2.2.2 Microclimate and microhabitat structure
To quantify microclimate and habitat structure, the air temperature (˚C)
and relative humidity (%) were measured at 0.5 h intervals over 24
hours using Hobo Pro RH/Temperature Data Logger (Onset Computer
Corporation, Pocassest, MA, USA) during our sampling period. Data
loggers were placed at 3 m intervals along each vertical transect,
starting 1 m above the ground. At each sampling point, the
photosynthetic photon flux density (PPFD) was recorded using a handheld
light meter (Quantum Lightmeter, Spectrum Technologies, Plainfield, 1L,
USA), and was further normalized by dividing by maximum light value
within each vertical transect to account for between-day variation in
lighting; the intent was to identify the relative (not absolute) light
environment of the forest canopy (Dial et al 2006). As microclimate was
only measured over a single 24-hour period for each transect, the
dataset does not capture daily microclimate variation at any single
vertical transect, but rather reflects the spatial variation in
microclimate across the vertical plane sampled. Although we measured
microclimate over a relatively short time, the daily variation and
vertical pattern in air temperature and relative humidity presented was
consistent with microclimate patterns from a nearby lowland primary
forest site over a longer time period (128 consecutive days measure in
Maliau Basin Conservation Area (4˚49’N, 116˚54’E), Hardwick et al.
2015).
We estimated one-sided total leaf area between sampling trays as a
measure of microhabitat structure at different sampling points for
transects 1 to 6 (T1-T6; Fig 2c). The leaf area within a sampling
interval was calculated by multiplying the number of leaf intersections
by the size of the base area of the interval which was 1
m2 (the area of the sample tray). We then used these
data to estimate leaf area index (LAI) over vertical intervals (sampling
methods described in Dial et al. 2004, 2006, and 2011; estimation
methods in Dial et al. 2006 and 2011). Conceptually, LAI refers to the
number of leaf layers above the ground surface that would be pierced by
a vertical line. For example, if LAI = 7, then there are, on average
seven leaf layers above a random point on the ground within that height
range; or 7 m2 of leaf area per m2of ground surface. We assumed (following MacArthur and Horn 1969) that
at any sample point in the canopy located at height z above the
ground, the foliage density was approximately equal in all directions.
Following this assumption at each height z , we systematically
measured horizontal distances (di ) with a laser
range finder to the nearest canopy element (foliage and stems) in 12
uniformly distributed azimuths every 2 m vertically from the ground to
the height of the horizontal traverse line supporting the vertical
transect. Using the n ≤ 12 distances to foliage at each sample
point, we found the mean distance (\(\overset{\overline{}}{d}\)) to
foliage, doubled the mean (assuming that the observer was on average
midway between foliage elements), then inverted it to find leaf
intersections per vertical meter at height z as
LAIz = 1/(2\(\overset{\overline{}}{d}\)). By multiplying
the LAIz by collection area (1m2) we
estimated the leaf area sampled within the interval.
Data analysesAll statistical analyses were conducted in R version 4.0.0 (R Core
Team, 2013)