Climate change

11.3.1 Global warming

The climate of the Earth has changed naturally many times since the origin of life at the beginning of the Cambrian. The present global warming (the temperature of the Earth has increased during the twentieth century by 0.76 °C) differs from those of the past in that it has almost certainly been initiated by the activities of humans. During the twentieth century the atmospheric concentration of carbon dioxide (CO2) rose by 35% from 280 parts per million (ppm) to 379 ppm in 2005. The concentrations of other greenhouse gases have also risen since 1750, the beginning of industrialization, notably methane (151% increase) and nitrous oxide (18% increase). While there is general agreement that climate is changing, the amount of change we can expect is not as certain. Links between changes in the Earth's atmosphere and global warming have been investigated by mathematical modelling. Although the final results are difficult to predict with accuracy, in 2007 the Intergovernmental Panel on Climate Change estimated that the long-term increase in surface air temperatures would by 2100 be between 1.8 and 4.0 °C with a best estimate of 2.5 °C. Houghton et al. (2001) contains more detail on predicted changes. The expected changes will not be felt uniformly over the whole globe. Northern latitudes are expected to show greatest temperature changes and in the UK, south-east England will show the greatest change in rainfall, with 20-30% less in summer and 10-15% more in winter than at present. Good news for the tourist industry but the increase in summer droughts is likely to be bad news for forests since we know that the droughts of 1976 and 1995 in the UK led to earlier defoliation of trees and shrubs as well as the death of drought-intolerant trees.

Climate change is not only a fact; its onset is rapid, and as the world becomes warmer the sea level is rising faster than had been expected. Sea level rose 0.1-0.2 m during the twentieth century and is projected to rise somewhere between 0.18 and 0.59 m by 2100 as the Arctic and Antarctic ice caps melt. The Thames barrier, built to protect London after the loss of 300 lives due to flooding in 1953, is the largest structure of its kind in the world. It had allowances for sea-level rise built into it, but the increasingly stormy weather and greater than expected sea-level rise caused it to be used many more times than was expected by 2003, when plans to raise its height by more than a metre had to be commissioned. Sea-level rise and the possibility of tsunami, huge and unusual ocean waves sweeping round the world, will have a considerable influence on both low-lying coasts and coastal forests.

When considering the influence of global warming on forests, it is important to bear in mind that since trees are long-lived, many forests reflect past climate changes over the last millennium. Gillson and Willis (2004) point out that trees that are more than 150 years old established during the Little Ice Age (1590-1850) and reflect a climate long gone. So if the old trees are failing to regenerate, or the forest is changing, it may be a reflection of changes in past climate rather than a present-day 'problem'. Due to this, some of the current changes we are seeing in forests can be hard to interpret. Nevertheless, it does seem that forests are already responding to global warming, especially in the northern hemisphere where the impact of climate change is largest. Many examples could be selected from the published literature to illustrate current and predicted changes, and the underlying mechanisms. The overall picture is complex given that the responses of forests and individual species occur at different spatial and time scales.

Some changes are readily seen on the small scale. On the basis of an investigation of a late-successional hemlock-northern hardwood forest, Woods (2004) suggests that climate change may induce changes in forest ecosystems by changing disturbance patterns, with many more instances of intermediate disturbance due to storms. An intermediate disturbance is one that has a marked influence on an ecosystem without destroying all the individuals present, as occurs when fire kills all the trees in a particular area. Tree mortality in the Dukes Research Natural Area in Michigan, before and

0 10 20 30

400 300 200 100 0

(a) Acer saccharum

Baseline mortality Storm mortality

(a) Acer saccharum

Baseline mortality Storm mortality

(b) Fagus grandifolia n

Diameter classes (cm)

(b) Fagus grandifolia

(c) Betula alleghaniensis iïVYwr a

(d) Tsuga canadensis

Diameter classes (cm)

Figure 11.3 Size structure by species in permanent plots in the Dukes Research Natural Area, an old growth mesic hemlock-northern hardwood forest in northern Michigan, USA. Ascending bars show distribution of live trees in 10 cm dbh (diameter at breast height) classes in 1992. Descending bars show mortality as percentage of size class; lighter shading indicates baseline (1992-2002) mortality, darker shading indicates mortality due to a storm in July 2002. Open-ended bars extend below the graphed range. The 'Recr' category includes stems reaching 5 cm between 1992 and 2002. (From Woods, 2004. Journal of Ecology 92, Blackwell Publishing.)

after a severe storm in July 2002 showed that it did not influence all tree species and all sizes of tree similarly (Fig. 11.3). Yellow birch Betula alleghaniensis was little affected by the storm but had a high baseline mortality (normal losses during the period 1992-2002). Losses of both types were low in eastern hemlock Tsuga canadensis, while the shade-tolerant maples, especially sugar maple Acer saccharum, and American beech Fagus grandifolia had similar patterns of baseline and storm mortality with storm mortality being higher for the larger stems.

The storm of 2002 thus reduced the dominance of the larger shade-tolerant trees, caused changes in canopy structure and distinctively affected the under-storey and ground flora by letting in more light. This work demonstrates that the consequences of rare, intermediate disturbances can differ markedly from what would be predicted by simply scaling up patterns resulting from frequent, less intense events. A feature of most climate-change scenarios is that the rare events will become more common. When these are added to the likelihood that severe disturbances such as fire and insect attack are also likely to become more common (see Dale et al, 2001 and Logan et al, 2003 for reviews), and changes in vegetation may exacerbate temperature increases (for instance, vegetation growing further north would absorb more heat - see Foley et al. 2003 for a review) the predicted large-scale effects of climate change become easier to believe.

In other cases, changes to the forest may be more extreme, involving the loss of key species or even whole forests. This is likely where the forest or species are closely tied to specific climatic conditions, show great sensitivity to climate or are currently living near the edge of their environmental tolerance. For example, in southern Germany predicted changes will result in hot dry summers but with a wetter spring and severe flooding. Beech Fagus sylvatica, an important forest tree species in this region, is drought and flooding sensitive and is already showing reduced growth and competitive ability especially as seedlings on extreme sites. Beech is likely to further suffer since it has limited capacity to respond to increased CO2 levels compared with other forest trees. It is predicted that if climate-change scenarios are correct then beech will disappear from southern Germany (Rennenberg et al., 2004). Tropical cloud forests, growing in altitudinal bands of cloud formation on mountains, rely on regular immersion in low-lying clouds for their existence and are a classic case of forests at high risk from climate change. Under current scenarios, there will be less cloud and it is likely to shift higher up the mountains. This will upset the delicate equilibrium of these very high biodiversity forests and will require species to move vertically (that is, if they have not already reached the top of the mountain) and between mountain peaks. Foster (2001) points out that while this has certainly happened in the past, mature cloud forest may take 200-300 years to develop and the suitable climate envelope may have moved on by then given the fast rate of climatic change predicted. Moreover, other climate change threats such as the incidence of typhoons may increase which would be particularly damaging to cloud forests since gaps in the canopy are very slow to regenerate.

To add spice to the arguments, recent discoveries imply that the climate of Europe could quite suddenly become much colder, as it has done in numerous past ice ages. The cause lies in the increasing dilution of the waters of the North Atlantic as the Greenland ice sheets continue to melt at an ever-increasing rate and more fresh water pours into the sea from Siberian rivers fed by higher

Figure 11.4 The great ocean conveyor current that joins warm surface currents (light grey) to cold ocean floor currents (dark grey). The flowing of this giant conveyor current is dependent upon warm water cooling by Greenland and sinking, but if too much melting ice mixes with the seawater it may become too buoyant to sink, causing the conveyor to halt irreversibly, with dire consequences to the climate of western Europe and beyond. Arrows show direction of flow.

Figure 11.4 The great ocean conveyor current that joins warm surface currents (light grey) to cold ocean floor currents (dark grey). The flowing of this giant conveyor current is dependent upon warm water cooling by Greenland and sinking, but if too much melting ice mixes with the seawater it may become too buoyant to sink, causing the conveyor to halt irreversibly, with dire consequences to the climate of western Europe and beyond. Arrows show direction of flow.

rainfall. The present mild climate of Britain and western Europe is mediated by the continual warming provided by the Gulf Stream and its continuation, the North Atlantic Drift. This water eventually cools, sinks to a great depth and is returned to the tropics by the so-called conveyor current (Fig. 11.4). The problem is that if this salt water is diluted it fails to sink as effectively and the conveyor current, which has slowed appreciably and faltered over the past few decades, could eventually halt as we now know it has in the past. The measured salinity of this water is falling and there is a real risk that the conveyor counter current, and consequently the Gulf Stream, will halt long-term, although recent predictions suggest this is unlikely to happen within the modelling time frame between now and 2100 (Houghton et al, 2001). If and when it does halt, however, Europe may again have a climate like that of Alaska, which is at similar latitudes, with sea ice off the coast of England. This would not be a local change: the climate of the whole world would be affected and the connected loss of monsoon rains would cause tropical forests to change into grasslands.

It has become accepted that atmospheric pollution is responsible for not just global warming but also another major effect, that of cooling. Global dimming, coined by Stanhill and Cohen (2001), causes less sunlight, between

9 and 30% in various parts of the world, to reach the surface of the Earth than would be the case if the atmosphere was not suffering from particulate pollution resulting from the emissions of power stations, surface vehicles and aircraft, as well as others such as forest fires. After the tragedy of 11 September 2001 there were virtually no flights by commercial aircraft in the USA. This diminution in pollution alone was enough to increase the local daily temperature range in that period by an unprecedented 1 °C.

Meteorologists have for many years, in some cases for as long as a century, kept a record of pan evaporation, the amount of water that has to be added to bring the open surface of water in an exposed vessel to the same level each day. Pan evaporation is influenced particularly by the amount of sunlight as the incoming photons virtually kick water molecules into the air, with relative humidity and wind force as subsidiary factors. Temperature has a still smaller effect. All over the world pan evaporation diminished during the twentieth century (by 22% between the 1950s and the 1990s in Israel) and a few scientists eventually realized that the cause was that atmospheric pollution changes cloud structure. Normal clouds consist of relatively few large droplets of water whereas polluted clouds consist of roughly ten times as many much smaller droplets centred around particles of ash, soot, sulphates, nitrates and other particulate impurities. While sunlight passes through normal clouds, these polluted clouds act as mirrors reflecting it back into space.

Global dimming is much stronger in the northern than the southern hemisphere and it has been responsible for a cooling effect so strong that it resulted in a further Sahel famine in 1984 (see Section 11.2 also), following failure of the annual monsoon for several years. If the Asian monsoon were to fail the effects on the world population would be even worse. Europe has made a big effort to cut down atmospheric pollution in recent years, causing a reduction in global dimming in areas downwind of it (see Roderick, 2006). Global dimming has the potential to cause huge crop losses, yet without it the effects of global warming would be far greater than they already are. Unless the countries of the world realize this and take effective action, the outlook for the forests of the world is bleak indeed.

11.3.2 Phenological changes: springs are getting earlier

Phenology is the study of natural phenomena recurring each year, especially in relation to climatic conditions (Section 4.6.1). Records of this type go back a very long way, indeed in Japan and China some of the dates of flowering of cherry and peach trees are known from the eighth century. The earliest known

year

Figure 11.5 First leafing time series for oak in Norfolk and Surrey, provided, respectively, by the Marsham and Combes records. A smoothed (LOWESS) line has been superimposed. (Courtesy of Tim Sparks, Centre for Ecology and Hydrology, Monks Wood.)

year

Figure 11.5 First leafing time series for oak in Norfolk and Surrey, provided, respectively, by the Marsham and Combes records. A smoothed (LOWESS) line has been superimposed. (Courtesy of Tim Sparks, Centre for Ecology and Hydrology, Monks Wood.)

surviving series in the UK was commenced by Robert Marsham FRS in 1736. After his death in 1798 records of the first leafing of the trees on his estate in Norfolk were continued by successive generations of his family. Figure 11.5 shows the Marsham first leafing dates for oak, the final records for the twentieth century being provided by Jean Combes, who has kept a summary of the leafing of oak, ash, lime and horse chestnut since 1947. Phenology is becoming increasingly important in an era of global climate change, and it is realized that phenological events, especially those in spring, can be extremely sensitive to climate fluctuations (attributable to more pronounced changes in winter and spring temperatures). A Royal Meteorological Society network kept phenological records for very many years (1875-1947). The UK Phenology Network, organized in 1998 with the support of the Woodland Trust, has effectively replaced it. The new network had more than 2000 registered recorders in 2005 and more than one million records in its database.

The season of spring, occurring as it does between winter and summer, technically begins on the day when the sun is over the equator (21 March in the northern hemisphere, 21 September in the southern). In practice, and particularly in the public mind, the start of the season is associated with particular biological events. In central and northern Europe it coincides with the arrival of the first skylark Alauda arvensis. This bird remains a resident in mild British winters, so in Britain it is the appearance of the first flowers, such as the snowdrop Galanthus nivalis and primrose Primula vulgaris, or the first budburst of various trees and shrubs, that are often used as the biological indicators of this important event. Data presented by Sparks (2000) and by Sparks and Smithers (2002) demonstrate quite clearly that, over very long periods, higher temperatures early in the year consistently led to earlier flowering in woodland herbs such as wood anenome Anemone nemorosa and earlier budburst in trees such as oak Quercus spp., hawthorn Crataegus monogyna and blackthorn Prunus spinosa.

There is thus no doubt that higher temperature in the early part of the year (or possibly a climatic variable correlated with temperature) causes phenolo-gical events, including first observations of particular insects such as the brimstone butterfly Gonepteryx rhamni, to occur earlier than they otherwise would. Observations since 1950 in Northumberland, and since the early sixties in Norfolk, show that first flowering dates for snowdrop have become increasingly early. When a smoothed line is applied to the data, which oscillate somewhat from year to year, it can be seen that whereas snowdrops in Northumberland flowered around 24 February in 1950, by 2000 they were doing so on 20 January. In warmer Norfolk this had advanced to the end of the first week of the year. Oak, which in Surrey was leafing around 2 May in 1950, was by 2000 doing so in the first week of April. The average date of first flowering of 385 British plant species was found by Fitter and Fitter (2002) to have advanced by 4.5 days during the past decade compared with the previous four decades. Of these, 16% of species showed an average advancement of 15 days in a decade, while 3% of species flowered significantly later in the 1990s than previously. There are fewer data for autumn events but these suggest an elongation of the growing season at that end of the year as well.

Anecdotal evidence suggests that the variation between years is also becoming more pronounced. The weather in the winter of 2002/3 in the English Midlands was quite extraordinary in this respect. Following an autumn so mild that the leaves of many trees were retained for much longer than usual, February was unusually cold with 21 frosty nights. The summer of 2003 on the other hand, was so very hot that Batsford Arboretum in Gloucestershire arranged tours demonstrating autumn leaf colours a fortnight earlier than usual. Moreover, the European winter of 2005/6 was very severe, greatly delaying foliage development of herbs, shrubs and trees. Climate warming is indeed associated with erratic short-term weather events, including wind storms.

The above data give clear evidence that warming is undoubtedly occurring and virtually all natural events, including the times at which birds nest, butterflies emerge, amphibians breed and birds migrate, are taking place earlier in the year. That much is certain, but it leads to the further question of how global warming will affect the distribution of species.

11.3.3 How will global warming influence the distribution of forest plants and animals?

As global temperatures increase, there will be a shift of species ranges towards the poles and towards higher altitudes based on species' climatic thresholds. In England acceptance of this factor has already led to change in the official attitude towards the planting of beech Fagus sylvatica (Section 10.4) with more being planted towards the north end of its range in northern England and Scotland. The USA Forestry Service, concerned about future changes in natural tree distribution, has devised a computer matrix simulation model called SHIFT whose function is to estimate potential migration of five major tree species due to global warming in the next 100 years (Iverson et al., 2004). The species concerned are persimmon Diospyros virginiana, sweetgum Liquidambar styraciflua, sourwood Oxydendrum arboreum, loblolly pine Pinus taeda and southern red oak Quercus falcata var. falcata. The model allows very long-distance events in which colonization resulting from seed dispersal by wind or animals could occur up to 500 km beyond the present distribution boundary. The abundance of these species near their range boundaries proved to have more influence than percentage forest cover in influencing migration rates. Though migration patterns for the five species differed, model outputs predicted migration of a generally limited nature for all of them over the next century. In this period there was a high probability of migration into zones 10-20 km distant from present boundaries, but very little of a greater migration attributable to climate change, though some remote outliers of particular species might result from rare long-distance dispersal of tree seeds.

Each species will respond to its own environmental needs and tolerances, so biomes and forest types as we know them may alter considerably as some species are left behind or lost and others are acquired (Walther, 2003 gives a more detailed review).

11.3.4 Carbon sequestration by forests

One strategy for reducing the effects of climate change is to remove CO2 from the atmosphere by locking up (sequestering) carbon in vegetation. Since forests cover around one-third of the Earth's land area and store more than 80% of terrestrial carbon, they are the prime candidate for absorbing extra carbon. Pastures on the other hand not only have a lower carbon density, they contribute both nitrous oxide and methane (from their livestock), which make a considerable contribution to global warming. The startling discovery by Keppler et al. (2006) that vegetation, especially tropical forests and grasslands,

Table 11.1. The amount of carbon (Gt) stored in the three main types of forest and the amounts found above ground in the vegetation and below ground in the soil. Pg or petagrams, 1015 g are the same as Gt, gigatonnes, 109 t or a thousand million tonnes. Some carbon is also found in other forest types, such as tropical savannas, which has been ignored here for simplicity.

Table 11.1. The amount of carbon (Gt) stored in the three main types of forest and the amounts found above ground in the vegetation and below ground in the soil. Pg or petagrams, 1015 g are the same as Gt, gigatonnes, 109 t or a thousand million tonnes. Some carbon is also found in other forest types, such as tropical savannas, which has been ignored here for simplicity.

Vegetation

Soil

Total

Forest type

Gt

%

Gt

%

Gt

%

Tropical

212

59

216

27

428

37

Temperate

59

16

100

13

159

14

Boreal

88

25

471

60

559

49

Total in forests

359

100

787

100

1146

100

Total in all terrestrial vegetation

466

2011

2477

Per cent in forests

77%

40%

46%

Source: (Data from Houghton et al., 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press.)

Source: (Data from Houghton et al., 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press.)

appear to produce around 149 000 Gt (gigatonnes) of methane annually (see Table 11.1 for a comparison of units), a potent greenhouse gas, led to speculation that forests may not be as valuable in fighting climate change as was thought. However, Frank Keppler and his colleagues have pointed out that methane production would reduce the value of carbon sequestration of new forests by less then 4%.

In Chapter 8 the concept of ecosystem productivity was explored. This shows that once a forest reaches maturity as much carbon is lost in respiration in a year as is captured by the growing vegetation. Thus, in theory, some carbon is withdrawn from the atmosphere as a forest grows, but once it is mature it becomes carbon neutral. Empirical evidence shows, however, that on a global scale temperate and northern forests are a net sink of carbon, i.e. they are sequestering extra carbon. The cause of this increase has been extensively debated and has been put down to two main possibilities: regrowth or enhanced growth. The initial idea was to attribute the extra sink to carbon dioxide fertilization, an increase or enhancement in forest growth stimulated by the extra carbon dioxide in the atmosphere, making the trees bigger. This makes intuitive good sense since CO2 is a key ingredient in photosynthesis and it has long been known that growth can be stimulated in glasshouses by pumping in extra CO2. Many experiments were carried out under field conditions to see if the same thing happened in the wild. It is now clear that while some trees and forests show increased growth this tends to be a short-term increase and within a few years, growth rates have fallen back to previous levels. The poor fertilization effect is usually because growth is limited by other environmental factors such as increasing temperatures and drought causing stress, and low nitrogen availability. As discussed in Chapter 8, nitrogen is often in short supply and, at least in temperate forests, is the main factor limiting growth. Carbon dioxide fertilization may initially lock up extra carbon in new wood and in the soil, but it will also lock up more nitrogen causing a further shortage and reduced growth. In the large areas of forests where nitrogen is an abundant pollutant (see Section 11.4.3 below) this problem should be removed but studies in eastern North America and Europe have shown that, despite extra growth, <10% extra carbon is sequestered in woody tissues (Nadelhoffer et al., 1999). Since leaves, twigs and fine roots are short-lived and then decompose, it is the carbon in woody tissue with a longer turnover time that is of value. Moreover, excessive nitrogen can lead to decline in forest growth and even wholesale death (see Section 11.4.3).

In conclusion, it seems that global forests are sequestering carbon not so much because of enhanced growth but due to regrowth (although the debate is not completely settled - e.g. Houghton, 2003). The net carbon sink (sequestration) by forests appear to be primarily due to afforestation (planting trees on new ground), reforestation (replanting former forests) and, most importantly, the recovery of existing forests from historical land uses (i.e. the forests are still growing back to maturity and so have a positive net ecosystem productivity). In 1996 the Intergovernmental Panel on Climate Change (IPCC) suggested that 700 million hectares of forestland might be available for sequestration globally, due to slowed deforestation (138 Mha), regeneration of tropical forests (217 Mha) and plantations and agroforestry (345 Mha). They suggest that over the period until 2050, sequestration rates could reach a maximum of 2.2 gigatonnes (Gt), compared to the net increase in atmospheric carbon of 3.3 Gt per year during the 1990s. Beedlow et al. (2004) provide a good readable overview of this subject and underlines that we must protect existing forests if we are to maintain the forest carbon pools and perhaps even sequester more carbon. Gains can also be made by using woody material, particularly forest thinnings, as biofuel to replace fossil fuels. Short-rotation coppice (SRC) based on poplar or willow varieties cut every 1-3 years could also be employed though its productivity is not that high. In Europe it is estimated that the total carbon pool (above and below ground) in a mixed hardwood forest after 150 years is in the order of 3241 ha-1 y-1 compared with 1621 ha-1 y-1 in a SRC. However, when carbon savings from not using fossil fuels are taken into account, the effective annual sequestration of carbon in SRC is 24-29 tonnes of CO2 per hectare compared to 6-71 CO2 ha-1 in mixed forest (Deckmyn et al., 2004). Growing enough biofuel to be

Box 11.1 Chronic Nitrogen Amendment Study at Harvard Forest, USA

Starting in spring 1988, a series oflong-term experiments was set up as part of the Harvard Forest Long-Term Ecological Research (LTER) project. Two adjacent stands were used: an even-aged red pine Pinus resinosa stand planted in 1926 and a 50-year-old mixed hardwood stand. The hardwood stand was dominated by black oak Quercus velutina and red oak Q. rubra with black birch Betula lenta, red maple Acer rubrum and American beech Fagus grandifolia. Within each, three main plots were established: control (with no extra nitrogen added), low N (50 kg of N added ha-1 y-1) and high N (150 kg N ha-1 y-1). An additional plot of low nitrogen with added sulphur was used for a few years but proved similar to just low nitrogen so the sulphur treatment was abandoned. The background rate of nitrogen deposition is 7-8 kg ha-1 y-1 (moderate by northeastern standards) so the additions of ammonium nitrate fertilizer (NH4 NO3) correspond to approximately 6 and 18 times the current background rate. Bear in mind that the natural deposition level, without any pollution, is around 1kg ha-1 y-1.

The two stands reacted to the nitrogen in different ways. After 14 years, the mortality of the pine trees (calculated as the fraction of biomass in dead trees) rose with nitrogen levels (control 12%; low N 23%; high N 56%) and was associated with reductions in foliage and wood production, leaf area (see photographs accompanying this box) and photosynthetic capacity. Complete mortality in the high N stand is likely in the near future. In the hardwood stands, however, mortality was lowest on low N treatment (control 27%; low N 19%; high N 49%) and growth of the surviving trees on the high N plots was 30% higher than on the control over 14 years and on the low N plots was 11% higher. Polyamine production (particularly putrescine) is known to increase in cells subjected to abiotic stress (such as low pH, nutrient stress, low temperatures). In the pine plots putrescine level in the foliage was higher in both nitrogen treatments as compared with the control plot. In the hardwood stand, it was higher only in the high N treatment (Minocha et al., 2000), giving additional evidence that the pine stand was under more stress. However, the high N input is having an effect in the hardwood stands because all the white birch, 73% of the red maple and 58% of the black birch had died by 2002, suggesting environmental stress. This strongly suggests that while any addition of nitrogen has been harmful to the pines, the hardwoods benefited from low N addition because the initial site conditions were limiting to growth. This was mirrored in above-ground production. In the pine stands the mean annual wood production was highest in the control and decreased 31% and 54% relative to the control for the low N and high N plots, respectively.

Losses of inorganic forms of nitrogen were high in the high N plots (higher in pines than hardwoods) but dissolved organic nitrogen (DON - see Section 8.4.2) showed no changes over time. This may well be due to inorganic nitrogen being

Forstkiste

Box 11.1 Chronic Nitrogen Amendment Study at Harvard Forest, USA. Chronic nitrogen plots in red pine (top to bottom): high nitrogen, low nitrogen, control. (Photographs taken December 2004 by Peter A. Thomas.)

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