Dendrochronological analysis of productivity and hydrology in two Louisiana swamps

Modified river flows and land subsidence have subjected many coastal swamp forests in the delta of the Mississippi River to greatly altered flooding, sediment, and nutrient regimes. These areas have become inundated to greater depth, duration, and frequency and either are connected to the river and receive drastically increased sediment and nutrients (riverine swamps) or have become disconnected from riverine flooding (stagnant swamps) and receive little or no sediment and nutrient input. To better understand how these changes are affecting ecosystems, dendrochronological techniques for baldcypress (Taxodium distichum L. Rich.) were used at three sites in each of two contrasting swamps to compare how productivity has been historically related to climate and hydrology. The historical responses of baldcypress radial growth to 111 years of climatic variables and 51 years of hydrological data were analyzed by separate and simultaneous multiple linear regression analysis of effects. The effect of flooding on growth of baldcypress depended on the temporal scale and type of floodwater. Seasonal flooding increased growth in flood years for both stagnant and riverine swamps but decreased growth in the ensuing year. Long-term, riverine flooding was positively correlated to growth, but long-term, stagnant flooding was negatively correlated to growth. Growth in both swamps increased at onset of greater flooding, then decreased after several decades of continued frequent flooding. Climatic controls on growth depended upon interactions with hydrology but correlations between growth and flooding variables were consistent and much stronger than climate. When climate was analyzed separately, warm winters and cool, rainy growing seasons were important for higher growth. When hydrology was included in simultaneous analysis with climate, cool, dry fall weather of the previous year was also important. A model of baldcypress growth that only includes hydrologic regime is insufficient to understand growth. Climate, stand history, hydrological regime and the interaction of these effects must be considered to accurately understand changes in growth