Watershed and Estuarine Controls Both Influence Plant Community and Tree Growth Changes in Tidal Freshwater Forested Wetlands along Two U.S. Mid-Atlantic Rivers

Round 1

Reviewer 1 Report

General Comments

This is an interesting and novel study that examines long-term effects of multiple variables on vegetation of tidal freshwater forested wetlands, an understudied ecosystem. The study appears to have been meticulously carried out the manuscript prepared with a lot of attention to detail.

The finding of greater basal area increase (BAI) during years with greater estuarine was level and salinity is surprising, but is also contradictory with lower BAI at transitioning sites  compared to fresher upstream sites. This contradiction should be more clearly explained throughout the paper so the reader is not left with the idea that higher salinity and tree growth will lead to larger and bigger trees – the observation of lower BAI at downstream sites disproves that. Instead there appears to be a physiological response of the trees to higher water level OR salinity periods that stimulates growth (e.g. many wetland plants grow taller when they are more flooded due to stimulation of ethylene production), but the sustained higher salinity at the downstream sites will lead to slower growth overall (as you found), smaller diameter trees and more open canopy.  The stimulatory physiological effects of increases in water level should be considered and teased apart more from salinity effects.

Specific comments

L30-31. Were salinity and estuarine water level correlated, or did they operate independently? Understanding this could help disentangle their effects on tree growth. I can imagine that droughts would occur that resulted in saltwater intrusion independently of estuarine water level, for example.

L32. The phrase  “with variable directionality of influence" is not needed because just before it says the nutrient influence is site specific, implying different responses at different sites.

L36. The statement “understory composition simplified” is vague – e.g. does it refer to diversity, structure? – and also inconsistent with the following phrase that herbaceous cover and richness increased.

L28. The finding that BAI was LOWER at the more saline downstream sites should be included here (and elsewhere) and distinguished from the annual BAI stimulatory events.

L38. The focus here and elsewhere seems to be on the surprising effect of salinity on growth in many place, but one might also think higher estuarine water level could stress plants if of long duration, or temporarily increase growth. Either way, the estuarine water level should be considered at least equally with salinity, and its potential role differentiated from that of salinity to the extent possible.

L92. The Duberstein et al. paper focuses on the southeast US but not the whole eastern U.S., e.g. including the study sites and other studies showing ash-dominated forests in the mid-Atlantic.

L173. The term “beginning with 2015” suggests increments will be calculated forward in time, and “recent year with an entire growing season” is not linked to the tree cores. So perhaps it would be good to rephrase this something like “…were calculated proceeding backwards in time starting with 2015, the most recent full growing season captured in the tree cores”.

L180. Explain “detrended”.

L213. So are these “flow-normalized concentrations” essentially accurate reflections of N and P loading rates to the forests? It might be worth explaining this clearly (e.g. concentration may go down during high flows but loading to forests is much higher).

L485-486. This sounds contrary to the abstract – here and throughout more closely link and explain both the lower average BAI at the ST sites vs. the higher BAI during periods of higher estuarine water and salinity level.

L519-523. Was estuarine water level correlated with salinity at these sites, and did the more saline years also have higher estuarine water level? If so, it would be hard to separate the salinity and water level effects.

L613-614. Here also bring in the lower overall growth at more saline than at freshwater sites to avoid people drawing unintended conclusions.

Author Response

Please see attachment with replies to both reviewer's comments.

Author Response File: Author Response.docx

Reviewer 2 Report

Title: Watershed and estuarine controls both influence tree growth and plant community changes in tidal freshwater forested wetlands along two U.S. mid-Atlantic Rivers

General Comments:

This study uses vegetation monitoring and annual basal area increment (BAI) data to assess longitudinal and temporal changes in composition and growth along tidal freshwater forest gradients, with a deeper assessment of drivers of BAI temporal trends. The paper is well-written, provides useful compositional characterization in these fairly understudied systems (compared to other wetland and riparian systems), and highlights complex drivers of tree growth. As these systems are particularly vulnerable to sea level rise and altered river flow regimes, such information will help to guide continued monitoring aimed at assessing changes in community composition and structure.  My only major comment regards organization.  That is, I suggest that all sections (abstract through results) match the organization used in the Discussion (and in Table 1), where contemporary longitudinal gradients in vegetation composition/structure data are presented first, followed by time-varying drivers of BAI.  In this way, the separation between contemporary spatial gradients (vegetation composition/structure) and temporal changes (only BAI) would be more explicit.  The two predictions could also be revised to better match this suggested order (prediction 1 about longitudinal gradients, prediction 2 about temporal changes in BAI and drivers thereof); see a detailed comment below.  Last, in following this new order, the authors may or may not find it more appropriate to include longitudinal trends in BAI within sections on such trends in vegetation composition/structure.    

Line 36: BAI was also assessed along the gradient.

Line 62: Delete "river flooding" to simplify and since it's related to "river discharge".

Lines 81-82: The use of "tidal forests" may be a bit misleading as mangroves are the other tidal forest that is not freshwater.  Further, it is obvious that freshwater species would differ in their response compared to salt-tolerant species.  I suggest revising this concluding statement to point out that these initial studies highlighted stressors which may vary longitudinally and thus as will their induced effects on composition and growth, linking it back to the topic sentence and to the subsequent paragraph.

Line 121-124: Without a more quantitative comparison with Rheinhardt's data, I suggest removing this sentence; having it here suggests such was done. Rather, include something like this in the Discussion where the more qualitative comparison is made.

Line 126: This prediction tends to mix drivers of annual BAI (statistically assessed) and characterization of longitudinal gradients in vegetation composition/structure.  Again, separating by longitudinal gradients and temporal drivers may help make the two predictions more direct to the assessments made.

Figure 1: Need to include definition of site labels in the caption.

Section 2.4:  Why was maximum estuarine water level and salinity included but not maximum river flow and nutrients?  What about other metrics, including variance and percentiles.  Not saying that such needs to be included but maybe a bit more justification for variables explored.

Line 287: How about longitudinal trends for two dominant species?  Their differences between rivers are noted, but seems some attention to their within-river trends would be useful.

Table 1: Why only one plot at MST?  This needs to be stated in methods.

Figure 2: For PLT and PSTs, shifts in TP flatlines is likely due to new detection limits or other methodologic artifacts, suggesting it should be removed as a variable here and, more importantly, in the models.

Figure 3: Less basal area but higher density at ST sites suggests smaller, maybe stressed trees.  Something to consider in the discussion.  Add "herbaceous" to percent cover in the caption.

Table 3: This will need to be polished and shortened if possible to provide the major findings for main body, with a potentially more exhaustive list/table for supplemental.

Author Response

Please see attachment with replies to both reviewer's comments.

Author Response File: Author Response.docx