Effects of time since fire
Whilst we find no effects of time since fire on species richness, the
species composition of tree/shrub, forb and graminoid communities
changes markedly following a fire. Increased species turnover
immediately after the fire is likely to be due to loss of species due to
mortality or their inability to tolerate novel environmental conditions
(including micro-climates, nutrient and light levels), and the
recruitment of new species that favour the altered environmental
conditions (Doherty et al., 2017; Kaewsong et al., 2022; Keeley et al.,
2005). Biodiversity should gradually recover after a fire event and
increasingly resemble the pre-fire community (Machida et al., 2021). In
forb communities, recovery tends to be close to completion ten years
after a fire event, but recovery of tree/shrub community composition is
incomplete after ten years. Moreover, there is no evidence for any
recovery in graminoids communities outside protected sites after ten
years. The long-term persistence of these compositional changes is
probably driven by multiple factors including the long-term legacy of
altered nutrient availability post-fire (Verma et al., 2019), fire
induced reductions in tree growth rates (Bucini & Hanan, 2007), and
(especially outside protected areas) altered land-use patterns following
fire events (Butsic et al. 2015). Our results underline the need to
avoid fire for at least ten years to allow forb communities to recover,
and longer for trees/shrubs – and to protect burnt locations from human
activities that could disturb regenerating vegetation. It is thus
notable that fire return rates in many tropical areas are already
shorter than ten years (Archibald et al. 2013), and in some locations,
such as central highlands of Vietnam, have increased in recent decades
primarily due to changes in human activity (Nguyen et al. 2023). Indeed,
the number of fires in the tropics has increased at ~5%
per annum since 2001 (Tyukavina et al., 2022), and are projected to
increase further due to both climate change and human activity across a
wide range of tropical regions and biomes (Wu et al., 2021; Li et al.,
2023). There is thus increasing probability that fires will fire return
intervals will decrease to the extent that plant communities will be
unable to recover community composition before the next fire occurs,
especially in the drier tropical biomes that currently have the shortest
fire return intervals (Archibald et al. 2013).