Plant community assembly from a trait-based perspective: Modeling stochastic and deterministic processes on Mount St. Helens

Background/Question/Methods

Ecologists are intrigued by how communities assemble following disturbance because it provides unique insight into the dynamic nature of ecosystems through time. The arrival of seeds of different species to disturbed areas undergoing succession depends on both deterministic (dispersal ability) and stochastic (movement of individual seeds) factors.  Similarly, local competitive dynamics depend on deterministic environmental filters on species traits, as well as demographic stochasticity. Traits corresponding to dispersal strategy, phenology, life history, and growth all influence community interactions at the neighborhood scale that ultimately influence local community composition and diversity.  Successional patterns in the distribution of species traits are expected to be more deterministic than successional patterns in species composition, but either pattern could be primarily deterministic or stochastic (i.e., nearly neutral) depending on rates of seed arrival. In 1980, Mount St. Helen’s erupted and reconfigured the landscape into an ecological blank slate. We used 30 years of plant species composition and abundance data along with associated trait data to develop an individual-based stochastic model of plant succession. This model examined how random versus deterministic processes dictate the trajectory of species composition and trait distributions as plant-available nitrogen builds over the course of primary plant succession due to biological nitrogen-fixation.  

Results/Conclusions

The model builds up from easily measured plant traits (e.g., seed size, leaf mass per unit area, leaf nitrogen concentration) to formulate the carbon balance of individual plants competing for nitrogen. The model predicts successional patterns that ranged from highly random (i.e., nearly neutral) to highly deterministic (niche-structured) depending on the degree of isolation (distance from seed source, which determines seed arrival rates). As expected, successional patterns in the distribution of plant traits were more deterministic than successional patterns in species composition.  In addition, we found that traits related to nitrogen and carbon acquisition and allocation, such as leaf nitrogen concentration and root:shoot allocation, were non-randomly spaced over the course of succession, which suggests a limiting similarity for local coexistence of successional types. However, non-random patterns became weaker as seed dispersal became more limiting. Linking observed and modeled plant trait distribution patterns gives us insight to potential mechanisms that affect succession and recovery following a disturbance. Using theoretical tools to link pattern to process will allow us to build upon our understanding of how communities assemble and also allow us to predict how ecological communities will respond to large-scale disturbance events.