Plant facilitation: The importance of diversity and seedling ontogeny

Background/Question/Methods

Both classical and contemporary ecological research has focused heavily on how competition can drive species specialization on resources (niche differentiation), and thus can explain species coexistence and the maintenance of species diversity.  Facilitation may also play a critical role in species coexistence, particularly in severe environments (the stress gradient hypothesis, SGH) where plant cover can ameliorate the severity of the environment.  However, facilitation may be more widespread than suggested by the SGH.  Facilitation may be especially important for the survival of newly established plants, which are small and are susceptible to mortality from desiccation and abiotic stress.  The increase in plant cover with increasing plant diversity means that the degree of environmental amelioration (relative humidity, temperature, shallow soil moisture) also increases with plant community diversity.  Here we use an experimental biodiversity gradient in central Minnesota to test the hypotheses that: (1) smaller seedlings benefit from community diversity due to amelioration of environmental stress and (2) larger seedlings are limited by community diversity due to increased competition intensity.  The biodiversity gradient is part of a larger experiment (BioCON), which was established in 1997 using common herbaceous species planted into 2 x 2 m plots at species richness mixtures of one, four, nine, or sixteen species.

Results/Conclusions

We used a two-way ANOVA (community species richness x tree size class on tree RGR) to test whether increasing plant community richness affected colonizing trees in different ways, depending on ontogenetic stage.  We found a significant interaction between species richness and tree size (F_{20, 275}=1.734, P=0.0381).  Further exploration of this relationship demonstrated that growth rates of the smallest tree seedlings were unaffected by plant community richness (either due to a lack of competition or facilitation, or competition intensity being balanced by equally strong facilitation intensity).  However, growth rates of larger seedlings were strongly limited by competition for resources, and competition dominated the plant-plant interactions at this ontogenetic stage.  These results suggest that competition intensity increases with plant size, while facilitation may be important only for smaller seedlings.