BELOW GROUND MARSH GRASS PRODUCTION AND DECAY ALONG A TIDAL/ELEVATIONAL
GRADIENT.
Organic matter accumulation in marsh sediments is dependent on the balance
between production and decay of belowground materials which, in turn, are
dependent on the plant species and the sediment properties. Plant species
distribution and dynamics vary greatly along transects from the creekbank to
the high marsh in response to differences in elevation and tidal inundation.
The combination of plant species interspecific and intraspecific differences
that occurs along an elevational/tidal gradient may result in differences in
organic matter production above- and belowground and in decomposition of the
organic matter.
We used a litter bag technique to compare root and rhizome decay of Spartina
alterniflora and Juncus roemerianus along an elevational/tidal gradient
(Fig. 1)
including low marsh (intermediate height-form S. alterniflora),
mid marsh (short height-form S. alterniflora), and high marsh (J.
roemerianus) locations. Experiments were carried out in the marsh
surrounding the upper portion of Phillips Creek which drains into the coastal
lagoon complex of the Virginia Coast Reserve. The marsh is located
behind a relict sand ridge, and the surrounding uplands are either farm lands
or pine-forest wood lots. The distribution of plants in Phillips Creek marsh
is very patchy: Spartina alterniflora is the dominant plant species near
the regularly flooded low marsh (intermediate form) and in the occasionally flooded
mid marsh (short form) while large patches of Juncus roemerianus are dominant
in the infrequently flooded high marsh areas. Transects were established along
level contours in the low, mid, and high marsh areas. Litter bags containing dead
roots and rhizomes of either S. alterniflora or J. roemerianus were inserted
vertically into the sediments at 30 cm intervals along the transects. Over
1000 litter bags were placed in the marsh on February 6, 1991.
Nylon litter bags (1 x 2 mm opening) were filled in 10-cm sections with 12 g
(wet weight) of dead roots and rhizomes per section. After filling, each
section was sewn closed to separate it from the adjacent section so that the
vertical distribution of decomposition and root growth could be examined.
Three bags of each root litter type were randomly selected for sampling from
each marsh area. Bags were collected approximately every two months and
returned to the laboratory where live roots and rhizomes were sorted from the
dead material. All organic materials were dried at 80 degrees C. Live and
dead roots were handled separately. The rate of litter decay was measured as
ash-free dry weight (AFDW) loss over time. Root productivity was calculated
as the difference between the minimum and maximum amount of live root
material in the bags during a growing season. Pore water sippers installed at
5 and 15 cm depths in the sediments were used to sample sediment pore water.
Salinity and pH, and the concentrations of NH4+,
PO43-, and H2S of the pore
water were also monitored in the low, mid, and high marsh areas.
Little difference in weight loss was observed between the mid marsh and high
marsh locations, but weight loss at the low marsh location may be more rapid
than at the two interior locations. Decay constants (k) were calculated using
an arithmetic and an exponential model for decay (Table 1).
The significantly greater values of k for Juncus are consistent with
the difference in the starting C/N ratios for the two plant materials (37:1
and 47:1; Juncus and Spartina respectively).
Root growth was highly variable, especially for the low and high marsh
locations where the total amount of live roots in the litter bags did not
exceed 300 g AFDW m-2. Root growth was much greater and less variable in the
mid marsh location than near the creek or in the high marsh (900 g AFDW m-2).
Aboveground plant standing stocks were greatest in the low and high marsh
areas as compared to the mid marsh location. However, total plant biomass
(above- and belowground) was significantly greater in the mid marsh because
short-form Spartina root growth was significantly greater than root
growth of either intermediate height-form Spartina or Juncus.
Sediment pore water chemical properties were very different among marsh
locations: mid marsh pore water salinities were lower on average than those
for the low and high marsh while NH4+ and
H2S concentrations were consistently greater than those for the
low and high marsh.
Based on one year of data from an ongoing 5-year experiment we can conclude
that:
- Regardless of the marsh location, the initial decay of Juncus roots
occurs more rapidly than that of Spartina. Differences in sediment
chemistry had no measurable affect on decay constants or on the amount of
organic material remaining in the litter bags after one year.
- Root growth is significantly different between marsh locations. This may
be related to differences in plant type or growth form or to differences in
sediment pore water chemistry.
- The differences in organic matter accumulation in these marsh sediments
are related to the type of plant and its ability to produce roots.
- The mid marsh location may represent a unique transitional area in salt
marshes as a result of its topographic position and tidal inundation
characteristics.
Table 1. Decay constants and percent AFDW remaining after
one year of decay.
-------------------------------------------------------------------
Arithmetic Exponential
______________ ______________
Root Type Depth % REMAIN.
in Location (cm) k r2 k r2 1 YEAR
-------------------------------------------------------------------
Sa in Low 0-10 -0.10092 0.77 -0.00128 0.77 59.0
10-20 -0.09689 0.65 -0.00122 0.65 60.8
Jr in Low 0-10 -0.09383 0.66 -0.00168 0.71 59.4
10-20 -0.12589 0.83 -0.00196 0.85 59.5
Sa in Mid 0-10 -0.09781 0.89 -0.00117 0.88 68.6
10-20 -0.09003 0.81 -0.00107 0.81 69.7
Jr in Mid 0-10 -0.1256 0.75 -0.00164 0.71 64.7
10-20 -0.1442 0.99 -0.00194 0.99 53.2
Sa in High 0-10 -0.07944 0.83 -0.00112 0.82 71.3
10-20 -0.07411 0.91 -0.00103 0.67 74.3
Jr in High 0-10 -0.05760 0.18 -0.00118 0.32 67.5
10-20 -0.06970 0.34 -0.00119 0.31 65.1
Sa in Pruned 0-10 -0.06996 0.76 -0.00082 0.78 70.9
Low 10-20 -0.08514 0.72 -0.00101 0.70 67.5
Jr in Pruned 0-10 -0.13630 0.96 -0.00248 0.96 45.9
High 10-20 -0.15103 0.71 -0.00208 0.85 52.9
Dried Sa in 0-10 -0.11172 0.85 -0.00145 0.82 60.1
Low 10-20 -0.11382 0.86 -0.00141 0.85 62.2
Dried Jr in 0-10 -0.14630 0.76 -0.00194 0.65 53.7
High 10-20 -0.11870 0.72 -0.00200 0.78 56.6
Sa in Low 0-10 -0.12906 0.90 -0.00163 0.88 59.7
Perched Marsh 10-20 -0.10744 0.89 -0.00135 0.91 63.2
Sa in Low 0-10 -0.06656 0.26 -0.00081 0.20 78.8
Island Marsh 10-20 -0.11973 0.72 -0.00162 0.68 50.1
-----------------------------------------------------------------
(Fig. 1)
including low marsh (intermediate height-form S. alterniflora),
mid marsh (short height-form S. alterniflora), and high marsh (J.
roemerianus) locations. Experiments were carried out in the marsh
surrounding the upper portion of Phillips Creek which drains into the coastal
lagoon complex of the Virginia Coast Reserve. The marsh is located
behind a relict sand ridge, and the surrounding uplands are either farm lands
or pine-forest wood lots. The distribution of plants in Phillips Creek marsh
is very patchy: Spartina alterniflora is the dominant plant species near
the regularly flooded low marsh (intermediate form) and in the occasionally flooded
mid marsh (short form) while large patches of Juncus roemerianus are dominant
in the infrequently flooded high marsh areas. Transects were established along
level contours in the low, mid, and high marsh areas. Litter bags containing dead
roots and rhizomes of either S. alterniflora or J. roemerianus were inserted
vertically into the sediments at 30 cm intervals along the transects. Over
1000 litter bags were placed in the marsh on February 6, 1991.