Data from: Indirect effects of global change accumulate to alter plant diversity but not ecosystem function in alpine tundra

1. Environmental change can affect species directly by altering their
physical environment and indirectly by altering the abundance of
interacting species. A key challenge at the interface of community ecology
and conservation biology is to predict how direct and indirect effects
combine to influence response in a changing environment. In particular,
little is known about how direct and indirect effects on biodiversity
develop over time or their potential to influence ecosystem function. 2.
We studied how nitrogen (N), winter precipitation (snow), and warming
influenced diversity and ecosystem function over six years in alpine
tundra. We used path analyses to partition direct effects of environmental
manipulations from indirect effects due to changes in the abundance of two
dominant plants. We hypothesize that 1) indirect effects will develop more
slowly but will become stronger than direct effects over time, and 2)
after six years, indirect effects will more strongly influence diversity
while direct effects will influence ecosystem function. 3. Indirect
effects of N on diversity were consistently stronger than direct effects
and actually developed quickly, prior to direct effects. Direct effects of
snow on diversity were detected in year two but then subsequently were
reversed, while indirect effects were detected in year four and grew
stronger over time. Overall in year six, indirect effects were much
stronger than direct effects. 4. Direct effects predominated for three of
four ecosystem functions we measured (productivity, N mineralization,
winter N availability). The only indirect effects we found were that N and
snow indirectly affected microbial biomass N by influencing Geum
abundance. Across all four ecosystem measures, indirect effects were
infrequent and weaker than direct effects. 5. Synthesis. Increasing
indirect effects on diversity over time indicate that short-term
experiments or monitoring of natural systems may underestimate the full
magnitude of global change effects on plant communities. Explicitly
accounting for changes in dominant plant abundance may be necessary for
forecasting plant community response to environmental change. Conversely,
weak indirect effects for ecosystem processes suggest that predicting
ecosystem function without knowledge of plant responses to global change
may be possible.