Leaf Shedding and Hydraulic Architecture

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In dry tropical forests some trees may have access to deep water sources and can maintain their water-use during drought. These trees do not have a great seasonal variation in their leaf fall (Meinzer et al. 1999). In other trees leaf shedding is an avoidance strategy in terms of the biological stress concept and like for savannas (Sect. 10.1.2.1) phenological cycles play an important role. Many dry tropical forest trees shed leaves at the onset of the dry season. A reduction of hydraulic conductance of the leaves precedes senescence and possibly causes senescence (Sobrado 1993; Brodribb and Holbrook 2003). Trees may flush new leaves before the onset of the rainy season protecting the young growth from herbivory (see Sect. 3.4.4.3)

by insects which emerge with the rains (Murali and Sukumar 1993). However, this is an overall somewhat simplified picture. In fact, there is a large range of phe-nological behaviours. A co-occurrence of tree species with phenological patterns ranging from deciduous to completely evergreen is often observed, and one can find species that are leafless for as much as 6 months standing near to ones that retain full crown foliage despite the near absence of rainfall for many months (Brodribb and Holbrook 2005; Diaz and Grandillo 2005). Leaves may be shed throughout the year and trees shed and flush leaves almost every month in a pattern associated with sporadic rainfall events (Diaz and Grandillo 2005). The phenological variability of leafless and fully leaved trees occurring side by side is not necessarily due to different rooting depths but can be explained by differences in the capability to use small episodic rainfall events especially with water wetting the leaf surfaces rather than coming from the soil (Diaz and Grandillo 2005).

Hydraulic architecture needs to be adapted. By shedding leaves drought-deciduous species avoid significant plant water loss during the driest and hottest months but they must cope with larger seasonal water potential fluctuations in their leaves and require a higher water transport efficiency which leads to seasonal occurrence of xylem embolisms (Sobrado 1993). The principle differences seen when deciduous dry rainforest trees are compared with evergreen ones are the following:

• deciduous species have larger xylem vessel diameters,

• therefore, deciduous species have lower wood density,

• deciduous trees are more vulnerable to xylem vessel cavitation or embolism blocking water transport,

• deciduous species have a higher water storage capacity.

These correlations can be found to be borne out in nature (Sobrado 1993; Choat et al. 2005). However, on the other hand there is also an overwhelming functional diversity and variation in hydraulic strategies, such as stem and leaf specific conductance and vulnerability to embolism, among dry forest species co-occurring side by side (Brodribb et al. 2002,2003). In some deciduous species drought-induced embolism is avoided prior to leaf shedding, whereas in others leaf shedding and xylem embolism are closely linked (Brodribb et al. 2002), where leaf hydraulic conductance and water potential are not correlated with leaf life spans (Brodribb and Holbrook 2005). Xylem cavitation of leaf veins can elicit a feed forward signal to stomata causing stomatal closing responses that may lead to depression of gas exchange (Brodribb and Holbrook 2004) and are one of the reasons for a midday-depression (see Sects. 5.2.2.1 and 10.1.2.3).

Among various other physiological reactions a high emission of isoprene gas has been noted, which may be linearly related to irradiance up to 2,500 | mol m-2 s-1 and constitute considerable parts of the entire carbon budget of the plants (Lerdau and Keller 1997). It is under the control of the endogenous circadian biological clock (Wilkinson et al. 2006). As a hydrophobic gas this has been discussed in relation to reduction of evapotranspiratory water loss. However, isoprene my also function as an antioxidant (Penuelas et al. 2005; Affek and Yakir 2002) protecting against excessive light and damage by heat and reducing oxidative stress (Sect. 4.1.2) (Sharkey and Yeh 2001).

In nutrient poor habitats of dry forests low soil water may amplify nutritional problems because more than actual nutrient availability water may control uptake of nutrients (Rentería et al. 2005). Before leaves are shed nutrients such as nitrogen are remobilized in the senescing leaves and stored in twigs, the N-resources of which may provide some N for later reconstruction of the canopy (Sobrado 1995). RuBISCO activity may be reduced by water stress (Parry et al. 2002).

5.2.2 Ecophysiological Responses of Plants with

C3-Photosynthesis and Crassulacean Acid Metabolism (CAM)

In their reactions to the interacting stress effects of drought and high irradiance the plants must optimise responses to the particular limitations given. This may lead to disadvantages; for example closing stomata at low water availability and high irradiance reduces water loss but also causes increased heating; CO2-uptake is prevented and this leads to the dangers of photoinhibition (Sect. 4.1.7). C3- and CAM-plants may respond by stomatal closure in the middle of the days when challenged by high irradiance, heating and transpiratory loss of water peaks, but this has very different implications in both modes of photosynthesis.

5.2.2.1 The Midday Depression of C3-Plants

The C3-bromeliad Pitcairnia integrifolia grows in the thornbush-forest of Trinidad and smaller adjacent islands. Its performance on a clear and very hot day demonstrates the implications of the strategy of midday stomatal closure in C3-photosyn-thesis (Luttge et al. 1986; Fig. 5.3). Photosynthetic CO2-uptake rose after dawn as light-intensity increased and reached the highest rate at about 09.00 h. During this time temperature increased from about 230C to about 360C, but leaf temperature remained very close to air temperature. Beyond that point stomata began to shut and had fully closed by noon, when leaf conductivity to water vapour, gH2O, was zero (not shown in Fig. 5.3). At this time, and until about 15.00 h irradiance had attained its highest level around 2,000 |mol photons m-2 s-1 and leaf temperature now increased much above air temperature with the highest value close to 52 0C and almost 8 0C higher than air temperature. If inhibition of CO2-uptake was only due to stomatal closure, one would have expected intercellular CO2-concentration () to have remained at low levels during this period. However, pCo rose and this shows that there were likely to be photoinhibitory responses occurring as well as the well documented change in carboxylation efficiency at this time. Later in the afternoon, when irradiance and temperatures declined again, stomata re-opened and pCo dropped, but CO2-uptake only reached less than a third of the rate attained in the morning. Hence, strain during the hottest time of the day was only partially elastic and had a strong plastic component. Only during the subsequent night water

Fig. 5.3 Daily course of CO2 uptake (JCO2), intercellular CO2 concentration (pco ), leaf temperature (Tl), air temperature (TA) and solar irradiance (L) for a plant of Pitcairnia integrifolia in Trinidad and photograph of a plant with the head of the porometer attached, which was used for measurements. (Lüttge et al. 1986)

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