Читать книгу Forest Ecology - Dan Binkley - Страница 22

Over the course of a day, the tulip poplar responds to the changing environment through a daily cycle just as strongly as an animal would. The uptake of CO₂ (and loss of water) begins as the sky brightens across the hillside in the morning, increasing as the intensity of sunlight increases (Figure 1.2). Rates may be highest near noon, decreasing if clouds develop, or if the air becomes so dry that the tree tightens the stomata to avoid losing too much water. Increasing temperatures in the afternoon drive up the capacity of air to hold water, resulting in a climb in the vapor pressure deficit. This deficit is a key force driving the water use by the tree. The tulip poplar would produce about 250 g of sugar on a sunny summer day (more than the average mentioned above for all days of the growing season), when the soil was moist, and transpiration could total 70 liters of water.

Not all processes in the tree shut down when the sun sets. Chemical reactions inside cells continue to renew thousands of biochemicals, generating and expanding new cells, and actively absorbing nutrient ions (such as nitrate and phosphate) from the soil. All of these processes require energy, most of which is supplied directly or indirectly from the sugars formed by photosynthesis. The oxidation of the sugar leads to substantial release of CO₂ from the tree; this “respiration” in all the tissues of a tree may equal half of the total photosynthesis that occurs on a sunny day.

FIGURE 1.2 The daily pattern of incoming sunlight (A) reflects the geometry of the Earth's tilt, the aspect and slope of a hillside, and the passing of clouds through the day. Temperature patterns (B) are driven in part by incoming sunlight, moderated by winds and evaporation of water (which cools the air). The combination of temperature patterns determines the capacity of air to hold water, and the vapor pressure deficit (C) tracks the difference between the current humidity of the air and the saturation point of the air. All these factors influence the rate of water use by the tulip poplar (D), though the connection to vapor pressure deficit is the most direct.

Source: Data from Chelcy Miniat.