Mangroves are coastal woody plants that are freeze intolerant, which severely limits their growth and distribution, therefore mangrove dominance is primarily restricted to the tropics. At their poleward limits, mangroves are intermixed with temperate dominating salt marsh species creating marsh-mangrove ecotones. Over the last 30 years, global changes are driving black mangrove (Avicennia germinans) stand expansion in the Northern Gulf of Mexico, with the reduction of freezing event frequency, duration, and severity invoked as the leading driver. In January 2018, Winter Storm Inga brought severe freezing conditions to the southern United States causing widespread damage to the local vegetation. We are investigating how this freeze event is impacting the black mangrove population within the Northern Gulf of Mexico marsh-mangrove ecotone. We are monitoring freeze damage and recovery at eight locations that span the Texas coast. The mangroves within the northern four sites, exhibited major freeze damage (93.6% of the total mangrove coverage), whereas the stands within the southern study sites, had minimal to no freeze damage (2.2% of the total mangrove coverage). Freeze damage consisted partial to complete defoliation, with minimal recovery after 3 months post-freeze. Some seedlings (this year’s newest recruits) survived in even the heavily freeze damaged sites, though average survivorship was much lower in sites that exhibited longer freezing conditions (19.7%) than sites that didn’t (64.0%). documented at all sites. These data give insight into how freezing temperatures affect black mangrove growth, recruitment, and survivability at their range limits, and the subsequent effects on mangrove range expansion or contraction. For example, major freeze damage may reduce reproductive output in mature trees in following years, thereby reducing mangrove recruitment. Therefore, a major storm event such as Winter Storm Inga may have the potential to pause mangrove expansion within the marsh-mangrove ecotone.

Reduced incidence and intensity of freezes has promoted the increasing dominance by the black mangrove, Avicennia germinans L., along much of the Texas coastal zone. Other factors, such as nutrients and interactions with pre-existing salt marsh species, may influence the rate or extent of dominance shift. In Texas, unlike similar latitudes in Florida, Avicennia tends to be short (<2 m, usually 1 m or less) and bushy (highly branched, high canopy:height ratio. We have an on-going field experiment initiated July 2017 in which mangroves imbedded in a marsh plant “matrix” are subjected to different fertilization treatments (Control, N, P, N+P, and Fe added approximately every 6 months) in the root zone. Response variables include: Target mangrove “tree” reproductive output, growth, side stem production; volunteer seedling mangrove survival and growth; and marsh plant size and robustness. Within 3 months, differences were noted in reproductive output. By the end of year 1, growth and branching were affected by fertilization treatments. Most response variables responded more to N and less to other fertilization treatments. Numbers of propagules per branch and per tree were much greater possibly because of increased propagule survival as indicated by the facts that flowering, bud production, and fruiting was already done or well underway at experiment initiation. This suggests that black mangrove expansion will be accelerated in nutrient-high regions.

Avicennia germinans is rapidly expanding along the Texas Coast. In some areas, small trees and shrubs form contiguous strips of mangrove forest along island and mainland coasts. In other areas, Avicennia occurs as individual trees within salt marsh dominated sites. I performed a manipulative common garden experiment to determine if survival and growth of propagules varied with maternal plant, natal site, dominant vegetation (Avicennia or Spartina) at natal site and diversity at the experimental site. Two levels of diversity treatments were applied: a) low –all six seedlings in a plot from the same maternal tree and b) high-plots with two individuals from each of three different maternal trees. Plots were assigned to blocks along a transect within the Batis zone at approximately the same tidal level. Results over the first 1.5 yrs are as follows. Seedling survival over the first six months was less than 50% and varied with maternal tree and natal site, but not diversity treatment–all seedlings from some maternal trees perished regardless of diversity treatment. Slight variation in elevation is important for seedling establishment; all individuals in one block located at a slightly lower elevation failed to establish and perished within the first three months. In contrast, mortality of established seedlings that were immersed for greater than one month after Hurricane Harvey was very low. Finally, there was no association between dominant vegetation and survival of seedlings. Data from this experiment suggest that the role of biodiversity in Avicennia seedling survival and growth is different from that recorded for Rhizophora. Survival and growth of seedling mangroves, both Avicennia (this study) and Rhizophora (earlier study) is complex and affected by environment (elevation) and factors such as maternal genotype.