Energy flow through forest ecosystems

Another way of looking at the dynamics of a forest is to investigate how energy flows through the system. This can give important insight into how a forest works as energy is always limiting and places stringent limits on the number of plants and animals a forest area can support. As discussed in Chapter 1, nutrients are cycled through an ecosystem while energy passes through, entering as sunlight and leaving as heat from respiration. Due to the complexity of forests, there have been few studies complete enough to give a good overview of energy flow through a forest.

One of the best examples comes from a study conducted at Hubbard Brook by James Gosz et al. (1978). In a typical year at this latitude in New Hampshire, the total energy input was measured at 1 254 000 kilocalories per square metre (kcal m-2). Of this, just 0.8% was 'fixed' by photosynthesis (gross primary production - GPP), of which 55% was used in respiration leaving 0.4% of the sun's arriving energy to be used for producing new plant growth (net primary production - NPP). This may seem very inefficient but if only the 4-month growing season (June to September) is considered, solar radiation was 480 000 kcal m-2, of which 10 400 kcal m-2 were fixed by photosynthesis in GPP (about 2%) which after respiration left a NPP of 4680 kcal m-2 or about 1% of incoming energy. This figure is a typical conversion rate of the sun's energy to NPP. Although only around 2% of the sun's energy is used in photosynthesis the rest is not wasted since another 42% is used in the growing season to heat up the environment and another 42% is used to evaporate water in transpiration, an essential element in a tree's growth. The remaining 14% of sunlight is reflected back to the sky.

Of the NPP (4680 kcal m-2) in a year, 26% (1199 kcal m-2) was stored in new growth. The remaining 74% went to tissues subsequently dealt with by decomposers with 65% (3037 kcal m-2) and 9% (437 kcal m-2) dying above and below ground, respectively. In most years less than an additional 1% (41 kcal m-2) was eaten by herbivores although this can rise to as much as 44% when, for example, defoliating caterpillars peak in numbers and strip the trees of leaves, and a good deal of what would otherwise fall as litter is eaten by the

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ingestion '

HIGH-DENSITY CATERPILLARS

SALAMANDERS

SHREWS

SALAMANDERS

SHREWS

Figure 8.5 Energy budgets for three of the main consumer organisms in the hardwood forests of the Hubbard Brook Experimental Forest of New Hampshire. The sizes of the strips through the boxes are proportional to the amount of energy passing through each process in kcal m~2 of ground in the forest. Energy ingested in the form of organic matter is either assimilated into the organism or lost in material egested from the body. This energy is then used for either respiration or put into new tissue through growth or reproduction. Much of the energy taken in by caterpillars is not absorbed but remains in the material that they egest as faeces. Shrews being 'warmblooded' use much of their energy in respiration (in maintaining their high body temperature) while the 'cold-blooded' salamanders put proportionately more energy into growth and reproduction. (Redrawn from data in Gosz et al., 1978. Scientific American 238.)

insects. When it is considered that of the 1% of energy transferred to herbivores normally only 1 — 10% of that (0.1 — 1% of NPP) is transferred to carnivores it can be appreciated why most food chains are fairly short. Figure 8.5 shows what happens to the energy consumed by different animals and reveals the great differences in energy use. Caterpillars can consume huge amounts of foliage but they are inefficient digesters and a large amount of the contained energy is lost in the faeces (egested) and, in the example given, only 14% of the energy ingested is assimilated; of that around 60% is used in respiration and 40% is incorporated into new tissue. Salamanders and shrews are important carnivores in the Hubbard Brook forests and show different strategies of energy use. The shrews being 'warm-blooded' (more accurately, homoeotherm-ic: keeping a high constant temperature) and active through the year are efficient at extracting energy from their food (90%) but use roughly 98% of their assimilated energy for respiration and only about 2% for growth and reproduction. By contrast, salamanders consume only about a sixth of the energy of a shrew and are almost as good as shrews at extracting energy from it (81%) but being 'cold-blooded' (or rather poikilothermic: with a body temperature tending to follow that of the surrounding environment) they use less energy to keep warm and around 60% of their assimilated energy goes on growth and reproduction. Salamanders are thus very good at transforming energy into increased biomass (see Box 7.5).

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