Online Resource 1
Plot fuel bulk density:
We calculated plot fuel bulk density (BD) as:
𝐵𝐷 =
oven-dried fuel mass (kg)
𝑓𝑢𝑒𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚3 )
We found no significant relationships between fuel mass and relative gradient position, or
vegetation structure predictors (Online Resource 3). As such we used the mean plot oven-dried
fuel mass (0.704 [0.040 SE] kg) from our subset of fuel transects in our BD calculations. This
fuel mass was calculated from the following six vegetation structure functional types: C4 grass,
C3 graminoids, fern, herbaceous dicots, and switchcane. These are vegetation structure functional
types that most readily burn along our ecotonal gradients.
We calculated the fuel volume for each plot (full dataset). First, we calculated the volume (m3)
for the following three vegetation structure functional types: C4 grass, fern, and switchcane. This
was calculated as:
𝑣𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑡𝑦𝑝𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚3 ) = ℎ𝑒𝑖𝑔ℎ𝑡 (𝑚) × 𝑎𝑟𝑒𝑎 (𝑚2 )
Area of the vegetation structure functional types was derived from the percent cover of 1 m2 plot
that they occupied. We then computed the volume of the fuelbed of the plot where height was the
fuelbed depth and area was calculated as the difference between the sum of the percent cover of
the three vegetation structure functional types and 100%.
In our BD calculations plot fuel volume was calculated as the weighted average of the volume of
the three vegetation structure functional types and the fuelbed.
Online Resource 2
Surface area to volume ratio (SV) calculations:
We used the surface area to volume ratio formulas proposed by van Wagtendonk (2006) for our
analyses.
The SV of cylindrical fuels (e.g. pine needles, stems) was calculated as:
𝑆𝑉 =
4
𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒
The SV of ‘flat’ fuels such as broadleaves was calculated as:
𝑆𝑉 =
2
𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒
van Wagtendonk JW (2006) Fire as a physical process. In: Sugihara NG, van Wagtendonk JW,
Shaffer KE, Fites-Kaufman J, Thode AE (eds) Fire in California’s ecosystems. University
of California Press, Ltd., Berkley and Los Angeles, CA, pp 38–57
Online Resource 3
Results of linear mixed-effect models evaluating total plot fuel mass (kg) by vegetation structure
functional type, or plot variable. Fixed effect: variable, Random effects: uncorrelated random
intercept (site) and random slope (variable by site)
Variable
Years since fire
No. of fires
Switchcane cover (%)
Deciduous tree cover (%)
Evergreen tree cover (%)
Plot cover (%)
Evergreen shrub cover (%)
Relative gradient position
C4 grass cover (%)
Fern cover (%)
Woody debris cover (%)
Deciduous cover (%)
Herbaceous dicot cover (%)
C3 graminoid cover (%)
Canopy closure (%)
Dead surface fuel depth (cm)
*
P < 0.05, NS = not significant
Estimate (SE)
0.131 (0.049)
-0.066 (0.041)
0.006 (0.006)
-0.003 (0.002)
-0.006 (0.004)
-0.001 (0.001)
-0.002 (0.002)
-0.098 (0.111)
0.003 (0.005)
0.001 (0.003)
-0.007 (0.012)
-0.001 (0.002)
-0.002 (0.007)
0.002 (0.011)
0.000 (0.001)
0.001 (0.015)
DF
18
20
3
89
91
93
11
34
12
4
82
82
88
81
82
85
t-value
2.7*
-1.6NS
1.0NS
-1.4NS
-1.3NS
-0.9NS
-0.8NS
-0.9NS
0.6NS
0.6NS
-0.5NS
-0.4NS
-0.3NS
0.1NS
-0.1NS
0.0NS
Marginal R2
0.112
0.048
0.027
0.018
0.016
0.008
0.008
0.007
0.007
0.003
0.003
0.001
0.001
0.000
0.000
0.000
Online Resource 4
Top Panel: Scatterplot of total plot fuel mass (kg) by relative gradient position. Bottom panel: Boxplots of
total plot fuel mass by relative gradient position. The solid center bar represents the median and the
asterisk depicts the mean. The letters represent a post-hoc Tukey’s HSD test. These letters are all the
same indicating that relative gradient positions are not significantly different in their mean plot fuel mass.
Online Resource 5
Boxplots of the pine needle proportion of total dead fuel mass by relative gradient position. The solid
center bar represents the median and the asterisk depicts the mean. The letters represent a post-hoc
Tukey’s HSD test. Relative gradient positions with different letters are significantly different.
Online Resource 6
Results of linear mixed-effect models evaluating total class fuel mass per plot (kg) or class
proportional fuel mass by relative gradient position. Fixed effect: relative gradient position,
Random effects: uncorrelated random intercept (site) and random slope (relative gradient
position by site)
Fuel class
Estimate (SE)
Total class fuel mass per plot
Herbaceous dicot
-0.024 (0.014)
Pinecone
0.226 (0.114)
Pine needles
-0.191 (0.048)
Deciduous tree
0.098 (0.082)
Evergreen tree
0.031 (0.022)
Fern
0.068 (0.054)
Deciduous shrub
0.053 (0.026)
Switchcane
0.068 (0.056)
Fine woody fuels
0.083 (0.063)
C4 grass
-0.029 (0.042)
Evergreen shrub
0.033 (0.130)
Bark
0.006 (0.021)
C3 graminoids
-0.003 (0.009)
Miscellaneous plant matter
0.006 (0.042)
Proportion of total fuel mass per plot
Pine needles
-33.237 (4.477)
Pinecone
12.751 (5.537)
Herbaceous dicot
-3.775 (2.516)
Deciduous tree
14.542 (12.478)
Evergreen tree
3.950 (2.793)
Switchcane
8.168 (5.588)
C4 grass
-6.773 (4.822)
Fern
5.333 (4.227)
Fine woody fuels
8.769 (4.582)
Deciduous shrub
9.369 (4.854)
C3 graminoids
-1.176 (1.246)
Evergreen shrub
-4.281 (9.529)
Bark
-0.295 (1.735)
Miscellaneous plant matter
0.104 (2.809)
***
*
P < 0.001, P < 0.05, NS = not significant
DF
t-value
Marginal R2
11
6
30
7
18
28
44
36
50
29
32
51
7
39
-1.7NS
2.0NS
-4.0***
1.2NS
1.4NS
1.3NS
2.0*
1.2NS
1.3NS
-0.7NS
0.3NS
0.3NS
-0.3NS
0.1NS
0.197
0.172
0.133
0.092
0.079
0.052
0.047
0.038
0.025
0.013
0.002
0.001
0.001
0.000
46
5
11
4
18
36
29
28
39
53
7
32
56
68
-7.4***
2.3NS
-1.5NS
1.2NS
1.4NS
1.5NS
-1.4NS
1.3NS
1.9NS
1.9NS
-0.9NS
-0.4NS
-0.2NS
0.0NS
0.366
0.225
0.158
0.127
0.087
0.053
0.051
0.051
0.049
0.046
0.008
0.006
0.000
0.000
Online Resource 7
Scatterplot of maximum fire temperature (°C; as indicated by temperature indicating paint at 20 cm above
ground level) by relative gradient position.
Online Resource 8
Results of linear mixed-effect models evaluating maximum fire temperature (°C; as indicated by
temperature indicating paint at 20 cm above ground level) by vegetation structure functional
type, site, microclimate, or plot variable. Fixed effect: variable, Random effects: uncorrelated
random intercept (site) and random slope (variable by site)
Variable
Estimate (SE)
Years since fire
43.63 (10.01)
Resultant
172.37 (59.13)
No. of fires
-19.14 (8.07)
Dead fuel depth (cm)
9.63 (4.34)
Canopy closure (%)
0.97 (0.47)
Evergreen tree cover (%)
-3.27 (1.38)
C4 grass cover (%)
1.63 (0.72)
Deciduous tree cover (%)
1.44 (0.81)
Switchcane cover (%)
-2.45 (1.19)
Relative humidity (%)
-1.69 (1.13)
Wind speed (m s-1)
66.14 (45.17)
Deciduous shrub cover (%)
0.92 (0.54)
Bulk Density (kg m-3)
-3.02 (1.98)
Soil moisture (%)
-160.84 (95.37)
Coarse fuel moisture (%)
-5.67 (4.20)
Wood debris cover (%)
-4.95 (3.52)
Plot cover (%)
0.45 (0.38)
Air Temperature (°C)
-3.59 (3.83)
-1
Precipitation throughfall (mm d )
-15.47 (13.39)
Wind direction (°)
-0.09 (0.11)
Fern cover (%)
-0.66 (0.72)
Evergreen shrub cover (%)
-0.59 (0.72)
Fuel temperature (°C)
-1.80 (3.06)
Vapor pressure deficit (kPa)
14.19 (24.70)
-2 -1
Photosynthetically active radiation (µmol m s )
0.01 (0.03)
Herbaceous dicot cover (%)
0.47 (2.31)
***
**
*
P < 0.001, P < 0.01, P < 0.05, NS = not significant
DF
35
42
37
116
104
149
146
20
144
78
54
141
152
133
125
142
79
50
152
21
148
140
81
40
149
6
t-value
4.4***
2.9**
-2.4*
2.2*
2.1*
-2.4*
2.3*
1.8NS
-2.1*
-1.5NS
1.5NS
1.7NS
-1.5NS
-1.7NS
-1.3NS
-1.4NS
1.2NS
-0.9NS
-1.2NS
-0.8NS
-0.9NS
-0.8NS
-0.6NS
0.6NS
0.2NS
0.2NS
Marginal R2
0.160
0.062
0.056
0.037
0.032
0.032
0.029
0.025
0.024
0.021
0.018
0.016
0.015
0.015
0.015
0.011
0.010
0.009
0.009
0.007
0.006
0.004
0.003
0.003
0.000
0.000
Online Resource 9
Conceptual diagram of three possible ecotonal gradients (depicted by the solid lines) by gradient
position and fire frequency at our study location. 1) If fires frequently burn the entire gradient,
we expect savanna upland and herbaceous (e.g. switchcane, fern) wetland. 2) If fires reliably
extinguish in the ecotone between the savanna and wetland, we expect savanna upland and
shrubby wetland 3) If fires are infrequent or absent along the entire gradient, we expect dry
forest upland and shrubby wetland.