PerspectiveClimate

Drylands in the Earth System

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Science  22 Jan 2010:
Vol. 327, Issue 5964, pp. 418-419
DOI: 10.1126/science.1184946

Arid regions (or drylands) cover about 45% of Earth's land surface; in most classifications of ecosystem types, they constitute the largest biome on the planet. Yet the global change literature is dominated by other ecosystems, particularly the humid tropics, with high deforestation rates and high biodiversity levels, and the Arctic regions, with high rates of warming and huge stocks of vulnerable carbon. Drylands are less studied because they seem to have low rates of biological activity and sparse biota. On page 451 of this issue, Rotenberg and Yakir (1) present evidence that contradicts this received wisdom. The dryland Yatir Forest in Israel takes up carbon at rates similar to those of pine forests in continental Europe.

For the past decade, Yakir and co-workers have studied carbon, water, and energy exchange in one of the world's driest forests. Rotenberg and Yakir now analyze how the Yatir Forest maintains productivity despite severe temperature and water stress. They argue that an adjustment of forest metabolism to ambient conditions reduces the impact of climate on carbon flux. Yatir's net carbon uptake [2.3 metric tons per hectare (t/ha)] is slightly higher than that of the average European pine forest (2 t/ha) and only slightly lower than the mean for all pine forests globally (2.5 t/ha).

How does a forest growing in a hot, dry environment sustain such high rates of carbon uptake? Several mechanisms contribute to the high levels of activity. First, although photosynthesis rates in this system are moderate relative to the range of fluxes observed globally, respiration is low (possibly because low soil moisture inhibits decomposition), resulting in a carbon storage efficiency 60% higher than the average of global data.

A second explanation lies in the timing of biological activity. The rates of carbon exchange in the Yatir Forest peak early in the spring, when temperatures are far below their midsummer highs. The local vegetation is adapted to achieve peak photosynthesis rates at springtime temperatures around 14°C and to be relatively dormant during the midsummer highs of 25°C and above. A series of sites from high northern latitudes through southern Europe also had peak carbon exchange rates at similar temperatures (16° to 18°C) [data cited in (1)]. However, in most ecosystems, peak photosynthesis rates occur near maximal temperatures.

Peak rates of carbon uptake are a key control over annual uptake; the other key control is the length of the growing season (when the system gains carbon) relative to the dormant season (when the system loses carbon) (2). The displacement of peak growth to early spring in the Yatir Forest results in a growing season length similar to other coniferous forest ecosystems, also contributing to Yatir's carbon uptake.

Rotenberg and Yakir expose an important set of emergent controls over carbon metabolism globally. The work reinforces the need to conduct research in extreme and marginal environments to expand the scale over which processes are observed.

The Yatir Forest from space.

The dark color of the forest contrasts with the surrounding, desertified landscapes. The Yatir, which covers about 30 km2, warms its local environment by absorbing incoming solar radiation, whereas the surrounding bright desert landscapes reflect more of the incoming radiation to space. Today, the Yatir shows up as a green anomaly in a vast desert landscape, but in biblical times, this entire region was forested.

PHOTO CREDIT: GOOGLE MAPS

The authors extend their analysis to consider other pathways through which forests influence climate and to elucidate the role of drylands in the overall surface energy budget of Earth's land surface. Since Bonan et al.'s seminal 1992 study (3), it has been known that tree cover affects the local radiation balance, with important consequences for climate. Forests are dark and absorb incoming solar radiation, converting it into energy for photosynthesis and heat and thereby causing local warming. Bonan et al.'s computer simulation explored eliminating the dark boreal forest cover and thereby exposing the bright, highly reflective snow. The bright surface reflects more of the incoming sunlight, cooling the surface. Rotenberg and Yakir now report a real-world analog to this computer-enabled thought experiment.

Desertification exposes the bright soil surface, which reflects sunlight, much as in the boreal simulation. At the same time, increased convection over dryland forests such as the Yatir cools the surface, reducing outgoing thermal radiation but increasing the radiative forcing on the overlying regional atmosphere and likely increasing air temperatures aloft. Whereas desert surfaces are hotter than vegetated ones, the atmosphere overlying the desert cools with altitude more quickly and is cooler overall. Paradoxically, desertification has thus likely contributed local cooling to off-set the global warming from the carbon release that occurs when dryland forests are cleared.

Although modern humanity has a hard time realizing it, the climate system has never been unchanging. Human activities have long been a driver of change in the Earth system and will continue to be for the foreseeable future (4). Rotenberg and Yakir's study of the arid Yatir Forest shows how recent desertification has affected local temperatures and global climate. It also provides a perspective on how humans and the climate system have interacted over millennia.

More than 3000 years ago, at the dawn of the human modification of the Earth system, the Israelites entered Canaan (modern-day Lebanon, Israel, and the Palestinian territories) and were commanded by Joshua to “go up into the forest country and clear an area for yourselves there” [Joshua 17:15; see also (5)]. Those early settlers released carbon as they cleared forests, thereby changing the albedo, affecting the surface energy balance, and altering the local climate. The modern-day Yatir Forest was planted in 1964 by the Jewish National Fund and—as Rotenberg and Yakir document—has substantially modified the local climate. A global program of dryland reforestation may initially cause regional warming as these new forests modify the surface energy balance, but will pay dividends in the long term as these forests become substantial global carbon sinks.

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