Trends of ECVs and their association
with
low-clouds
Beyond low-cloud trends, our results also indicate that most TMCFs
exhibit shifts in their climate to warmer environments. Such changes are
described mainly by increases in the surface temperature (i.e., average,
minimum, and maximum), dew point, and pressure.
On a decadal basis, the observed
increases in average temperature (0.31 K per decade) might suggest that
these ecosystems are warming at higher rates when compared with tropical
forest regions (0.26 K per decade) (Malhi and Wright, 2004) or global
estimations (0.2 K per decade) (Allen et al., 2018). Warming rates could
be even stronger than global when TMCFs are detailed at the
macro-ecological level as occurs in Neotropic and Indomalayan TMCFs
(0.35 and 0.31 K per decade, respectively). However, we acknowledge that
the comparison among other studies should be performed with caution,
given the differences in methods.
Although it is expected that the warming of TMCFs also leads to
increases in evapotranspiration and presumably PET (Still et al., 1999),
its association with ΔCF was not as important as VSWC. It may be
considered that increases in PET driven by lowland warming correlate
with cloud formation upwind (Still et al., 1999). However, we evaluated
the ΔPET - ΔCF association as an in-situ factor without
considering the potential movement of moist masses. On the other hand,
it could be expected that the variability in VSWC would be more
consistent with changes in CF, as changes in clouds have the potential
to drive mist interception and water availability. Previous studies have
shown similar patterns where high soil moisture in lowland and
mountainous areas is related to cloud base heights and their regimes
(Lawton et al., 2001; Nair et al., 2008; Ray et al., 2006). At some
Neotropical TMCFs (e.g., Monteverde, Costa Rica), negative trends in
VSWC and precipitation could be tied together with increases in the
number of dry days (Pounds et al., 2006, 1999). An increasing number of
dry days in a year is likely to reduce annual averages of VSWC; thus,
increases in their frequency over time may also lead to VSWC declines.
The coherent variation of VSWC with dry days, low-clouds, or
precipitation may suggest the former is an indirect indicator of cloud
dynamics which could be used to for the local monitoring the of TMCFs
forest health. The reliability of VSWC as an indicator of forest health
at the TMCF’s should be evaluated in detail by future studies.
Overall, our results appear congruent with the climatic mechanisms
associated with changes in clouds at the TMCFs. Our observed increases
in surface temperature and its implication in negatives ΔCF could
exemplify the warming effects on cloud formation as well as the rising
of cloudbanks. Likewise, our observed increases in temperature are
likely to lead increases evapotranspiration which may implies decreases
in cloud formation. Despite this, there is an important variance that
our PLSR model does not explain according to the coefficient of
determination. Differences among
TMCFs associated with orographic effects, proximity to water bodies, El
Niño Southern Oscillation (ENSO) effects, lowlands land use changes can
also to contribute to the observed ΔCF (Lawton et al., 2001; Nair et
al., 2011; Still et al., 1999). For instance, ENSO effects — that have
shown to drive the TMCFs dry seasonal moisture (Anchukaitis and Evans,
2010) — are likely to have a higher impact on the TMCF’s cloudiness
that strictly dependent on water masses from the ocean than those that
strictly depend on lowland forest conditions. As such, future studies
should disentangle which drivers are closely related to local cloud
trends.
Broad implications and future
directions
The interconnected web of life in TMCFs is intricately tied to cloud
formation, making it imperative to address global changes to preserve
these vital ecosystems and safeguard their biodiversity and invaluable
services. Our results reveal that the fingerprints of global change are
already having a profound impact on the cloudiness of TMCFs. Altered
cloud patterns are likely to disrupt the delicate balance of these
ecosystems, which may lead to significant consequences for biodiversity
and ecosystem services. With a
changing climate affecting the frequency of clouds and the ecosystem
balance, species that rely on constant mist and humidity may need to
adapt to warmer conditions or migrate to higher elevations (Feeley et
al., 2020, 2011). The previous could be particularly important for
species in those sites where declines in low-clouds occur at higher
rates. Therefore, it is crucial for countries and conservation agencies
to consider the rate of change in cloudiness as a key climatic factor in
their preservation efforts and to develop appropriate management
strategies. For instance, South American countries such as Colombia,
Venezuela, and Bolivia, as well as Central American countries like
Honduras and Costa Rica, should recognize the reduction in low-clouds as
a significant threat to their TMCFs, given the pronounced declines
observed in these regions and the number of sites affected (Fig. 5).
However, we need to highlight that our findings are restricted to the
evaluated sites described as TMCFs (Aldrich et al., 1997a), which may
not represent the much broader TMCF distribution.