Disentangling elevation, annual flooding regime and salinity as hydrochemical determinants of halophyte distribution in non-tidal saltmarsh

Abstract

Background and aims: Hydrological disconnection, especially in a Mediterranean climate, creates coastal saltmarshes with an annual cycle of flooding that are unlike tidally inundated systems. Winter rainfall produces long, continuous hydroperiods, alternating with continuous exposure caused by evaporation in warm, rain-free summers. We aimed to distinguish the effects of elevation, hydroperiod and salinity on annual and perennial halophytes in such a system.

Methods: We recorded vegetation and sediment salinity in permanent quadrats on a marsh in the Doñana National Park, Spain, over seven consecutive years with widely differing rainfall. Elevation was determined from LIDAR data and the duration of the annual hydroperiod from satellite imagery. The independent effects of collaterally varying elevation, hydroperiod and salinity on species distribution were examined using generalized linear models and hierarchical partitioning.

Key results: Both hydroperiod and salinity were inversely related to elevation but interannual fluctuations in rainfall facilitated discrimination of independent effects of the three collaterally varying factors on halophyte distribution. Perennial distribution was strongly structured by elevation, whereas many annual species were more sensitive to hydroperiod. The independent effects of salinity varied according to individual species' salt tolerance from positive to negative. Thus life-history and, in the case of annuals, phenology were important in determining the relative impact of elevation and hydroperiod.

Conclusions: The consequences of elevation for halophyte distribution in seasonally flooded saltmarshes are fundamentally different from those in tidal marshes, because protracted and frequent flooding regimes require different adaptations, and because of the unpredictability of flooding from year to year. These differences could explain greater species diversity in non-tidal marshes and the absence of key saltmarsh species prominent in tidal marshes. The vegetation of non-tidal marshes will be particularly susceptible to the more extreme annual cycles of temperature and rainfall predicted for Mediterranean climates.

Keywords: Coastal marsh; Mediterranean climate; flooding regime; hydroperiod; life history; marsh embankment; remote sensing; salt tolerance.

Fig. 1.

(A) The distribution of elevation (relative to Spanish Hydrographic zero) of 170 permanent sample points on a grid covering the Doñana non-tidal salt marsh; (B) boxplot showing variation in hydroperiod from year to year (2004–2010) at the 170 sample points (different letters indicate significant differences between years in Mann–Whitney tests, P < 0.05).

Fig. 2.

The relationships between elevation and (A) mean hydroperiod per sample point (n = 170); (B) hydroperiod in all the sample points for every year (n = 1190); (C) mean electrical conductivity in surface soil (EC (1:1) 0–2 cm) per sample point (n = 19); (D) electrical conductivity in surface soil (EC (1:1) 0–2 cm) in all the sample points for every year (n = 101); (E) mean electrical conductivity in subsurface soil (EC (1:1) 8–10 cm) per sample point (n = 19); and (F) electrical conductivity in subsurface soil (EC (1:1) 8–10 cm) in all the sample points for every year (n = 101).

Fig. 3.

The distribution of abundance of perennial species and bare ground in relation to (from left to right): elevation, hydroperiod, surface salinity (soil EC (1:1) 0−2 cm) and subsurface salinity (soil EC (1:1) 8−10 cm). Cover values are means; n = 1065 for elevation and hydroperiod; n = 101 for salinity measurements.

Fig. 4.

Contour lines showing the relationship between subsurface salinity (soil EC (1:1) 8–10 cm) and elevation and (A) the probability of occurrence and (B) the percentage cover (arcsine-transformed) of perennial species (Suaeda vera, Arthrocnemum macrostachyum, Juncus subulatus, Bolboschoenus maritimus and Schoenoplectus litoralis). Contours show the predicted probability from generalized additive models. Darker colours indicate higher occurrence or cover. Species are ordered as in Fig. 3.

Fig. 5.

The distribution of abundance of annual species in relation to (from left to right): elevation, hydroperiod, surface salinity (soil EC (1:1) 0–2 cm) and subsurface salinity (soil EC (1:1) 8–10 cm). Cover values are means. n = 1065 for elevation and hydroperiod; n = 101 for salinity measurements.