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Forest Responses to Global Warming
Volume 6, Number 7: 12 February 2003

You've all heard the claim, again and again and again: CO2-induced global warming will be so great and so rapid that many of earth's trees will not be able to migrate towards cooler regions (poleward in latitude or upward in altitude) rapidly enough to avoid extinction (Woodwell, 1989; Overpeck et al., 1991; Dobson, 1992; Root and Schneider, 1993; Dyer, 1995). This prediction is based on the assumption that tree growth rates rise from zero at the cold limit of their natural ranges (their northern boundaries in the Northern Hemisphere) to a broad maximum, after which they decline to zero at the warm limits of their natural ranges (their southern boundaries in the Northern Hemisphere). Loehle (1998), however, convincingly demonstrates that this assumption is only half correct: it properly describes tree growth dynamics near a Northern Hemispheric forest's northern boundary, but it is an inaccurate representation of tree growth dynamics near the forest's southern boundary.

Loehle notes, for example, that in the Northern Hemisphere - to which we will restrict our discussion for purposes of simplicity - trees planted north of their natural ranges' northern boundaries are only able to grow to maturity within 50-100 miles of those boundaries. Trees planted south of their natural ranges' southern boundaries, however, often grow to maturity as much as 1000 miles further south (Dressler, 1954; Woodward, 1987, 1988). In fact, Loehle reports that "many alpine and arctic plants are extremely tolerant of high temperatures, and in general one cannot distinguish between arctic, temperate, and tropical-moist-habitat types on the basis of heat tolerances, with all three types showing damage at 44-52C (Gauslaa, 1984; Lange and Lange, 1963; Levitt, 1980; Kappen, 1981)."

What Loehle finds from his review of the literature and his own experience with U.S. trees, is that as temperatures and growing degree days rise from very low levels, boreal tree growth rates at some point begin to rise from zero and continue increasing until they either plateau out at some maximum value or drop only very slowly thereafter as temperatures rise still higher and growing degree days continue to accumulate. Trees from the Midwest, by comparison, do not begin to grow until a higher temperature or greater accumulation of growing degree days is reached; but their growth rates rise much higher than those of the colder-adapted boreal species before they too either level out or begin to decline ever so slowly. Last of all, southern species do not begin to grow until even higher temperatures or growing degree day sums are reached, after which their growth rates rise highest of all before leveling out and exhibiting essentially no decline thereafter.

In light of these observations, it can readily be appreciated that although the northern range limit of a particular tree species is indeed determined by growth-retarding cool growing seasons and frost damage, the southern boundary of a tree's natural range is not determined by temperature per se, but by competition between the northern species and more southerly-adapted species that have inherently greater growth rates.

Whenever significant long-term warming occurs, therefore, earth's coldest-adapted trees are presented with an opportunity to rapidly extend their ranges northward in the Northern Hemisphere. Individual trees at the southern limits of their ranges, however, feel no need to go anywhere. As time progresses, they may at some point begin to experience pressure from some of the "stronger" nearest-neighbor southern species trying to invade their territory; but this potential challenge is by no means assured of quick success. As Loehle describes it:

Seedlings of these southern species will not gain much competitive advantage from faster growth in the face of existing stands of northern species, because the existing adult trees have such an advantage due to light interception. Southern types must wait for gap replacement, disturbances, or stand break up to utilize their faster growth to gain a position in the stand. Thus the replacement of species will be delayed at least until the existing trees die, which can be hundreds of years... Furthermore, the faster growing southern species will be initially rare and must spread, perhaps across considerable distances or from initially scattered localities. Thus, the replacement of forest (southern types replacing northern types) will be an inherently slow process (several to many hundreds of years).

In summing up the significance of this situation, Loehle says that "forests will not suffer catastrophic dieback due to increased temperatures but will rather be replaced gradually by faster growing types."

Another possibility is that northern or high-altitude forests will not be replaced at all by southern or low-altitude forests. Rather, they may merge, creating new types of forests of greater species diversity, such as those of the warmer Tertiary, when in the western U.S. many montane taxa regularly grew among mixed conifers and broadleaf schlerophylls (Axelrod 1994a, 1944b, 1956, 1987), creating what could well be called super forest ecosystems, which Axelrod (1988) has described as "much richer than any that exist today." Possibly helping warmer temperatures to produce this phenomenon during the Tertiary were the higher atmospheric CO2 concentrations of that period (Volk, 1987), as suggested by Idso (1989); and that condition is something that is also expected to occur in the future.

In conclusion, these real-world observations make it clear that any global warming that might occur in the future will not create an irresistible impetus for the warm-limit boundaries of forests to move poleward or upward. In addition, a further increase in the air's CO2 content, which is almost assured, would prevent such a phenomenon from occurring anyway; for as noted in the title of the paper of Taub et al. (2000), "growth in elevated CO2 protects photosynthesis against high-temperature damage." In fact, not only does elevated CO2 protect plants at high temperatures, it actually makes them prefer warmer environments; for as Idso and Idso (1994) demonstrated in their massive review of the literature, and as has been discussed by Long (1991) and Cannell and Thornley (1998), the optimum temperature for plant growth typically rises by several degrees C in response to a 300-ppm increase in the air's CO2 concentration, as does the temperature at which death is induced (Idso et al., 1995).

Truly, the climate-alarmists have got things 180 degrees out of phase with reality on this one. Higher temperatures, especially if accompanied by higher atmospheric CO2 concentrations, will not only not reduce the species richness of earth's forests, they will enhance it.

Sherwood, Keith and Craig Idso

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