Appendix A
Climatic water deficit Calculations
Climatic water deficit is the amount of water by which potential evapotranspiration exceeds
actual evapotranspiration, which we estimated using a Thornthwaite-type water balance model
(Thornthwaite, 1948; Dingman, 2002) following the equations provided in Lutz et al. (2010),
which incorporates monthly precipitation and temperature, heat load, soil water holding capacity,
and day length. For temperature and precipitation data, we used 30-year averages (1980-2010)
from the PRISM climate dataset (800 m resolution) for each of our sites. We used our field
measurements of slope and aspect to calculate heat load. We used our soil available water
capacity data to calculate water holding capacity by assuming a soil depth of 2 m. While soil
water holding capacity is used to calculate climatic water deficit, soil water holding capacity had
a very marginal effect on climatic water deficit due to the course (monthly) timescale of the
model, which is why soil available water capacity was used as an additional environmental stress
predictor variable. We quantified the degree to which soil water holding capacity influenced
climatic water deficit at our study sites by comparing climatic water deficit using our calculation
of soil water holding capacity for each site (which varied from 30 mm to 300 mm) to climatic
water deficit using a constant soil water holding capacity of 200 mm for all sites. The correlation
between climatic water deficit with water holding capacities allowed to vary and climatic water
deficit with soil water holding capacities held constant was high (Pearson’s r = 0.97),
highlighting the negligible effect that soil water holding capacities had on climatic water deficit
in our water balance model.
While elevation isn’t a variable used to calculate climatic water deficit, elevation was highly
negatively correlated with climatic water deficit (Pearson’s r = -0.94) due to the differences in
monthly precipitation and temperature at varying elevations. In our study area, higher elevations
receive more annual precipitation and have cooler temperatures, which results in a smaller
climatic water deficit.
Surface piñon cone density methodology
We wanted to estimate a long-term (~10 year, i.e. post overstory tree mortality) index for seed
availability at each site. To do this, we followed a similar protocol as Gworek et al. (2007) and
counted the number of piñon cones beneath the canopy of all live piñon trees at each site. We did
not use this estimate of seed availability in our analysis because it was highly correlated with live
piñon basal area (Pearson’s r = 0.88, Fig. S1), and because we hypothesized that live piñon
basal area is likely as good of a metric of seed availability given the limitations with our cone
counting methodology. In particular, we did not count dead cones that were still on tree branches
and we also didn’t count cones outside of the tree canopy, which likely resulted in an
underestimate of cone density in areas with steep slopes.
Table S1. Average live tree density (tress ha-1) in 2000 (predrought) and 2014 (postdrought) in each size class (based on basal trunk
diameter [BTD]) across our study sites. The range is included in parentheses.
BTD Size
Class
5 - 20 cm
20 – 30 cm
30 – 40 cm
> 40 cm
Oneseed Juniper
2000
2014
54 (0 - 238)
51 (0 -231)
39 (0 - 291)
39 (0 - 291)
26 (0 - 100)
23 (0 - 91)
37 (0 - 150)
35 (0 - 110)
Piñon Pine
2000
2014
110 (0 - 333) 38 (0 -140)
18 (0 - 100)
5 (0 - 40)
7 (0 - 33)
2 (0 - 20)
4 (0 - 30)
1 (0 - 30)
Ponderosa Pine
2000
2014
19 (0 - 290) 12 (0 - 130)
19 (0 - 320) 17 (0 - 260)
16 (0 - 190) 14 (0 - 170)
14 (0 - 170) 10 (0 - 80)
Alligator Juniper
2000
2014
3 (0 - 70)
3 (0 - 70)
1 (0 - 50)
2 (0 - 50)
1 (0 - 30)
1 (0 - 30)
1 (0 - 40)
1 (0 - 40)
Table S2. Results of the post-hoc pairwise comparison of the effects of different nurse plants on
juvenile piñon survival. P-values were adjusted using the Tukey HSD method.
Pairwise Comparison
Dead piñon – Live juniper
Dead piñon - Live piñon
Dead piñon – Live ponderosa
Dead piñon – Live shrub
Live juniper - Live piñon
Live juniper - Live ponderosa
Live juniper - Live shrub
Live piñon - Live ponderosa
Live piñon - Live shrub
Live ponderosa - Live shrub
β
log(odds)
-1.35
-0.78
-1.62
-0.45
0.57
-0.27
0.90
-0.84
0.33
1.18
SE
z value
P -value
0.30
0.47
0.51
0.38
0.39
0.43
0.30
0.54
0.47
0.51
-4.49
-1.67
-3.21
-1.19
1.46
-0.64
3.04
-1.56
0.71
2.30
0.001
0.45
0.01
0.76
0.59
0.97
0.02
0.52
0.95
0.14
Figure S1. The relationship between 30 year mean annual climatic water deficit (mm) and
elevation (m) at our study sites.
Figure S2. The relationship between piñon basal area and surface piñon cone density (A.) and
total piñon canopy cover (B.) at our study sites.
Figure S3. The relationship between the three predictor variables that were not included in our
final regression model and new piñon recruitment.
Figure S4. Proportion of interspace (A.) and nursed (B.) juveniles that survived (grey) and died
(black) across sites with varying climatic water deficits. Above each bar includes the sample size
(i.e. the number of juveniles within each climatic water deficit bin).