Introduction
Dissolved organic matter (DOM) derived from vegetation and soils
accounts for up to 90% of the carbon (C) found in surface water
(Aitkenhead-Peterson, McDowell, & Neff, 2003) and DOM character
regulates microbial C processing in aquatic environments (Benner, 2003;
D’Andrilli, Junker, Smith, Scholl, & Foreman, 2019; Lennon & Pfaff,
2005). While encompassing less than 1% of the global land surface,
inland waters regulate carbon cycling from landscape to continental
scales (Battin et al., 2009; Cavallaro et al., 2018; Cole et al., 2007;
Raymond et al., 2013), and headwater streams drain 70% of North
American land area (Colvin et al., 2019). Changes to forest land cover
that affect terrestrial DOM character will influence aquatic ecosystem
metabolism (Cawley, Yamashita, Maie, & Jaffe, 2014; Lajtha & Jones,
2018; Williams, Yamashita, Wilson, Jaffe, & Xenopoulos, 2010), and
deciphering these relationships are crucial for predicting C cycling in
inland waters.
Vegetation responses to land management can alter watershed hydrology
and biogeochemistry for decades to centuries (Chantingy, 2003; Lee &
Lajtha, 2016; Rhoades, Hubbard, & Elder, 2017; Troendle & King, 1985;
Wilm & Dunford, 1948). In the mountain ranges of the American West,
where the short growing season and cold, dry climate limit forest
regrowth, the consequences of timber harvesting on snowpack, streamflow,
and nutrient export as well as on potential wildfire behavior and
susceptibility to insect outbreaks are long-lasting (Stottlemyer &
Troendle, 1999; Rhoades et al., 2017; Wilm & Dunford, 1948). Timber
harvesting decreases canopy interception and evapotranspiration
resulting in more snowpack water storage, and increased soil moisture
and streamflow (Troendle & King, 1985) with effects that remain
measurable for more than 50 years. Timber harvesting also shifts the
species composition of these forests and their development trajectory
for a more than a century (Collins, Rhoades, Hubbard, & Battaglia,
2011; Lotan & Perry, 1983).
Harvest of subalpine forests comprised of subalpine fir (Abies
lasiocarpa ), Engelmann spruce (Picea engelmannii ) and lodgepole
pine (Pinus contorta ) typically regenerate into lodgepole
pine-dominated stands (Collins et al., 2011). While the effects of land
cover, age, and species change on terrestrial nutrient cycling and
organic matter decomposition are well studied (Chantigny, 2003 for a
review), their consequences on adjacent aquatic ecosystem C cycling are
not. The chemical characteristics of litter in regenerating forests
alter hillslope DOM inputs relative to those in old-growth forest (Beggs
& Summers, 2011; D’Andrilli et al., 2019), likely regulating the
reactivity of DOM exported to streams at watershed scales. The highly
polyphenolic and protein-like DOM released from lodgepole pine litter in
regenerating stands, for example, is more reactive than that generated
by old-growth, mixed-conifer forests (Beggs & Summers, 2011; Yavitt &
Fahey, 1984 & 1986). Connecting the effects of land cover type on the
reactivity of DOM inputs from the forest floor to the reactivity of
hillslope DOM exports will increase understanding of the effects of land
cover change on watershed-scale C cycling.
The consequences of land cover and tree species shifts on
watershed-scale DOM dynamics benefit from techniques that differentiate
DOM reactivity of hillslope inputs and exports. Analysis of fluorescing
components of DOM (FDOM) has become a relatively routine, low-cost
approach to characterize the relative abundance of biologically reactive
organic molecules (Smith et al. 2018). Correlation between fluorescing
and non-fluorescing DOM molecules of similar chemical structures and
biological reactivities expands the utility of FDOM as an index of
overall DOM quality (Stubbins et al., 2014). Optical characterization
can be used in conjunction with heterotrophic microbial oxygen
consumption assays to identify which DOM components are microbially
created or utilized (D’Andrilli et al., 2019).
In this study we compare DOM inputs from litter leachate and the
concentration, character and biological reactivity of DOM exports in
subsurface flow between adjacent old-growth subalpine and second-growth
pine forests. Early work at these sites documented that clear cut
harvesting resulted in lasting increases in snow accumulation,
subsurface discharge, and nutrient export (Reuss, Stottlemyer, &
Troendle, 1997; Starr, 2004; Troendle & Reuss, 1997). The amount of DOC
exported from forests is determined by O horizon C content, with
old-growth forests exporting more DOC compared to regenerating ones
(Cawley et al., 2014; Lajtha & Jones, 2018; Pacific, Jensco, &
McGlynn, 2010). However, DOC reactivity is affected by C character and
processing within subsurface flow paths (Lehmann & Kleber, 2015).
Because old-growth forests generally have deeper O-horizons composed of
more recalcitrant DOM (Jandle et al., 2007; Johnson, Johnson,
Huntington, & Siccama, 1991), we expect greater flux but lower
biological reactivity of DOC from these forests. Conversely, we
hypothesize that DOM exported from pine-dominated, second-growth forest
to contain proportionally more biologically reactive DOM typical of the
water-soluble proteins and polyphenolics found in pine needle litter
(Beggs & Summer, 2011). Western conifer forests often require a century
to regrow to their pre-harvest stand structure (Burns & Honkala, 1990),
so the extent of old-growth forest cover has decreased during more than
a century of timber harvesting and most forests are in intermediate
stages of recovery (Anderson‐Teixeira et al., 2013; Hurtt et al., 2011;
McDowell et al., 2020). This study evaluates how forest change following
clear-cut harvesting alters the reactivity of DOM from the landscape to
aquatic ecosystems.