4.1. Effects of Water Level on Sap Flux Rates and Response to Atmospheric Drivers
As hypothesized, we observed significantly higher sap flux rates in black ash than in codominants. However, the difference between black ash and non-black ash sap flux rates was greatest at low water levels, and lowest at high water levels, which is contrary to the second component of this hypothesis. This response was likely driven by physiological differences between black ash and non-black ash in response to low water levels or changes in water level.
This study did not examine physiological differences or physiological responses between species in these sites though adaptations have been observed in black ash and red maple (see above). Seedling studies have shown that wetland species are known to adapt in a variety of ways, including stomatal closure, hypertrophied lenticels, aerenchyma tissue formation, adventitious root growth, and reproductive resilience. In addition to species-level adaptations, the growing conditions of an individual stem may also lead to the improved tolerance of inundation. Individuals from wet-site populations, or those exposed to early inundation or continued wet soil conditions, recovered more quickly and fully from inundation treatments [57
]. This suggests that beyond species selection for canopy replacement, stock source and early growing conditions may need to be considered to best match the current ecohydrologic function of black ash. Likely, it is a collection of these species- and individual-level adaptations that leads to black ash dominance on wet sites in the Great Lakes region and elsewhere, as Mistch and Rust [60
] suggested for riparian tree growth.
Few studies have directly assessed the role of the water table on sap flux or transpiration. Using a similar study design to our own, McJannet [36
] found that broad-leaved paperbark (Melaleuca quinquenervia
(Cav.) S. T. Blake), a wetland-adapted species, showed no change in transpiration with inundation. Bald cypress (Taxodium distichum
(L.) Rich), another species that exhibits canopy dominance in sites with regular and prolonged inundation, significantly increased sap flux during periods of inundation [61
]. The results from both broad-leaved paperbark and bald cypress are in contrast to the response of black ash in this study, where a reduction in water levels caused a significant increase in sap flux rates, while inundation led to suppressed sap flux. Further research on black ash in sites less prone to inundation would help determine if the observed high sap flux rates at low water levels are the persistent state for black ash or if these rates occur only as an adaption for recovery from periods of inundation. Broad-leaved paperbark is native to eastern Australia and Oceania [8
] and cannot be considered as a replacement species for black ash. The native range of bald cypress extends north to southern Illinois, USA [62
], but individual cold hardiness varies, and planted individuals can survive as far north as Hayward, WI, USA [63
]. No research has been conducted on bald cypress as a canopy replacement species for black ash.
We did not observe a suppressed response to atmospheric drivers in codominants relative to black ash as water levels increased, as we hypothesized. At all water levels, black ash and red maple responded similarly to Dz, and at low and mean water levels, black ash showed a significantly stronger response than yellow birch. While black ash and red maple both showed a significantly stronger positive response to Dz with decreasing water levels, the response in red maple did not lead to significantly higher sap flux rates. This suggests that red maple is adapted to respond to changing water levels, but the response is less vigorous than that of black ash.
Atmospheric conditions and energy availability are well-known drivers of sap flux [43
]. In contrast, the results of studies on the effect of soil moisture have been more mixed, with much of the existing research focused on soil moisture deficit as a limiting factor of transpiration [43
]. Specifically, within a wetland-upland transition in northern Wisconsin, Traver et al. [69
] report that atmospheric drivers were more important than edaphic conditions for understanding spatial patterns of transpiration. However, significant changes in water table position, such as those observed in these systems following the removal or death of black ash [14
], can be expected to have a greater impact than the scale of soil moisture variation often studied, as inundation can lead to the suppression of transpiration and growth, even in wetland-adapted species [27
A study of black ash in Minnesota found that across sites with varying soil moisture regimes, mean sap flux and response to Dz
were greater in sites with greater soil moisture [71
]. The authors identified differences in sapwood depth and area in the black ash populations among sites, suggesting that individual adaptation or site-induced selection may play a role in the results. The design of our study, and the mixed-modeling approach in our analysis, allow us to test the effect of water level within populations of individuals rather than among populations. It is difficult to compare within-population trends to among-population trends when previous work has shown that adaptation to wet sites and inundation occurs at the species and individual levels (see above), so that systemic variations may occur at different levels.
4.2. Persistence of Hydrologic Change
The loss of black ash has been shown to lead to reduced water level drawdown during the growing season and an earlier water level rebound in the fall following senescence [15
]. As a result of the inverse relationship between ash sap flux and water level, the rate of drawdown will be further reduced late in the growing season when water levels have receded from their spring peaks. The lack of an increase in sap flux in response to reduced water levels in non-ash species will result in the sustained reduction of water level drawdown, leading to earlier water level rebound in the fall. The end result will be persistently higher water levels, both intra- and inter-annually, even if a similarly stocked forest canopy of codominants becomes established. However, differences in microsite survivorship suggest that a less dense forest canopy may be expected.
Black ash consistently occupies the hollows in a hummock and hollow landscape, evidenced by black ash’s significantly lower relative root collar elevations. The data presented here cannot confirm that non-black ash prefer hummocks in these sites, but observed water levels were never above non-black ash root collars. The prevalence of black ash in the hollows, paired with a significant increase in the survivorship of non-black ash seedlings planted on hummocks [18
], suggest that low-lying microhabitats in these sites may not be suitable for the future growth of mature individuals of these species, precluding a significant area from potential restocking efforts. Following infestation, increased water levels will reduce the prevalence of higher elevation microsites available for seedling establishment, further increasing the potential for a less dense canopy.
Changes in the forest canopy will likely influence site hydrology in ways not examined in this study. Changes in forest canopy closure or composition can be expected to change precipitation and energy inputs to the site. As part of the ongoing research efforts to understand black ash stands and anticipated changes brought on by EAB, the impacts of simulated EAB infestation on throughfall [12
] and changes in radiative energy reaching the shrub layer and vegetative ground cover have been examined. Davis et al [12
] found that the loss of the ash canopy without a mature canopy replacement increased forest throughfall, though these results were confounded by high canopy heterogeneity. The effect of changes to throughfall may be masked by other inputs, as sourcewater analysis of these wetlands found groundwater contributions throughout the growing season [15
]. A reduction of radiative energy reaching the forest floor following EAB-induced mortality relative to the complete removal of black ash will have the effect of suppressing the understory growth response and reducing the potential evaporative demand from the soil surface and pooled water. In this way, a non-black ash canopy with lower relative transpiration could have an even stronger negative effect on growth and evapotranspiration, further contributing to the persistence of hydrologic changes in black ash wetlands if seasonal canopy water use is less than black ash.