Abstract
AbstractClimate change exposes ecosystems to strong and rapid changes in their environmental boundary conditions mainly due to the altered temperature and precipitation patterns. It is still poorly understood how fast interlinked ecosystem processes respond to altered environmental conditions, if these responses occur gradually or suddenly when thresholds are exceeded, and if the patterns of the responses will reach a stable state. We conducted an irrigation experiment in the Pfynwald, Switzerland from 2003–2018. A naturally dry Scots pine (Pinus sylvestris L.) forest was irrigated with amounts that doubled natural precipitation, thus releasing the forest stand from water limitation. The aim of this study was to provide a quantitative understanding on how different traits and functions of individual trees and the whole ecosystem responded to increased water availability, and how the patterns and magnitudes of these responses developed over time. We found that the response magnitude, the temporal trajectory of responses, and the length of initial lag period prior to significant response largely varied across traits. We detected rapid and stronger responses from aboveground tree traits (e.g., tree‐ring width, needle length, and crown transparency) compared to belowground tree traits (e.g., fine‐root biomass). The altered aboveground traits during the initial years of irrigation increased the water demand and trees adjusted by increasing root biomass during the later years of irrigation, resulting in an increased survival rate of Scots pine trees in irrigated plots. The irrigation also stimulated ecosystem‐level foliar decomposition rate, fungal fruit body biomass, and regeneration abundances of broadleaved tree species. However, irrigation did not promote the regeneration of Scots pine trees, which are reported to be vulnerable to extreme droughts. Our results provide extensive evidence that tree‐ and ecosystem‐level responses were pervasive across a number of traits on long‐term temporal scales. However, after reaching a peak, the magnitude of these responses either decreased or reached a new stable state, providing important insights into how resource alterations could change the system functioning and its boundary conditions.
Highlights
Climate change including changes in temperature and precipitation regimes chronically alter resource availability in forest ecosystems (Grossiord et al, 2020; Schuldt et al, 2020; Will et al, 2013) with far reaching biological consequences such as shifts in ecosystem structure and functioning (Estiarte et al, 2016; McDowell et al, 2020; Richardson et al, 2018; Williams et al, 2012)
Our results indicate that the initial response of the trees to increased water supply was from above-ground compartments and trees took longer (≥14 years) time to significantly increase their fine root biomass (Fig. 2 and 6)
Irrigated trees had higher fine root biomass in 2012 and 2014, our analysis identified that this higher fine root biomass was not significantly different compared to trees in control treatment (Fig. 2)
Summary
Climate change including changes in temperature and precipitation regimes chronically alter resource availability in forest ecosystems (Grossiord et al, 2020; Schuldt et al, 2020; Will et al, 2013) with far reaching biological consequences such as shifts in ecosystem structure and functioning (Estiarte et al, 2016; McDowell et al, 2020; Richardson et al, 2018; Williams et al, 2012). In forests, molecular and physiological responses to changes in resource availability or environmental conditions, usually occur immediately (Martin-StPaul et al, 2013; Timofeeva et al, 2017), while ecosystem-level responses such as changes in tree mortality and species composition take longer as they depend on altered competitive interactions among multiple species (da Costa et al, 2018; Korell et al, 2021) It is largely unknown whether any individual tree- and ecosystem-level change will persist and reach a new stable state or will return to the initial condition over longer time periods (Smith et al, 2015). Lack of responses over extended time periods have been reported (Felsmann et al, 2015; LeBauer & Treseder, 2008), which could occur when only such resources are altered that are not limiting tree growth or other functions (Leuzinger et al, 2011)
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