3.2 Comparison of Dry-Down Curves and Leaf Wetness Duration
between Vegetation Types
After rain ceased, the period of dry-down for all crop types tended to
follow a logarithmic decay (see Figure 2). Over the period from
saturation to fully dry, there was a very good fit to the linearized log
trends of the back-transformed data for all dry-down events
(R2 ≥ 0.93), and the slopes differed greatly between
vegetation types (p < 0.05).
The closed-canopy forest stayed, on average, wetter longer than any
other vegetation type (see Table 1 and Figure 3, p < 0.05).
Closed-forest leaf wetness dried at a relatively consistent rate after
the initial 100 minutes, when the rate became less steep. Based on this
observed shift in drying rate for the forest, a separate analysis of the
first hour after the event was conducted. When examining the initial
drying period of one hour, a linear fit was most appropriate. Closed
forest had a less negative slope than all other locations, –0.52%
wetness lost per minute compared to –1.22 ± 0.07% per minute. Finally,
forests took ~250 minutes longer to reach the level of
dryness observed in crops after only one hour (see Figure 2 and 3).
Papaya retained wetness longer at a lower level after the initial period
of similar drying, instead of continuing to dry at a uniform rate. When
comparing the three crops to each other individually, there is an
apparent inflection point before which the non-transformed slopes of the
three crops were –0.12, –0.11, and –0.10% per minute, respectively,
all very steep with similarly high goodness of fit (R2= 0.93, 0.94, 0.87 for papaya, taro, and sweet potato). However, at 50
minutes, papaya begins to have a slope of –0.02% per minute, less than
1/7 of the rate of change as during the first hour (Figure 2), and LWD
extends much longer than for the shorter canopy crops. Drying rates were
remarkably similar between taro and sweet potato (p < 0.05,
see Figure 3), which is surprising considering sweet potato is a
ground-cover crop and taro had a more distinctive canopy up to 1.3 m in
height.
The mean LWD in closed mature forest of 339 ± 174 minutes was 3.6 times
greater than crops, which averaged 94 ± 37 minutes, and more than 2.6
times the duration of open forest (129 ± 68 min, see Figure 3). Papaya
LWD behaved more similarly to open forest than to the lower crops
despite being similar in height to the crops, drying almost twice as
slowly (LWD = 137 ± 51 min) as taro and sweet potato (LWD = 73 ± 23 min,
all with p < 0.01). The dates and time of day for each
dry-down event as well as the event duration are listed in Table 1 by
vegetation type.
As height increased, the slope of the
dry-down curve flattened, as is expected if the higher canopy and
inter-canopy space retain moisture longer after a dry-down event. We
found strong evidence that height could be used to predict LWD
(R2 = 0.94, see Figure 3). For short-stature
vegetation around 1 m, a 1 m increase in height extends dry down rates
by 18%, whereas for taller vegetation around 10 m, a 1 m increase in
height only extends dry down rates by 0.17% per minute (Figure 4).
Discussion
In very wet tropical systems, the use of LWD as an additional variable
to study the impact of deforestation on available water in the forms of
canopy water storage capacity, intercepted rainfall, and leaf
evaporation appears to have merit, as it relates to canopy properties
that these other variables do not fully capture. A five-fold longer LWD
in forest than crop fields is consistent with previous findings,
demonstrating that LWD varies depending on position in canopy and
species tested (Sentelhas et al., 2005). Although we noted significant
differences in LWD between short and tall statured crops, we were unable
to discern a difference between small crops using comparisons between
only a few rain event dry-down cycles. However, we report a predictive
relationship that can better differentiate how small changes in canopy
heights, even in short-statured crops, can lead to incremental changes
in LWD.
Our results also demonstrate that
LWD can contribute to the larger question about the impact of land use
on hydrology, particularly in very wet systems with high interception
fractions. Premontane tropical forests undergo frequent wet-dry cycles,
thus the fraction of the water budget affected by interception is
relatively high compared to temperate regions. At this site, vegetation
below the forest canopy was wet up to 85% of the time (Aparecido et
al., 2016). Increased LWD is indicative not only of higher surface area
of leaves intercepting precipitation, which translates to less water
reaching the ground surface and more potential for water storage on the
canopy, but also the aerodynamic properties that allow air in the canopy
to mix, which lets leaves re-wet repeatedly throughout an event and
between events that occur sequentially. LWD may therefore be a key
covariant with I and E that could improve predictions of water budgets
in these forests and determine the effects of deforestation on stream
flow more precisely.
Tropical deforestation has been found to impact stream flow dynamics. In
Mexico, for example, deforestation of cloud forest was associated with
more erratic flows during the dry season (García Coll, 2002). Bruijnzeel
and Scatena (2011) also concluded that conversion of lower montane cloud
forests will likely considerably increase runoff locally due to low
cloud water interception. The shorter LWD we observed in the crop fields
show similar findings.
We further examined wetness differences between crop types by comparing
RH data from sensors in the crop boundary layer. We observed the
apparent conundrum that papaya humidity was lower, but it still took a
long time to dry. It is reasonable that the leaf surface properties held
water longer, supported by the fact that the first hour dried similarly
as taro. Although leaf wetness sensors do not mimic leaf surface
properties, the extended drying times in papaya suggest the leaf
surfaces might trap water and release it more slowly than in taro, which
has smoother and waxier leaves. Both leaves are much larger than sweet
potato leaves. Furthermore, because papaya plants are more
heterogeneously spaced in wide rows, the turbulent mixing ratio is
higher in the space surrounding that crop. That could explain why
usually papaya interspaces were less humid than taro’s or potato’s.
Increased turbulent mixing within more widely spaced crops may
counteract longer LWD to some extent, as seen in Bailey and Stoll (2013)
and Bailey et al. (2014). Sweet potato and taro have lower roughness
coefficients than the taller and structurally more complex papaya,
creating a more defined boundary layer than in papaya, which acts as a
pocket of humid air at the canopy (Raupach, 1994). Nonetheless, we
postulate that this dampening boundary layer effect combined with
greater surface area of papaya leaves could explain the net effect of
longer LWD in papaya than shorter crops. Further work is needed to
understand the relative importance leaf properties (size, texture),
canopy properties (shape, spatial orientation), and total surface area
on the water budget.
Our results demonstrate how the spatial orientation of plants in tall
canopies can affect drying rates. Even though within-crop biomass
differences were much smaller than biomass differences between forests
and crops, LWD differed by less than an order of magnitude. Asdak et al.
(1998) found that rainfall interception loss decreased with reduced
canopy cover in progressively more open forest areas, which approximates
our result from open canopy forest conditions. Thus, forest
fragmentation is likely to have an effect on evaporation through not
only decreased interception, but also faster drying rates. We found
statistically indistinguishable trends in LWD between papaya and open
canopy forest, suggesting that even modest canopy gaps can effectively
alter the hydrologic characteristics of otherwise intact forests. This
has interesting policy implications, because it suggests that
agroforestry, even of thin short-lived trees like papaya, may provide
similar hydrological services to that of open forests. This is an
important issue to highlight, as there are many programs focusing on
agroforestry in the tropics (Mercer, 2004).