Habitat fragmentation impacts how plants survive, grow, and reproduce. So does a variable climate. Taken together, these two can alter the way plants function in unexpected ways – impacting demographic rates, population dynamics, and more. For example, plant progeny from fragmented habitats across the globe show decreased genetic diversity and outcrossing rates, and increased inbreeding and correlated paternity. And as global temperatures increase, population are more likely to need connected habitat in order to facilitate range shifts.
Scott, E.R., Uriarte, M. and Bruna, E.M. 2022. Delayed effects of climate on vital rates lead to demographic divergence in Amazonian forest fragments. Global Change Biology 28(2): 463-479.
One of the biggest regions to be impacted by both fragmentation and climate change is the Amazon. In Brazil, the Biological Dynamics of Forest Fragments Project (BDFFP) is a decades-old, experimentally fragmented landscape within the larger tropical forest that is currently experiencing unprecedented deforestation rates. The BDFFP has shown how fragmentation and edge effects impact forest dynamics and composition, and how species within forest fragments show complex responses to changes in landscape composition and configuration.
New research from the BDFFP sheds light on how precipitation extremes affect plant demography, using a decade of annual surveys of an understory herb (Heliconia acuminata) in both fragmented and continuous forests. These surveys coincided with two severe droughts, and provided a rare case of long-term plant demography data from an experimentally fragmented forest across climate extremes. Demographic data included survival, growth and flowering, which were then compared to climate conditions over time (measured as standardized precipitation evapotranspiration index, SPEI).
There are two main results to highlight. First, the demographic response to climate events were delayed up to three years, a time lag that might have been missed had the study lasted only a few years or taken place in a different time window. The reason for this lag might have several underlying causes, all of which underscore the complexity of potential individual responses. Plants showing a delayed response to climate may be the norm rather than the exception. In this case, being able to collect ten years of data led to a more complete – and more complex – picture than would have emerged otherwise.
Second, responses in fragmented forests were more variable and wide-ranging. Extremes in precipitation – both drought and unusually wet conditions – affected growth and survival more in fragmented than continuous forest. It’s likely that demographic responses to climate extremes in forest fragments are much more complex than previously thought, and that forest fragments are much more susceptible to changes in climate. It’s also possible that climate and fragmentation interact to impact plant demography – but as of now, there are no statistical tools that can effectively test for that synergy. Understanding these interactions becomes more important as forest fragmentation increases in the tropics and around the globe.
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