Читать книгу Forest Ecology - Dan Binkley - Страница 41

Most forest organisms have distinct cycles through seasons and years. Plants germinate, grow, blossom, and produce seeds. Temperatures are fundamentally important in the timing of these aspects of physiology (called “phenology”). Buds on spruce trees in Sweden may withstand winter temperatures of −40 °C, and then buds break and new needles form in the spring. At a site in northern Sweden, the average date of budbreak is June 3 (Figure 2.17), but budbreak may occur a couple weeks sooner in warm springs, and a couple weeks later in cold springs. Budbreak depends more on temperatures than on the days on a calendar, and a common way to gauge the overall warmth of a season is with growing degree days. The idea of growing degree days is that the biological response to several days of moderate temperatures might equal the response from only one or two days of high temperatures. The cumulative tally of warmth first requires a baseline temperature, such as 5 °C for an average daily temperature, which is considered too cold to lead to a biological response such as budbreak. The average daily temperatures that are higher than 5 °C contribute to the tally. Three days of 10 °C would add 15 (=3 × 5) growing degree days, and three days at 15 °C would add 30 growing degree days. The timing of budbreak of spruce trees in Sweden shows much less variation from year to year when examined in terms of the sum of growing degree days than when based on calendar days.

Cold temperatures slow the growth of plants, and many animals too (especially those whose body temperatures reflects surrounding environmental temperatures). Chemical reactions are slow in the cold, and cold locations have shorter growing seasons that also limit growth. It's easy to imagine that trees in a northern boreal forest would likely grow more slowly than trees in a temperate or tropical climate, but the effects of temperature are also large across smaller gradients. Tropical climates are generally warm throughout the year, but of course some locations are warmer than others. Across the Tropics, an increase of average annual temperature from 20 to 22 °C is associated with a 22% increase in wood production (Figure 2.18; revisiting one of the studies used in the Preface to discuss statistics). This is a very sensitive response compared to differences in rainfall. For example, increasing rainfall from 500 mm yr⁻¹ (a dry tropical forest) to 2000 mm yr⁻¹ (a tropical rain forest) is associated with only a 14% increase in growth if temperatures are the same. The value of extra water for forest growth increases substantially, though, with increasing temperatures. This description of the pattern in Figure 2.18 implies that because the trend in forest growth is associated with the amount of precipitation that the difference is driven by precipitation. In fact, correlation patterns should not be assumed to show cause and effect; and implication of a causal connection needs to be done carefully (as noted in the Preface). Is it possible that the soils differ substantially along with precipitation, and that soil differences are the actual causes of the pattern in growth? Or is it more likely that rainfall drives the pattern in growth?

The influence of temperature on forest growth may be even stronger on plantations where intensive silviculture reduces the influence of other factors that might limit growth, such as competing understory vegetation, low nutrient supplies, and genetics that do not optimize growth. Eucalyptus plantations in Brazil are managed on short rotations, and high rates of growth (trees may exceed 30 m height in six years) achieve forest volumes that would take decades in other regions (Chapter 7).