Resilience of palm populations to disturbance is determined by interactive effects of fire, herbivory and harvest

Abstract

Summary 1.Little is known about the interactive effects of multiple forms of disturbance -natural or anthropogenic -on plant population dynamics. This limits our ability to understand the drivers of these dynamics and effectively manage plant populations in the face of changing disturbance regimes. 2. Fire, grazing and harvest of wild plants are three widespread, commonly co-occurring land management activities in the tropics that contribute to altered disturbance regimes. In this study, we investigate the effects of and interactions among these activities on mountain date palm (Phoenix loureiroi) populations in the Western Ghats, India with data from 14 palm populations, including over 2300 palm ramets monitored from 2009 to 2011. 3. We construct integral projection models (IPMs) of palm population dynamics based on the response of palm individuals to fire and varying intensities of grazing and leaf harvest and carry out Markov Chain Monte Carlo simulations to understand the implications of different scenarios of disturbance for population persistence. 4. We find that palm individuals and populations were resilient to low levels of fire, grazing and harvest. There are also important interactions among these forms of disturbance: intensities of harvest and grazing are highest in populations one to two years after fire, and the intensity of harvest is lower in heavily grazed populations. 5. These interactions mean that the combined effects of multiple forms of disturbance differ from what would be assumed from simply adding their separate effects. While simulations show that sustained high intensities of harvest and grazing would have severely negative consequences for palm populations, interactions between these two forms of disturbance prevent palm populations from actually experiencing such extreme conditions. On the other hand, because intensities of grazing and harvest both increase moderately following fire, their combined negative effects on population growth rates are exacerbated by fire. Thus, multiple, interacting disturbances determine the resilience of these palm populations and the sustainability of their management. 6. Synthesis. This study illustrates the importance of explicitly incorporating realistic interactions among multiple forms of disturbance when evaluating plant populations' resilience to changing disturbance regimes and the sustainability of plant harvest.

Population modelling and genetics of a critically endangered Madagascan palm Tahina spectabilis

“…Previous studies have found that it is only the smallest size classes of seedlings in palms that are vulnerable to fire as the apical meristem is often well protected by leaf bases in juvenile stages (McPherson & Williams, 1998). Thus, our model accounts for the chance of fire leading to reduce survivorship of the seedling stages consistent with the levels reported in other studies (Arneaud et al, 2017; Mandle et al, 2015; McPherson & Williams, 1998). Our models suggest that fire alone is unlikely to lead to extinction of T. spectabilis populations.…”

“…Our population models that accounted for both reproductive failure and seedling mortality mimic the potential impacts of fire, grazing, harvesting, and insect attack and show that T. spectabilis is potentially tolerant to the combined effects of these. Mandle et al (2015) also found the Phoenix loureiroi palms in India were resilient to low levels of fire, grazing, and harvesting.…”

“…Extrinsic factors may have a major impact on the survival of the T. spectabilis population, as they can cause stochastic effects at the demographic level (Sodhi & Ehrlich, 2010) by exacerbating stress, and thus leading primarily to the death of vulnerable individuals. Many palms such as T. spectabilis inhabit fire prone environments and mature palms of many species are known to survive fires due to the distribution of meristematic tissue throughout the stem (Arneaud et al, 2017; Mandle, Ticktin, & Zuidema, 2015; McPherson & Williams, 1998). Previous studies have found that it is only the smallest size classes of seedlings in palms that are vulnerable to fire as the apical meristem is often well protected by leaf bases in juvenile stages (McPherson & Williams, 1998).…”

Do the structures of macaw palm fruit protect seeds in a fire-prone environment?

“…While arborescent monocots do not have bark (Tomlinson 2006) that confers protection from fire (Pausas 2015), there is no superficial meristematic cambium, which would otherwise be vulnerable to fire, pathogens, and trauma (Tomlinson 2006). In fact, because there is no damage to the apical meristem, palm species, including A. aculeata, might resist, or even benefit, from fire of low intensity, due to the compensatory effects of increased flowering and leaf production by individuals (Mandle et al 2015), or at the community resilience level (Souza & Martins 2004;Mandle et al 2015).…”

Towards a mechanistic understanding of the synergistic effects of harvesting timber and non‐timber forest products

“…; Gaoue & Ticktin ; Baldauf et al . ; Mandle, Ticktin & Zuidema ). Matrix models are stage‐ or age‐ structured models developed using field data on life (st)age survival, growth and fertility to parametrize a set of difference equations that are simplified in a matrix form (Caswell ).…”