Peligro en la demora

results presented here are an amalgamation of measurements made over the duration of one year. This has obvious drawbacks since seasonal variations in NR activities and soluble amino acid pools can be large and significant (Chapter 4). However, it was thought essential to look at the results as a whole, in order to preserve general trends in foliar NR activity of climax and pioneer species. Without the amalgamation of annual d a t a , gross errors could have crept into the results. For instance, conclusions made about foliar NR activity in climax and pioneer species (section 3.3.3) would havè been quite different if just one month had been selected for the study. In July, for example, the pioneer Betula pubescens was exhibiting an activity of approximately 1 0 0 pkat g“^ f.wt.

while the climax species. Ilex aquifolium, had activities in the range of 200-400 pkat g"^ f.wt. Such results lend themselves to the conclusion that climax species assimilate

nitrate at rates approximately three times greater than pioneers. This may well be the case in July, but mean NR activities for these two species over the course of a year show that birch reduces nitrate at a rate 1 . 6 times greater

than the climax species, holly.

The use of average annual rates of foliar nitrate reduction (i) 'smooths' variation and results in emphasizing general mean activities of climax and pioneer woody species; (ii) encompasses a very large sample number, producing results that are more statistically viable; (iii) stresses the range of activities possible in climax and pioneer species, and (iv) takes account of the fact that at certain times of year, climax species may be assimilating nitrate at a higher rate than pioneers. If only one month had been chosen for this s t udy, production of f igures 3.1 and 3.2, would have been more-or-less impossible, and the results, difficult to interpret. However, it is, of course, wrong to completely ignore seasonal differences when interpreting NR data. For this reason seasonal variation forms the subject of Chapter 4.

3-3-2 NR activitv in temperate and tropical forests-

Histograms summarising the frequency distribution of foliar NR activity of woody plants at the three sites were produced by assigning NR activities to one of seven activity classes

Table 3.1 Nitrate reductase activity classes.

Class Nitrate reductase activity

______________________________ (pkat g~^ f.wt.l._______ 1 < 2 5 2 2 6 - 5 0 3 51 - 100 4 101 - 250 5 251 - 500 6 501 - 1000 7 > 1000

Problems arise when comparing average the annual NR data with results found in the literature. Frequently, other workers appear to have concentrated their studies on one month, or even less, of a particular year. Further, often, no mention is made of the time of year at which studies were performed. Thus, for reasons discussed earlier (section 3.3.1), comparisons between data sets should be treated with caution.

A wide range of NR activities were found in woody species at the three sites (Figure 3.1). The mean(±SE) NR activities of 186.76 ± 28.34, 201.30 ± 34.38, and 196.38 ± 45.66 pkat g"^ f.wt. (for Bencroft Wood, Boxhill and Tooting Graveney Common respectively) were normally distributed about the mean values.

Interesting comparisons can be made between data from English temperate woodlands and data from three Australian tropical forests (Fig 3.1; data from Stewart at al., 1988). The histograms showing the tropical Australian data are skewed to low values in comparison with the normally distributed values for the temperate sites. It is evident that over 80% of the species assayed by Stewart et al. (1988) had activities less than 1 0 0 pkat g"^ f.wt., with very few species having

% Frequency

1 2

(a) Bencroft Wood

3 4 8 NR A ctivity Cleee % Frequency * Frequency 1 2 (b) Boxhill 3 4 6

NR A ctivity Claee NR A ctivity Cleee

(c) Tooting Graveney Common

% Frequency 60 60 40 30 20 NR A ctivity Cleee * Frequency 60 40 30 20 10 NR A ctivity Cleee (d) Mount Glorious, Australia (e) Brigalow, Australia

% Frequency

3 4 6

NR A ctivity Cleee (f) Lamington, Australia

Figure 3.1 Frequency distribution of NR activity in the leaves of woody species growing in English and Australian* woodland communities. (*Data from Stewart et a l . . 1988).

activities in classes 5 to 7 (i.e. >100 pkat f.wt. ; Figure 3.1).

The differences in the distribution curves of temperate and tropical foliar NR activities can be explained by looking at differences in soil nitrogen availability at such sites. In most temperate forests, uptake occurs primarily from the surface soil organic layer where the highest rates of mineralization are found. In addition, rapid rates of fine- root turnover in this zone provides a better environment for uptake (Cole, 1981). In tropical forests, however, a zone of organic accumulation typically does not develop because of high rates of decomposition. Without a specific zone of mineralization, no consistent supply of inorganic nitrogen compounds exists in the soil solution (Cole, 1981; Gessel et al., 1979). The low concentration and supply of enzyme substrate in ancient tropical soils results in NR activities skewed to low values (Figure 3.1). In contrast, normally distributed foliar NR activities at the English woodland sites (Figure 3.1) suggests the prevalence of a more varied supply of inorganic nitrogen than is found in tropical soils.

The median activity class among woody species studied at the three English sites was 101 - 250 pkat g"^ f.wt. (class 4), with activities greater than 50 pkat g"^ f.wt. occurring in 82.8% of the species. This distribution of NR activity is similar to results obtained from herbaceous species studied by Lee and Stewart (1978) and Stewart and Orebamjo (1983). They observed that in both temperate and tropical groups of

herbaceous plants the median activity class was 138.8 - 277.78 pkat f.wt. (classes 4 to 5) with 92% of the species having activities greater than 55.6 pkat g"^ f.wt (> class 3).

The present results can be compared with those of Smirnoff et al. (1984). They studied woody dicotyledons and found that the median activity class was 58.3 - 138.9 pkat g"^ f.wt. with activities greater than 55.6 pkat g~^ f.wt. occurring in 73% of the species. Smirnoff et al. (1984) studied 555 species of woody plants, whereas the present study concentrated on less than 50 species, sampled with a high degree of repetition in order to obtain statistically viable results. The lower median activity class among the woody dicotyledons studied by Smirnoff et al. (1984) is perhaps due to a higher proportion of climax species being sampled than in the present study, - climax species having a tendency to metabolise nitrate in their root systems more frequently than in their leaves (see section 3.3.3). Also, many of the species studied by Smirnoff et al. (1984) were tropical species which often exhibit lower leaf NR activities than temperate species (Stewart and Orebamjo, 1983; Stewart, et al., 1988; 1990; 1992).

In conclusion, a wide range of NR activities are prevalent in temperate woody species. Many species (often in the climax group, e.g. Carpinus betulus and Quercus robur; section 3.3.3), exhibit medium to low NR activities in their leaves. However, the occurrence of high leaf NR activities is only a little less frequent in woody species than in herbaceous species. Most of the herbaceous species with high activities

are ruderals (Lee and Stewart, 1978; Stewart and Orebamjo, 1983). Some trees and shrubs with high NR activities are pioneer species (for example Sambucus nigra and Betula s p p . ; see section 3.3.3 below) occurring early in succession or after disturbance and are therefore comparable with herbaceous ruderals.