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Effects of herb ivory and flooding on reforestation of baldcypress taxodium distichum [L.] saplings planted in caddo lake, texas.

Abstract.--The effects of herbivory and flooding were examined on survival and growth of planted baldcypress (Taxodium distichum (L.) Rich.) saplings at three sites in Caddo Lake, TX, over a 4-yr period. There were two flood regimes (shallow periodic and deep continuous), where half of the saplings in each flood regime were protected by tree shelters to prevent herbivory. By the end of the first year, over 80% of saplings survived with half of saplings classified as healthy. By the end of the fourth year, only half of the saplings were alive and one-third were healthy. At all three sites, the combination of no protection and continuous flooding resulted in a significant number of missing saplings. Likewise, most unprotected saplings in periodic flooding were missing by the end of the study. Saplings clipped by herbivores showed about 50% chance of recovery, but many of the sprouts were of poor quality. Protected saplings in tree shelters achieved significantly greater survival and height growth.

The success of reforestation of baldcypress (Taxodium distichum [L.] Rich.) depends on early survival and growth. This swamp species, common to the wetlands along the Gulf of Mexico coastline, tolerates inundation with occasional exposure for seedling regeneration (Williston et al. 1980). The main requirements for successful germination and establishment of baldcypress are abundant seed supply, adequate moisture during germination, lack of herbivory, and an extended dry period to allow growth tall enough to survive and remain above flood events (Mattoon 1916; Demaree 1932). Baldcypress seedlings can endure partial shading, but direct overhead light ensures normal growth (Fowells 1965; Burns & Honkala 1990). When conditions are favorable, baldcypress will regenerate, grow, and survive for hundreds of years.

In the past (1890-1925), there has been extensive logging that depleted vast virgin baldcypress stands (Conner et al. 1986; Myers et al. 1995), whereby natural regeneration of baldcypress may not have been adequate to restore or maintain existing stands (Conner et al. 1986). There is little recent natural regeneration reported in Louisiana (Conner et al. 1986), Florida (Gunderson 1977), or Georgia (Hamilton 1982).

Artificial reforestation by plantings has been investigated in Louisiana (Conner 1988; Conner et al. 1993; Myers et al. 1995; Krauss et al. 2000; Souther & Shaffer 2000) and South Carolina (Conner 1993). In addition to controlling for critical hydroperiod changes, where seedling mortality is dramatic when inundated beyond 2 wk (Brandt & Ewel 1989), research has focused on combating detrimental herbivory effects (Blair & Langlinais 1960; Conner et al. 1986; Platt & Brantley 1991). Subsequently, numerous tree shelters that protect against herbivory have been designed and extensively studied by Myers et al. (1995) and Conner et al. (2000), where greater success in baldcypress seedling survival and growth was noted.

Baldcypress stands have been studied at Caddo Lake, a 10,850-ha lake located on the northeast Texas--Louisiana border. The Texas portion of the lake is dominated by dense stands of baldcypress ranging from ~ 100 to 350 yr in age (Keeland & Young 1997). The younger stands are composed of mostly even-aged trees that apparently became established when the lake was drained during the early 1900s before dam construction in 1914. Many new seedlings have been observed during late summer each year, but there has been little recruitment into the sapling stage for approximately 100 yr or more (Klimas 1987; Keeland & Young 1997; King et al. 1998). The few saplings that were observed often showed evidence of resprouting after being clipped repeatedly by herbivores.

Klimas (1987) was the first to report a lack of young baldcypress trees at Caddo Lake, suggesting that constant flooding nearly eliminated or greatly reduced germination and early seedling survival. This was subsequently confirmed by Keeland and Young (1997). King et al. (1998) studied the densities of beaver (Castor canaciensis Kuhl) lodges and nutria (Myocastor coypus Molina) populations at Caddo Lake to assess the effects of herbivory. Most herbivore damage observed was not intensive (bark stripping and gnawing on stem lobes) and presumed to have little effect on mature trees. The lack of small trees and saplings, however, may be the result of herbivory, shading, or flooding. The extent of herbivory damage to seedlings or saplings can range from missing (or uprooted) and standing dead to clipped and resprouted (Myers et al. 1995; Conner et al. 2000).

The goal of this study was to investigate the effects of herbivory and hydrologic regime on the survival and growth of baldcypress saplings at Caddo Lake planted with and without tree shelters. Specific Objectives included: (1) testing for differences in the types of damage to the saplings among two protection levels (with and without tree shelters) and two flood regimes (periodic and continuous flooding), as well as examining trends over time, and (2) testing for differences in sapling height among flooding and protection levels over time.

METHODS

Study areas.--Three sites at Caddo Lake were chosen for study (Fig. 1). The Starr Ranch site is located in the Goose Prairie area on the Caddo Lake National Wildlife Refuge. Starr Ranch is bounded by baldcypress stands, bottomland hardwood forest, and open water, and represents one of the few areas on the lake with an unforested slope extending upland from the water line that was cleared as a campsite. The other sites are just above the Caddo Lake water line, and they arc surrounded by dense bottomland hardwood forests: Duck Pond, a beaver pond located on the Caddo Lake National Wildlife Refuge south of Starr Ranch, and Wiley Pond, a man-made pond located on the southeast tip of Potters Point on Caddo Lake in Louisiana. At both pond sites, areas were cleared for this study. Soils at the sites are composed of alluvial loams with Duck Pond of Sardis-Mathiston complex, Wylie Pond of Gore silt loam, and Starr Ranch of Guyton-Cart complex.

[FIGURE 1 OMITTED]

At each study site, baldcypress saplings were planted in areas of either periodic or continuous flooding. The periodically flooded areas are inundated during the winter and spring to average depths of 24 cm, but exposed during the late summer and fall. The continuously flooded areas have an average ( [+ or -] SD) water depth of 85 ( [+ or -] 27) cm throughout the entire year.

In the spring of 1995, approximately 300 saplings, locally grown in Louisiana in 10 cm tree-pot containers, were planted 1 m apart in a grid system at the sites (Table 1). At each site, there were ~ 50 saplings in each flood regime area, where half of the saplings (in alternating positions) were protected against herbivory with TUBEX [R] tree shelters (Treessentials, St. Paul, MN). The tree shelters were constructed of corrugated plastic sheets formed into a tube and secured to wooden stakes with cable ties. Each shelter was 122 cm tall to ensure coverage of the bark and sometimes the lower portions of the crown of each sapling. Unprotected saplings were marked with pin flags.
Table 1. Number of saplings in each flood regime and protection level
combination persite at Caddo Lake planted at the beginning and surviv
-ing at thc end of the study.

                          Periodic Flooding   Continuous Flooding

                          Planted  Surviving  Planted  Surviving
                                    4 years             4 years

Starr Ranch  Protected       25        6        28        11
             Unprotected     27        1        27         0

Duck Pond    Protected       25       23        24        16
             Unprotected     25        9        26         4

Wiley Pond   Protected       25       22        27        26
             Unprotected     24       19        26        18


Initially, all saplings were healthy at approximately 3 yr of age and 175-200 cm tall. Types of sapling damage from herbivory, flooding, or other causes and sapling heights were recorded yearly during September from 1995 to 1998, and during April 1996. Damage types were categorized into six classes as clipped, resprouted (after being clipped), dieback, dead (standing), missing (entire sapling), or healthy (none of the above). For any type of damage, no attempts were made to identify the herbivore involved, as described in the protocol in King et al. (1998). Sapling heights were measured to the nearest centimeter for live stems only.

Statistical analysis.--Analysis of variance was applied to test for differences in damage classes among flood regimes and protection levels at the sites and through time using the Poisson probability distribution function in a log-linear model (PROC GENMOD). The trend of changes in damage classes over time was examined with a linear model analysis for time (PROC CATMOD). To determine whether there were changes in damage classes for the growing season vs. winter, the frequency of saplings in each damage class from fall 1995 to spring 1996 was compared with spring 1996 to fall 1996 (PROC FREQ). Differences in sapling height among the sites, flood regimes, and protection levels were tested by applying a nonparametric, repeated-measures analysis of variance (PROC GLM on ranked scores). All analyses were performed using SAS (a = 0.05), version 8 SAS Institute Inc. (1999).

RESULTS Sapling damage.--The pattern of sapling damage among flood regimes and protection levels through time differed significantly among sites (P= 0.0251). Sapling damage at Duck Pond and Wiley Pond were similar, whereas at Starr Ranch most saplings did not survive (Table 1). There were significant differences between protected vs. unprotected saplings at Duck Pond (P < 0.0001) and Wiley Pond (P < 0.0001).

The overall proportion of healthy saplings declined to 50% by the fall of 1995 and to 35% by 1998 (Fig. 2). The proportion of saplings in each of the damage classes differed significantly from each other within a year (P < 0.0001) and the years were significantly different within a damage class (P < 0.0001). The first year (1995) was different (P < 0.0001) from the other 3 yr that statistically did not differ from each other. The damage status changed in 49% of the saplings over the winter (1995-1996), whereas significantly fewer saplings (13%) changed during spring 1996 (P = 0.001).

[FIGURE 2 OMITTED]

Clipped saplings were found in all sites and differed significantly among years ( P < 0.0001; Figs. 3-5). During 1995, 19% of the stems were clipped by either beaver or nutria, but decreased to only 2% in 1996 and none by the end of 1998. Most clipped saplings (93%) were unprotected, and most of these (71%) were in continuous flooding. Few protected saplings (< 7%) in continuous flooding were clipped. Of the 57 saplings that were clipped in 1995, 35% resprouted and were of poor quality, 16% were dead, 39% were missing, and only 10% grew into healthy saplings by the end of the study.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

A large amount of resprouting occurred in 1996 (P <- 0.0001) following the relatively high levels of clipped stems in 1995, and again in 1998. (46 resprouted, P < 0.0001; Figs. 3-5). Most resprouted saplings (90%) observed in 1998 were listed as healthy. Many unprotected saplings (~80%) in continuous flooding at Wiley Pond were clipped and repeatedly resprouted, in contrast to 23% of the unprotected saplings in periodic flooding at Wiley Pond (Fig. 3). Resprouting was significantly more frequent for the unprotected saplings (P < 0.001), albeit very few protected saplings were clipped.

Saplings experienced significant dieback in tree shelters (P < 0.000l; Figs. 3-5). Overall, dieback occurred in 12% of all stem tips at all sites during the first year (1995) but was < 5% for subsequent years (Fig. 2). Most saplings with dieback (89%) recovered in 1996 and by 1998, 63% of dieback stems were reclassified as healthy saplings. The majority of dieback occurred in protected saplings in continuous flooding (~80%, P < 0.0001).

Standing dead sterns appeared to be nonspecific throughout all years, sites, flood regimes, and protection levels. The number of dead stems was initially low and then slightly increased (6%-10%) during the study. For example, at Wiley Pond, the largest numbers of dead stems were in both the continuously flooded, unprotected and periodically flooded, protected saplings (Fig. 3), whereas the opposite occurred at Duck Pond, with more dead stems incontinuously flooded, protected and periodically flooded, unprotected saplings (Fig. 4). At Starr Ranch, most dead stems were protected saplings found in continuous flooding (Fig. 5).

In many cases, a sapling found clipped or dead at the end of one season may be missing in subsequent years, but healthy saplings also disappeared between measurements. The number of saplings reported as missing in 1995 (14%) increased significantly to 37%39% in subsequent years (P < 0.0001; Fig. 2).

The proportion of healthy saplings in different flooding and protection levels varied among sites (P < 0.0001). At Wiley Pond, the majority of protected saplings in both periodic flooding (> 60%) and continuous flooding (> 80%) were classified as healthy during the entire study (Fig. 3). The majority of unprotected saplings in periodic flooding (> 60%) remained healthy until 1998 when ~ 40% were clipped and resprouted, whereas most unprotected saplings in continuous flooding (~80%) were clipped early in 1995. At Duck Pond, most healthy protected saplings were found in periodic flooding (> 80%), but were fewer and declined in continuous flooding (< 60%; Fig. 4). There were few healthy unprotected saplings in both continuous flooding (< 30%) and periodic flooding (< 10%), as most were missing at the beginning in 1995. Survival was lowest at Starr Ranch when compared with the other two sites (Fig. 5). Healthy protected saplings were lower (< 60% in 1996) and declined to ~ 20% in both flood regimes by 1998. All unprotected saplings, except for one in periodic flooding, were missing by the end of the study.

Sapling height.--Overall, sapling heights differed significantly among sites (P = 0.0251), with Starr Ranch exhibiting the shortest (190.7 [+ or -] 5.5 cm), Duck Pond intermediate (227.8 [+ or -] 9.6 cm), and Wiley Pond the tallest saplings (241.6 [+ or -] 7.3 cm) by the fall of 1998 (Fig. 6). Significant differences in the heights of surviving saplings were also noted among protection levels (P < 0.0001) and flood regimes (P = 0.0002). Protected saplings in periodic flooding at Duck Pond were the tallest (291.7 [+ or -] 6.5 cm; P < 0.0001).

[FIGURE 6 OMITTED]

After an initial decline in average height (also known as negative height growth) due to dieback or being clipped by herbivores, the saplings generally displayed a trend over time of increasing height growth through 1998 (P < 0.0001), but the growth response through time was not consistent among sites (P = 0.0003) or in protection levels (P = 0.0299; Fig. 6). There were significant interactions of time with site--protection (P = 0.0005) and flood regime--protection (P < 0.0001). Because of the high number of missing saplings, stem heights for unprotected saplings at Starr Ranch could not be statistically evaluated. The few surviving saplings at Starr Ranch showed little variation in average height throughout the study, whereas saplings at both Duck Pond and Wiley Pond generally showed increasing height in protected saplings but initial decline followed by increasing height in unprotected saplings.

DISCUSSION

There were significantly fewer damaged saplings found in protected tree shelters during the 4 yr of this study at Caddo Lake, concurring with other studies that have shown higher survival and better growth of seedlings in tree shelters (Potter 1988; Keyser 1989; Allen & Boykin 1991; Conner 1993; Allen 1995; Jones et al. 1996; West et al. 1999; Conner et al. 2000). Tree shelters generally provide a better growth environment for seedlings and reduce herbivory.

During this study, there was an extensive herbivore population throughout the Caddo Lake area. Historically, beaver populations were greatly reduced and nearly exterminated throughout most of North America due to overtrapping by the early 1900s (Novak 1987; Obbard et al. 1987). Nutria was introduced in Louisiana in 1937 and spread to Texas soon after (Swank & Petrides 1954). This lack of herbivores at the time of the draining of Caddo Lake and its dam construction in 1914 was beneficial for the extensive establishment of young baldcypress stands that are now ~ 100 yr in age.

Management practices since 1950 have resulted in dramatic furbearer population recoveries (10-fold; Novak 1987). King et al. (1998) reported ~ 230 beaver lodges occupied by either beaver or nutria at Caddo Lake, with densities of one lodge per 47.4 ha. Beaver lodges were approximately 350 m from planting sites. Although Keeland et al. (1996) and King et al. (1998) suggested that seedling survival was largely affected by competition and flood regime, extensive herbivore activity was observed at all three sites as well as at numerous other locations across Caddo Lake during this study. Unprotected saplings in areas of continuous flooding, including many areas not part of this study, were clipped and resprouted. Similarly, Blair and Langlinais (1960) reported that the initial and most extensive damage by nutria was in flooded areas, whereas Conner (1988), Platt and Brantley (1991), and Myers et al. (1995) verified extensive damage to unprotected plantings.

Protected saplings were also damaged by herbivores but to a lesser degree in the Caddo Lake study. Herbivores were able to chew through the wooden stakes supporting the tree shelter, as also observed by Myers et al. (1995). Conner and Toliver (1987) found that all baldcypress seedlings in tree-shelter tubes made of tight-weave netting were destroyed within 3 mo by nutria. Tree shelters constructed of netting may not be as effective as the solid plastic shelters used in this study or in the Myers et al. (1995) study. Herbivore density can also affect seedling survival (Conner & Toliver 1987). Greater damage was observed in plots with more herbivore feeding/resting mounds. Consequently, although tree shelters arc generally effective at reducing damage to seedlings, the effectiveness can be variable depending on site conditions, herbivore numbers, and the type of shelters used.

Although baldcypress seedlings and saplings can readily produce sprouts, the long-term viability of these sprouts is low (Conner 1988; Spencer et al. 2001; Chambers et al. 2005). Conner et al. (1986) reported prolific sprouting of baldcypress stumps after logging activities, yet most of the sprouts died in the following years. Similar findings were reported for pondcypress (Tavodium ascendens Brongn.) by Ewel et al. (1989), Ewel (1996), and Randall et al. (2005). Spencer et al. (2001), however, reported that baldcypress stands in their study exhibited stems originating as coppice only in the stumps of young trees affected by beavers. In this Caddo Lake study, baldcypress saplings clipped by herbivores had < 50% chance of survival over the 4-yr period, but only ~ 10% of the clipped saplings were of good quality.

An initial decline in the average height was observed at Caddo Lake and is generally referred to as negative height growth (Palmiotto 1993; Thompson & Schultz 1995; Garriou et al. 2000; Trowbridge et al. 2005) that is usually due to dieback or being clipped by herbivores. The significantly high amounts of dieback (80%) in tree shelters in this study during the first growing season may have been caused by transplant shock as discussed by Allen et al. (2001), especially for saplings subjected to the stress of flooding. Transplant shock occurs because of the failure of the root system to accommodate when transitioning from regular watering schedules to stressed conditions. Visible tip dieback is caused by the inability of the root system to supply water to the rest of the plant (Kozlowski & Davies 1975; Harris & Bassuk 1995; Barton & Walsh 2000).

Kjelgren et al. (1997) and Gerhold (1999) also noted an increase in shoot dieback and mortality for some species grown in tree shelters, attributed to a lack of acclimation to winter caused by the protection of the shelters. The average low winter temperature was above freezing during the 4 yr at Caddo Lake, so it is unlikely that reduced winter hardiness contributed to dieback.

Dieback and mortality could be also related to modified temperatures inside the tree shelters. During the summer, higher temperatures (2-16[degrees]C) within tree shelters have been reported as compared with ambient air temperatures (Kjelgren 1994; Kjelgren & Rupp 1997; Swistock et al. 1999; Bellot et al. 2002). Since ambient summer temperatures at Caddo Lake may reach ~ 37[degrees]C, any additional increase in temperature could severely damage saplings (47[degrees]C results in complete leaf loss). Higher temperatures may retard growth of seedlings enclosed in tree shelters, resulting in water deficit and reduced trunk diameters (Kjelgren 1994; Kjelgren & Rupp 1997). The tree shelters in this study covered only the stems of saplings that were specifically selected as taller than the shelters, which may imply less susceptibility to heat damage, especially for saplings in standing water. Recovery of most of the stems with dieback by the end of the second growing season suggested that the saplings had adjusted to their new environment or any transplant shock.

The greater height growth of saplings in tree shelters observed at Caddo Lake agrees with the results of Potter (1988), Lantagne et al. (1990), Allen and Boykin (1991), and Lantagne (1992). Conner et al. (2000) suggested that the increase in height for seedlings enclosed within shelters was not only due to reduced herbivory but also to the microclimate or greenhouse effect (increased carbon dioxide and relative humidity levels, decreased transpiration loss, etc.) inside the shelter. This greenhouse effect probably did not influence height growth at Caddo Lake since the average height of unprotected, not-clipped saplings was equivalent to protected saplings.

Saplings may become missing due to abiotic as well as biotic forces. In general, saplings in flooded waters can be removed by wind, wave action caused by boats and wind, floating debris, or by encroachment of other vegetation, such as water lily (Nymphaea L.), American lotus (Ne/umbo lutea Willd.), and water hyacinth (Eichhornia crassipes (Mart.) Solms). The missing saplings at Caddo Lake were predominately unprotected and in continuous flooding. Wind and wave action could uproot unprotected saplings as well as protected saplings, since the tree shelters were anchored into the root ball of each sapling instead of into the surrounding soil. Furthermore, the spatial pattern of missing saplings appeared to not indicate vandalism or other human intervention. Because the unprotected saplings were planted among the protected saplings (in alternating positions), herbivory is the more likely explanation for the missing unprotected saplings.

CONCLUSIONS

In conclusion, this study highlights the widespread impact of herbivory on baldcypress regeneration at Caddo Lake. Many dense, even-aged stands of baldcypress became established across Caddo Lake about 100 yr ago when water levels were low before dam construction; however, very little baldcypress recruitment has since occurred on the lake. The high water levels maintained by the Caddo Lake dam hinder baldcypress germination and overtop established seedlings (Klimas 1987), leading to high seedling mortality.

In addition, the denseness of the baldcypress stands limits the amount of exposed land for recruitment. The results of this Caddo Lake study indicate that the activity of herbivores such as beaver and nutria is further constraining the survival and growth of the established seedlings and saplings. Flood regime can also affect the intensity of herbivory because of the tendency to feed in areas of continuous flooding rather than periodic flooding. Tree shelters have been shown to be effective at reducing damage, but may still be overcome by the herbivores. Without control of the herbivore population or protection of developing seedlings, very little recruitment beyond the seedling stage can be expected at Caddo Lake. As the current young baldcypress stands mature and deteriorate, the combination of herbivory and flooding could become a potential problem by preventing the establishment of new stands in the future.

DISCLAIMER

This study was funded by the US Geological Survey, the US Bureau of Reclamation, and the Caddo Lake Institute, Inc. Reference to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the US Government or any agency thereof.

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ROD-D at: dalerg@usgs.gov

Bobby D. Keeland (1), Rassa 0. Draugetis-Dale (1), Roy Darville (2) and John W. McCoy (1)

(1) USGS, National Werlands Research Center lafayette. Louisiana 70506

(2) East Texas Baptist University Marshall, Texas 75670
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Author:Keeland, Bobby D.; Draugelis-Dale, Rassa O.; Darville, Roy; McCoy, John W.
Publication:The Texas Journal of Science
Date:Feb 1, 2011
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