Site description and experimental design
This research was conducted in situ at the two sites of the
B4WarmED experiment in northern Minnesota, USA established in 2008. The
sites are located at the Cloquet Forestry Center (CFC, 46°40’46” N,
92°31’12” W, 382 m a.s.l.) near Cloquet, MN and the Hubachek Wilderness
Research Center (HWRC, 47°56’42” N, 91°45’29” W, 415 m a.s.l.) near
Ely, MN in the ecotone of the boreal-temperate forest. The research
sites are characterized by a mean annual precipitation and temperature
(1980–2019) of 824 mm and 4.9°C for the CFC and 715 mm and 2.8°C for
the HWRC (based on nearby weather stations). At each site 24 research
plots 3 m in diameter were established, half in relatively open areas
that were recently cleared (open canopy) and half in the understory
(closed canopy) of existing stands of ≈70 years old mixed
aspen-pine-birch with scattered fir, spruce, and other species; in both
sites on coarse-textured upland soil. The study includes an incomplete
factorial of sites, canopy types, warming and rainfall manipulation,
which we analyzed as two overlapping factorial experiments. One
experiment consisted of two sites, two canopy conditions (closed and
open), and two temperature treatments (ambient and elevated), replicated
in three blocks per canopy condition per site. In addition, rainfall was
manipulated but only in open canopy plots; thus the second experiment
consisted of two sites, one canopy condition (open), two temperature
treatments (ambient and elevated), and two rainfall manipulation
(ambient and reduced), replicated in three blocks per site.
An open air (chamberless) warming treatment was implemented
simultaneously for the above- and belowground part of the plot via an
integrated microprocessor-based feedback control system (Rich et
al., 2015), designed to maintain a fixed temperature differential
between ambient and warmed plots. Infrared ceramic heaters mounted above
each plot in an octagonal pattern were used for the aboveground warming
of plant surfaces, while resistance-type warming cables were buried (10
cm deep and spaced 20 cm apart) to achieve belowground warming. For more
details about the project warming methodology see Rich et al.,(2015) as well as in Reich et al., (2015), Sendall et al.,(2015), and Reich et al., (2016). Aboveground temperature on each
plot was measured at mid-canopy height (i.e., roughly the average for
all planted tree seedlings in each plot). For temperature measurements
below ground (i.e., soil temperature), we used soil temperature probes
randomly inserted on each plot at the depth of 10 cm. During the
mid-summer and daytime periods, across all 11 years, average temperature
differentials between treatments specifically for the above ground were
slightly different than the target of 3.3°C (Tables 1 and 2, Figures S1
and S2) but as they were close for the full period of warming
treatments, we call the warming treatment +3.3°C throughout the paper.
The summer rainfall reduction treatment began in 2012 via rainout
shelters installed only in the open canopy, on randomly selected plots
across warming treatments at both sites. Rainout shelters were used to
reduce both total summer rainfall and the number of rain events in each
year from June 1st to September 30th(for details on rain shelter design and implementation see Stefanskiet al., (2020)). To minimize shading of tree seedlings, rainout
shelters were typically deployed during overcast condition or at night
shortly before and closed shortly after (typically 0.5-1h) the rain
event. Over the course of seven seasons, rain shelters were deployed for
an average total time of ~8% of the entire rainfall
reduction period (i.e., June 1st – September
30th). In each growing season, about half of this time
occurred during night hours (for more information about treatments see
Table 1 and Figures S1 and S2). Across the seven years of the summer
rainfall removal, we saw an average reduction of 40.7% summer rainfall
as compared to ambient plots (Table 1 and Figures S1 and S2). That
translated to a reduced mean summer rainfall of 269.5 mm (±15.5 SE) and
222.9 mm (±11.6 SE) as compared to ambient mean realized summer rainfall
of 454.5 mm (±26.4 SE) and 376.8 mm (±19.9 SE) at the CFC and HWRC sites
respectively. Consequently, our rainfall treatments were representative
of relatively wet summer (~70thpercentile wettest) and rather dry summer
(~10th percentile driest) for ambient
and reduced rainfall respectively as compared to the broader temporal
context of the 100 year of the weather record (1912-2011 availiable for
the CFC site). Soil moisture on the research plots was monitored over
the course of this research using water reflectometers (Model CS616 from
Campbell Scientific). Soil Volumetric Water Content (VWC -
cm3 water/cm3 soil) was measured
across 0-30 cm soil profile on an hourly basis through all years. The
reduction of soil moisture by warming treatment is a result of increased
evapotranspirative demand due to elevated temperatures that reduces soil
moisture on a continuous manner as we warm 24/7 from early spring until
late fall. In contrast to warming, rainfall reduction treatment is
implemented on a cyclic demand basis that allows replenishing soil water
in between rainfall removal events (see Table 1 and Figure S2 for more
details).
Over the course of this experiment between both the open and closed
canopy we grew seedlings of 17 native and four invasive tree species (a
total of 21) in different combinations among years and canopies (for
details see Tables 3 and S2). Seedlings were sourced from local
ecotypes, well suited for the research sites typical environmental
conditions. We planted one or two year old seedlings produced by MN DNR
(Minnesota Department of Natural Resources) nurseries into an existing
matrix of native vegetation. The chosen species represent dominant tree
species from the boreal-temperate ecotonal region of northern Minnesota.
Newly planted cohorts were given one year (≈14 months) to acclimate
after transplant before any gas exchange measurements were performed
except for the 2012 and 2013 cohorts when plants were measured in the
same growing season following spring planting (but see below on
requirements of foliage selection for the measurement). The observations
reported in this study were made throughout all years of the
experimental operation from 2009 to 2019 on different cohorts of
seedlings that ranged from two to eight years of age (See Tables 3 and
S2 for more details).