Soil respiration (

Soil respiration (

One obstacle to robust measurements of

Plant root respiration constitutes

This study examines the effect of tree proximity on measured

the amount of basal area close to

this effect would occur in the growing (leaf on) season but not in the dormant (leaf off) season, because root respiration is much higher during the growing season; and

this effect would be stronger during drier times of year, because trees might maintain access to deep soil moisture (Burgess et al., 1998) and thus continue respiring even when the surface soil is dry.

This study was conducted in a mid-Atlantic, temperate, deciduous forest at
the Smithsonian Environmental Research Center (SERC) in Edgewater, MD, USA.
Three sites were chosen along Muddy Creek, a stream draining into an arm of
the Chesapeake Bay. Each site was separated by

Study site characteristics of each site along Muddy Creek,
including trees per hectare, cumulative basal area, main soil types, and
dominant tree species by percent of basal area. Values are the mean

Soil respiration measurements were taken using an infrared gas analyzer
(IRGA; LI-8100A, LI-COR Inc., Lincoln, NE) with a 20 cm diameter opaque soil chamber
attached. Measurements were taken every 10–14 d from April 2018 to April 2019. The IRGA measures concentrations every second over a one-minute period
and calculates the

We recorded distance from the soil collar, diameter at breast height (1.37 m), and species of each tree within a 15 m radius of each soil
respiration measurement point (Fig. 1a). Dead trees were included in the
dataset but only account for

Respiration data were checked visually for artifacts or unusual outliers,
but we did not exclude any data a priori. Data were then combined with the proximity
measurements described above based on collar number. We used a linear
mixed-effects model to test for the influence of BA

Our secondary hypotheses, that the effect of BA

We used the spatial variability between collars within individual plots to
estimate the number of samples required for a robust estimate of the

We measured

Sample size required to estimate soil respiration with a particular error (delta, left column, fraction of mean flux) for different statistical power values. Values are the mean (standard deviation) between plots. Power is the probability that the test rejects the null hypothesis when a specific alternative hypothesis is true and informally connotes the degree of confidence that the measurement is within some delta value of the true mean.

There was large variability in the basal area and number of trees close to
the measurement collars (Fig. 3). The mean number of trees within 1, 5, and 10 m distance were 1, 6, and 21 trees (with respective nearby
basal areas of 0.0002, 0.24, and 0.88 m

Cumulative basal area for each soil collar (

The linear mixed-effects model using temperature, soil moisture, and basal
area within 5 m (BA

Residuals of a soil respiration model, incorporating temperature and soil moisture as independent variables, versus cumulative tree basal area within 5 m, by site. Each point is an individual observation (see Fig. 2). Regression lines are shown for each site; the black line is the overall trend. Note that five extreme points are out of the plot but are accounted for in the regression lines.

There were strong differences between the driest and wettest thirds of the
data, but our hypothesis that any basal area effect would be strongest in
the driest time of year was not supported. In the driest third of the data,
neither BA

Our a priori choice of 5 m for the basal area test was one of many possible choices and could potentially bias the results, as the actual extent of tree roots
at these sites is unknown. Refitting the main statistical model and
calculating variable importance metrics across a wide range of distances,
however, showed that basal area and its interactions with

Summary of the linear mixed-effects model testing the main hypothesis of
the effect of nearby tree basal area on soil respiration (the dependent
variable). Terms tested include soil temperature at 5 and 20 cm (

Test of robustness of results, run at various distances from soil
respiration measurement collars (

The

Sample size requirements to estimate annual

Many studies have examined whether autotrophic respiration (

Mechanistically, these findings could be explained by a number of processes.
When substrate supply from root exudates is higher during the growing
season,

There is also abundant evidence that soil moisture influences temperature
sensitivity: Suseela et al. (2012), for example, found that

We hypothesized that any BA

Tree basal area within 5 m of our

A number of limitations should be noted in our study design and execution.
First, this was not a fully spatially explicit analysis; we did not map the
collars relative to each other or construct a full spatial map of the
forest stands (Atkins et al., 2018). Such mapping can be useful to examine
the

Second, this study tested the effect of basal area on

We found that measurement collars with higher tree basal area within 5 m had
a significantly higher temperature sensitivity of

All code and data necessary to reproduce our results are available in our
online GitHub repository (

The supplement related to this article is available online at:

This study was designed by BBL and SCP. All fieldwork and data analysis was performed by SCP, except for the statistical analysis, which was written by BBL. NGM, JPM, and JCS provided feedback on the study design, analysis, and interpretation of results. SCP wrote the paper in close collaboration with all authors.

The authors declare that they have no conflict of interest.

This research was supported by the Smithsonian Environmental Research Center. We thank Alexey Shiklomanov for pointing out a crucial mistake in our statistical code.

This research has been supported by the Pacific Northwest National Laboratory (Laboratory Directed Research and Development, grant no. DE-AC05-76RL01830) as part of the PREMIS initiative.

This paper was edited by Michael Weintraub and reviewed by Daniel Epron and two anonymous referees.