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Keywords:

  • leaves;
  • stems;
  • LMA;
  • sapwood density;
  • disturbance response;
  • plant ecological strategies

Drought can result in loss of leaves and desiccation of roots and twigs. Whole plants may die in severe drought. Plants may manage losses, for instance by shedding leaves as drought progresses (Orshan, 1954). Drought deciduousness is conspicuous where soil moisture availability is distinctly seasonal, such as in forests and savannas of wet-dry tropics, mediterranean shrublands, annual grasslands, and some temperate deserts (Nilsen & Muller, 1981; Comstock et al., 1988; Borchert, 1994; Williams et al., 1997). This leaf loss occurs to different degrees between species, and covers the range from evergreen to fully deciduous (Williams et al., 1997). In cases where drought is less predictable but still recurrent, how might plants respond to soil moisture deficits that cause significant damage and threaten plant survival? Here we present a conceptual model of drought damage and recovery patterns with observations of leaf loss, stem damage and regrowth after severe drought in a glasshouse for 19 dicotyledonous species from semiarid south-eastern Australia. We then examine evidence and ideas from the literature supporting the processes we propose. Because the observations were not designed to test the model, they should be regarded as illustrating it rather than as rigorously testing it.

A conceptual model for patterns of drought damage and regrowth

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

We propose that following severe drought, the observed pattern of loss, damage and regrowth is the outcome of two processes in opposite directions, leaf loss proceeding from base to apex and stem damage from apex to base (Fig. 1). Depending upon the rates of these two processes, qualitatively different patterns can be observed. In species with high rates of leaf loss compared to stem damage, basal leaf loss is observed. Conversely, in species with low rates of leaf loss and comparatively high stem damage rates, apical stem damage is observed. In species where the two rates are coordinated each of basal leaf loss and apical stem damage may be seen.

image

Figure 1. Conceptual model of drought damage and regrowth patterns. Leaf loss progresses from base to apex, faster in low leaf mass per area (LMA) species. Stem damage progresses from apex to base, faster in low sapwood density species. Depending on species’ combination of these traits different patterns of damage and regrowth can occur. When rate of leaf loss is high relative to stem damage, basal leaf loss is seen. When rate of stem damage is high relative to leaf loss, apical damage is seen. Live tissues heavily shaded, dead tissues lightly shaded. Triangle is live apical bud, circle indicates dead apical bud. New leaves are smaller ellipses.

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We propose that the rates of these two processes (leaf loss proceeding from base to apex and stem damage progressing apex to base) are related to traits reflecting structural investment and expected lifespan of leaves and stems: leaf mass per area (LMA) (Reich et al., 1992; Westoby et al., 2000) and sapwood density (Hacke et al., 2001a). Specifically, low LMA should be associated with high rates of leaf loss and evidence of basal leaf loss. Low sapwood density should be associated with high rates of stem damage and evidence of apical damage. Because these processes are expected to proceed concomitantly, the values of LMA relative to sapwood density may be more closely linked to damage and regrowth patterns than either trait alone. Although we describe mechanisms, we do not wish to claim we have proven these links. Rather we hope that others will be motivated to test and improve the model.

Experimental drought

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Plants from 19 perennial dicotyledonous species from semiarid south-eastern Australia were raised from seed and grown in potting mix in 13 cm diameter pots (soil volume ∼1.3 l), watered daily and supplied ample nutrients. At the time of drought treatment, plants ranged in age from a couple of months to approx. 1 yr. The difference in age was partly a function of varied germination time and the age of plants was roughly proportional to expected lifespan, so the youngest plants were short-lived perennial Asteraceae.

Soil drought was imposed by weighing pots each day and adding water to c. half the mass that had been lost in the previous day. Plants were closely observed for signs of drought stress (combinations of colour change, wilting and leaf drop depending upon the species). When damage was incurred, plants were transferred to a different glasshouse for re-watering so as not to raise the humidity of the drought treatment glasshouse. Sample sizes for surviving plants were small and varied among species (surviving plants mean 4.7 per species, range 2–11, from total plants mean 8.7 per species, range 5–12; Table 1). Several species were likely to die by the time they started to show signs of drought stress, for example, Eutaxia microphylla, hence it was difficult to obtain drought-recovering plants.

Table 1.  Responses to severe glasshouse drought and plant traits for 19 species from semiarid south-east Australia. Nomenclature follows Harden (1990). Clipped plants did not experience drought
SpeciesFamilyB: Base-up leaf loss*E: Each of basal & apical loss*T: Total leaf lost*A: Apex- down loss*Drought mortality* (n)Sapwood density (D, mg mm−3)Leaf mass per area (LMA, mg mm−2)LMA-D#Proportion sprouting after clipping (n)
  • *

    Numbers of plants. # LMA relative to sapwood density as difference between standardised, log-transformed LMA and sapwood density, see text.

Rhagodia spinescensChenopodiaceae4   3 (7)0.870.069−0.800.0 (8)
Maireana pyramidataChenopodiaceae5   4 (9)0.840.093−0.130.0 (10)
Enchylaena tomentosaChenopodiaceae5   3 (8)0.950.060−1.640.5 (10)
Dodonaea viscosa cuneataSapindaceae1 1 4 (6)0.790.071−0.060.2 (10)
Olearia pimelioidesAsteraceae1 2 2 (5)0.910.053−1.480.5 (10)
Eutaxia microcephalaFabaceae12  8 (11)0.850.063−0.760.0 (10)
Vittadinia trilobaAsteraceae14  2 (7)0.770.024−1.410.5 (10)
Dodonaea viscosa spatulataSapindaceae211 4 (8)0.860.100−0.190.4 (10)
Atriplex semibaccataChenopodiaceae311 3 (8)0.840.072−0.460.3 (10)
Minuria leptophyllaAsteraceae416 1 (12)0.830.044−1.101.0 (10)
Calotis cuneifoliaAsteraceae133 1 (8)0.720.043−0.090.5 (10)
Senna artemisioidesFabaceae142 4 (11)0.920.150−0.100.6 (10)
Vittadinia cuneataAsteraceae 22 4 (8)0.610.042 1.040.3 (11)
Eucalyptus populneaMyrtaceae  7 1 (8)0.720.140 1.590.9 (10)
Einadia nutansChenopodiaceae  417 (12) 0.039 0.4 (11)
Brachyscome ciliarisAsteraceae  412 (7)0.590.040 1.220.9 (10)
Hakea tephrospermaProteaceae   29 (11)0.830.430 2.151.0 (10)
Casuarina cristataCasuarinaceae   39 (12)0.650.240 3.100.5 (10)
Bossiaea walkeriFabaceae   35 (8)0.860.120 0.070.7 (10)

Damage and regrowth responses

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

For plants that recovered, repeat observations were available of locations of leaf loss and leaves remaining, apical bud loss and position of new leaf growth. This was restricted to observations of primary stems. Where no single stem was clearly the main axis, a lineage from base to most distal stems was considered. Individual plants were classified into four groups, based on observations made during this study (see also Fig. 1).

Apical group (A): plants that had clearly lost the apical bud and apical leaves, but had leaves remaining at the base. These plants had to sprout from basal or axillary buds below the location of damage to the stem as revealed by loss of colour, turgor and resilience to bending.

Basal group (B): plants that had clearly lost leaves at the base of the stem, but either had not lost the apical bud or had not lost all apical leaves. These plants could recommence growth from the apical bud upon re-watering.

Each End group (E): plants that had lost leaves at base and apex of the stem, but had some leaves remaining subapically. These plants sprouted subapically, below the dead portion of stem, revealed by loss of colour, turgor and resilience to bending.

Total group (T): plants that survived total leaf loss, through basal and/or subapical sprouting.

Species were then classified according to the responses of individual plants. A broad definition (species were classified to either of Basal or Apical if any individual was observed displaying that response), and a narrow definition (a species was classified as Basal or Apical if it exhibited only that response) of response were used. Species that had plants with only Each End or Total loss could not be assigned to either Basal or Apical loss.

In six of 19 species, plants recovered either with only basal leaf loss (three species) or with only apical damage (three species) (Table 1). No species that included plants showing only basal leaf loss also included other plants that showed only apical damage. Similarly, no species that included plants recovering with only apical damage had other plants recovering with only basal leaf loss (Table 1). Several species that included plants with only basal leaf loss also had other plants recovering from damage at both ends (on the same plant) or from total leaf loss (Table 1).

So, in response to severe drought, species displayed consistent patterns of damage and regrowth after drought ranging from basal leaf loss and continued apical growth to apical stem damage and subapical or basal sprouting. Next we turn to the question of whether traits are related to these patterns.

Plant traits

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Two to five fully expanded leaves from upper parts of each of five individual plants that had not experienced drought were collected from each species for measurement of LMA. Leaves were scanned with an optical scanner and areas calculated using Delta-T Scan (Kirchhof & Pendar, 1993). After drying for at least 24 h at ∼65°C, dry masses were measured and LMA calculated as mass over area. For sapwood density of current season twigs, specific gravity was measured by displacement volume after removing periderm according to the protocol of Hacke et al. (2000). Because patterns of damage might be expected to result from the interaction of leaves and stems, we assessed the relative difference between LMA and sapwood density. For this, both variables were first log10 transformed and then standardised to distributions with mean zero and standard deviation one, inline image(eqn 1) (Sokal & Rohlf, 1995: p105). The LMA relative to density was then calculated as LMAstand– Dstand (eqn 2).

Figures presented here are based upon the broad definition of response groups, but with the narrowly defined groups highlighted (Fig. 2). Statistics are only presented for the narrowly defined groups, as these were considered most representative of the drought response patterns of interest. Trait values for the basal and apical groups were compared in preplanned contrasts (Sokal & Rohlf, 1995).

image

Figure 2. Trait values and their relation to location of damage or leaf loss due to experimental glasshouse drought for 19 semiarid south-eastern Australian species. Filled symbols are species that only had plants displaying that pattern of loss and regrowth, open symbols are species for which at least one plant showed that pattern.

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LMA and sapwood density in relation to drought damage and regrowth patterns

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Sapwood density (D) ranged roughly 1.5-fold and LMA ranged 20-fold among species (Table 1). Species that showed only apical damage tended to have higher LMA than species that had only basal leaf loss (t(16) = 2.724, P= 0.015, Fig. 2(a)). Sapwood density values for the apical damage group extended to lower values than the basal loss group although the two groups did not differ significantly (t(15) = 1.271, P= 0.223, Fig. 2(b)). LMA and sapwood density were uncorrelated (r = 0.15, P= 0.56, n= 18). The apical damage group had higher LMA leaves relative to sapwood density than did the basal loss group (t(15) = 3.024, P = 0.009; Fig. 2(c)). Moreover, for all species that displayed any evidence of apical loss, the LMA relative to sapwood density was positive, and for all species with any evidence of basal loss, LMA relative to sapwood density was negative (Table 1).

These data indicate that the drought damage and regrowth patterns were related to LMA and sapwood density in the manner proposed in our conceptual model. However, it is not so much the absolute values of LMA and sapwood density that are important but rather their relative values. We now examine each of the processes and supporting evidence and ideas from the literature in greater detail.

Leaf turnover and leaf lifespan

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

What is the significance of the spatial pattern of leaf loss, and why should LMA be related to the rate of leaf loss? New leaves are added at the distal end of the new shoot, and under drought stress old leaves are shed from the base of the stem (Pook, 1985; Comstock & Ehleringer, 1986; Davidson & Reid, 1989). In terms of economics of investment, younger leaves have higher expected future value than older leaves (Westoby et al., 2000). Younger leaves might also occupy superior positions to older leaves for photosynthesis, being further toward the surface of the canopy. Hence, older basal leaves should be shed preferentially to younger apical leaves.

Slow leaf turnover (long leaf lifespan) is associated with high structural investment measured as LMA (Reich et al., 1992; Westoby et al., 2000; Wright & Cannon, 2001). Hence, in species of higher LMA and longer leaf lifespan, loss of leaves from base to apex would be expected to progress more slowly. This means, all else being equal, a decreased probability of basal leaf loss for higher LMA species, as observed in this study.

Hydraulic failure

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Why should stem damage progress from apex to base? Water potentials become more negative with distance from the stem base and drop sharply at branching points (Zimmermann, 1978, 1983; Hacke & Sperry, 2001). As embolisms accumulate with progressive drought, the number of embolisms should increase with distance from the stem base (Comstock & Sperry, 2000). Thus distal regions of stems would be expected to exhibit total loss of conductance first and this should progress from apex to base, although hydraulic failure at roots could also occur (Comstock & Sperry, 2000; Hacke et al., 2000; Hacke & Sperry, 2001; Davis et al., 2002). Cavitation in short-lived ‘cheap’ tissues such as twigs may protect main stems and roots by ‘segmentation’ (Zimmermann, 1978, 1983; Sperry, 1995; Pockman & Sperry, 2000; Rood et al., 2000). Indeed, hydraulic failure in petioles may well be the mechanism of leaf shedding under drought proceeding up the stem (Zimmermann, 1978, 1983; Sperry, 1986; Tyree et al., 1993). Exactly why it proceeds up the stem is unclear; perhaps the accumulated minor cavitation events over time cause older petioles to be more vulnerable (Hacke et al., 2001b).

Determinate twigs and flowering

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Although progressive leaf loss from base to apex is the norm in most species, determinate shoots are also common, where whole leaf sequences or distal twigs may be lost (Gray & Schlesinger, 1981; Nilsen & Muller, 1981; Gill & Mahall, 1986; Comstock et al., 1988). Determinate and sympodial growth is often associated with irreversible switching of apical meristem function from growth to flowering (Bell, 1991). However, flowering is not the mechanism that leads to stem death. Six species had flowered by the time of treatment (Brachyscome ciliaris, Vittadinia triloba, V. cuneata, Calotis cuneifolia, Atriplex semibaccata, Einadia nutans). In Atriplex semibaccata and Einadia nutans, flowering was restricted to the short lateral branches. In the remaining species, all from the Asteraceae, the apex had sometimes switched to reproduction. Hence, flowering was conflated with drought for these plants. This may have led to increased representation of species in the each end group for Vittadinia triloba, V. cuneata and Calotis cuneifolia (Table 1), and of the single Brachyscome ciliaris in the apical loss group, but did not alter the substance of these results. It is not clear to what extent hydraulic failure and other mechanisms can be disentangled as explanations for the majority of species with determinate shoots. However, dieback of shoots similar to the apical stem damage seen here has been convincingly attributed to hydraulic failure of stems for some species (Rood et al., 2000; Davis et al., 2002). We suspect that the apical stem damage observed in this experiment is a function of stem hydraulic failure, though we have no data to prove this.

Stem construction

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Why should stem hydraulic failure be related to sapwood density? The mechanism of drought-induced loss of conductance has been widely attributed to air seeding resulting in transfer of embolisms from air-filled to water-filled vessels mediated by interconduit pits (Sperry, 1995; Hacke & Sperry, 2001). But how do pits relate to sapwood density? The segmentation hypothesis and explicit use of the word ‘cheap’ by Zimmermann (1983), hint at an argument on the grounds of construction costs (Hacke et al., 2000). The vulnerability of xylem to conductance loss was well correlated with the strength of vessel walls to resist implosion due to large negative pressures within the vessels, across a wide range of North American woody species (Hacke et al., 2001a). Wall strength is, in turn, directly related to sapwood density (Hacke et al., 2001a). The check-valve function of interconduit pits (that mediate air-seeding) should be coordinated with wall strength such that resistance to implosion is greater in species that do not embolise extensively until water potentials are very negative.

Because low sapwood density stems are less resistant to total conductance loss, all else being equal, they would be expected to show evidence of more extensive apical stem damage and a consequent failure to regrow from that region of dead stem. Further, the rate with which stem conductance loss progresses from apex to base should be faster in species with lower sapwood density.

Positive correlation of LMA and stem density might be expected. To maintain long-lived leaves through dry seasons, xylem conductivity must be maintained, requiring safe stem construction reflected in high stem density (Sobrado, 1993; Kolb & Davis, 1994; Ackerly, 2003). Ackerly (2003) found a positive relationship between LMA and stem density across 20 species of chaparral shrubs, and Sobrado (1993) found higher stem density in two evergreen compared to four deciduous tropical trees. Both these authors used whole wood, which includes non-conductive tissue, but whole wood density and sapwood density are likely correlated (here r2 = 0.36, n= 18 species).

Was regrowth after drought related to sprouting after clipping?

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Sprouting after clipping (not exposed to drought) was assessed on a separate sample of plants by clipping above the cotyledonary node. The maximum number of plants that sprouted (i.e. ignoring subsequent mortality of sprouted plants) was expressed as a proportion of treated plants. Species that suffered apical damage tended to have higher sprouting ability following clipping (t(16) = 2.404, P = 0.029; Fig. 2(d)), meaning that species that respond to drought with apical loss need to have at least moderate (basal) sprouting ability.

The ability to recover from total leaf loss induced by drought was common (11 of the 19 species), and was related to species’ sprouting ability following clipping (Fig. 3). As a measure of species’ ability to recover following total leaf loss due to drought, the number of plants recovering with total leaf loss was divided by the sum of the number of plants with total leaf loss and the number of dead plants [T/(T + D)]. Across all species there was a triangular relationship, such that species that were poor sprouters after clipping did not recover well from drought-induced total leaf loss (Fig. 3). Species that were good sprouters after clipping ranged widely in how well they recovered from total leaf loss due to drought.

image

Figure 3. Proportion of plants that recovered after total leaf loss due to drought compared to proportion sprouting after clipping. Calculated as number displaying recovery with Total leaf loss, divided by the sum of plants recovering with Total leaf loss and dead plants [T/(T + D)]. Circles represent species that mostly recovered with Apical damage, upside-down triangles are species that mostly recovered with Basal leaf loss, squares are species that mostly recovered with damage at Each end. Solid line is linear regression for all species (recovery = 0.043 = 0.503 × clip sprouting, r2 = 0.26, P= 0.026, n= 19).

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Apical bud maintenance and sprouting

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

Maintaining the apical bud should benefit further growth once the drought has passed, especially if height growth is an important aspect of the plant's strategy. So why would a species consistently show apical loss? Presumably it has to do with the cost of maintaining a water column to apical meristems through drought (Westoby, 1980). This cost may be incurred through resistant stems as outlined above, or through deep root systems (Davis et al., 1999; Davis et al., 2002).

Having to sprout subapically after apical loss and then continue upward has three disadvantages: structural discontinuities in the stem due to growing past dead tissue (Midgley, 1996); a time cost in gaining leaf area and height (Davidson & Reid, 1989); possible entry of pathogens through the dead branches (Davis et al., 2002). On the other hand, if lateral expansion or space filling is more important than height growth, then dying back at the tips and sprouting subapically is no disadvantage and actually improves filling of space. This may be the case for semiarid species, such as those studied here. Plants that are prone to losing their apices to drought may require good sprouting ability (see Davidson & Reid, 1989). Sprouting is useful for recovery from a variety of hazards that result in major loss of above-ground biomass (Bond & Midgley, 2001), but is not commonly associated with drought. In locations subject both to recurrent drought and to recurrent fire however, ongoing selection might lead to tandem solutions to these hazards.

Caveats to results and their generality

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

The difference in ages of plants would definitely affect the rate of soil drying. Though this should not affect the pattern of damage. The size of plants may affect the ability to detect pattern. In small plants with short hydraulic pathways, the whole plant may desiccate more-or-less simultaneously. In large plants, the patterns of damage may be clearer as there is more space and time for the progress of damage to be noted. Finally the age of leaves relative to their potential lifespan may have a large effect. In species with long leaf lifespan leaves (approx. 2 yr), a 1-yr-old plant has no ‘old’ leaves to shed. For these reasons it would be good to see whether these patterns hold in adult plants in the field.

When response groups were defined narrowly, statistically significant patterns were detected. However, when defining the groups more broadly such that more species were included in the Basal and Apical groups, the trends persisted, giving confidence in the generality of the patterns for the species set. Species that only showed partial basal leaf loss were exclusively in the Chenopodiaceae. This family of leaf-succulent plants are known to be poor sprouters and biogeographically tend to be associated with conditions of summer drought and clayey, often saline, soils. The structural support of these leaves is due largely to water, and the implications for leaf lifespan, particularly with respect to drought, are unclear (Vendramini et al., 2002). In the apical group, Bossiaea walkeri has photosynthetic stems and bears leaves only as a juvenile and on recovery from damage. Casuarina cristata has very small leaves closely appressed to the stem. Whole needles (distal stems ensheathed by scale-like leaves) may be abscised or may grow into stems and the leaves simply lose photosynthetic capacity and become part of the periderm. These two species serve to suggest that responding to drought solely through apical stem damage is an unusual strategy. This reinforces our earlier expectation that basal leaf loss and apical maintenance should be the norm. However, solely basal leaf loss and solely apical stem damage are the extremes, between which lie a range of possibilities that we expect to be related to strategies of growth, architecture and coping with disturbances. We propose that patterns of damage and regrowth are important aspects of plants’ ecological strategies in locations subject to periodic drought and that links between carbon gain strategies and disturbance responses may reward further research.

Acknowledgements

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References

For assistance in the glasshouse we thank A. Downing, J. Grubba & A. Stock. For comments on earlier drafts we thank D. Ackerly, W. Allaway, E. Garnier, A. Nicotra, I. Wright and 3 anonymous referees. PAV was supported by a Land & Water Australia Postgraduate scholarship. This is publication no. 385 of the Research Unit for Biodiversity and Bioresources, Macquarie University.

References

  1. Top of page
  2. A conceptual model for patterns of drought damage and regrowth
  3. Experimental drought
  4. Damage and regrowth responses
  5. Plant traits
  6. LMA and sapwood density in relation to drought damage and regrowth patterns
  7. Leaf turnover and leaf lifespan
  8. Hydraulic failure
  9. Determinate twigs and flowering
  10. Stem construction
  11. Was regrowth after drought related to sprouting after clipping?
  12. Apical bud maintenance and sprouting
  13. Caveats to results and their generality
  14. Acknowledgements
  15. References
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