Box 72 Pulsed resources

Forest research tends to concentrate on normality, in this case how much dead material normally falls in a forest. However, there is growing interest in resource pulses, periodic large increases in food for organisms. This is seen in mast years when an unusually heavy seed crop is produced (see Section 4.4). In terms of decomposition, Zackrisson et al. (1999) observed that in a mast year of Norway spruce Picea abies in northern Sweden, an extra 5-10 kg of nitrogen was added per hectare to what is normally a nutrient-poor soil. Seeds that failed to germinate decomposed more rapidly than needle litter and released 65-75% of their nitrogen in the first growing season, which was linked by Zackrisson and colleagues to better growth of Scots pine Pinus sylvestris seedlings. They suggest that this nutrient pulse from the dead seeds is a significant factor in the strongly pulsed natural tree regeneration found in these northern forests.

Insect defoliation can also produce a concentrated pulse of nitrogen, redistributing it from the trees to the soil microbes in the droppings or frass (Lovett et al., 2002) with potential long-term consequences for reproduction and growth as the trees compete with the microbes to regain nitrogen. In eastern USA there are a number of periodical cicadas which swarm in spectacular numbers for up to a month every 13-17 years (including the 17-year cicada Magicicada septendecim), most recently in 2004. Densities of adults can reach over two and a half million per hectare over hundreds of square kilometres. While the havoc these plant-feeding insects can cause is well known, it seems they may have benefits when they die. Yang (2004) provides compelling experimental evidence that adding 120 dead cicadas per square metre in hardwood forests led to an increase in soil bacteria and fungi within a month with increase in soil ammonium of 412% over 30 days and 199% increase in soil nitrate over 100 days. Moreover, he has shown that this results in 12% more nitrogen in foliage and 9% bigger seeds in the American bellflower Campanula americanum, a common understorey plant within the cicada's range. It may also explain the small increase in growth seen in pines after a cicada event, given that pines rarely suffer cicada damage (Koenig and Liebhold, 2003).

There is an overall increase in annual above-ground litter-fall from the poles to the equator (Vogt et al., 1986); with broad averages of 0.1-5 tonnes per hectare (t ha-1) in boreal forests, 3-10tha-1 in temperate deciduous forests and 5-15 (even 30) tha-1 in tropical forests. There is an apocryphal story about a park keeper who wanted only evergreen trees in his park because they shed fewer leaves. Fortunately for him, that appears partly true. In temperate and northern regions, evergreen forests produce slightly less litter than deciduous forests (a smaller proportion of the leaves are replaced each year) but this difference is pronounced (and even then there is a good deal of variation

Table 7.1. The fate of the biomass grown (net primary production) in one year in 23- and 180-year-old Pacific silver fir Abies amabilis stands in the Washington Cascade Mountains. In these stands the dominant fir is mixed with western hemlock Tsuga heterophylla, mountain hemlock Tsuga mertensiana and noble fir Abies procera. The understorey is dominated by broadleaved deciduous shrubs with a mostly deciduous field layer. Note that the figures for living material are after the detritus has been taken away, thus above-ground the vegetation produced 4.31 haT1 of living material plus 2.11 haT1 that subsequently died in the year. T = trace.

23-year-old stand 180-year-old stand thaT1yT % of total t haT1yT % of total

Above-ground biomass added thaT1yT % of total t haT1yT % of total

Above-ground biomass added

Tree

4.3

23.3

2.3

13.8

Shrub stems

0.06

0.3

T

T

Total living

4.3

23.6

2.3

13.8

Detritus production

Falling trees and litter

1.8

9.9

2.2

12.9

Field layer litter

0.3

1.8

0.05

0.3

Total detritus

2.1

11.7

2.2

13.2

Total above-ground

6.5

35.3

4.6

27.0

Below-ground biomass added

Total living (big roots)

1.8

9.7

0.7

4.2

Total detritus

10.0

55.0

11.5

68.7

Total below-ground

11.8

64.7

12.2

72.9

Total primary production

18.3

100.0

16.8

100.0

Source: Data from Grier et al., 1981. Canadian Journal of Forest Research 11.

Source: Data from Grier et al., 1981. Canadian Journal of Forest Research 11.

and overlap) only at high latitudes away from the tropics. Note that the difference is between evergreen and deciduous rather than between coniferous and broadleaved; the amount of litter produced by a deciduous larch forest and a deciduous oak forest is of the same order of magnitude.

The next question to ask is how much of the forest's new growth in a year ends up as dead material. Grier et al. (1981) looked at annual litter-fall in a Pacific silver fir forest (Table 7.1) and found that in a young stand (23 years old) about one-third of the above-ground biomass grown in a year ended up as litter (2.1 of 6.51 ha-1). In an old stand, 180 years old, around half of the year's growth fell as litter (2.2 of 4.61 ha t1). Almost the same weight of litter was produced in both stands but the young stand was growing more vigorously and a greater amount of the year's growth was kept in living tissue.

Despite its importance, litter-fall is not necessarily the main source of necro-mass. Grier et al. (1981) also looked at the amount of 'detritus' (as they chose to call it) produced each year below-ground from dying roots. In the young and old stand, respectively, below-ground detritus was 10.0 and 11.51 ha -1 in the year, compared with around 21 ha-1 above-ground (Table 7.1). So in both stands more than three-quarters (around 83%) of the detritus was produced below-ground. Adding together these above- and below-ground figures gives a total detritus production of 12.1 and 13.71 ha-1 of dead material produced in the year in the young and old stands, respectively. This is two-thirds (66.6%) of total production that year in the young stand; in the old stand it was more than three-quarters (82%) - Table 7.1. Eventually, in a mature stand, that has reached its maximum accumulation of organic matter, decomposition will theoretically be 100% of the year's growth (see Chapter 8).

The exact amount of litter produced each year in the same stand can vary considerably due to factors such as weather, numbers of flowers and fruits produced (e.g. mast years), leaf production and factors such as pests and diseases or storm damage killing part or all of a tree. In temperate deciduous trees, around 75% of above-ground litter is composed of leaves, 10% flowers and fruits, and 15% branches and twigs although there can be tremendous variation within these figures.

There can also be considerable seasonal variation in litter-fall. The most conspicuous seasonality is in temperate deciduous trees dropping the majority of their leaves in the autumn. Some leaves can also be lost at other times of the year due to disease, insects or inclement weather. Added to this is the seasonal shed of bud scales in spring, flower parts (normally in spring or summer) and immature fruits in summer. Temperate evergreen species are less synchronous in litter shedding; some shed at irregular times through the year (e.g. Norway spruce Picea abies) while others peak in spring (e.g. holly Ilex aquifolium of Europe and holm oak Quercus ilex of the Mediterranean region), summer (e.g. cypresses Cupressus species) or autumn (e.g. many pines). Tropical evergreen trees are more complicated since individuals of a species may shed their leaves together or as individual trees or even a branch at a time; shedding may also be at irregular intervals or, if regular, not necessarily linked to the calendar year.

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