Stand age controls canopy and soil rainfall partitioning in slash pine forests

Changing forest structure with stand development is predicted to influence rainfall partitioning, and thus impact forest management and conservation strategies. Extensive pine silviculture in Florida also makes changes in rainfall partitioning with stand attributes consequential for regional water planning. We used weekly measurements of rainfall partitioning and soil water infiltration in two contrasting Pinus elliottii (slash pine) forest ecosystems in north Florida (USA), to quantify stand age and structural development impacts on forest hydrological function. Over 52 weeks - 45 with storms - we observed that stemflow and litterflow were significantly higher, and litter and total interception significantly lower, in the young stand (∼ 10 years since planting) than the mature stand (∼ 21 years since planting) (p <0.05). This was despite negligible differences in leaf area index and other crown structure parameters. We observed substantial differences in trunk storage capacity and trunk saturation point, with greater storage and thus reduced stemflow generation in the mature stand. We show that forest litter stores significant precipitation (7.3 % vs. 10.6 % of annual rainfall for young and mature pine stands, respectively) suggesting total interception predictions that neglect litter storage significantly underestimate actual rates, and overestimate root water availability. We further show that ignoring stemflow in small trees (i.e., young stands) underestimates total stemflow (3.4 % vs. 7.2 % of incident rainfall), leading to an overestimation of canopy and total interception in young stands. We used a reformulated Gash analytical model to obtain weekly estimates of canopy rainfall interception. Our most robust results were observed when we used in-situ partitioning parameters and dynamic canopy cover measurements. However, using literature-derived canopy ecohydrological parameters for southern pines also yielded acceptable predictions given dynamic canopy measurements, suggesting informative generality for application to new settings. Using static canopy measurements yielded consistently inferior results, suggesting that tracking canopy properties is more important for accurate interception predictions than local partitioning parameter estimation. Our results highlight the significance of forest maturity as a factor altering water storage capacity and rainfall partitioning in planted pine stands.