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10. Warm temperate forest (fairly tall, many broadleaved evergreen/semi-deciduous angiosperm trees but moisture-requiring conifers also tend to be abundant).

(Corresponds to subdivision of Olson et al. temperate broadleaved forest).

References directly cited in these pages (does not at present include secondary citations)


Introduction

Warm temperate forests occur where summers are warm, winters are mild without extended periods of snow cover or severe frost, and where there is no marked dry season (Walter 1971). The large east Asian warm temperate forest zone is dominated by evergreen or semi-deciduous angiosperms with various moisture-requiring conifers also present. The south-eastern USA forests presently have a high proportion of large pines as well as evergeen and deciduous angiosperm trees. However, during the early-to-mid Holocene, deciduous angiosperm trees seem to have been much more abundant in these North American forests (Delcourt & Delcourt 1987). In Australia, the small areas of warm temperate forest that occur are dominated by large evergreen eucalypts, with tree ferns and broad-leaved conifers also present. The New Zealand and Andean warm temperate forests have a high proportion of large moisture-requiring conifers and various evergreen angiosperms (Walter 1971), merging into what can reasonably be described as evergreen cool temperate forests. Thus, the warm temperate forest biome is very varied and obviously difficult to define in terms of a single carbon storage value. However, it has a distinct tendency to be taller than its cooler temperate equivalent, with more very massive trees. For the sake of simplicity it has been necessary here to impose what appears to be a fairly representative value for these several rather different ecosystems. The definition used here to distinguish 'warm temperate' from 'cool temperate' forest is very ad hoc, and based on the assumption that ecologists 'know it when they see it' in the present-day world or in the pollen record. It can be taken as roughly approximating to the Holdridge definition of a mean annual biotemperature of above 12C (N.B. biotemperature is not identical to mean annual temperature).

Almost wherever they occur, the warm temperate forests have supported agriculure and forestry for hundreds or thousands of years, and have been very extensively cleared or cut. Thus, the forest we see at present is nearly all in various stages of recovery from agricultural clearance, cutting or replanting within the last few centuries. In the SE USA, the forest has been further depleted by the loss of its large chestnut trees and elms to introduced fungal pathogens. There can be no doubt that the 'youthfulness' of many of these forests means that soil, vegetation and litter carbon storage are well below what they would generally have been in their primaeval state several thousand years before.

Generally, it seems that natural fires would have had little effect on broad scale carbon storage in these forests, as the moist climates (including high summer air humidity) tend to prevent the vegetation from burning. Christensen (1978) suggests that occasional crown fires were important in determaining forest structure and species composition throughout the south-eastern USA forests, even in the moist cove forests of the southern Appalachians. Though natural and anthropogenic leaf litter fires in winter time are a well documented phenomenon in the Appalachian forests, there does not seem to be any observational or historical evidence to back up this idea that crown fires were significant, even on the time scale of centuries. Nevertheless, fires do occur within this vegetation type and may have been important in the suppressing carbon storage in certain restricted areas. Litter and grassy understory layers may have burned frequently in the pre-settlement longleaf pine forests of the Florida panhandle, southern Alabama and southern Louisana, maintaining a fairly open forest or even patchy savanna vegetation with tall pines separated by grassy areas (Christensen 1978, Ware et al. 1993).

Storage (tC/ha) Ecosystem component
36 tC/ha Dead standing trees, coarse woody litter, leaf litter and other debris
190 tC/ha Above and below-ground vegetation
145 tC/ha Soil organic carbon
371 tC/h Total carbon storage

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Litter

The figures suggested here for the primaeval state emphasise the role of anthropogenic disturbance in lowering the amount of coarse woody debris, assuming that it would have been a significant reservoir in non-anthropogenic forests. However, there are relatively few data available on this.

Storage (tC/ha) Location Author(s)
36 tC/ha (1.) Coarse woody debris Harmon pers. comm.

(1.) On the basis of his field experience, in Asia & North America, in which primary forests are found to contain large amounts of fallen and standing dead wood.

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Vegetation

Storage (tC/ha) Location Author(s)
100 tC/ha Temperate broadleaved forest Olson et al. (1982)
216 tC/ha Japan. Managed forests >100 yrs Cannell (1982)
229 tC/ha SE USA C19 inventoried forests Delcourt & Harris (1980)
150 tC/ha Old-growth Nothofagus, NZ Cannell (1982)
190 tC/ha (5.) Himalayan warm temperate forests Singh et al. (1995)
150-230 tC/ha (6.) 120-year old south-central USA pine forest Birdsey (1996)

(1.) Olson et al. do not distinguish between warm temperate and cool temperate forests. Figures of 60 (low) - 100 (medium) -140 (high) tC/ha are suggested by Olson et al. (1983). Many of these figures cited are from Cannell's book, and cross-checking shows that these figures are mostly from extensively disturbed and recently planted sites which would be expected to be low in biomass.

(2.) Cannell (1982) presents various items of data for Japan from individual site studies (forest stands over 100 years old, but mostly harvested and replanted within the past couple of centuries); Some of these sites are apparently from cool temperate deciduous forests, others from warm temperate evergreen forests, but the locations of the sites are not given. Multiplied by 0.475 to convert from biomass to carbon, the total above and below-ground carbon storage per hectare is 152, 163, 150, 162, 290* and 345* for the various samples (* the starred figures are from samples of forest with a characteristic warm temperate evergreen composition of tree genera, such as Castaneopsis; other samples are more ambiguous and may in fact be cool temperate). The overall mean for these samples of Japanese temperate broadleaved evergeen and deciduous forest is 216 tC/ha.

(3.) Delcourt & Harris (1980) calculate this value on the basis of extensive late 19th century Government forest inventory studies (basal area of each tree species) in virgin forests in the south-eastern USA; 229 tC/ha in vegetation (including soil and litter at 327 tC/ha, i.e soil and litter C= 98 t/c/ha). They are critical of the higher figure of Whittaker in IBP field studies (500-600 t/ha aboveground biomass alone) as having been based on cove forests protected from the natural regime of disturbance by wind-throw etc. Delcourt & Harris calculate an overall carbon loss of 42 GtC from SE USA forests between 1750 and 1960. Delcourt & Harris' figures are obtained on the basis of extrapolation from extensive late 19th century Government forest inventory studies (which give figures for estimated timber stock of each tree species, per unit area) in virgin forest stands remaining in the south-eastern USA (Sargent 1884). Delcourt & Harris (1980) suggest an overall carbon loss of 42 GtC from SE USA forests (comprising the range of forest types of the states of Alabama, Arkansas, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, South Carolina, Tennessee, Virginia and west Virginia) between 1750 and 1960. Biomass may however have lower for areas where Sargent's inventory reports that long-leaf pines dominated (see below), in a belt along the extreme southern coastal plain.

(4.) Cannell (1982) presents data for Lowland Nothofagus stands, >300 yrs old. 155 & 145 tC/ha: Mean for the two N.Z. sites = 150 tC/ha. These values are rather lower than the other values cited above. They may refect the relatively cool upland climate in which these South Island forests were growing and despite their evergreen habit should probably be regarded as cool temperate (e.g. the warmer-climate Kauri [Agathis] forests are much taller with very large trees). Also, in many areas these Nothofagus forests apparently occur as a secondary replacement of the origonal forests that were present before extensive forest disturbance following Maori colonization about 1000 years ago (McGlone 1994), and furthermore may now represent a relatively low biomass form occurring on the depleted soils. Hence they should probably be seen as somewhat lower than the mean for old-growth forests in northern New Zealand.

(5.) Based on a sample of 33 forest stands at a range of altitudes in the Himalayas. Mean above and below ground biomass approx 380-420 t/ha, or x 0.475 = 180 - 200 tC/ha. This includes many samples that would be regarded as 'cool temperate', but the overall mean appears to remain the same for warm and cold temperate. Sample areas appear to have been selected in a representative way, avoiding areas subject to intense grazing or recent severe ground fire. However, only accessable areas were sampled, which may tend to select for sites on deep soil. Values were deduced from bole areas, based on equations derived from weighing sample trees.

(6.) Birdsey extrapolates from younger stands using stand accumulation models to suggest overall carbon storage for a stand age of 120 years, which may in fact be much less than the actual stand age of many south central (Tennessee, Alabama etc.) forests before European settlement of North America. The fact that this is based on a model opens up the possibility of errors in the estimated carbon storage accumulation over time.

Other figures which Birdsey puts forward for 120-year old stands on the basis of extrapolation from observed young stand carbon storage are; south-central (Tennessee, Alabama etc.) oak-pine forest, 204 tC/ha; south-central oak-hickory, 180 tC/ha; south-central bottomland hardwoods 232tC/ha,

Conclusion;

The available evidence suggests that in the absence of dense agricultural human populations, warm temperate forests in various parts of the world have tended towards a much higher carbon storage than is generally seen at present in surviving forest areas. In virtually all the regions in which these forests occur, very large trees would have been abundant, with larger scale natural disturbance events (fire, landslides, wind destruction of forest stands) unlikely to play an important role in suppressing overall carbon storage. It seems likely that warm-temperate forests have a higher biomass carbon storage than cool-temperate forests, although the Himalayan data discussed above suggest that cool temperate forests may be equally high in biomass. A relatively conservative figure of 180 tC/ha is used here for above and below ground vegetation, thought the actual figure in the early-to-mid Holocene may well have exceeded 250 tC/ha.

An exception to the general rule that pre-agricultural warm temperate forests were filled with very large trees may be the southern Gulf coastal plain of the USA. On the Florida panhandle, southernmost Alabama and SW Louisana, a tall but rather open pine forest may have existed before European settlement, maintained by frequent burns of the grassy understory (Ware et al. 1993), probably encouraged by indians (early to mid Holocene forests in this region included a high proportion of deciduous tree species). Since crown fires are very rare in this relatively humid climate, it can be assumed that in its presettlement state, forest would have been fairly high in biomass relative to the present - perhaps around 120-150 tC/ha in carbon - though not as high as the other southeastern forests mentioned above.

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Soil

Storage (tC/ha) Location Author(s)
161 tC/ha (1.) Warm broad-leaved forests Zinke et al. (1984)
132 tC/ha (2.) Warm conifer forests Zinke et al. (1984)
214-383 tC/ha (3.) Taiwan forests Chen & Hseu (1997)

(1.) Note that this is warm temperate not subtropical, based on the Holdridge (1967) classification. Based on a global total of 85 standardised soil samples. SE of 19.2. Also note that many of these forests will have been disturbed by humans, tending to give lower carbon storage values than would otherwise occur.

(2.) From global total of 330 soil samples. SE of 7.7.

(3.) From a systematic study of forest soils of Taiwan (71 samples) to 1m depth, for a range of soil types. Histosoils (peats) are not included here.

Conclusion; On the basis of the Zinke et al. data, an overall figure of 145 tC/ha as organic soil carbon is suggested (this being the mean of the two average figures for warm temperate forest types obtained by Zinke et al. 1984). This is a relatively conservative value compared to the study by Chen & Hseu, who find much higher carbon storage for all of the wide range of warm temperate forest soils which they sampled. Even these figures may in fact be rather lower than for the early-to-mid Holocene state, owing to inclusion in the Taiwan survey of many previously anthropogenically disturbed forest areas.