Tree growth and recruitment in a leveed floodplain forest in the Mississippi River Alluvial Valley, USA

Forest Ecology and Management 334 (2014) 85–95

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Tree growth and recruitment in a leveed floodplain forest in the Mississippi River Alluvial Valley, USA Hugo K.W. Gee a,⇑, Sammy L. King b, Richard F. Keim a a b

Louisiana State University Agricultural Center, 225 School of Renewable Natural Resources, Baton Rouge, LA 70803, USA U.S. Geological Survey, Louisiana Cooperative Fish and Wildlife Research Unit, LSU Agricultural Center, 225 School of Renewable Natural Resources, Baton Rouge, LA 70803, USA

a r t i c l e

i n f o

Article history: Received 9 March 2014 Received in revised form 12 August 2014 Accepted 13 August 2014

Keywords: Floodplain Hydrology Canopy disturbance Dendrochronology Quercus lyrata Celtis laevigata

a b s t r a c t Flooding is a defining disturbance in floodplain forests affecting seed germination, seedling establishment, and tree growth. Globally, flood control, including artificial levees, dams, and channelization has altered flood regimes in floodplains. However, a paucity of data are available in regards to the long-term effects of levees on stand establishment and tree growth in floodplain forests. In this study, we used dendrochronological techniques to reconstruct tree recruitment and tree growth over a 90-year period at three stands within a ring levee in the Mississippi River Alluvial Valley (MAV) and to evaluate whether recruitment patterns and tree growth changed following levee construction. We hypothesized that: (1) sugarberry is increasing in dominance and overcup oak (Quercus lyrata) is becoming less dominant since the levee, and that changes in hydrology are playing a greater role than canopy disturbance in these changes in species dominance; and (2) that overcup oak growth has declined following construction of the levee and cessation of overbank flooding whereas that of sugarberry has increased. Recruitment patterns shifted from flood-tolerant overcup oak to flood-intolerant sugarberry (Celtis laevigata) after levee construction. None of the 122 sugarberry trees cored in this study established prior to the levee, but it was the most common species established after the levee. The mechanisms behind the compositional change are unknown, however, the cosmopolitan distribution of overcup oak during the pre-levee period and sugarberry during the post-levee period, the lack of sugarberry establishment in the pre-levee period, and the confinement of overcup oak regeneration to the lowest areas in each stand after harvest in the post-levee period indicate that species-specific responses to flooding and light availability are forcing recruitment patterns. Overcup oak growth was also affected by levee construction, but in contrast to our hypothesis, growth actually increased for several decades before declining during a drought in the late 1990s. We interpret this result as removal of flood stress following levee construction. This finding emphasizes the fact that flooding can be stressful to trees regardless of their flood tolerance and that growth in floodplain trees can be sustained provided adequate soil moisture is present, regardless of the source of soil moisture. However, future research efforts should focus on the long-term effect of hydrologic modification on stand development and on how hydrologic modifications, such as elimination of surface flooding and groundwater declines, affect the vulnerability of floodplain forests to drought. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Geomorphic and hydrologic alterations of river systems are globally widespread (Pinter et al., 2006; Poff et al., 2007; Bormann, 2010) and have profound implications to their associated floodplain forests. Flooding is a defining disturbance for floodplain forests that strongly influences seed dispersal, seed germination, seedling establishment, and stand structure and ⇑ Corresponding author. Postal address: #7, 2323 Oakmoor Dr SW, Calgary, AB T2V 4T2, Canada. Tel.: +1 587 224 4371. E-mail address: [email protected] (H.K.W. Gee). http://dx.doi.org/10.1016/j.foreco.2014.08.024 0378-1127/Ó 2014 Elsevier B.V. All rights reserved.

composition (Streng et al., 1989; Pierce and King, 2007; Nilsson et al., 2010; Rodríguez-González et al., 2010). Regeneration of floodplain forest species is also affected by light availability and few species are considered tolerant of both low light and frequent flooding (Niinemets and Valladares, 2006). Furthermore, these disturbances interact because flooding reduces vegetation in the ground layer and can reduce tree stem densities (Hanberry et al., 2012; Smith et al., 2013), thus increasing resource availability for more flood-tolerant but less shade-tolerant seedlings. Spring flooding can also be particularly detrimental to newly germinated lightseeded species relative to heavy seeded species, as the light-seeded

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species lack the reserves necessary for resprouting (Streng et al., 1989). Tree growth is also affected by hydrologic processes (Tardif and Bergeron, 1993; Rodríguez-González et al., 2010; Smith et al., 2013), although the relationship among tree growth and hydrologic processes varies among studies. Depending upon the species and flood characteristics, tree growth can be increased or decreased by flooding (Kozlowski, 1984). Few studies have examined growth response to long-term (>30 years) hydrologic and climatic variability on floodplains with altered flood regimes and most have focused on a single species and site (Palta et al., 2012; Keim and Amos, 2012). Far fewer studies, however, have evaluated the effects of levee construction on tree growth (Predick et al., 2009) or stand establishment (Gergel et al., 2002) in the floodplain, particularly their long-term effects. Levee construction completely eliminates overbank, riverine flooding and most geomorphic disturbances on the floodplain, resulting in precipitation, local runoff, and groundwater as the only sources of soil moisture. Levees are a common flood control measure and exist along many rivers throughout the world (Gergel et al., 2002; Opperman et al., 2009; Steinfeld and Kingsford, 2013), including the Mississippi River (USA). The Mississippi River and Tributaries project is the largest flood control project in the world and contains 6095 km of authorized embankments and floodwalls (Camillo, 2013). The Mississippi River Alluvial Valley (MAV) once supported about 10 million ha of floodplain forest (commonly known as bottomland hardwoods; BLH), but flood control facilitated broad-scale conversion to agriculture and only about 25% remains (Fredrickson, 2005). Most of the remaining forest is hydrologically altered by dams, levees, and/or channelization (Interagency Floodplain Management Review Committee, 1994). The consequences of hydrologic management for the structure and composition of floodplain forests are not well understood, in part because of the complexity of disturbance processes that define these systems. There is some evidence that decreased flood disturbance in the MAV has promoted species that are tolerant of shade but not of flooding (Foti, 2001; Hanberry et al., 2012), but it also may be that concurrent decreased severity of disturbances from logging have been responsible (Oliver et al., 2005). Isolating the effects of hydrologic alterations on stand development requires more work to identify how flooding and light availability interact. In this study, we used dendrochronological techniques to reconstruct tree recruitment and canopy disturbance patterns over a 90year period in a forest of the overcup oak-water hickory (Quercus lyrata-Carya aquatica) type (Wharton et al., 1982; Hodges, 1997) in the MAV subject to a major change in flood regime caused by a flood control levee. We chose two study species with differing tolerances for shade and flooding: overcup oak, which is highly tolerant of flooding and moderately intolerant of shade, and sugarberry (Celtis laevigata), which is weakly tolerant of flooding and very tolerant of shade (Hook, 1984; Meadows and Stanturf, 1997). We hypothesized that sugarberry is increasing in dominance and overcup oak is becoming less dominant since the levee, and that changes in hydrology are playing a greater role than canopy disturbance in these changes in species dominance. Furthermore, we expect that overcup oak growth has declined following construction of the levee and cessation of overbank flooding whereas that of sugarberry has increased. 2. Methods 2.1. Study area Concordia Parish in east-central Louisiana (USA) is a low-lying area that historically experienced overbank or backwater flooding

almost annually in winter and spring (US Army Corps of Engineers 1990). Richard K. Yancey Wildlife Management Area (YWMA) is in Concordia Parish near the confluences of the Red, Black, and Mississippi rivers and historically received deep, prolonged flooding for several months annually. In 1940, work was completed raising the mainline levee of the Mississippi River to protect the eastern portion of the parish from extreme floods (Louisiana Board of State Engineers, 1940). In 1955, the US Army Corps of Engineers completed a ring levee (Red River backwater levee) around the parish for further flood protection from the Red, Black, and Tensas rivers (Louisiana Department of Public Works, 1955). Following completion of the ring levee, riverine flooding ceased although rainwater and seepage water were impounded by the levees (US Army Corps of Engineers 1990). In 1988, a pump system was installed on Wild Cow Bayou north of the management area to further reduce surface flooding (US Army Corps of Engineers 1990). Study stands were in closed canopy forests at YWMA. The site was logged prior to acquisition by the Louisiana Department of Wildlife and Fisheries (LDWF) in 1973; no records exist on the volume or species composition of these harvests (D. Locascio, LDWF, pers. comm.) although we assume that timber harvest techniques were consistent among stands. We selected 3 stands in the northern portion of YWMA at varying elevations: Stand 1 (31°150 N, 91°450 W) at 10.2–11.4 m above mean sea level (msl), Stand 2 (31°180 N, 91°410 W) at 10.9–11.6 msl, and Stand 3 (31°160 N, 91°430 W) at 10.7–12.0 msl. All sites are on broad flats with poorly-drained clay soils of the Sharkey series (thermic Chromic Epiaquerts). Below these surface clays, which are up to 18 m thick, are point-bar sands and gravels at Stand 1 and backswamp silts and clays at stands 2 and 3 (Saucier, 1994). Mean annual precipitation is 140 cm and the growing season is approximately 220 days from March to September (Martin et al., 1988). 2.2. Hydrology There are no long-term data on surface and subsurface water levels at the study stands. To determine short-term hydrologic characteristics, we measured surface and subsurface water levels in each stand. Monitoring stations consisted of co-located surface and subsurface water measurements using self-logging capacitance water level probes (accuracy = 0.8 mm; Odyssey™; Dataflow Systems PTY Ltd, NZ) that electronically recorded water levels every 10 min from 1 Jan to 28 Dec 2009. Surface probes were mounted in 3.2 cm diameter  1.5 m length PVC pipes secured to trees and subsurface probes extended to a depth of 2 m within 3.2 cm diameter  3.0 m length perforated, screened PVC pipes. We filled well bores outside pipes with sand to prevent clogging of screens and filled the top 0.3 m with bentonite clay to prevent infiltration of precipitation. At each monitoring station, we calculated the number of flood days during the growing season (March–September) and number of days the water table was within the root zone during the growing season (top 1 m of soil; Phipps, 1979). 2.3. Vegetation sampling We conducted a forest inventory to determine community composition and structure at each stand. We established a total of 30 sampling plots at each stand on 125-m grids. Plots consisted of a 0.04-ha plot (11.3-m radius) for all live trees P10 cm diameter at breast height (DBH) and concentric 0.01-ha plot (5.6-m radius) for trees P5 cm DBH. We identified all trees to species and classified trees into four crown classes (dominant, codominant, intermediate, and suppressed) (Meadows et al., 2001). We measured the heights of 2–3 dominant or codominant trees at each sampling plot using a Haglöf Vertex IV Hypsometer (height resolution = 0.1 m).

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We calculated a modified importance value for each species (Curtis and McIntosh, 1951) by crown class at each stand:

Importance value ¼

Relative density þ Relative dominance  100% 2 ð1Þ

where relative density is proportion of total stem density and relative dominance is proportion of total basal area. Stand density was quantified as a stocking percentage according to guidelines published by Putnam et al. (1960) and quantified by Goelz (1995) for southern BLH forests, which can be used as a measure of inter-tree competition. Full (100%) stocking corresponds approximately to the stand density at which competition mortality begins, based on a detailed analysis of similar guidelines for baldcypress-water tupelo (Taxodium distichum-Nysssa aquatica) by Keim et al. (2010). We used generalized linear models to test statistical hypotheses that species importance values are related to sampling plot elevation (a fine-scale proxy of flooding), stand, and the interaction between elevation and stand (elevation⁄stand). We used a log link function as the linearizing transformation and normal probability distribution for interpretation of individual models for each species. To determine sampling plot elevation, we georeferenced (position accuracy 5–8 m) each plot center using the global positioning system and referenced coordinates at plot centers to elevation data from the publicly-available Louisiana statewide elevation survey measured by light detection and ranging (LIDAR) with a root mean square error of 9.8–12.5 cm (Watershed Concepts, 2006). We calculated elevations at plots centers as the mean of a 5 m  5 m area to compensate for coarse position accuracy of the GPS. 2.4. Recruitment patterns We collected two increment cores from at least one tree from each 10 cm diameter class (minimum 5 cm DBH) at each stand to sample the full range of diameters for each species if individuals were present. In total, we sampled 125 overcup oak trees 10– 109 cm DBH (median 37 cm) and 122 sugarberry trees 6–67 cm DBH (median 20 cm). Cores were at 50 cm height unless basal flaring or signs of rot required coring higher. Cores were air dried, mounted, and sanded with increasingly finer grit sandpaper until tree-ring boundaries were visible under a microscope (Stokes and Smiley, 1996; Orvis and Grissino-Mayer, 2002). We determined ages of increment cores by tree-ring counts and cross-dated ring widths for each species using narrow signature years (Yamaguchi, 1991). To improve the accuracy of tree age determination, we used a pith offset estimation technique that matches the growth pattern from cores where the pith was hit to those where the pith was missed (Villalba and Veblen, 1997). We adjusted estimates of tree ages of all cores by adding the mean time to reach the coring height based on a stem analysis conducted at 10 cm increments from the root collar to 1.4 m on 8 overcup oak saplings (mean 3 years to 50 cm and 5 years to 140 cm) and 10 sugarberry saplings (mean 2 years to 50 cm and 6 years to 140 cm) (Gee, 2012). We estimated ages of all remaining trees that were not cored by modeling the age-diameter relationship in cored trees using a third-order polynomial regression model constrained to intercept zero for each species (Condit et al., 1993; Loewenstein et al., 2000). 2.5. Tree growth We collected increment cores from overcup oak trees (Stand 1 – 26 trees; stand 2 – 21 trees; stand 3 – 22 trees) to determine the effects of altered hydrology on tree growth; the ages of sugarberry

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trees were inadequate for pre- and post-levee growth analyses. Trees were sampled from the dominant and codominant crown classes to most accurately reflect growth dynamics of the whole stand instead of local competition effects (Fritts, 1976). Two increment cores were obtained at breast height (1.37 m) from each tree to assist in cross-dating. Tree cores were air dried, mounted, and sanded with increasingly finer grit sandpaper until tree-ring boundaries were visible under a microscope (Stokes and Smiley, 1996; Orvis and Grissino-Mayer, 2002). All tree-ring widths were measured to the nearest 0.01-mm using a Velmex Unislide positioning stage (Velmex Inc., Bloomfield, NY) under a dissecting microscope. Ring widths from the two cores were averaged to produce a tree-ring series for each tree. Series were cross-dated to assign exact calendar years using narrow signature years (Yamaguchi, 1991), and the software COFECHA was used to statistically corroborate cross-dating (Holmes, 1983). Stand-level chronologies were constructed using all tree-ring series that fell within a 99% confidence interval of the stand mean (Holmes, 1983). Investigating the effect of hydrologic modifications on growth patterns is complicated by multiple factors affecting tree growth. We modeled ring widths based on a modification of Cook (1987) by Keim and Amos (2012) as, R(t) = f (A(t), C(t), H(t), e(t)), where R is ring width in year t; A is the expected age-related variation in ring width; C is the effect of annual variation in climate; H is the effect of variations in water level; and e is error not accounted for in other terms. To remove growth trends associated with tree age (A) while preserving variation that may be related to climate (C) or hydrology (H), we modeled A using the Regional Curve Standardization (RCS) method (Briffa et al., 1992; Esper et al., 2003). The RCS method models A by cambial age as a ratio of observed ring width to the mean ring width for a large number of trees from the region. To create the regional curve, all 137 tree-ring series were aligned by cambial age (Esper et al., 2003) and we fitted a generalized exponential regression model (Hugershoff curve; Bräker, 1981) to the data. Tree-ring series are autocorrelated because tree rings integrate responses to climatic variation for several years (Fritts, 1976). We modeled this short-timescale autocorrelation by fitting an autoregressive-moving average (ARMA) model to the residuals of each RCS series using the software ARSTAN (Cook and Holmes, 1986). Final chronologies were constructed by adding ARMA-detrended residuals back into each RCS series. To examine relationships between growth and climate (C), we used Pearson’s correlations between residual tree-ring chronologies and climate over the full time period. Climate variables were obtained using interpolations from station data by Daly et al. (1994). Climate data were monthly midpoint temperature and total monthly precipitation. To examine relationships between tree growth and hydrology (H), we used Pearson’s correlations between residual tree-ring chronologies, water level, and Palmer Drought Severity Index (PDSI; an index of soil moisture) for periods before and after hydrologic modifications. Water level data were mean monthly river stages from river gauges on the Mississippi (Natchez, Mississippi) and Atchafalaya (Simmesport, Louisiana) rivers. Monthly PDSI data were from the Natchez, MS NOAA National Climatic Data Center (NCDC, 2011). Annual growth utilizes energy produced in the ring of year formation and stored energy from the previous year (Fritts, 1976), so correlations with climatic and hydrologic variables were obtained from months from the beginning of the previous growing season to the end of the current growing season (March–September). To determine the combined effect of levee construction and drought on tree growth, we used Student’s t-tests to make comparisons of tree growth before and after levee construction during

H.K.W. Gee et al. / Forest Ecology and Management 334 (2014) 85–95

years with at least moderate drought (defined as years with PDSI 6 2.0) during the growing season (March–September). We evaluated statistical procedures at 0.05–0.10 level of significance using Statistica 6.1.

We identified canopy disturbances in increment cores used for growth analysis at each stand. We used the methods of Nowacki and Abrams (1997) to classify a stand-wide canopy disturbance event as P25% of sampled trees with a P25% increase in radial growth for at least 10 years, based on a 10 year running mean (minimum sample size = 2).

0.5

Water level (m)

2.6. Canopy disturbance patterns

1.0

Stand 1 11.6-m above msl 11.6-m above msl 11.5-m above msl

18

16

0.0

14

-0.5

12

-1.0 10 -1.5

Daily stage (m above msl)

88

8 -2.0

3.1. Hydrology

0.5

Hydrographs for monitoring stations from 2009 indicate brief surface flooding after precipitation events and extended flooding when water tables reached the surface (Fig. 1). Flood depth was <0.1 m at each station (Fig. 2) and only the lowest-elevation stations at stands 2 and 3 had more than 30 growing season flood days (Fig. 2). The majority of flooding was from temporarily ponded rain, and water tables never reached the surface at most stations. The number of growing season flood days had a significantly negative relationship with elevation (b = 9.9 days per m, p = 0.05). The duration of at least partial saturation in the root zone (<1 m) during the growing season was highly variable among stands (Stand 1 – 74–80 days; Stand 2 – 32–94 days; Stand 3 – 70–212 days) as well as within stand 3 (Fig. 2). The water table was within the root zone longer on the lowest elevation stations in each stand. Subsurface water levels declined due to evapotranspiration during the spring at least one month before the recession of the Mississippi River from its spring peak (Fig. 2) except at the lowest elevation station at Stand 3, where the water table remained relatively constant (Fig. 1). At the end of the year (non-growing season), longer-duration flooding occurred following frequent precipitation events that caused the water table to rise above the forest floor at the lower elevation monitoring stations at stands 2 and 3 (Fig. 1). Growing season PDSI in 2009 (0.2) was similar to the mean for the post-ring levee period (0.1; range = 4.2 to 3.7) (NCDC, 2011), so we inferred that hydrographs are representative of normal years in the post-levee period.

0.0

The three stands had similar density (stocking varied by only 6% among stands), but tree size and stem density varied (Table 1). Overcup oak was the most important species in the overstory of all stands, and the most important species in the understory was sugarberry (Stand 1), overcup oak (Stand 2), or swamp privet (Forestiera acuminata) (Stand 3) (Fig. 3). The shorter trees and high importance of swamp privet and water locust (Gleditsia aquatica) are consistent with hydrological data indicating Stand 3 is the wettest of the study stands. 3.3. Canopy disturbance patterns A series of stand-wide canopy disturbances occurred of varying lengths for each stand (Fig. 4). The first period of canopy disturbance lasted 10–14 years for each stand. The most recent period of disturbance lasted 21–42 years and overlapped with completion of the mainline (1940) and ring levees (1955). Peak disturbance was in the 1910s for Stand 1, the mid-1940s for Stand 2, and the

11.4-m above msl 11.4-m above msl 11.3-m above msl 11.2-m above msl

18 16

14

-0.5

12

-1.0 10 -1.5 8 -2.0

Water level (m)

0.5

Stand 3 11.2-m above msl 10.5-m above msl 10.2-m above msl

18

16

0.0

14

-0.5 12 -1.0 10 -1.5

Daily stage (m above msl)

1.0

8 -2.0

Total daily (cm)

3.2. Stand characteristics

Stand 2

Daily stage (m above msl)

1.0

Water level (m)

3. Results

10.0

Precipitation

7.5 5.0 2.5 0.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Fig. 1. Daily surface and subsurface water levels recorded at monitoring stations established along an elevation gradient (identified in m above msl) at Stands 1, 2, and 3 at Yancey Wildlife Management Area in 2009. Precipitation at Marksville, Louisiana (bottom graph) and stage on the Mississippi River at Natchez, Mississippi (gray line in upper graphs) are presented for comparisons. Horizontal dashed lines indicate forest floor at 0 m and horizontal dotted lines indicate maximum rooting depth at 1 m.

mid-1960s for Stand 3. Canopy disturbances have been less common at each stand after the 1960–1970s, and none have occurred since the mid-1980s (Fig. 4). 3.4. Recruitment patterns In general, the overcup oak trees were older than the sugarberry trees. The 125 overcup oak trees we sampled ranged from

89

80

60

70

Importance value (%)

70

50 40

Stand 1 Stand 2 Stand 3

30

2

R = 0.35 p = 0.04

50 40 30

0 80

0 10.8

11.0

11.2

11.4

11.6

11.8

Table 1 Stand characteristics at Yancey Wildlife Management Area, Louisiana.

60 50 40 30 20

Basal Area (m2/ha)

258 446 370

101 103 107

24.0 25.6 19.3

Goelz (1995). mean of dominant and codominant trees.

27 to 318 years old (median 92 years) and the 122 sugarberry trees we sampled ranged from 20 to 60 years old (median 42 years). Overcup oak was found at all sampling plots at each stand. Overcup oak had high importance values in the dominant and codominant crown classes at all stands, but had relatively low importance values in the suppressed crown class with the exception of Stand 2 (Fig. 3). There were overcup oak trees in all diameter classes, but only Stand 2 had high stem densities of small trees <30 cm DBH (Fig. 5). The majority of overcup oak trees cored at each stand (71%) germinated before the Mississippi River mainline levee (Fig. 6). All 17 cored overcup oak trees that germinated after completion of the ring levee were <30 cm DBH. The age-diameter relationship for overcup oak was good at all stands (Table 2) suggesting that uncored stems <30 cm DBH also germinated after completion of the ring levee. Importance values of overcup oak stems <30 cm DBH (after mainline levee) were highest at Stand 2 (b = 27.03, p = 0.05) with higher importance values at lower elevations (b = 2.47, p = 0.05) at this stand. Plot-level comparisons of tree ages with disturbance patterns indicate that overcup oak was more likely to germinate following localized or widespread canopy disturbances at all stands (Stand 1 – 7 of 7; Stand 2 – 10 of 12; Stand 3 – 6 of 7). Sugarberry was found at the majority of sampling plots at each stand (Stand 1 – 28 of 30; Stand 2 – 17 of 30; and Stand 3 – 24 of 30). Sugarberry was most important in the intermediate and suppressed crown classes (Fig. 3) with high stem densities in the smaller diameter classes (Fig. 5). All sugarberry trees cored germinated after the Mississippi River mainline levee (Fig. 6) and most stems (93%) germinated after completion of the ring levee. Reconstructing age distribution of sugarberry using the age-diameter relationship was limited by relatively weak models (Table 2), but we infer that most uncored sugarberry also became established after the ring levee. Sugarberry is very shade-tolerant but also responds well

0 80

Stand 3

70 60 50 40 30 20 10 0

Most tolerant

Highly tolerant

Moderately tolerant

Sugarberry

28.1 26.8 28.8

Cedar elm

1 2 3

10

Honeylocust

Height (m)b

Green ash

Stocking (%)a

Persimmon

Density (stems/ha)

Swamp privet

Stems P 10 cm DBH

Importance value (%)

Stand Stems P 5 cm DBH 28.5 27.1 29.3

Stand 2

70

Overcup oak

10.6

Waterlocust

10.4

Water hickory

10.2

Baldcypress

10.0

Fig. 2. Number of flood days recorded during the growing season (March– September 2009) at eleven monitoring stations at three stands at Yancey Wildlife Management Area, Louisiana.

a

60

10

10

Elevation (m above msl)

b

Stand 1

Dominant and codominant Intermediate Suppressed

20

20

Importance value (%)

Number of flood days during growing season

H.K.W. Gee et al. / Forest Ecology and Management 334 (2014) 85–95

Weakly tolerant

Fig. 3. Importance values of species by flood tolerance (Hook, 1984) in 3 crown classes at Yancey Wildlife Management Area, Louisiana.

to release from suppression (Johnson, 1975), which can result in high variability in the age of similarly sized trees. Importance values of sugarberry were higher (b = 2.15, p < 0.001) at higher elevations at all stands. Sugarberry was more likely to germinate in undisturbed areas at Stand 1 (13 of 19) and Stand 2 (17 of 26).

3.5. Overcup oak growth Growth response of overcup oak to river stage and PDSI varied by stand both before and after levee construction (Fig. 7). Growth had a significant, positive relationship with stage of one or both rivers (Atchafalaya and Mississippi rivers) during late summer months prior to levee construction (Fig. 7), but PDSI did not have a significant relationship with growth during the pre-levee period. After levee construction, growth had a significant, positive relationship with stage on the Atchafalaya River in early spring (April)

90

H.K.W. Gee et al. / Forest Ecology and Management 334 (2014) 85–95

100

Stand 1

4. Discussion

90 80

1907-1920

1941-1962

70 60 50 40 30 20

Percentage (%) of trees indicating a disturbance event

10 0 100

Stand 2

90 80 70

1906-1915

1921-1963

60 50 40 30 20 10 0 100

Stand 3

90 80

1927-1936

1943-1976

70 60 50 40 30 20 10

1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

0

Year Fig. 4. Percentage of trees of any species indicating a disturbance event at three stands in Yancey Wildlife Management Area, Louisiana.

at stands 2 and 3 (Fig. 7) and throughout the growing season (April–July, September) at stand 1 (Fig. 7). During the post-levee period, growth also had a significant, positive relationship with PDSI from late in the non-growing season to the end the growing season (January–September) at stand 1 (Fig. 7) and during the early and late growing season (March–April, July–September) at stand 2 and stand 3 (Fig. 7). Growth in years with at least moderate drought PDSI 6 2.0 was not significantly different (t = 2.16– 2.45, p = 0.81–0.88) between pre- and post-levee periods at all stands. In contrast to our prediction, overcup oak growth actually increased for several decades after ring levee establishment before declining during a drought in the late 1990s (Fig. 8). In fact, of the years with above-average growth, a far greater number occurred in the post-levee period for all 3 stands (Stand 1 pre-10 of 53 years; post 30 of 52 years; Stand 2 1 of 53 years; post-41 of 52 years; Stand 3 pre 4 of 53 years; post-37 of 52 years).

Our results indicate a profound impact of levee construction on stand establishment and development. The radical changes in surface flooding patterns changed the species composition of newly established trees from flood-tolerant overcup oak to flood-intolerant sugarberry. Furthermore, the spatial distribution of the species was also altered. During the pre-levee period, overbank and backwater flooding were common and facilitated the establishment of overcup oak at all stands across the full elevation range; the less flood-tolerant sugarberry was absent. After the levee, however, overcup oak establishment was restricted to canopy openings associated with forest harvest operations on lower elevations at Stand 2 and sugarberry started to occupy higher elevations in undisturbed areas (D. Locascio, LDWF, pers. comm.). Our hydrologic data indicate that precipitation-driven surface flooding is deeper and longer duration at lower elevations which is similar to previous studies on hydrologically disconnected floodplains (Lewin and Hughes, 1980; Gergel, 2002). Our results strongly support the hypothesis that flood-tolerant species become less common in a post-levee environment and that flood-intolerant species increase in abundance. Shifts in forest composition in response to hydrological alterations have been previously documented (King et al., 1998; Johnson and Waller, 2013; Alldredge and Moore, 2014), although few studies have evaluated long-term responses to hydrologic alterations, fewer have evaluated compositional responses to levees (Gergel et al., 2002), and none that we are aware of have used tree-rings to quantify long-term establishment responses to altered hydrology resulting from levees. The mechanisms behind the compositional change are unknown, however, the cosmopolitan distribution of overcup oak during the pre-levee period and sugarberry during the post-levee period, the lack of sugarberry establishment in the pre-levee period, and the confinement of overcup oak regeneration to the lowest areas in each stand after harvest in the post-levee period indicate that species-specific responses to flooding and light availability are forcing recruitment patterns. In general, oaks (i.e., Quercus spp.) require frequent disturbance, such as fire or flooding, for establishment and insufficient disturbance frequency or intensity can limit regeneration through competition in the understory and midstory (Lorimer et al., 1994). As was also reported by McCarthy and Evans (2000), canopy disturbances enhanced establishment of moderately shade-tolerant overcup oak although this response was still generally limited to the lowest elevation sites where surface ponding was more frequent and prolonged. In contrast, shade-tolerant sugarberry was able to establish without canopy disturbances. Seedling establishment is one important process that is likely altered by hydrologic alteration. Overcup oak seeds may need immersion in water to activate biochemical processes needed for germination (Pierce and King, 2007), which would result in decreased establishment after levees eliminated overbank flooding. In contrast, sugarberry produces large seed crops nearly every year (Burns and Honkala, 1990; Kennedy, 1990) and the seeds can remain viable in the soil for up to 5 years (Meadows et al., 2006), which may have contributed to their expansion in the post-levee period. The greater impact of flooding on stand development, however, may be due to the direct and indirect effects of flooding on seedling survival. Streng et al. (1989) found that flooding was more detrimental to newly established light-seeded species (similar to sugarberry) than heavy-seeded species because light-seeded species lacked the reserves to reestablish after flood events. Furthermore, frequent flooding limits ground cover thereby increasing the amount of light reaching the forest floor (KüBner, 2003) and facilitating the establishment of less shade-tolerant species, such as overcup oak.

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QULY

CELA 100

Stand 1

100 80

60 60 40

40

20

20 0

0

120

100

Stand 2

100

Stand 2

80

80 60 60 40

40 20

20

0

0 100

120

Stand 3

100 80

Stand 3

80 60

60 40 40 20

20

80+

70-79

60-69

50-59

40-49

20-29

80+

70-79

60-69

50-59

40-49

30-39

20-29

5-9

10-19

Diameter (cm)

30-39

0

0

10-19

Stem density (stems/ha)

Stand 1

80

5-9

Stem density (stems/ha)

Stem density (stems/ha)

120

Diameter (cm)

Fig. 5. Diameter distributions of overcup oak (QULY) and sugarberry (CELA) at Yancey Wildlife Management Area, Louisiana.

The lack of data on species removed during forest harvest operations and historic stand density at the study sites complicates efforts to determine the relative effect of hydrologic modifications on recruitment patterns (Trémolières et al., 1998). It is unlikely that flood-intolerant species such as sugarberry were preferentially removed by prior logging, so the absence of pre-levee sugarberry in the current stand suggests conditions were not suitable for establishment of sugarberry prior to levee construction. Current species such as overcup oak and sugarberry were likely not preferentially logged because they are not preferred species for forest products (Meadows and Stanturf, 1997). Furthermore, we infer from hydrologic history that conditions did not favor flood-intolerant species. The location of this site at the confluence of the Mississippi, Red, and Atchafalaya rivers suggests that it received prolonged, deep flooding in most years prior to levee construction (Mann et al., 1912) and would not have been an ideal site for sugarberry. Overcup oak growth was also affected by levee construction, but in contrast to our hypothesis, growth actually increased for several decades before declining during the late 1990s. We interpret this result as removal of flood stress following levee construction. Palta et al. (2012) also found an increase in baldcypress growth following a reduction in flooding after dam construction on the Savannah River floodplain, which they also attributed to alleviation of pre-dam flood stress. Thus, in our study, the prolonged pre-levee flooding of this site apparently allowed for overcup oak growth at a rate less than its maximum. This finding emphasizes the fact that flooding can be stressful to trees regard-

less of their flood tolerance and that growth in floodplain trees can be sustained provided adequate soil moisture is present, regardless of the source of soil moisture (Smith et al., 2013). Thus, when surface flooding is removed from a floodplain, precipitation and shallow groundwater can sustain or even increase tree growth. During the post-impoundment period, climate and stage of the Atchafalaya River were correlated with growth, which suggests a possible groundwater linkage with the Atchafalaya. Spatial (and temporal) variability in groundwater depth can be high in floodplains (e.g., Fig 1, Stand 3) because of soils, microtopography, and heterogeneous groundwater connectivity (Jung et al., 2004; King et al., 2012; Newman and Keim, 2013). Floodplain forests with groundwater declines and surface flooding removal, however, could be particularly vulnerable to mortality events during severe droughts (Stromberg and Tiller, 1996; Cunningham et al., 2011). Hodges (1997) described natural patterns of coupled site- and stand development in floodplain forests of the region, whereby forest composition progresses towards less flood tolerant species as sediment deposition raises the floodplain elevation and flooding decreases. Our study indicates that levee construction essentially shortcuts this process by immediately eliminating overbank flooding and favoring less flood tolerant species. Although recruitment may respond fairly rapidly to the elimination of overbank flooding from levee construction, growth of moderately flood-tolerant to flood-tolerant species may be sustained by precipitation and groundwater in the root zone, so that there may be a delayed transition to drier communities. For instance, trees have persisted for more than a century following reductions in overbank flooding in

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QULY

20

CELA

15

Stand 1

Stand 1

15 10 10 5 5

0

0

20

15

Stand 2

Stand 2

15

Stems

10 10 5 5

0

0

20

Stand 3

15

Stand 3

15 10 10 5 5

<1895 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

0 <1895 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

0

5-year period

5-year period

Fig. 6. Establishment-date distributions of overcup oak (QULY) and sugarberry (CELA) at Yancey Wildlife Management Area, Louisiana.

Table 2 Age-diameter models of overcup oak and sugarberry at Yancey Wildlife Management Area, Louisiana. Site Stand 1 Stand 2 Stand 3 Stand 1 Stand 2 Stand 3

Age-diameter model, A= Overcup oak 0.0003D3  0.05D2 + 3.9D 0.0002D3  0.02D2 + 3.2D 0.0007D3  0.10D2 + 5.0D Sugarberry 0.002D3  0.2D2 + 4.3D 0.002D3  0.2D2 + 4.7D 0.001D3  0.1D2 + 4.1D

Coefficient of determination (r2)

p-Value

Sample size

Population size

0.69 0.64 0.48

<0.001 <0.001 <0.001

43 43 39

136 267 135

0.25 0.32 0.33

<0.001 0.002 0.001

42 40 42

100 40 71

humid floodplain forests in Europe (Trémolières et al., 1998) and semi-arid floodplain forests in North America (Howe and Knopf, 1991). At YWMA, forest communities will continue to be dominated by moderately shade-tolerant species (overcup oak) in the dominant and codominant crown classes in the short-term, but if current successional trajectories continue, these species will be replaced by shade-tolerant species (sugarberry) in the long-term. In the short-term, moderately shade-tolerant species will likely replace themselves in the dominant and codominant crown classes on sites where they have high importance values in the intermediate crown class. Unlike floodplain forests in the nearby Gulf Coastal Plain (Battaglia et al., 1999), natural patterns of canopy disturbance in the MAV typically produce small openings (<2 ha), which may limit recruitment of less shade-tolerant species (Denman and Karnuth, 2005; King and Antrobus, 2005; Oliver et al., 2005). At

YWMA, stand density is in the range of competitive mortality (Goelz, 1995) and falls within the range of other studies with small canopy openings (Denman and Karnuth, 2005; Predick et al., 2009; Lockhart et al., 2010) and suggests that large canopy openings may not be produced without some form of management. In the absence of large canopy openings, the most shade-tolerant species (sugarberry) will eventually replace moderately shade-tolerant species (overcup oak) in the dominant and codominant crown classes. Large canopy openings may enable continued recruitment of moderately shade-tolerant species (overcup oak) into the upper crown classes. In summary, broad-scale hydrologic modifications have altered flood regimes in most floodplains throughout the world (Tockner and Stanford, 2002), which affect forest stand development. In our study, a flood-tolerant overcup-water hickory forest was disconnected from riverine flooding by a ring levee, which led to

* *

*

*

*

*

+

*

+

* * +

*

Mississippi River stage (after ring levee)

Mississippi River stage (before ring levee) *

Atchafalaya River stage (after ring levee)

*

* *

*

*

*

*

+

+

Mississippi River stage (after ring levee)

*

Mississippi River stage (before ring levee)

Mississippi River stage (after ring levee)

+

+ +

*

+

+ +

PDSI (after ring levee)

*

*

*

* *

*

*

* *

* +

+

+

+

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

* +

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

+ *

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

*

PDSI (after ring levee)

PDSI (before ring levee)

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

+ *

PDSI (after ring levee)

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

+ + +

PDSI (before ring levee)

Month

Month

Month

Month

Month

Month

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5

Correlation coefficients

* +

*

Atchafalaya River stage (before ring levee)

Correlation coefficients

Stand 3

Atchafalaya River stage (after ring levee)

Correlation coefficients

Atchafalaya River stage (before ring levee)

-Mar -Apr -May -Jun -Jul -Aug -Sep -Oct -Nov -Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Correlation coefficients Correlation coefficients

Stand 2

Atchafalaya River stage (after ring levee)

H.K.W. Gee et al. / Forest Ecology and Management 334 (2014) 85–95

Correlation coefficients

Stand 1 0.8 0.7 Atchafalaya River stage (before ring levee) 0.6 * + + + 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.8 Mississippi River stage (before ring levee) 0.7 0.6 + * + 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.8 PDSI (before ring levee) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5

Fig. 7. Correlation between annual radial growth of overcup oak from stand 1, 2, and 3 at Yancey Wildlife Management Area (Louisiana) with mean monthly stage on the Atchafalaya River (Simmesport, Louisiana), Mississippi River (Natchez, Mississippi) and Palmer Drought Severity Index (PDSI; Natchez, Mississippi) before (N = 52) and after the ring levee (N = 53). Negative months on x-axis indicate previous year. Asterisks (⁄) indicates significant coefficients at a = 0.05 and plus (+) indicates significant coefficients at a = 0.10.

93

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Standardized tree-ring index

2.5

2.0

Stand 1 Stand 2 Stand 3

1.5

1.0

0.5

0.0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year Fig. 8. Standardized tree-ring index at Yancey Wildlife Management Area, Louisiana. Horizontal dotted line at 1.0 indicates normalized mean.

expansion of less flood tolerant sugarberry. Broad-scale hydrologic modifications influenced forest recruitment patterns, but this was mediated by canopy disturbances that enhanced recruitment of relatively flood tolerant species. Growth of overcup oak was enhanced by the removal of surface flooding and growth switched from being correlated with river stage in the pre-levee period to climate in the post-levee period, thus emphasizing that flooding can be a stress to even flood tolerant species and that adequate soil moisture for floodplain tree growth can be sustained by groundwater and precipitation. Future research efforts should focus on the long-term effect of hydrologic modification on stand development and on how hydrologic modifications, such as elimination of surface flooding and groundwater declines affect the vulnerability of floodplain forests to drought.

Acknowledgements Funding was provided by the Arkansas Game and Fish Commission, Louisiana Department of Wildlife and Fisheries, U.S. Fish and Wildlife Service, and the U.S. Geological Survey Louisiana Fish and Wildlife Cooperative Research Unit. We appreciate the assistance of numerous field and laboratory technicians. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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