COS 159-4 - Projected impacts of climate change and habitat fragmentation on population dynamics of a tropical understory plant

Background/Question/Methods

Climate change is predicted to increase the frequency, intensity, and duration of drought in the Amazon. Extreme drought events result in increased tree mortality and reductions in growth. However, the impacts of drought on understory species (comprising up to __% of forest biomass) are less well known. We have previously shown that drought has immediate and delayed impacts on the survival, growth, and flowering of an understory herb (Heliconia acuminata, Heliconaceae). Additionally, drought has different impacts on plants in forest fragments compared to those in contiguous forest plots.

We used integral projection models (IPMs) to create demographic models that account for seasonal and lagged effects of drought on plant vital rates. We then used these lagged effects IPMs to project populations into the future using outputs from global circulation models (CMIP6) under four climate change scenarios (SSPs) in order to answer the following questions: Will populations in forest fragments fare significantly worse than those in contiguous forest under climate change? 2) Are the impacts of fragmentation and climate change likely to be interactive; i.e. is the difference between contiguous forest and forest fragment population growth more pronounced under more extreme climate change scenarios?

Results/Conclusions

We selected 8 CMIP6 models (out of 14 that contained variables of interest) that met our model skill criteria. The CMIP6 models varied in their projections for the study region with some showing drying conditions across the year and others with drier dry seasons but wetter or relatively unchanged wet seasons. All CMIP6 models predicted increasing drought frequency (# months with SPEI < -1) and all but one model predicted disproportionate increases in the number of dry season droughts.

Using lagged effect IPMs to project population trajectories resulted in population growth in contiguous forests and population decline in forest fragments across all CMIP6 models and scenarios. Although there was variation among CMIP6 models in projected population trajectories, the rank order of SSPs within each climate model was fairly consistent. The climate change scenario associated with the greatest warming trends (SSP5-8.5) resulted in greater population growth in contiguous forest and a greater population decline in forest fragments compared to the least extreme scenario (SSP1-2.6). Habitat fragmentation had a greater effect on population trajectories than changes in SPEI under climate change, demonstrating the importance of preserving contiguous tropical forest for H. acuminata.