stand structure and mortality at the community level
At Wartonokadri, early in the drought, mean stem density (living and dead, ≥ 10 cm d.b.h.) was 627 ± 105 trees ha−1 (mean ± SD, n = 5), of which 610 ± 100 trees ha−1 were alive, i.e. standing dead trees were 2.6% ± 1.7 (Table 1). The basal area of all living and dead stems was 32.4 ± 7.0 m2 ha−1 and of living stems alone was 31.5 ± 6.4 m2 ha−1. Wartonokadri was not significantly different from Sungai Wain in either stem density (t-test, F = 0.37, P = 0.86, d.f. = 21) or basal area (F = 2.37, P = 0.85, d.f. = 21). In the Wartonokadri plots 4 months after the drought, the percentage of dead trees rose to 11.4 ± 1.7%, corresponding with an 8.8% mortality over an 11-month period (Table 1). By 22 months after the drought dead stems had nearly doubled, resulting in 22.2 ± 2.4% dead trees (19.6% mortality).
Table 1. Average percentages of dead trees above 10 cm d.b.h. per permanent sample plot at various assessments in the Wanariset Wartonokadri forest and Sungai Wain research forest. Two estimates are given based on the varying confidence with which stem death was indicated (see methods)
|Treatment||Average date||Time (months)||Cumulative percentage dead trees|
|Since end of drought and fire||Since last observation||Total (Average ± SD)||Unambiguous (Average ± SD)|
|Wanariset Samboja Wartonokadri (5 plots, 599 trees)|
|Drought||September 1997|| ||–|| 2.6 ± 1.7|| |
|August 1998|| 4||11||11.4 ± 1.7|| |
|February 2000||22||19||22.2 ± 2.4|| |
|Sungai Wain forest, unburned (9 p.s.p., 3221 trees)|
|Drought||December 1998|| 8||–||18.5 ± 5.6||16.4 ± 5.7|
|January 2000||21||13||26.3 ± 5.0||23.6 ± 5.4|
|Sungai Wain forest, burned (9 p.s.p., 2985 trees)|
|Drought x fire||December 1998|| 8||–||64.2 ± 12.2||60.3 ± 12.0|
|January 2000||21||13||79.0 ± 10.2||75.7 ± 11.0|
|Sungai Wain forest (9 paired p.s.p.)|
|Exclusively fire||December 1998|| 8||–||47.8 ± 14.5||41.8 ± 14.6|
|January 2000||21||13||55.4 ± 11.7||49.3 ± 12.4|
At Sungai Wain, 8 months after the drought, the stem density (dead and living trees ≥ 10 cm d.b.h.) was 599 ± 82 ha−1 (average ± SD, n = 9), of which 487 ± 62 ha−1 remained alive. Assuming that about 2.6% were dead prior to the drought (cf. Wartonokadri), we estimate pre-drought densities of 584 ± 79 ha−1. Eight months after the drought, mortality in the unburned Sungai Wain plots was similar to Wartonokadri, with 18.5 ± 5.6% dead trees. Mortality remained high, resulting in 26.3 ± 5.0% dead stems after 21 months, a mortality of 9.4 ± 3.6% among trees alive during the first census. By then total basal area was 29.6 ± 1.9 m2 ha−1, of which living stems contributed 21.9 ± 2.4 m2 ha−1. We estimated the live basal area to be 28.8 ± 1.9 m2 ha−1 before the drought.
Unburned patches occurred in four ‘burned’ plots and amounted to 2.1% of the total area in the 0.4-ha plots and to 4.4% in the 1.8-ha plots. One plot provided half the area that escaped the fire. Such areas appear positively associated with valleys and low slopes (ignoring spatial non-independence: χ2 test on 10 × 10 m, n = 356, and on 20 × 20 m, n = 297, subplots give P < 0.001 and P = 0.008, respectively). However, all site types had a high probability of burning. These patches are not excluded in our main analyses.
In Sungai Wain mean post-burn mortality was over three times unburned levels, and resulted in 192 ± 69 (≥ 10 cm d.b.h.) living stems ha−1 after 8 months and 113 ± 57 ha−1 after 21 months. Of surviving stems in the burned forest, 6 ± 11% grew on unburned ground. Trees alive at the first census had a mortality of 38.5 ± 8.7% in the following 13 months. Paired plot comparisons indicate a relatively consistent fire-dependent mortality of 47.8 ± 14.5% at the first census and 55.4 ± 11.7% at the second.
Stem density decreases with d.b.h. After drought, this pattern is accentuated, after fire the distribution becomes flatter (Fig. 3). Variation in basal area between the plots is influenced by the scattered presence of larger trees (> 80 cm d.b.h.). Two burned plots (14 and 16) were outliers in terms of basal area, because large stems locally occurred at a high density. The total basal area for stems > 80 cm d.b.h. was, respectively, 21.7 and 18.1 m2 ha−1 in these plots, but averaged only 7.6 m2 ha−1 for the other seven (as in Fig. 3d). Plot-by-plot summaries are provided in Appendices S1 and S2 (see Supplementary material).
Figure 3. Stand structure in unburned (a, b) and burned (c, d) plots. (a) and (c) show mean per-plot density of living stems per d.b.h. class (stems ha−1) (average + SD, n = 9), and (b) and (d) show the basal area per d.b.h. class (m2 ha−1) (average + SD, n = 9). White bars = stand structure at the onset of the drought; light grey bars = 8 months after the end of the drought; dark grey bars = 21 months after the end of the drought. Hatched sections indicate stems for which survival is ambiguous. Two burned plots were excluded from the basal area calculations (see Methods).
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While the percentage of dead trees increases with d.b.h. in the unburned forest, the opposite holds in burned forest (Fig. 4). Below 10 cm d.b.h., burned forest mortality was above 80%. Below 5 cm d.b.h., it approached 100%. All t-tests between the percentages of dead trees in the unburned and burned plots after 21 months by 10-cm d.b.h. intervals up to 60–69.9 cm were significant at P < 0.005. At a larger diameter no significant difference were detected (pooling all stems over 70 cm gives P = 0.3).
Figure 4. Average percentage tree mortality per d.b.h. class (average ± SD, n = 9) in the Sungai Wain forest. (a) Unburned, (b) burned, and (c) ‘fire mortality’. White bars = 8 months after the end of the drought; shaded bars = 21 months after the end of the drought. Hatched sections indicate stems for which survival is ambiguous. ‘Fire mortality’ is calculated by pairwise subtraction of the unburned from the burned mortality for each plot pair. Stems below 10 cm d.b.h. were monitored only once. Where values were not available ‘na’ is shown.
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In the unburned forest, delayed mortality between 8 and 21 months was similar across size classes. In the burned forest, mortality remained negatively related to d.b.h. Tree-falls were dominated by dead stems everywhere. Tree-fall frequency in the burned forest was 28% of the standing dead stems and 7.4% of living trees by the second year. Living tree-falls were lower in unburned forest (3.6%). Tree-falls (living and dead) decreased with size only in the burned forest.
Size-dependent mortality means that stem density, basal area and biomass summarize stand differences in distinctive ways. When all dead stems ≥ 10 cm d.b.h. were considered the unburned forest had 32% of the density compared with burned forest, but 54% of the basal area and 64% of the biomass, while for larger stems (≥ 40 cm d.b.h.) the respective estimates are both higher and more similar (68, 74 and 78%, respectively; see Fig. 5; note that we excluded plots to the north of the site as large stems are unevenly clumped in these samples). Our estimates suggest that before the drought the (Wartonokadri) forest contained around 7.3 ± 2.2 tonnes ha−1 (95% confidence) of above-ground biomass as dead standing trees, after 21 months the drought-stricken (Sungai Wain) forest held 133 ± 30 tonnes ha−1 of dead stems, while the equivalent number for burned forest was 207 ± 50 tonnes ha−1.
Figure 5. Comparison of estimated standing dead stems density (a), basal area (b), and above-ground biomass (c) (equation 2 of Ketterings et al. 2001) in the six southerly plot pairs before (estimated using Wartonokadri observations, see Methods) and 21 months after drought and drought and fire. The contribution of stems between 10 and 40 cm d.b.h. are white, and those larger than 40 cm are black. Error bars are 95% confidence intervals based on per-plot sample variance treating each plot as independent.
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Twenty-one months after the drought, dead stems species−1 (≥ 30 cm d.b.h.) ranged from 5% to 64% (Table 2). Dense timbered Eusideroxylon zwageri was especially resistant, while the emergent Koompassia malaccensis suffered considerable mortality. Variation was not significantly correlated with wood density when analysing across these species (Fig. 6, Pearson correlation coefficient = −0.16, P = 0.65, n = 10) but K. malaccensis mortality appears to be an outlier; such selective drought dieback of this species is neither previously reported nor consistent with the relative abundance of large stems despite previous droughts. Excluding K. malaccensis yields a significant negative correlation.
Table 2. Species specific mortality after drought and fire of trees above 30 cm d.b.h. in Sungai Wain forest 21 months after the end of the fire and drought. The percentage dead trees after ‘drought’ and ‘drought + fire’ are given as the average percentage dead trees per plot (n = 6). To calculate the additional mortality caused by fire, the mortality after drought per plot is subtracted pairwise from the mortality after ‘drought + fire’ for each pair of adjacent plot. Negative values in this second last column occur as an artefact caused by sample noise
|Wood density (g cm−3)||Drought||SD||Drought + fire||SD||Fire||SD|
|Artocarpus anisophyllus Miq.||Moraceae||0.72||20||27||92||17|| 67||24|
|Dipterocarpus confertus Sloot||Dipterocarpaceae||0.80||15||18|| 8||10||−14||19|
|Dipterocarpus cornutus Dyer||Dipterocarpaceae||0.84||14||13||29|| 6|| 15||18|
|Drypetes kikir Airy Shaw||Euphorbiaceae||1.00||13||14||31||30|| 27||18|
|Eusideroxylon zwageri Teijm. & Binn||Lauraceae||1.07|| 5|| 6||39|| 5|| 30||10|
|Gironniera nervosa Planch.||Ulmaceae||0.60||30||23||59||39|| 29||39|
|Koompasia malacensis Maing. Ex Benth||Caesalpiniodae||0.93||64||24||92||12|| 29||24|
|Madhuca kingiana (Brace) H.J. Lam||Sapotaceae||0.79||17||19||59||38|| 42||38|
|Shorea laevis Ridl.||Dipterocarpaceae||0.93||23||18||44||14|| 29||21|
|Shorea ovalis (Korth.) Blume||Dipterocarpaceae||0.54||28||22||20||27|| −5||33|
Figure 6. Species-specific mortality of trees > 28 cm d.b.h. vs. specific wood density. The apparent high mortality outlier is Koompassia malaccensis.
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The percentage of dead trees per species varied from 11 to 91% in burned forest. In Dipterocarpus confertus and Shorea ovalis stems over 30 cm d.b.h. seemed unaffected by the fire. For most species, the fire mortality component was around 25%, though Artocarpus anisophyllus reached 67% (Table 2). When assessed by basal area (≥ 10 cm d.b.h.), the five most dominant families in unburned forest are, in order, Dipterocarpaceae, Lauraceae, Sapotaceae, Euphorbiaceae and Myrtaceae. After fire (second census) Euphorbiaceae moved ahead of Sapotaceae. However, relative density (stems ha−1) showed that after the fire Dipterocarpaceae moved from third to first place while the other top six families maintained their rank (Euphorbiaceae, Sapotaceae, Myrtaceae, Myristicaceae, Lauraceae, Burseraceae). Palms (≥ 10 cm d.b.h.) were only 3 ± 4% dead after the drought and 10 ± 11% dead after the fire.
fire mortality and bark thickness
Log(d.b.h.) and log(bark thickness) (all species combined) fitted a linear regression (Fig. 7a). The percentage mortality per d.b.h. class attributed to fire (i.e. corrected for drought mortality) declined linearly with average bark thickness of trees in that class (Fig. 7b). A log-log regression again provided a good fit to all 14 species (mean R2 ranging from 0.70 to 0.96%). Differences in tree densities over 10 cm d.b.h. in the unburned and the burned forest were examined using backwards-stepwise regression. Explanatory variables included the per-species intersect and gradients from the d.b.h. vs. bark thickness log-log regressions and the 95% d.b.h. limit (n = 14, for all fitted models F < 2.52, P > 0.13, NS). Also, a similar regression, using the same explanatory variables, against the percentage fire mortality of nine species above 30 cm d.b.h. showed no significant relationship (F < 0.52, P = 0.57, NS).
Figure 7. The relationship between d.b.h., bark thickness and exclusive fire mortality. (a) The relationship between log d.b.h and log bark thickness (cm) for 16 species combined. Regression line: y = 0.669x – 1.21 (R2 = 0.788). (b) Average bark thickness (± sd) per 10 cm d.b.h. class (of 16 species combined, as in Figure 5a) and the percentage mortality per d.b.h. class (mean ± sd) caused exclusively by fire. Regression line: y = −56.3x + 82.1 (R2 = 0.892).
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