CAM plants: their importance in epiphyte communities and prospects with global change

Abstract

Background and Scope The epiphytic life form characterizes almost 10 % of all vascular plants. Defined by structural dependence throughout their life and their non-parasitic relationship with the host, the term epiphyte describes a heterogenous and taxonomically diverse group of plants. This article reviews the importance of crassulacean acid metabolism (CAM) among epiphytes under current climatic conditions and explores the prospects under global change. Results and Conclusions We question the view of a disproportionate importance of CAM among epiphytes and its role as a “key innovation” for epiphytism but do identify ecological conditions under which epiphytic existence seems to be indeed contingent on the presence of this photosynthetic pathway. Possibly divergent responses of CAM and C3 epiphytes to future changes in climate and land use are discussed with the help of experimental evidence, current distributional patterns, and the results of several long-term descriptive community studies. The results and their interpretation aim at stimulating a fruitful discussion on the role of CAM in epiphytes, under current and under altered climatic conditions in the future.

Climate change and low species drought tolerance will negatively impact plant richness in the Neotropical seasonally dry forests

“…Our results suggest that drought tolerance is particularly critical for more restricted families, such as the Capparaceae, a relevant taxon in the evolution of the NSDF (Mercado-Gomez et al 2020). Similarly, functional traits of Cactaceae species are poorly understood, but it is already known that they developed a CAM metabolism that is highly related to drought tolerance (Zotz et al 2023). As a result, these species may be among those that will persist in future dry forest communities under the GCC scenarios.…”

Atmospheric CO2 decline and the timing of CAM plant evolution

“…CAM plants are widely noted to perform better at elevated CO 2 , with improvements in carbon gain and growth that often exceed that of C 3 species ( Cui et al ., 1993 ; Cui and Nobel, 1994 ; Graham and Nobel, 1996 ; Winter et al ., 1997 ; Zhu et al ., 1999 ; Drennan and Nobel, 2000 ; Zotz et al ., 2010 , 2023 ; Wagner and Zotz, 2018 ). This observation could lead to a conclusion that high CO 2 enhances CAM photosynthesis such that low CO 2 would not be necessary for CAM evolution.…”

“…However, the improvement of CAM plant performance in elevated CO 2 is due more to increased C assimilation during the C 3 phases of the diurnal CAM cycle and, for facultative CAM plants, increased C gain when the plants are operating in the C 3 mode ( Fig. 6 ; Winter et al ., 1997 , 2014 ; Zhu et al ., 1999 ; Ceusters and Borland, 2010 ; Zotz et al ., 2023 ). Responses of four representative CAM plants are shown in Fig.…”

“…As we move forward, rising CO 2 should remove an important facilitating agent for CAM success, and it is possible that CAM species will become restricted and outcompeted by C 3 plants. In epiphytic floras, for example, more aggressive growth of C 3 species in elevated CO 2 might be expected to crowd out CAM species, particularly since many of the CAM plants use obligate CAM with weak C 3 phases; however, the evidence for this possibility is not strong ( Zotz et al ., 2023 ). Elevated CO 2 is unlikely to harm CAM species directly, given their flexibility and propensity to rely increasingly on C 3 metabolism in higher CO 2 , and one might hypothesize CAM being able to move to more extreme epiphytic microsites as their WUE improves.…”

“…Elevated CO 2 is unlikely to harm CAM species directly, given their flexibility and propensity to rely increasingly on C 3 metabolism in higher CO 2 , and one might hypothesize CAM being able to move to more extreme epiphytic microsites as their WUE improves. Before any of this happens, however, there are far more serious threats to the CAM flora that must be addressed, namely from habitat destruction, run-away fire cycles, over-harvesting of CAM plants for horticultural uses and invasive species outbreaks ( Grace, 2019 ; Sage and Stata, 2021 ; Zotz et al ., 2023 ; Hultine et al ., 2023 ). CAM photosynthesis is not unique in feeling the heat of anthropogenic global change, as the C 4 flora and much of the C 3 flora are under similar threats ( Sage, 2020 ).…”