Carbon Balance & Shade Tolerance

Front 1

Ecophysiology

Back 1

"experimental science that seeks to describe the physiological mechanisms that underlie ecological observations"

--consider the physiological context of shade tolerance. Because of differences in shade tolerance between species, changes in available light largely drive forest succession

Front 2

Photosynthetic capacity

Back 2

peaks when the leaves reach full size

begins to decline soon afterwards

Front 3

relationship between leaf longevity and leaf nitrogen content per unit dry mass

Back 3

Negative

Although evergreen leaves, being usually somewhat thick or succulent, may have a higher cost of formation per unit area than deciduous leaves, they last longer.

Front 4

What season is best for photosynthesis

Back 4

Spring is likely best for photosynthesis because of the longer days and more intense light (the sun is higher in the sky) than in the autumn.

Spring ephemerals and other understory species (like salmon berry that leafs out before the red alder) can also accomplish a great deal at this time.Tree seedlings may also benefit at this time because they tend to break bud a bit faster (i.e.,they have a lower heat sum) than do older trees.

Front 5

Dark respiration

Back 5

linked to photosynthetic activity in that it is highest just after dusk and lowest just before dawn.

reflects the availability of respiratory substrates

Front 6

maintenance respiration

Back 6

--Ignoring diurnal trends and the effects of temperature and stress, respiration rate remains fairly constant from day to day

-- reflects the energy requirement needed to keep things in order.

Front 7

Growth Respiration

Back 7

during spring flush respiration rates may be 3-10 times higher, and photosynthetic capacity is very low in expanding leaves. The extra respiration is in support of growth

---> Young plants and tissues in general have relatively high rates of respiration. Respiration rates are also higher during leaf senescence in deciduous trees.

Front 8

Respiration and tissue types

Back 8

Flowers and fruitshave high rates of respiration. In fact, much of the respiration in climacteric fruits is by way of the cyanide-resistant alternative path. Roots respire more actively than stems but not more than leaves. In branches and tree trunks most respiration isconfined to the living bark, the cambium and the sapwood

Front 9

Summer respiration

Back 9

respiration rates average about 10% of the gross photosynthetic rate [note: gross photosynthesis -respiration = net photosynthesis]

But respiration continues at night and, unlike photosynthesis, it occurs throughout the plant. There is also some respiration in winter, under the snow. Consequently, total respiratory losses amount to 25-50% of fixed carbon.

Front 10

The influence of temperature on respiration and photosynthesis

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Rates of photosynthesis and respiration (and growth) increase as temperature increases, until reaching an optimum range. At higher temperatures, rates of both processes decrease rapidly, but this happens at a lower temperature for photosynthesis than for respiration. Hence, net photosynthesis has a more narrow temperature optimum

Front 11

heat limit

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Prolonged high temperatures above the heat limitfor net photosynthesis can reduce growth by stimulating respiration too much.

Front 12

The influence of temperature onCO2 and O2

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the solubilities of CO2and O2 both decrease with temperature, there is a greater effect on CO2

increased temperatures promote photorespiration by increasing the ratio of dissolved O2relative to CO2

CO2 enrichment increases the thermo-tolerance of net carbon assimilation

Front 13

The influence of temperature on species

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---Tropical species have higher temperature optimal for photosynthesis than temperate species. ---Temperate and boreal species can continue to photosynthesize at considerably lower temperatures than tropical species

---Within limits, plants can acclimate to new temperature optima as appropriate

---Evergreens that photosynthesize in both summer and winter may show pronounced shifts in temperature optima