Nine Years of Irrigation Cause Vegetation and Fine Root Shifts in a Water-Limited Pine Forest

Abstract

Scots pines (Pinus sylvestris L.) in the inner-Alpine dry valleys of Switzerland have suffered from increased mortality during the past decades, which has been caused by longer and more frequent dry periods. In addition, a proceeding replacement of Scots pines by pubescent oaks (Quercus pubescens Willd.) has been observed. In 2003, an irrigation experiment was performed to track changes by reducing drought pressure on the natural pine forest. After nine years of irrigation, we observed major adaptations in the vegetation and shifts in Scots pine fine root abundance and structure. Irrigation permitted new plant species to assemble and promote canopy closure with a subsequent loss of herb and moss coverage. Fine root dry weight increased under irrigation and fine roots had a tendency to elongate. Structural composition of fine roots remained unaffected by irrigation, expressing preserved proportions of cellulose, lignin and phenolic substances. A shift to a more negative δ13C signal in the fine root C indicates an increased photosynthetic activity in irrigated pine trees. Using radiocarbon (14C) measurement, a reduced mean age of the fine roots in irrigated plots was revealed. The reason for this is either an increase in newly produced fine roots, supported by the increase in fine root biomass, or a reduced lifespan of fine roots which corresponds to an enhanced turnover rate. Overall, the responses belowground to irrigation are less conspicuous than the more rapid adaptations aboveground. Lagged and conservative adaptations of tree roots with decadal lifespans are challenging to detect, hence demanding for long-term surveys. Investigations concerning fine root turnover rate and degradation processes under a changing climate are crucial for a complete understanding of C cycling.

Responses of sapwood ray parenchyma and non‐structural carbohydrates of Pinus sylvestris to drought and long‐term irrigation

“…; Herzog et al . ), resin duct production (Rigling et al . ), cell lumen diameter and cell wall thickness (Eilmann et al .…”

“…; Herzog et al . ). We measured NSC reserves and rays in 5‐year blocks of tree‐rings in the stem sapwood of 40 mature trees.…”

A decade of irrigation transforms the soil microbiome of a semi‐arid pine forest

“…; Herzog et al . ). The plots were aligned side by side along a channel fed by the Rhone River, from where water was taken to irrigate four randomly selected plots (further termed ‘irrigated’).…”

“…) and greater fine root biomass (Herzog et al . ). Furthermore, irrigation caused significant shifts in plant community composition and increased vegetation cover (Herzog et al .…”

“…) as well as ectomycorrhizal fungi such as Basidiomycota as a consequence of a shift in vegetation (Herzog et al . ) and increased symbiosis to protect the plants from desiccation and starvation (Brunner et al . ).…”