Potential moss cover responses to climate warming and their implications for tree seedling recruitment in high-elevation montane forests

Background/Question/Methods

Complex plant-plant interactions are among the fine-scale processes that affect forest regeneration but remain poorly understood, particularly so in the context of plant responses to changing climate in climatically sensitive montane forests. In northeastern United States, montane forests have experienced gradual climate warming and decreasing acidic deposition during recent decades. Forest regeneration in these high-elevation ecosystems occurs in the form of fir waves where moss cover is abundant and plays important role in facilitating tree seedling establishment and recruitment. We studied how the reported negative impacts of warmer climate on moss cover, specifically the observed reduction in moss cover by 1 to 5% per each 1ºC of maximum daily temperature, may impact seedling establishment of the dominant high-elevation tree species, balsam fir (Abies balsamea). Further, we tested whether fir seedling density has declined in recent decades by comparing current seedling density estimates with those from several decades ago that preceded the most recent climate warming. Finally, we calculated potential weak (minimum) and strong (maximum) responses of the moss cover to warming under alternative future IPCC climate scenarios (“RCP2.6 – conservative”, “RCP4.5 – moderate”, and “RCP8.5 – extreme”) to estimate the corresponding seedling recruitment potential of balsam fir.

Results/Conclusions

Our results suggested that fir seedling density declined between 1970’s and 2010’s. When considering weak responses of moss cover to changing climate, fir seedling recruitment potential did not seem to be affected. However, strong responses of moss cover to changing climate under the most extreme future climate scenario suggested that both moss cover and fir recruitment potential could be considerably reduced relative to the current state. In this most extreme case, moss may cover only 32.8 ± 0.23 % (mean ± SE) of the ground surface in these high-elevation forests by 2100 (a decline of ~25% relative to the current moss cover); this decline in moss substrate may reduce fir seedling recruitment potential by ~22%, compared to 2010’s. Our findings indicate that this threatened climate-sensitive high-elevation ecosystem could potentially suffer decline in fir tree seedling establishment solely due to the negative effects of declining moss substrate in the relatively more extreme (but plausible) climate warming scenario. Since the abundance and composition of forest tree seedling bank affects forest dynamics and ecosystem-level properties such as productivity, fine-scale plant-plant interactions, including those between the moss layer and tree seedlings, need to be explicitly considered when modelling forest ecosystem responses to changing climate.