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.