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\begin{document}

\title{Long-term hydrological and biogeochemical datasets from a Mediterranean
forest site (Montseny, NE Spain)}



\author[1]{Anna Avila}%
\author[1]{Ferran Rodà}%
\affil[1]{Centre de Recerca Ecologica i Aplicacions Forestals}%


\vspace{-1em}



  
  \date{June 18, 2024}


\begingroup
\let\center\flushleft
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\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
In humid regions, the chemical flux and cycling of elements is
intimately linked to the hydrologic cycle. This insight opened in the
late s. XX a worldwide avenue for the use of small watersheds as
ecological units to study the hydrological and biogeochemical
functioning of ecosystems at the small catchment scale. The Montseny
catchment research, starting in 1978, initially addressed the forest
response to acid rain. But continuous recording for about 4 decades in
two small catchments allowed to describe the changes in streamwater
chemistry related to changes in atmospheric deposition (with particular
emphasis to S, N and P deposition), to climate change and to the inputs
of African dust. Further research and new hypothesis testing may take
advantage of the collected data series in these long-term study sites at
a Mediterranean site. This is the motivation for the publication of the
quality-checked original stream and atmospheric deposition chemistry
files whose links accompany this paper.%
\end{abstract}%



\sloppy


\textbf{Long-term hydrological and biogeochemical datasets from a
Mediterranean forest site (Montseny, NE Spain)}

Avila, Anna\textsuperscript{1}; Rod\selectlanguage{ngerman}à, Ferran \textsuperscript{1}

\emph{\textsuperscript{1} CREAF, Campus Universitat Autònoma de
Barcelona, E08193 (Cerdanyola del Vallès), Catalonia, Spain}

Keywords: terrestrial ecology, forest ecosystem functioning, long-term
trends, streamwater chemistry, atmospheric deposition, Mediterranean
catchment, holm oak, \emph{Quercus ilex} , Montseny, La Castanya

\textbf{Introduction and antecedents}

Hydrological and biogeochemical studies in the Montseny holm oak
(\emph{Quercus ilex} ) forests started in 1978 with funding from a
Hispano-American project that allowed the onset of terrestrial ecology
studies in Spain. Later funding was provided by a suite of projects from
the Spanish Government and the European Commission. This research
followed the small catchment approach developed at Hubbard Brook
(Bormann et al. 1979) and, sustained over several decades, has been
instrumental in showing the responsiveness of forested ecosystems to
atmospheric deposition changes (Avila and Rodà 2012; Bernal et al. 2013;
Avila et al. 2020) and the uniqueness of stream N response to deposition
in Mediterranean forests as compared to temperate forests (Templer et
al. 2022). The effects of forest management on (1) stream chemistry has
been modelled (Neal et al. 1995) and that of climate change on stream
chemistry (Avila et al. 1996) and on the habitat suitability for an
endangered amphibian species as well (Ledesma et al. 2019). Mineral
dust, originated in North Africa (Avila et al. 2007) has been shown to
be an important contributor of nutrients and alkalinity to these forest
ecosystems (Avila et al. 1997, 1998). Further research and new
hypothesis testing may take advance of the collected data series in
these long-term study sites. This is the motivation for the publication
of the quality-checked original stream and atmospheric deposition
chemistry files of well curated small catchments at Montseny whose links
accompany this paper.

\textbf{Site description}

Two catchments, named TM9 and TM0, were instrumented in La Castanya
valley (Montseny mountains; 41,774853 °N / 2,351109 °E) in the early
1980s within a broad-scope study on forest ecosystem functioning (Rodà
et al. 1999). A gauging station was built at each site, consisting of a
stilling pond and a V-notch weir. At TM0 the V-notch had a 120° angle,
while the smaller TM9 weir had a 60° opening.

La Castanya valley is 40 km to the NNE of Barcelona and 27 km from the
Mediterranean coast. Climate is subhumid mesomediterranean: annual
precipitation is around 900 mm with a typical summer drought, but
interannual variability is very high. The bedrock is a metamorphic
phyllite, with quartz, chlorite, albite and muscovite as major minerals.
Two catchments were instrumented in Torrent de la Mina stream, a major
stream draining La Castanya valley: TM0, the main catchment and TM9, a
small tributary (Fig. 1, Table 1). Relief is steep and there are some
rock outcrops in the upper slopes of TM0 and in the TM0 stream channel.
TM9 is located in the lower slopes of TM0 and is totally covered by holm
oak. In TM0, two distinct physiographic units can be distinguished:
holm-oak and beech forests on very steep slopes and heathlands and
grasslands in the upland plateau (Fig. 1). Soils in the slopes of TM0
are colluvial, stony, well drained, and spatially heterogeneous due to
the rugged topography. Typically, they are shallow with a 0-5-cm organic
layer and depths varying between 0.25 to 1.5 m (Hereter, 1990, Hereter
and Sánchez 1999). They are classified as Entisols (Lithic Xerorthents)
or Inceptisols (Typic, Lithic or Dystric Xerochrepts; Soil Survey Staff
1992).

Table 1. Site characteristics\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
\begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Catchment\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Area (ha)\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Min. altitude (m)\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Max. altitude (m)\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Mean slope of main stream (\selectlanguage{ngerman}°)\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Prevailing direction of main stream\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Vegetation\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
Vegetation\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
TM9\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
5,9\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
710\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1036\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
35\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
N\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
100\% holm oak\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
100\% holm oak\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
TM0\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
200,4\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
650\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1343\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
26\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
NE\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
52\% holm oak 15\% beech 30\%heathland and grassland 3\% rock
outcrops\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig 1. Site characteristics of the gaged catchments (TM9 and TM0,
diamonds). LC1, LC2 and LC3 (stars) indicate the location of the
atmospheric deposition collectors for different periods: LC1 equipped
with bulk deposition and throughfall collectors for the period
1978-1980; LC2 with bulk deposition from 1983 to 2000 and throughfall
for 1996-1997; LC3 with bulk and wet only deposition for the period
2002-2019 (see Table 2) and with throughfall in 2011-2013.

\textbf{Methods}

3.1 Stream water flow and chemical data

Hydrological and stream chemistry data (which includes water
temperature) were monitored in the two catchments for a non-continuous
record in the period 1983-2019 (Table 2).

Table 2. Time periods and instrumentation used for hydrological
recording and sampling.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Catchment & Period & Instrument & recording method\tabularnewline
TM9 (60\selectlanguage{ngerman}° V notch) & 1983-1997 & WeatherMeasure stage level & continuous
recording on paper\tabularnewline
TM0 (120\selectlanguage{ngerman}° V notch) & 1990-2002 & OTTstage level record & continuous
recording on paper\tabularnewline
TM0 (120\selectlanguage{ngerman}° V notch) & 2007-2010 & OTTstage level record & continuous
recording on paper\tabularnewline
TM0 (120\selectlanguage{ngerman}° V notch) & 2010-2018 & Schlumberger pressure probe & data
logger 15 min data\tabularnewline
\textbf{Streamwater chemistry} & & &\tabularnewline
TM9 & 1983-1997 & grab sample & chemical analysis\tabularnewline
TM0 & 1983-2019 & grab sample &\tabularnewline
\bottomrule
\end{longtable}

In each field visit, manual reads of the limnimetric scale located at
the weir were taken and were used to calibrate the instrument flow
recordings. Discharge (D\textsubscript{i,} in L) was calculated for
successive small steps tracking the variation of the hydrographs (Fig.
2).

D\textsubscript{i} =
q\textsubscript{i}*(t\textsubscript{i+1}-t\textsubscript{i}) +
{[}(q\textsubscript{i+1}-q
\textsubscript{i})*(t\textsubscript{i+1}-t\textsubscript{i})/2{]},

being q\textsubscript{i} the instantaneous stream flow (in L/s at TM9
and in m3/s at TM0) and q\textsubscript{i+1} , the stream flow for the
next reading.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. Hydrograph decomposition tracking flow variation.

Grab samples of stream water were collected at each visit (approximately
weekly) several meters upstream from the respective stilling ponds in
high-density polyethylene 250 mL bottles. More frequent samples were
obtained during storms from 1983 to 1990 at TM9, collected with an
automatic sampler (Avila et al. 1992).

3.2 Atmospheric deposition data

Atmospheric deposition was monitored from 1978 through 2019 with some
interruptions. The location and the mode of sampling collection varied
in this period as described in Figure 1. Parallel bulk and wet
deposition sampling allowed comparison between the two methods: high
correlation between them was found for the different ions, BD exceeding
WD by around 37\% for base cations (Izquierdo and Avila 2012).

3.3 Analytical methods

Stream water and atmospheric deposition samples were taken to the
laboratory the same day of collection. Sample processing and analysis
are described in Avila (1996), Rod\selectlanguage{ngerman}à et al. (1993) and Avila et al.
(2020).

Annual mean concentrations were volume-weighted for all ions except for
H\textsuperscript{+}. At Montseny, with weekly rainwater pH values
varying between 4.0 and 8.0, alkalinity/acidity is the conservative
property to average. Volume-weighted mean alkalinity was then
transformed to mean pH with the equation: pH = log (alk) + 5.2 (units in
\selectlanguage{greek}µ\selectlanguage{english}eq L-1) which is derived from the equilibrium equations of the
CO\textsubscript{2} -carbonate system (Stumm and Morgan, 1981).

Deposition samples with African dust were filtered and chemical (ICP-MS
and ICP-AOE), mineralogical (X-Ray diffraction; Avila et al. 1997) and
genomic analysis of the particulates in the filters was later performed.
DNA was extracted using the Mobio PowerSoil DNA Isolation Kit (Mobio
Laboratories). PCR and high-speed multiplexed SSU rRNA gene Illumina
MiSeq sequencing were carried out for 16S and 18S rRNA genes (https://
rtsf.natsci.msu.edu/).

Element input or export from the catchment was calculated as the sum of
the element inputs or exports for each sample in the period of interest
(e.g. month, year). Inputs are calculated as the product of bulk or wet
deposition concentrations by precipitation. Export for a sample
S\textsubscript{i} is calculated as the product of the element
concentration in the sample (S\textsubscript{i} concentrations in \selectlanguage{greek}µ\selectlanguage{english}eq/L)
by the water drained in the period starting at the point midway from
S\textsubscript{i} to the previous sample S\textsubscript{i-1} and
ending at the point midway from S\textsubscript{i} to the next sample
S\textsubscript{i+1} (water yield in mm = L/m2). This mode of
calculation has been found adequate for weekly or biweekly stream
sampling frequencies, as is here the case (Rekolainen et al. 1991,
Swistock et al. 1997). Drainage is also expressed per unit catchment
area (drainage in mm) in the hydrology files.

\textbf{Principal results}

4.1 Streamwater chemistry

The Torrent de la Mina streamwaters are well buffered, with
Na\textsuperscript{+} and Ca\textsuperscript{2+} co-dominating as main
cations and alkalinity as the main counterpart, both in TM0 and TM9.
Mean pH was 7,47 and 7,54 for TM0 and TM9 respectively. Solute variation
at TM9 was explained as the mixing of groundwater and deep-soilwater
components, contributing 70 and 30\% of annual runoff, respectively
(Pi\selectlanguage{ngerman}ñol et al. 1992, Avila et al. 1996). Both catchments responded
rapidly to changes in atmospheric deposition: an alkalinity increase was
found associated to declining sulfate deposition (Avila and Rodà 2012).
This responsiveness may be explained by a low residence time of water in
these catchments compared to other geographical areas (Bernal et al.
2013). Nitrogen was strongly retained in the catchments, with only 2\%
of total N deposition being exported as inorganic N in streamwater.
However, higher nitrate export occurred during the wettest months
(November and March). Moreover, this pattern showed an increasing trend
in time, suggesting the onset of N saturation in the catchments (Avila
et al. 2020).

Torrent de la Mina data have been used for modelling the stream
chemistry response to forestry practices (Neal et al. 1995 ) and to
climate change (Avila et al. 1996). More recently, the stream
hydrological response to climate change scenarios from IPCC and to land
use changes was modeled in the context of LIFE-Tritó project. The aim
was to predict the vulnerability of the critically endangered Montseny
newt (\emph{Calotriton arnoldi} ) to expected habitat changes in future
scenarios (Ledesma et al. 2019). In this work, the combination of
hydrological modelling, climatology and species ecology knowledge
provides an example of the use of data gathering to help in the
management of an endangered species.

Atmospheric deposition

Rainwater in Montseny is rarely acidic, since sulfate and nitrate acid
anions are largely neutralized by base cations and ammonium (Rodà et al.
1993). Annual mean pH values ranged from 4,92 to 7,18 with a median of
6,79. All annual mean pH values below 5,50 occurred before 1990.
Frequent episodes of African dust transport (Avila and Peñuelas 1999)
are important contributors of alkalinity and of dissolved and
particulate elements to this site (Avila et al. 1997, 1998; Castillo et
al. 2017). Back trajectory analysis has revealed that the main
atmospheric transport pathway to Montseny is from the Atlantic Ocean,
delivering dilute and circumneutral rainwaters while acid episodes
predominated in local and European trajectories (Izquierdo et al.
2012a). Nitrogen deposition was higher with air masses coming from
Mediterranean/ south-east European provenances (Izquierdo et al. 2014).
Significant declining trends of S deposition (since the 1980s; Avila and
Rodà 2002) and N deposition (starting in the mid-2000s) have been found
(Aguillaume et al. 2016; Avila et al. 2020).

The role of African episodes as contributors of phosphorus deposition in
Montseny and in the near Mediterranean coast has been analysed by
Izquierdo et al. (2012b) and Longo et al. (2014). Because of concern on
the negative eutrophying effects of N deposition, particular attention
has been devoted to characterise the N inputs to Montseny (Rodà et al.
2002, Avila et al. 2002, García-Gómez et al. 2018). Throughfall sampling
and leaf washing experiments were undertaken at different times along
the recording period to distinguish dry deposition from canopy exchange
at the canopy level (Rodrigo et al. 2003, Avila et al. 2017). Lately,
the role of the leaf microbiome was assessed using a multiple isotope
approach (involving \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{17}O in NO\textsubscript{3}\textsuperscript{-} in wet
deposition and throughfall) and quantification of amoA genes: microbial
nitrification contributed but the NO3 dominant flux in throughfall was
atmospheric (Guerrieri et al. 2020).

The Montseny catchments have shown distinctive characteristics compared
to more temperate catchments, such as: \textbf{(1)} the hydrology is
strongly controlled by evapotranspiration, which accounts for 2/3 of the
incoming precipitation (Pi\selectlanguage{ngerman}ñol et al. 1999), \textbf{(2)} nevertheless,
there is a quick hydrologic response, as residence time of water in the
catchment is estimated around 4-5 months (Bernal et al.
2013),\textbf{(3)} inorganic N is strongly retained in the system, with
nitrate being the only inorganic nitrogen form detected in stream waters
(detection limit = 0,5 \selectlanguage{greek}µ\selectlanguage{english}eq/L) mostly during stormflows; as a
consequence, 98\% of DIN deposition is retained in the catchments (Avila
et al. 2020). The lack of relationship between DIN inputs and outputs in
the Montseny catchments contrasts with the finding that N deposition is
a strong driver of stream N export in temperate catchments (Templer et
al. 2022). On the other hand, similarly to other catchments worldwide
and corresponding to a general declining sulfate deposition trend,
sulfate concentrations in Montseny stream waters have decreased, while
alkalinity has increased (Avila and Rod\selectlanguage{ngerman}à 2012) providing well buffered
waters for these catchments lying on silicate bedrock.

\textbf{Data sets and availability}

We provide two datasets of open data for further exploration and
hypothesis testing. Files are provided for: (1) stream water flow and
stream water chemistry, doi:10.5281/zenodo.7228249; and (2) atmospheric
deposition, doi: 10.5281/zenodo.7228266.

\textbf{Acknowledgements}

In a study covering such a long-time span, a multitude of researchers
and field and laboratory technicians have collaborated. We provide here
a list (in alphabetic order) and apologize for any involuntary omission:
Laura Aguillaume, Jordi Bartolomé, Cristina Belillas, Susana Bernal,
Keith Beven, David Bonilla, Miquel de Cáceres, David Carreras, Sonia
Castillo, Lluís Comas, Antoni Escarré, Helena Estevan, Miguel Escudero,
Lluís Ferrés, Irene Fraile, Carles Gené, Carlos A. Gracia, Ester
Granados, Víctor Granda, Rebeca Izquierdo, Xavier Mayor, Carme Melción,
Colin Neal, Noemi Perez, Josep Piñol, Roger Prat, Xavier Querol, Angela
Ribas, Anselm Rodrigo, Jordi Serrano, Jaume Terradas and Claret Verdú.

Funding was initially provided by a Hispano-American project which was
followed by several projects from the Spanish Government and the
European Commission.

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