Contrasting effects of flooding on tree growth and stand density determine aboveground production, in baldcypress forests

Stand production, a common indicator of site productivity, is often low in flooded wetland forests. However, it remains unclear whether the low production reflects flood stress limiting tree growth, flood disturbance limiting tree density, or both. Here, we investigate how tree growth and stand density vary in baldcypress forests, across gradients of flooding by both fresh and saline waters. We used three investigations to conduct this analysis: (a) a meta-analysis of previously reported tree- and stand-production measurements; (b) an analysis of tree cores from 204 baldcypress trees in 16 sites; and (c) intensive stand surveys and growth measurements across two freshwater flooding gradients in southern Louisiana. The meta-analysis (a) showed that stand production generally varies linearly with stand density, and that detrimental effects of flooding on tree-level growth are only consistently evident in saline systems. The tree-core investigation (b) showed that tree basal area growth was largely a function of tree sapwood area, but basal area growth per sapwood area (i.e. sapwood efficiency, a measure of how efficiently trees grow per growing space occupied) was higher in the more flooded sites of freshwater gradients and lower in the more saline sites of salinity gradients. The intensively studied sites (c) showed that deeper flooding was associated with sparse stands with lower production; however, trees in those sparser sites had higher sapwood efficiency, implying that the benefits of reduced competition were greater than any potential physiological limitations caused by that deeper flooding. Our findings generally show that sparser stands with saline flooding contained poorly growing trees, but sparser stands with freshwater flooding generally did not. These dichotomous effects of freshwater flooding on tree growth versus stand density demonstrate the need to conceptualize production as a dual function of disturbance effects on stand structure and environmental limitations on individual trees, each with potentially different limiting factors.