Abstract
The forest-floor litter layer can retain substantial volumes of water, thus affecting evaporation and soil-moisture dynamics. However, litter layer wetting/drying dynamics are often overlooked when estimating forest water budgets. Here we present field and laboratory experiments characterizing water cycling in the forest-floor litter layer, and outline its implications for subcanopy microclimatic conditions and for estimates of transpiration and recharge. Storage capacities of spruce needle litter and beech broadleaf litter averaged 3.1 mm and 1.9 mm respectively, with drainage/evaporation timescales exceeding 2 days. Litter-removal experiments showed that litter reduced soil water recharge, reduced soil evaporation rates, and insulated against ground heat fluxes that impacted snowmelt. Deadwood stored ~0.7 mm of water, increasing with more advanced states of decomposition, and retained water for >7 days. Observed daily cycles in deadwood weight revealed decreasing water storage during daytime as evaporation progressed and increasing storage at night from condensation or absorption. Water evaporating from the forest-floor litter layer modulates the subcanopy microclimate by increasing humidity, decreasing temperature and reducing VPD. Despite the relatively small litter storage capacity (<3.1 mm in comparison to ~102 mm for typical forest soil rooting zones) the litter layer alone retained and cycled 18% of annual precipitation, or 1/3 of annual evapotranspiration. These results suggest that overlooking litter interception may lead to substantial overestimates of recharge and transpiration in many forest ecosystems.
Introduction
Interception and retention of precipitation are important for forest ecosystems and their water budgets. While the effects of canopy interception on rates of evapotranspiration and subcanopy precipitation (throughfall) are well known, less attention has been paid to interception and retention processes in the forest-floor litter layer, including forest-floor deadwood and fruit bodies (but see Gerrits and Savenije, 2011; Van Stan et al., 2017; Klamerus-Iwan et al., 2020). Forest floors comprise organic litter at various stages of decay (e.g., leaves, needles, bark, seeds, deadwood, fruit bodies) and short subcanopy vegetation (e.g., grasses, forbs, mosses, and low shrubs) overlying the soil surface (Gerrits and Savenije, 2011). Unlike precipitation intercepted by forest canopies, which is exposed to conditions that can drive rapid evaporation (Stewart, 1977), water intercepted by the forest floor may be retained for longer periods and evaporate more slowly (Baird and Wilby, 1999). Forest-floor litter layers are mostly root-free, and thus water fluxes are primarily controlled by gravitational forces, direct evaporation and the overall physical storage properties of the litter, rather than plant water uptake (Klamerus-Iwan et al., 2020). Although the absolute volumes that can be stored in the forest-floor litter layer are small (typically a few millimeters), the overall fraction of total annual precipitation that is temporarily retained in this layer, or potentially evaporated from this layer back to the atmosphere, can be significant (Gerrits and Savenije, 2011; Van Stan et al., 2017). This fraction could be especially large in precipitation regimes that are dominated by frequent low-intensity events separated by dry periods. Ultimately, almost all precipitation falling to the forest floor must travel through the litter layer, so storage, transport and evaporation processes taking place there can alter total evapotranspiration fluxes, plant-available water dynamics, and the rates and chemical composition of soil water recharge. The key questions are: how large are litter-layer storages, how long can they store, evaporate, and release water, and thus how important are they for the forest water cycle?
Whereas canopy interception losses have been characterized across numerous forest sites (Yue et al., 2021), only a few previous studies have evaluated the storage and retention capacity of the forest-floor litter layer, and most of these studies have been focused on the leaf component (e.g., Gerrits and Savenije, 2011; Klamerus-Iwan et al., 2020). Laboratory experiments on forest litter samples have been conducted to quantify their water storage properties (e.g., Walsh and Voigt, 1977; Putuhena and Cordery, 1996; Sato et al., 2004; Guevara-Escobar et al., 2007; Li et al., 2013; Ilek et al., 2021). Fewer studies (to our knowledge) have measured the water retention capacity of litter in situ (e.g., Brechtel, 1969; Thamm and Widmoser, 1995; Schaap and Bouten, 1997; Gerrits et al., 2007). Likewise, only a few studies have characterized and measured water cycling through deadwood, bark or fruit bodies (e.g., Harmon and Sexton, 1995; Błońska et al., 2018; Woodall et al., 2020; Van Stan et al., 2017; Levia et al., 2004), although those studies found deadwood to be a potentially large and important storage in the forest water cycle. Deadwood has been estimated to be approximately 34 m3 per hectare (Lachat et al., 2019) or 3.5% to 5.6% of total Swiss forest biomass (Hararuk et al., 2020), with substantial increases in recent decades due to changes in forest management (Lachat et al., 2019).
There is need for better estimates of how much, and for how long, water is stored in litter and deadwood of different types and decay conditions, as well as further research on how litter influences evaporation, energy balance, and microclimate. In this paper we report on a series of field and laboratory experiments to quantify forest-floor water fluxes in a temperate mixed forest, guided by two main research questions:
How much water is retained in the various forest-floor litter types (i.e., leaves, needles, spruce cones, deadwood), and for how long?
How does this litter-layer storage affect the forest water cycle, that is, how does it affect rates of soil water recharge and subcanopy evaporation, and what are its implications for humidity, temperature, and vapor pressure deficit in the subcanopy atmosphere?
Study site and methods
Our research site is located in an experimental forest in Zurich, Switzerland, recently established as part of the “Waldlabor Zurich” initiative (www.waldlabor.ch). The Waldlabor (”forest laboratory” in German) is a 1.5 km2 temperate mixed forest area at the edge of the city, with a mean annual temperature of 9.3 °C and mean annual precipitation of 1134 mm. Our research site is situated in the 0.3 km2 Holderbach catchment at the eastern edge of the Waldlabor, at a mean elevation of 510 m a.s.l. Since March 2020, we have measured all relevant climate variables approximately 150 m outside the forest with a compact all-in-one weather station (Meter Group - Atmos41), as well as temperature and relative humidity (using Sensirion SHT31 sensors) at different heights on two towers within the forest, all at 10‑minute resolution (see Figure 1). These small towers are located under spruce (tower 1) and beech canopies (tower 2), where we measured relative humidity and temperature at heights of 20, 95, 170, 245, 320, 395, 470 cm, and 50, 100, 200, 400 cm, respectively.