PEAT
DEPTH AS A CONTROL ON MOSS WATER AVAILABILITY DURING DROUGHT
P.A. Moore*, B.D. Didemus, A.K. Furukawa, and J.M.
Waddington
School of Earth, Environment and Society, McMaster University, Hamilton,
ON, L8S 4K1
*Corresponding author: Paul Moore,
paul.moore82@gmail.com
Key words: peatland, water table, tension, Sphagnum moss, peat,
moisture stress
Short running title: Peat depth and moss water availability
ABSTRACT
Peatlands are globally important long-term sinks of carbon, however
there is concern that enhanced moss moisture stress due to climate
change mediated drought will reduce moss productivity making these
ecosystems vulnerable to carbon loss and associated long-term
degradation. Peatlands are resilient to summer drought moss stress
because of negative ecohydrological feedbacks that generally maintain a
wet peat surface, but where feedbacks may be contingent on peat depth.
We tested this ‘survival of the deepest ’ hypothesis by examining
water table position, near-surface moisture content, and soil water
tension in peatlands that differ in size, peat depth, and catchment area
during a summer drought.
All shallow sites lost their WT (i.e. the groundwater well was
dry) for considerable time during the drought period. Near-surface soil
water tension increased dramatically at shallow sites following water
table loss, increasing ~5–7.5× greater at shallow sites
compared to deep sites. During a mid-summer drought intensive field
survey we found that 60%–67% of plots at shallow sites exceeded a 100
mb tension threshold used to infer moss water stress. Unlike the shallow
sites, tension typically did not exceed this 100 mb threshold at the
deep sites. Using species dependent water content - chlorophyll
fluorescence thresholds and relations between volumetric water content
and water table depth, Monte Carlo simulations suggest that moss had
nearly twice the likelihood of being stressed at shallow sites (0.38 ±
0.24) compared to deep sites (0.22 ± 0.18). This study provides evidence
that mosses in shallow peatland may be particularly vulnerable to warmer
and drier climates in the future, but where species composition may play
an important role. We argue that a critical ‘threshold’ peat depth
specific for different hydrogeological and hydroclimatic regions can be
used to assess what peatlands are especially vulnerable to climate
change mediated drought.