Tropical Montane Cloud Forests (TMCFs) are ecosystems located at mid to
upper elevations of low mountain systems with high humidity regimes
owing to mist or cloud immersion (Bruijnzeel et al., 2011b). The
functioning of these ecosystems is driven by the frequency of clouds and
the conditions created around them (i.e., solar radiation, horizontal
rain, and soil moisture) (Goldsmith et al., 2013; Oliveira et al.,
2014). Given their upwind location at high elevations, TMCFs are
characterized as filters for capturing atmospheric water (Bruijnzeel,
Mulligan et al., 2011). This confers a vital ecosystem service that
supports watersheds for human settlements and prevents erosion on
mountainous terrain (Bruijnzeel, Scatena et al., 2011; Oliveira et al.,
2014). In many instances, TMCFs have also been considered hotspots of
biodiversity and endemism, as they host many species restricted by their
microclimates and topography (Gentry, 1992; Karger et al., 2021).
However, many of these species are endangered, given their restricted
distribution and limited protection of these ecosystems (Betts et al.,
2017; Karger et al., 2021).
Current studies suggest that the TMCF’s climate will likely change in
coming decades because recent lowland deforestation and global warming
can impact their cloud dynamics (Foster, 2001; Helmer et al., 2019;
Lawton et al., 2001; Ponce-Reyes et al., 2012). Lowland deforestation
can affect cloud dynamics by changing the boundary layer over land (Wang
et al., 2009) increasing the orographic cloud base height upwind (Lawton
et al., 2001) and reducing cloud formation (Chagnon, 2004; Smith et al.,
2023). Likewise, temperature increases can impact cloud dynamics by
increasing the base height and evapotranspiration regimes (Still et al.,
1999). Changes in cloud regimes, and thus the TMCF’s climate, will
likely trigger losses in biodiversity and ecosystem services. For
instance, decreases in the abundance of birds, reptiles, and amphibians
have been associated with increases in temperature and their effects on
cloud-base lifting in these ecosystems (Pounds et al., 2006, 1999).
Thus, knowing how cloud regimes have been changing and to what magnitude
is critical for the future conservation of these ecosystems.
Although Helmer et al. (2019) predicted that the frequency of clouds at
TMCFs will likely decrease in coming decades, it remains unclear whether
changes are currently occurring and, if so, how severe they are. Current
cloud studies based on simulations or observations in TMCFs have focused
on describing their geographic extent (Los et al., 2021; Wilson and
Jetz, 2016) or projecting their future climate or extent according
climatic scenarios (Helmer et al., 2019; Ponce-Reyes et al., 2012;
Rojas-Soto et al., 2012) but not on investigating recent trends in
cloudiness. Surprisingly, characterization and monitoring of clouds over
TMCFs are rare, with minimal direct observations.
This study highlights the recent climate changes faced by TMCFs and
their potential implications for the conservation of this unique
ecosystem. Here, we compare current trends in low-cloud fractions (CF)
and Essential Climatic Variables (ECV) of 521 TMCFs with tropical areas.
In doing so, we hyp\ref{339889}othesized that recent traces of global warming over
the last two decades have led to a decrease in low-clouds on TMCFs. We
evaluated this using low-cloud fraction (CF) estimations from ERA5
reanalysis (Hersbach et al., 2020) between 1997 and 2020 and their
patterns among biogeographic realms. Overall, CF is defined by ERA5 as
the proportion of an area covered by clouds at the lower level of the
troposphere, a level at which clouds may occur with a pressure greater
than 0.8 times the surface pressure (Hersbach et al., 2018). It is
likely that the presence of clouds at low levels represents the
phenomenon of cloud immersion in TMCFs; thus, they could be used as a
potential descriptor of the cloudiness that prevail in these ecosystems.