Metapopulation dynamics of the mistletoe and its host in savanna areas with different fire occurrence

Mistletoes are aerial hemiparasitic plants which occupy patches of favorable habitat (host trees) surrounded by unfavorable habitat and may be possibly modeled as a metapopulation. A metapopulation is defined as a subdivided population that persists due to the balance between colonization and extinction in discrete habitat patches. Our aim was to evaluate the dynamics of the mistletoe Psittacanthus robustus and its host Vochysia thyrsoidea in three Brazilian savanna areas using a metapopulation approach. We also evaluated how the differences in terms of fire occurrence affected the dynamic of those populations (two areas burned during the study and one was fire protected). We monitored the populations at six-month intervals. P. robustus population structure and dynamics met the expected criteria for a metapopulation: i) the suitable habitats for the mistletoe occur in discrete patches; (ii) local populations went extinct during the study and (iii) colonization of previously non-occupied patches occurred. The ratio of occupied patches decreased in all areas with time. Local mistletoe populations went extinct due to two different causes: patch extinction in area with no fire and fire killing in the burned areas. In a burned area, the largest decrease of occupied patch ratios occurred due to a fire event that killed the parasites without, however, killing the host trees. The greatest mortality of V. thyrsoidea occurred in the area without fire. In this area, all the dead trees supported mistletoe individuals and no mortality was observed for parasite-free trees. Because P. robustus is a fire sensitive species and V. thyrsoidea is fire tolerant, P. robustus seems to increase host mortality, but its effect is lessened by periodic burning that reduces the parasite loads.

槲寄生是一类空中半寄生植物，它们栖息于被不适生境环绕的适宜生境斑块（即寄主树木）中，可被建模为集合种群（metapopulation）。集合种群被定义为：在离散生境斑块中，通过定殖与灭绝过程的动态平衡得以维持的细分种群。本研究旨在利用集合种群研究方法，探究巴西三处稀树草原区域内的健壮鹦鹉槲寄生（Psittacanthus robustus）及其寄主厚叶萼囊花（Vochysia thyrsoidea）的种群动态；同时探究火灾发生情况的差异对这两个物种种群动态的影响——研究期间有两处区域遭遇火灾，另一处则实施了防火保护。本研究以半年为间隔对种群进行监测。健壮鹦鹉槲寄生的种群结构与动态符合集合种群的预期判定标准：① 槲寄生的适宜生境以离散斑块形式存在；② 研究期间存在局域种群灭绝事件；③ 此前未被占据的斑块发生了定殖事件。所有区域的已占据斑块比例均随时间推移呈下降趋势。槲寄生局域种群的灭绝存在两种不同诱因：在无火区域中为斑块生境灭绝，在火烧区域中则为火灾直接致死。在一处火烧区域中，已占据斑块比例出现最大幅度下降，其原因是一场火灾杀死了所有寄生植物，但并未造成寄主树木死亡。厚叶萼囊花的最高死亡率出现在无火区域，该区域内所有死亡的寄主树木均附着有槲寄生个体，而未被寄生的树木未出现死亡情况。由于健壮鹦鹉槲寄生为火敏感物种，而厚叶萼囊花具有耐火性，因此健壮鹦鹉槲寄生似乎会加剧寄主树木的死亡，但定期火烧可通过降低寄生负荷来削弱这一效应。