Beach pinweed (Lechea maritima) COSEWIC assessment and status report: chapter 10

Limiting Factors and Threats

Climate Change and Sea-level Rise

There is now a general scientific consensus that human-caused climate change is occurring and will result in increased global temperature, causing sea-level rise and increased storm frequency and severity (Houghton et al. 1996, Shaw 2001, Kont et al. 2003, Environment Canada 2006). Parkes et al. (2006) document a statistically significant increase in severe windstorms in the southern Gulf of St. Lawrence since the 1980s with impacts including more frequent flooding as well as increased erosion and breaching of coastal barrier dunes (Environment Canada 2006). O’Carroll et al. (2006a) have documented a net decrease in the amount of beach and dune habitat on New Brunswick’s southeast coast during the period 1944 to 2001 (though that analysis did not include any beach pinweed sites). Data is not available for Prince Edward Island dune systems, but anecdotal evidence suggests many barrier dunes are narrowing as the seaward face is moving inland (R. Curley, PEI Department of Environment, Energy and Forestry).

Field observations show that ideal beach pinweed habitat is flooded by storms very infrequently. Increased storm frequency and severity combined with higher sea levels would likely change some beach heather–pinweed communities to American beachgrass-dominated communities that are less suitable or unsuitable for pinweed. Storm erosion effects are also not necessarily limited to those areas subjected to direct flooding, as erosion at lower elevations can destabilize the dune crests above and cause changes their plant communities.

Several beach pinweed sites have already been notably modified in recent years by heavy storm activity, with some loss of pinweed habitat and population observed. The northern section of the Bouctouche Dune and many sections of the South Kouchibouguac, North Richibucto and South Richibucto dunes were affected by very high tides and significant storm surges in October and January 2000 (Shaw 2001, Environment Canada 2006). Erosion by wave action opened breaches, locally eliminated vegetation and lowered foredune topography. Affected areas have become more susceptible to storm overwash and many have been further modified nearly every subsequent year (D.M. Mazerolle, pers. obs.). On the northern section of the Bouctouche Dune, pinweed density declined over roughly 25% of the population area and some patches were lost under sand deposition caused by overwash flooding (D.M. Mazerolle, pers. obs.). A tendency towards lower density was also observed on the North Richibucto Dune in sections having recently been subjected to storm disturbance (D.M. Mazerolle, pers. obs.).

Relevant projections of future relative sea level (factoring both sea level rise and natural land subsidence) and storm surge frequency are available only for southeastern New Brunswick pinweed populations, which happen to be among the most susceptible because of their lower dune heights. Projections suggest that sites at 3.1m, 3.2m and 3.3m elevations will be impacted by storm disturbance 4.5, 5.5 and 3.8 times more often by 2100 (Parkes et al. 2006). Environment Canada (2006) used digital elevation maps (DEMs) to determine areas flooded by storm surges of 4 m above current sea levels [note also that sea level rise by 2100 is projected at 53-60cm±30 cm (Forbes et al. 2006)]. The areas that would be flooded represented roughly 90% of the Bouctouche pinweed population and roughly 50% of the South Richibucto and South Kouchibouguac populations. Sediment budget calculations also show that the Bouctouche Dune has an especially limited capacity to counter effects of erosion and is therefore the most threatened by climate change-related impacts (O’Carroll et al. 2006). Most pinweed populations away from southeast New Brunswick are at slightly higher elevations or are protected by higher foredunes and would likely be less affected.

Dunes are naturally dynamic systems, and all dune plant species are adapted to some extent to cope with dune movement and sand deposition. It is not possible for coastal geomorphologists to predict total dune area through the future, even on the best-studied dunes in the region (Dominique Bérubé, NB DNR, pers. comm.) and it is impossible to rule out the possibility that beach pinweed habitat and populations will simply move with shifting dune locations caused by sea level rise. However, significant portions of existing southeastern New Brunswick occurrences are within an elevation range that could see pinweed habitat loss or degradation due to increased storm frequency and intensity within 90 years. The extent to which impacts will be felt over the next 30 years, representing the upper estimate for generation time multiplied by three, is not possible to determine.


Limited Habitat/Habitat Specialization

Beach pinweed in the Gulf of St. Lawrence has a very restricted geographic range and only occurs in a very specific habitat type on large, stabilized dune systems. Habitat specificity likely plays the largest role in the rarity of this taxon. Barrier dune habitats occupy a total of 3377 ha in Prince Edward Island, representing only 0.6% of the province (Prince Edward Island DAF 2003). The proportional total for New Brunswick is much smaller. Surveys indicate only a small portion of dune area offers suitable habitat for the species, and the known population sites represent the majority of areas containing suitable habitat in the southern Gulf of St. Lawrence.


Off-road Vehicle and Pedestrian Traffic

The recreational use of all-terrain vehicles (ATVs) in natural areas is undergoing rapid growth throughout North America (see Groom et al. 2007), with the number of recreational riders now approaching 45 000 in New Brunswick alone (NB ATVTF 2001). ATV use in coastal ecosystems such as dunes and wetlands is prevalent in New Brunswick and Prince Edward Island despite prohibitive laws and other deterrents. On dunes, new trails can quickly develop after just a few passes of an ATV and even the more remote island sites can be accessed by ATVs over ice in winter. Thus ATV impacts are a potential threat to most beach pinweed populations.

Recently used off-road vehicle trails were observed, however, in beach pinweed populations only at the South Richibucto Dune and one of two portions of the Fox Island population. At Cabot Beach, a single ATV trail crossed the width of the dune near pinweed populations but did not appear to affect any plants. Old vehicle tracks are evident within both Bouctouche Dune populations and at South Richibucto Island, but ATV access to these areas is now limited because of the interpretive centre and boardwalk opened at Bouctouche in 1997 and a deep channel that split the South Richibucto Dune in 1995.

ATV impacts were most notable at the South Richibucto Dune population, a popular destination for camping, hunting and recreational ATV use where traffic is common on the foredune and interdune areas. Here vehicle tracks passed through several large patches of beach pinweed and have caused several blowouts. Beach pinweed plants were observed at the edge of tracks but were generally absent within them, indicating a loss of individuals (D.M. Mazerolle, pers. obs. 2003). Even at South Richibucto Dune, however, overall ATV impacts at present are still quite small relative to the whole population. It is also important to note that in some situations, such as on Fox Island and at the southernmost edge of the South Richibucto Dune population, beach pinweed is present in and on the immediate periphery of vehicle tracks where vehicular disturbance seems to create suitable microsites for germination.

Pedestrian trampling causing destruction of beach pinweed plants was observed at the South Kouchibouguac Dune and Bouctouche Dune populations, both very popular beach destinations, but overall effects on these populations appear very small relative to the entire populations. Most other sites except for Cabot Beach Provincial Park are likely too remote and infrequently visited for trampling to be a significant factor, although occasional camping or unauthorized construction of cabins could certainly affect individual occurrences. Beach heather–pinweed habitats are sometimes selectively targeted for camping and cabin sites because they have a lower density of tall, rough, American beachgrass.


Isolation and Fragmentation

New Brunswick and Prince Edward Island populations are separated by more than 55 km of open water and unsuitable habitat, so genetic interchange between the provinces seems very unlikely. Each of the five regions of occurrence identified in Table 1 are isolated from any of the other regions by at least 19 km, thus making a rescue effect unlikely between regions. Genetic interaction between the populations within a region of occurrence is much more plausible, with a number of populations separated only by short distances of unsuitable or marginal habitat on the scale of a few kilometres. There is no information on the population genetics of beach pinweed, but reduced gene flow in small, isolated populations can lead to genetic drift, inbreeding depression and loss of genetic variability (Karron 1991, Newman and Tallmon 2001, Oostermeijer et al. 2003) potentially leaving populations at greater risk of extinction (Barrett and Kohn 1991).

Anthropogenic and natural (but climate-change influenced) disturbance may be increasing fragmentation of pinweed populations through breaching of barrier dunes, loss of suitable habitat and conversion of marginal American beachgrass habitat to even less suitable bare sand overwash areas. At the South Richibucto Dune population, a combination of ATV traffic and storm disturbance caused the opening of a massive breach and led to the formation of the “South Richibucto Island” (Éric Tremblay, Kouchibouguac National Park, pers. comm., Dominique Bérubé, NB DNR, pers. comm.). The plants on this newly created island are now separated from other occurrences by approximately one kilometre of open water and unsuitable habitat and it is uncertain to what extent propagule exchange is possible.


Hybridization

Hybridization can increase competitive pressure on both parent species and lead to increasingly diluted genomes (Levin et al. 1996). According to Hodgdon (1938), L. maritima commonly hybridizes with other pinweeds including narrowleaf pinweed, which is also found in New Brunswick and Prince Edward Island in a variety of open, dry habitats. Hybrids produced may appear as intermediates or bear close resemblance to either parent plant and are frequently hard to detect (Hodgdon 1938). The two species were in fairly close proximity on Hog Island, Cabot Beach Provincial Park and on the northern section of the Bouctouche Dune, but no evidence of hybridization was found. There is thus no direct evidence of a threat posed by hybridization.


Development

New Brunswick and Prince Edward Island have seen a considerable increase in the development of coastal areas during the late twentieth century (NB DELG 2001, Prince Edward Island DAF 2003), resulting in habitat loss and fragmentation (Stewart et al. 2003).

Although the encroachment of residential development in coastal areas does affect dune habitat in New Brunswick (D.M. Mazerolle, pers. obs.), pinweed population sites are for the most part in protected and/or relatively inaccessible areas and are not likely to be at risk. High levels of recreational activity during summer months do pose a problem for some populations, as the beaches and dunes of protected areas are often a primary focus for regional tourism. Pedestrian trampling of pinweed patches was observed, for example, on the South Kouchibouguac Dune and on the Bouctouche Dune (D.M. Mazerolle pers. obs.), and is a potential issue at Cabot Beach.

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