured in climax species generally, suggest that nitrification is suppressed in areas of the communities studied, and that such species may be utilizing ammonium as their main source of inorganic nitrogen.

3.3.4 Trends in a m ino acid concentrations in relation to