Land cover change affects the amount and reactivity of DOM exported from
old growth and regenerating forests in headwater ecosystems
Abstract
Headwater forest ecosystems of the western U.S. generate a large portion
of the dissolved organic matter (DOM) transported across North America.
Land cover changes that alter forest structure and forest species
composition affect the quantity and composition of DOM transferred to
aquatic ecosystems. Clear-cut harvesting effects ~1% of
the forest area of North America annually, leaving most forests in
varying stages of successional regrowth, and the total area of
old-growth forest decreasing. The consequences of this widespread
management practice on watershed carbon cycling remain unknown. We
investigated the role of land cover change from old-growth subalpine
forest to lodgepole pine dominated second-growth on the character and
reactivity of DOM hillslope exports. We evaluated inputs of DOM from
litter leachates and export of DOM collected at the base of trenched
hillslopes during a three-year period (2016-2018) at the Fraser
Experimental Forest in northcentral Colorado, USA. Dissolved organic
carbon (DOC) and total dissolved nitrogen (TDN) were higher in lateral
subsurface flow draining old- versus secondary-growth forest.
Fluorescence spectroscopy showed that the DOM exported from the
old-growth forest was more heterogeneous and aromatic and that
proteinaceous, microbially processed DOM components were more prevalent
in the second-growth forest. Biological oxygen demand (BOD) assays
revealed much lower microbial metabolism of both DOM inputs from litter
leachate and subsurface exports from old-growth forest. Old-growth and
second-growth forests are co-mingled in managed ecosystems, and our
findings demonstrate that the influence of species composition on DOM
inputs can affect the reactivity of DOM transferred from terrestrial to
aquatic ecosystems.