Seasons As Well As Vegetation Distribution Are Changing

For a long time, naturalists and gardeners have recorded dates of flowering and leafing of the plants around them. These records happen to provide another interesting measure of responses to climate change.

In Europe, it is quite evident that the seasonal patterns in vegetation have been shifting in response to warmer temperatures. In Britain, for example, a long tradition of amateur natural history has ensured an abundance of information on the detailed distribution and behavior of plants, stretching back many decades. Temperatures in England are on average 1 °C warmer than they were 50 years ago: in response, certain wild flowers are now blooming about a week and a half earlier, and autumn leaf fall has been getting later.

For instance, a study of the long-term records of the respected botanist R.S.R. Fitter in England since the 1950s has shown that many plants (about 16% of a sample of 385 species) have been flowering several days later during the last decade, compared with their long-term average over the previous four decades. His study would probably have shown even more striking trends if the earlier data had been divided up into individual decades so the 1950s could be compared with the period since about 1995 when warming has been most dramatic. Not all plants in Fitter's study showed the same trend: about 3% of the sample actually flowered significantly later during the last decade, indicating the complexity and the diversity of plant responses to warming.

Another study across Europe by Annette Menzel and colleagues looked at the records of 542 plant species between 1971 and 2000. They found that 70% of these species had followed a trend towards earlier leafing out, flowering or fruiting during this period, and only 3% had followed the opposite trend in becoming later. On average, spring seemed to be getting earlier by about 2.5 days per decade. Autumn leaf fall was also starting later in many species (see below), also likely as a response to warming, and adding to the longer growing season.

There are similar indications of a trend towards earlier flowering in North America—for example, in the flowering times of cultivated trees and shrubs. One study compared flowering times of hundreds of plants growing at the Arnold Arboretum in Boston between 1980 and 2002, with old records of flowering times of the exact same individuals between 1900 and 1920. In the last two decades, the plants flowered 8 days earlier on average than they did back then. This seems to be a response to the warming of about 1.5°C in average temperature that shows up in the climate record for Boston. A similar trend has been noted amongst wild trees in the same general region: the time of maple sugar sap flow in northern New England has moved forward in the year by at least a week and a half since the 1960s.

Satellite studies that compare the timing of a "green wave" of emerging leaves in spring are finding evidence that the change in seasons is more dramatic than the more localized ground-based observations suggest. Franz Badeck and colleagues of Potsdam in Germany conclude that the satellites suggest a consistent trend all across the northern hemisphere's mid and high latitudes, with spring getting between 4 and 8 days earlier with each decade that has passed since the 1970s.

Across Europe, records kept by naturalists show that the leaf color change into autumn has become 0.3-1.6 days later each decade since the 1950s. In some areas the length of the total growing season between leafing out and leaf fall has increased by up to 18 days over the past 50 years. Satellite images of the timing of autumn leaf color change seem to confirm that autumn has been getting later over the past two decades.

Because climate can be fairly fickle from one year to the next, a colder-than-average spring nowadays can still be later than a warm spring in the 1940s or 1950s. But it is the long-term average trend that we should pay attention to, and in terms of averages there is no doubt that the length of the growing season has increased during that time.

There are various other examples of ongoing change in vegetation seasonality that seem to be a result of warming in the mid and high latitudes. It isn't occurring everywhere, and where it is occurring it often fluctuates and even temporarily reverses for a few years. But the predominant pattern in vegetation looks like a response to increasingly warm climates.

One must also bear in mind the possibility that part of this change in seasonal patterns in vegetation might be due to the increasing direct CO2 fertilization effect (see Chapter 8). Conceivably, it might in some ways tend to mimic the effects of temperature increase, with earlier leafing out and later leaf fall. On the other hand, one experiment on the seasonal growth patterns of grasses under increased CO2 showed that when they were CO2-fertilized some species flowered later in the season, not earlier! If this is more generally true of plants, climate change might be pushing them in one direction (earlier flowering) while increasing CO2 is pushing them in another (later flowering).

Another important thing to consider is that part of the trend we see might be due to the fact that cities have got bigger over time. Bigger cities have in themselves

Figure 3.12. The heat-trapping effect of large cities alters the time of arrival of spring. In Tokyo, Japan, cherry trees flower earlier in the warmer central districts than on the outskirts. Contours mark differing average flowering times in spring.

Figure 3.12. The heat-trapping effect of large cities alters the time of arrival of spring. In Tokyo, Japan, cherry trees flower earlier in the warmer central districts than on the outskirts. Contours mark differing average flowering times in spring.

tended to pour out and trap more heat, producing their own local climates (this is known as the "urban heat island effect"), and plants that grow within urban areas will be affected by this. In some big cities, such as Tokyo (Figure 3.12), flowering times of trees are more than a week earlier in the center than in the outskirts. Surely, if cities have grown then the warming effect on local climate will have grown too, producing a change in the seasonal rhythms. However, even far out in the countryside, away from any growing city, the trend in vegetation over the last few decades seems to be much the same. This implies that the changes in plant seasonality are not solely due to this "heat island" effect.

Trends in seasonal timing observed in wild vegetation and in cultivated plants are in a sense reassuring. They show that plants are flexible enough in their biology to respond to climate change fairly rapidly, at least up to some point. In some areas of the world, observations of changes also indirectly bolster the evidence that global warming is really occurring, because few long-term climate station data exist there. The plants are in effect acting as weather stations, helping to show up what appears to be a global warming trend!

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