The páramos form a tropical alpine grassland biome located mostly in the
Northern Andes (between 11°N and 8°S) and parts of Central America
(Figure 1), above the tropical mountain forest biomes (⁓3000 m a.s.l.)
and below the cryosphere (⁓5000 m a.s.l.) (Josse et al., 2009; Luteyn,
1999). Their delineation is not always clear because of deforestation
and increasing encroachment of the lower páramos for agricultural
purposes (López Sandoval and Valdez, 2015; Tovar et al., 2013a) and
ongoing influence of global-warming that induced glacier retreat
(Morueta-Holme et al., 2015).
R. G. M. Hofstede et al., (2003) estimated that the páramos occupy an
area of around 35000 km2 of the tropical Andes.
Geographically, the largest extent of páramos occurs in Colombia and
Ecuador and smaller, disconnected patches exist in the Andes of
Venezuela (Páramos de Mérida) and Costa Rica (Cerro Chirripó). The
southern limit of the páramos is known as the Jalca grasslands
(Sánchez-Vega and Dillon, 2006; Tovar et al., 2012), a transition
vegetation to the Puna biome of the central and south Peruvian Andes
(Cuesta et al., 2017; Ochoa-Tocachi et al., 2016a). However, ecosystems
with very similar biogeographical and hydrometeorological
characteristics occur as far south as Bolivia, where they are known as
the Páramo Yungeño (Jørgensen et al., 2014).
The páramos are highly fragmented because of the complex topography of
the Northern Andes and underwent in the past changes in isolation and
connectivity (Flantua and Hooghiemstra, 2018). Glacial and interglacial
periods raised and lowered respectively the upper forest line leading to
changes in connectivity that have directly impacted the flora of the
region (Flantua and Hooghiemstra, 2018; van der Hammen, 1982;
Hooghiemstra and Van der Hammen, 2004). These changes are one of the
main drivers of the high levels of endemism and species diversification
making the páramos the fastest evolved biome among biodiversity hotspots
(Madriñán et al., 2013).
The vegetation of the páramos (Figure 2) is dominated by tussock grasses
(Calamagrostis, Stipa and Festuca sp.) with scarce forest patches
(e.g., Polylepis sp.) with transitions to acaulescent rosettes
(e.g., Werneria nubigena, Hypochaeris sessiliflora) and cushions
plants (e.g., Azorella sp., Plantago rigida) at higher elevations
(Luteyn, 1999; Ramsay and Oxley, 1997). The total number of plant
species recorded in the páramos is 3595 distributed among 540 genera of
which 14 are endemic (Sklenar and Balslev, 2005). The páramos have the
highest number of plant species among tropical alpine flora (Sklenář et
al., 2014) and are fundamental part of the habitats of emblematic
species such as the Andean condor (Vultur gryphus) and the
spectacled bear (Tremarctus ornatus).
Precipitation at the páramo is known for its remarkably spatial-temporal
variability (Buytaert et al., 2006d; Celleri et al., 2007) caused by the
interaction between various synoptic climate processes and the complex
topography. The mean annual precipitation between 2000 and 2014
calculated from the global precipitation product CHIRPS (Climate Hazards
Group InfraRed Precipitation with Station Data) varies between 150 and
4090 mm yr-1 (Figure 3A) with mean maximum values
reported in Costa Rica and minimum in Perú (Table 1). They are usually
referred to as “wet” ecosystems (Buytaert et al., 2006a; Padrón et
al., 2015)even though rainfall can be well below 500 mm
yr-1 in some of the drier páramos, such as those of
Chimborazo in Ecuador (Clapperton, 1990; Saberi et al., 2018). However,
rainfall regimes between 1000 and 2000 mm yr-1 are
more common, with values exceeding 3000 mm yr-1 in
some páramos on the Amazonian slopes of the Andes. The analysis of
rainfall extremes, evidenced that the southern component of tropical
airflow is important for the distribution of wet convection, leading to
a high intensity of precipitation in the Andean mountains. The mountains
at the other hand, dampen the airflow on a large-scale, enabling local
hygrothermal gradients to control extreme precipitation anomalies
(Pineda and Willems, 2018).
Compared to rainfall, the temperature range of the páramos is much more
homogeneous, since this is one of the distinctive characteristics of
their biogeographical niche (Tovar et al., 2013a). Average temperatures
range around 10ºC in the sub-páramos to close to 0º on the upper fringes
of the super-páramos, which borders the cryosphere (Buytaert et al.,
2006b).
The low latitude of páramos limits seasonal temperature variability,
which is predominantly induced by the seasonality of precipitation and
related cloud cover. Contrastingly, day-night temperature variations can
be extreme and are a direct result of the high altitude and Equatorial
position, which gives the páramos among the world’s highest influx of
shortwave radiation (Buytaert et al., 2006b).
From the actual evapotranspiration (ETa), 51% of the total annual
rainfall according to Carrillo-Rojas et al. (2019), only the component
of interception loss can be considered important at the páramos. Despite
the predominance of low-intensity rainfall and frequent fog, the
vegetation can lead to exceptionally high interception rates. For
instance, Ochoa‐Sánchez et al. (2018) reported interception values by
tussock grass in páramos between 10% to 100% of total rainfall, with a
maximum storage capacity of 2 mm. Cárdenas et al. (2017) estimated that
inputs from fog and drizzle can represent between 7% and 28% (120 and
212 mm yr−1) of the rainfall in Colombian páramos. In
areas covered by fog, horizontal precipitation, and cloud water,
interception amounts between 10% to 35% of the total precipitation
have been reported for forested catchments (Bruijnzeel, 2004; Pryet et
al., 2012), although Bonnesoeur et al. (2019) indicate that values
between 2% and 8% may be more common. Actual evapotranspiration (ETa)
shows values of 646 mm yr−1 (Buytaert et al., 2007a)
and 723 mm yr−1 (Córdova et al., 2015). However, when
data are limited errors are thought to be as high as 30%
(Carrillo-Rojas et al., 2016; Córdova et al., 2015). Carrillo-Rojas et
al. (2019) improved evapotranspiration estimates for páramos using
Eddy-covariance, reporting a value of 635±9 mm yr−1for their study site in southern Ecuador. The same authors reported
water deficit during specific periods and highlighted the role of air
humidity variation in the control of the hydrological system.
Furthermore, Wouter Buytaert & Beven, (2011) emphasized the importance
of non-stationary hydrological processes such as changing
evapotranspiration, infiltration and routing due to vegetation growth.
In Figure 3B to 3D, indexes derived from TRMM-3b43 (Tropical Rainfall
Measuring Mission) and MODIS (Moderate Resolution Imaging
Spectroradiometer) were presented for the study region
(Arciniega-Esparza et al., 2020 (in prep)). For the Seasonal
Precipitation Index (SPI), high values represent a longer dry season
compared to a rainy season and low values the absence of precipitation
seasonality. The southern páramos therefore present a prolonged dry
season, with the highest indexes of evaporation (EI) and aridity (AL),
50% and 100% respectively, higher than the subsequent ones (Table 1).
Mid-ranges of EI were observed for most of the region with minimums in
Ecuador and Colombia. In addition, low tendencies of aridity (AI) in the
central and northern páramos can be observed.
The high altitudinal position of the páramos compared to the elevation
of most human settlements and cities makes them convenient “natural
water towers” (Messerli et al., 2004) from which water can be sourced
by gravity. The hydrological response of páramos is strongly related to
their soil conditions and extent of wetlands (Mosquera et al., 2015). W.
Buytaert et al. (2005) revealed that the probability of water stress
occurrence in wet paramo soils is reduced due to their hydraulic
conductivity, which prevents soil moisture to drop below 60 vol%.
Páramos’ soils are therefore important regulators of runoff production
(Harden, 2006). The most organic-rich soils primarily located at the
foot of the hillslopes and in the bottom of the valley are commonly
covered by cushion plants and present nearly saturated conditions
(Buytaert et al., 2005) while the more freely draining soils are
situated on the hillslopes under a cover of tussock grass. The
rainfall-runoff response is mainly controlled by the variable extent of
the saturated zone in the valley bottom (Correa et al., 2017). Water
from valley soils compose around 40% of the runoff in a headwater
catchment in the Ecuadorian páramos. During rainier periods, the
contributing area expands, thus increasing the connectivity with lateral
flow from hillslopes and therefore its contribution to the channel
network. In drier conditions, only the deep soil horizons from the
hillslopes seems to be hydrologically connected (Correa et al., 2017)
and water from these horizons drains via the riparian area into streams
(Crespo et al., 2011). A particular characteristic of most of the
studied high Andean catchments is the presence of underlying impermeable
bedrock that minimizes deep infiltration and greatly limit the
groundwater contributions (Buytaert et al., 2007a). Nevertheless, some
regions also present deep permeable soils, sustain important aquifers
(Buytaert et al., 2006a; Favier et al., 2008) and proof the influence of
shallow groundwater on stream generation especially when soil moisture
decreases (Correa et al., 2017, 2018; Favier et al., 2008).
Runoff ratios (ratio between annual precipitation and annual discharge)
between 0.50 and 0.70 have been reported in natural wet páramos
(Buytaert et al., 2007a; Ochoa-Tocachi et al., 2016a) reaching even
values of 0.9 during rainfall-runoff events (Correa et al., 2016). The
water yield increases with the amount of wetland, likely because of
saturation excess flow (Mosquera et al., 2015). Additionally, Buytaert
& Beven (2011) highlighted the importance of threshold-triggered
hydrological processes, such as disconnected water storages within the
catchment microtopography. In addition, threshold-driven hydrological
processes, such as disconnected water storage within the microtopography
of the catchment, play a crucial role in the runoff generation and
catchment response (Buytaert and Beven, 2011).
Figure 3
Table 1