Mangroves forests have historically been found in tropical and subtropical climates. However, in recent decades with rising average temperatures and milder winters associated with climate change, mangroves have gradually extended their ranges poleward into temperate latitudes [1] [2] . The black mangrove, Avicennia germinans, for example, has been expanding its range in the Gulf of Mexico extending as far north as the coasts of Texas, Louisiana, and Florida [3] .

Mangroves in higher latitudes are faced with a variety of biotic and abiotic factors that limit their establishment, productivity, and resilience. Some of these factors include the rate of propagule dispersal and predation [4] and deep freeze events where air temperatures reach −6.3˚C to −7.6˚C or below [4] [5] . The Gulf of Mexico experiences a freeze regime that has historically been a major limiting factor in mangrove range expansion and has contributed to range contraction as freezes induce mass die-off events [6] [5] . One such event was caused by winter storm Uri in 2021 where air temperatures as low as −23˚C resulted in a mass die-off of A. germinans off the coast of Texas where we conducted our field study [7] .

As mangroves attempt to establish further into the temperate zone, they must also adapt to the seasonal variations in temperature, humidity, and sunlight. This is in contrast with the tropics which experience variation in precipitation while maintaining consistently high temperatures, humidity, and sunlight levels year-round. Mangroves are known to be resilient in response to salinity, water pollution, and tidal undulation [8] - [10] . Their continued expansion, establishment, and population recovery after die-off events will depend in part on their ability to adapt their water regulation and stress response strategies to keep pace with the increased seasonal environmental variation and endure extreme weather events exacerbated by climate change (i.e. freezes, droughts).

The purpose of this study was to increase our understanding of the plasticity of the hydraulic traits of black mangroves (Avicennia germinans) near Port Aransas, Texas to understand the resilience of the black mangrove forest ecosystems to environmental fluctuations as well as to analyze their post-disturbance recovery. We did so by examining their physiological responses to environmental forcings across the 2024 summer growing season, particularly in how they regulate water loss while maintaining photosynthesis. The timing of this study and local climatology was such that the environment moved towards drier conditions as the summer progressed creating higher osmotic and atmospheric stress on the coastal vegetation.

All plants must manage their water content to survive. Water in plants influences their structural support, transport of materials, growth, productivity, and repair [11] [12] . Low water content puts vegetation at risk of potentially fatal embolisms which are obstructions in water flow caused by the buildup of air bubbles in the xylem [13] [14] . Trees manage water content in the root zone through water uptake and at the leaf level by controlling water loss to the atmosphere through their stomata. At the leaf level, closing stomata reduces water loss via transpiration at the expense of reducing atmospheric carbon uptake [15] [16] . Water regulation strategies vary by species and are influenced by xylem physiology, rooting habit, size, age, environment, disturbance history, and ecological conditions [17] - [19] . While most terrestrial plants are dependent on soil moisture for refilling their water content after transpirational loss during the day [20] , mangroves grow in soils that are often fully saturated. Salinity, rather than soil saturation, therefore, is the greater limiting factor to the mangroves’ ability to recover after water loss [18] [21] - [23] .

Many studies have been conducted analyzing mangrove water regulation in response to salinity stress [18] [21] - [23] . However, few studies have been conducted analyzing plasticity in mangrove hydraulic traits in relation to a variety of ambient field conditions and disturbance histories. In this study, we focused on water regulation strategies of pre- and post-freeze black mangroves at the leaf level.

Mangroves respond to changes in their environment by adapting their water regulation strategies at the leaf level by opening and closing stomata to maximize carbon uptake while attempting to minimize water lost to transpiration. We hypothesize that as temperature and VPD increase, increasing the evaporative forcing of the atmosphere, the transpiration, stomatal conductance, and assimilation rate will also decrease as the leaves close their stomata to prevent excessive water loss. We predict that leaf water potential will become more negative during these times corresponding to greater plant stress. We also predict that saplings (younger, established post-freeze) will have a higher WUE than juveniles (older, established pre-freeze) due to the saplings’ narrower xylem vessels [24] and lower salt tolerance [23] leading to lower water content.

Several studies have shown differences in traits and physiological responses to the environment between age classes in different species of mangroves. Osland et al. (2015), for instance, found that after a freeze event in 2014 in Florida, adult A. germinans sustained less damage than juvenile mangroves, but seedlings remained mostly undamaged [25] . Kodikara et al. (2017) found that after the 15^th to 20^th week post-establishment, there is a shift toward higher optimal salinity for growth for five mangrove species, including two Avicennia species [23] . Cisnero de la Cruz et al. (2021) observed a difference in xylem vessel size and density between adult and seedlings in the mangrove species Rhizophora mangle and found that leaf water potential was higher in saplings than in adults [24] .

We conducted this study on the surviving population of A. germinans affected by winter storm Uri in 2021. The living trees we measured were young survivors of the freeze (juveniles) or post-freeze regenerations (saplings). This study therefore provides an understanding of the hydraulic trait plasticity of the surviving mangrove population about three years post-disturbance as well as points to potential differences between trees that survived the freeze as saplings and those that were established after.

Understanding how age class, especially the sapling stage, might affect hydraulic strategies will also improve our understanding of their establishment capabilities and ability of the population to recover after ecosystem disturbances. As mangroves encroach into new wetland ecosystems, we can use this data to predict their success in competition with the resident dominant forb vegetation. Changes in wetland vegetation habitats could induce wildlife composition changes [26] .

Evapotranspiration also plays a significant role in atmospheric cooling by converting incoming solar radiation to latent heat rather than sensible heat. Vegetation cover therefore greatly influences regional heat fluxes and energy balances [27] [28] . With mangroves covering about 147,000 km² of the earth surface according to the GMW v4.0 world map developed by the Global Mangrove Watch [29] , understanding how these trees adapt their transpiration rates in response to environmental fluctuations will contribute to greater accuracy in modeling regional climate and weather patterns in areas with large mangrove forest cover.

While mangroves are continuing to expand their range into the temperate zone, mangrove ecosystems globally are facing considerable threats to their existence from natural disasters, climate change, coastal development, aquaculture, and over-exploitation among other threats [30] [31] . According to an assessment by the IUCN Red List of Ecosystems, 50% of mangrove ecosystems in the assessment were categorized as Vulnerable, Endangered, or Critically Endangered [32] . Mangrove forests provide many ecosystem services including phytoremediation, which in some cases, protect more vulnerable habitats, such as coral reefs, from pollution damage [9] [33] . They also provide habitats and nurseries for marine and terrestrial wildlife [34] , act as large carbon sinks [35] - [37] and protect against coastal erosion and flooding [10] [38] . Understanding questions of mangrove hydraulic trait plasticity and resilience in the face of changing conditions will inform conservation efforts and priorities.

In total, we took 159 measurements: 40 in May, 31 in July, 39 in September, and 49 in October. Variation in sample sizes between months are due to instrument failure or excluded observation errors. Data samples from all metrics (E, A, g_sw, LWP) failed either the equal variance and/or normality assumption for the ANOVA test. Therefore, we proceeded to analyze differences using the Kruskal-Wallis test and subsequently, the Dune’s Test Pairwise Comparison.

Between juveniles and saplings, there was no significant difference in average LWP for May, July, or September. In October, there was a significant difference (p-value < 0.05) in average LWP during predawn but not during midday. Midday LWP values were on average lower (more negative) than those of predawn, signifying lower water availability in the leaf as the day progressed in all months ( Figure 2 ).

For predawn LWP, the results of the Kruskal-Wallis test were an H-statistic of 13.16 and a p-value < 0.005 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that LWP predawn was significantly lower in October than in the other three months ( Figure 2 , panel A).

For midday LWP, the Kruskal-Wallis test produced an H-statistic of 23.78 and a p-value < 0.001 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that midday LWP was significantly lower in July than in the other three months ( Figure 2 , panel B).

There was no significant difference in midday transpiration rate between juveniles and saplings for all four months. May and July saw similar rates of midday transpiration around 0.002 mol m⁻² s⁻¹, while September saw a spike up to around 0.007 mol m⁻² s⁻¹. October transpiration values returned to similar levels seen in May and July at 0.001 mol m⁻² s⁻¹ on average ( Figure 3 , panel A).

For midday transpiration, the Kruskal-Wallis test produced an H-statistic of 24.94 and a p-value < 0.001 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that midday transpiration was significantly higher in September than in the other three months. October midday transpiration was also significantly lower than May ( Figure 3 , panel A).

There was a similar trend in stomatal conductance to transpiration across the four months with May, July, and October experiencing similar average stomatal conductance rates around 0.76 mol m⁻² s⁻¹ and a spike in September at an average of 0.28 mol m⁻² s⁻¹. As with transpiration, there was no significant difference in stomatal conductance between age classes for all four months ( Figure 3 , panel B).

For midday stomatal conductance, the Kruskal-Wallis test produced an H-statistic of 27.81 and a p-value < 0.001 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that midday stomatal conductance was significantly different between May and July, May and October, July and September, and September and October. Between May and September there is a near significant difference with a p-value of 0.067 ( Figure 3 , panel B).

Assimilation rate, however, followed a slightly different trend than seen in both transpiration and stomatal conductance. Both juveniles and saplings experienced a spike in CO₂ assimilation in September, but while sapling assimilation decreased again in October (though not to the degree of change seen in transpiration or stomatal conductance), juvenile assimilation remained at a similar rate as seen in September. Therefore, in October, juveniles had a significantly higher CO₂ assimilation rate around 16 µmol m⁻² s⁻¹ than saplings at 8 µmol m⁻² s⁻¹ while there was no significant difference between age classes in May, July, or September ( Figure 3 , panel C).

For midday assimilation, the Kruskal-Wallis test produced an H-statistic of 19.19 and a p-value < 0.001 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that midday assimilation was overall significantly higher in September and October than in May and July ( Figure 3 , panel C).

We calculated water use efficiency (WUE) for all measurements. We found no significant difference in midday WUE between juveniles and saplings for all months. WUE remained stable at around 2.5 g of CO₂/g of H₂O for May, July, and September but spiked to an average of 17.8 g of CO₂/g of H₂O in October ( Figure 3 , panel D).

For midday WUE, the Kruskal-Wallis test produced an H-statistic of 21.26 and a p-value < 0.001 signifying a significant difference between two or more months. A Dune’s Test Pairwise Comparison showed that midday WUE was significantly higher in October than in the other three months ( Figure 3 , panel D).

The raw hourly temperature, VPD, and PAR data from ERA5 failed all assumptions of equal variance and normality.

The Kruskal-Wallis test for temperature in May, July, September, and October yielded an H-statistic of 1280.69 and a p-value < 0.001, indicating a highly significant difference in temperature between two or more months. The Dunn’s Test Pairwise Comparison showed temperature varied significantly between all four months. A comparison of the mean temperature for each month showed the highest temperature in July and the lowest in October ( Figure 4 ). The average, maximum, and minimum temperature for each month is shown in Table 1 .

The Kruskal-Wallis test for VPD yielded an H-statistic of 432.08 and a p-value < 0.001 indicating a significant difference in VPD between two or more months. The Dunn’s Test Pairwise Comparison showed VPD varied significantly between all four months except between July and September. VPD increased gradually from the lowest in May to the highest in October ( Figure 4 ). The average, maximum, and minimum VPD for each month are shown in Table 1 .

The Kruskal-Wallis test for PAR yielded an H-statistic of 81.82 and a p-value < 0.001 indicating a significant difference in PAR between two or more months. The Dunn’s Test Pairwise Comparison showed PAR varied significantly between May and September and May and October. The average, maximum, and minimum PAR value for each month is shown in Table 1 .

According to the linear regression analysis between VPD and transpiration, VPD explained about 20 percent of the variation in transpiration rate with a p-value < 0.001. There was a significant negative trend (y = −0.002x + 0.006) between VPD and transpiration ( Figure 5 , panel A).

VPD explained about 31 percent of the variation in stomatal conductance with a p-value < 0.001. There was a significant negative trend (y = −0.314x + 0.510) between VPD and stomatal conductance. May experienced the greatest variation in stomatal conductance while in all other months stomatal conductance remained relatively consistent ( Figure 5 , panel B).

VPD explained about 12 percent of the variation in assimilation rate with a p-value < 0.001. There was a significant positive trend (y = 4.327x + 3.992) between VPD and assimilation rate ( Figure 5 , panel C).

VPD explained about 22 percent of the variation in LWP with a p-value < 0.001. There was a significant negative trend (y = −1.826x − 2.338) between VPD and LWP ( Figure 5 , panel D).

According to the linear regression analysis between temperature and transpiration, temperature explained about 2 percent of the variation in transpiration rate with a p-value of 0.17 signifying no significant trend between temperature and transpiration ( Figure 6 , panel A).

Temperature explained about 15 percent of the variation in stomatal conductance with a p-value < 0.001. There was a significant negative trend (y = −0.051x + 1.643) between temperature and stomatal conductance ( Figure 6 , panel B).

Temperature only explained about 0.05 of the variation in assimilation with a slightly significant p-value of 0.02 and a positive trend (y = 0.666x – 10.706) ( Figure 6 , panel C).

Temperature explained about 21 percent of the variation in LWP with a p-value < 0.001. There was a significant negative trend (y = −0.418x + 7.810) between temperature and LWP ( Figure 6 , panel D).

According to the linear regression analysis for PAR, PAR did not explain any of the variation in transpiration, stomatal conductance, or LWP ( Figure 7 , panel A, B, D).

PAR explained only 10 percent of the variation in assimilation rate. With a p-value < 0.01, there was a significant positive trend (y = 0.003x + 5.928) between PAR and assimilation rate ( Figure 7 , panel C).

We attempted to understand how different age classes of Avicennia germinans adapt their hydraulic strategies to maintain efficient photosynthesis under environmental fluctuations. Over the measurement period, atmospheric VPD increased corresponding to decreasing precipitation and high temperatures ( Figure 4 ; Table 1 ). Therefore, the atmosphere exerted an increasingly negative water potential that tended to pull water out of the leaves through the stomatal openings. By measuring transpiration, assimilation, and stomatal conductance, we observed how the trees regulated their photosynthetic inputs in response to these increasingly stressful conditions.

Juvenile and sapling mangroves did not differ significantly in the majority of their water regulation and gas exchange strategies. Although we found that juveniles exhibited higher assimilation rates during the highest VPD month (October), they were still consistent with saplings in their WUE across all VPD conditions observed. We saw the highest WUE and greatest plant stress (LWP) during the highest VPD conditions. Hydraulic plasticity and high WUE, especially in the younger age class, may be an essential adaptation to continue to survive, expand, and establish in an increasingly changing climate where potential drought will bring prolonged atmospheric and osmotic stress. Overall, WUE was consistent between the pre- (sapling) and post-freeze (juvenile) age classes suggesting a lack of lingering freeze damage and the potential for population recovery after disturbance.

We found that VPD had a greater effect on transpiration, stomatal conductance, assimilation, and LWP, than temperature or PAR. We observed that under low VPD conditions (> 2 kPa), transpiration and stomatal conductance decreased while CO₂ assimilation increased with increasing VPD. This is in line with the study by Ulatowski and Matheny (2025) that showed that A. germinans increase their WUE under short term VPD stress [48] .

We conclude that A. germinans are resilient to some seasonal shifts in temperature and VPD by adjusting their hydraulic strategies on monthly time scales. We also conclude that the A. germinans are resilient to extreme freezing events by recovering a healthy, hydraulically plastic population through surviving saplings. With predictions of increasing drought brought on by climate change [58] [59] , survival of mangrove forests will depend on this ability to adapt to stressful environmental conditions. As mangrove populations push their ranges further into the temperate zones, their plasticity in water regulation will be tested by seasonal changes in environmental conditions. Understanding the hydraulic strategies of these vulnerable yet resilient trees will help us to prepare for foreseeable threats to their existence by climate change or human activity.

AMM, CG, CC, and MUU were supported by the Department of Energy Terrestrial Ecosystem Science Program Grant (DE-SC0020116) and in part by NSF CAREER Award 2046768.