Differential changes in the reproductive cycle of two temperate plants in response to experimental warming

The evidence of a changing climate influencing plant phenology has grown exponentially over the last 25 years with the majority coming from the examination of flowering records. Given each part of the reproductive cycle influences the preceding and succeeding phases it is not yet clear how changes in one phase may influence another.

Elevated temperature environments allow changes in reproductive phenology to be assessed in greater detail (rather than only using first flowering date), and also allow greater replication to better understand population variability of plastic responses to increased mean temperature.

We examined the differences in the timing of first bud appearance, first flowering, last flowering, seed dispersal, and height of inflorescence in Cynoglossum suaveolens (Sweet Hound's Tongue) and Coronidium scorpioides (Button everlasting) in both an elevated temperature and control (outside) environment (n = 20 plants per treatment). In addition the differences in the seed weight of C. suaveolens and floral disc diameter in C. scorpioides between the two environments were also compared. These are two co-occurring perennial understory species.

The glasshouse mean temperature was on average 5.6°C warmer than the controls. Plants received the same irrigation regime and were watered to field capacity every 1 – 3 days, depending on weather, to standardize any soil moisture interactions with the phenophases.

We found that for both species those subjected to warmer temperatures reached each phenological stage significantly earlier (P < 0.001). For C. suaveolens, the differences in magnitude in the first three phases were similar: bud appearance (54.3 days), first flowering (60.0 days), last flowering (52.9 days). The difference in seed dispersal between the two environments was 38.6 days. In C. scorpioides the largest difference was in first flowering (56.6 days); followed by bud appearance (50.1 days) and last flowering (38.7 days) and as with C. suaveolens seed dispersal had the least difference (20.9 days).

These differences in turn were reflected in the total durations with glasshouse durations beings significantly longer: 67.5 versus 44.9 days in C. suaveolens and 104.1 versus 75.0 days in C. scorpioides. For both species the duration from bud appearance to first flowering was significantly longer (P < 0.05) with the flowering and seed development period significantly shorter in the control (P < 0.001). Also for both species the average inflorescence height was significantly taller (P < 0.01) in the warmer environment: C. suaveolens 205.8 ± 47.5 mm compared to 137.7 ± 43.9 mm; C. scorpioides 283. 0 ± 65.6 mm compared to 212.0 ± 48.0 mm. The warmer environment also resulted in significantly heavier seed weight in C. suaveolens (5.25 ± 0.46 mg compared to 3.21 ± 1.53 mg) and significantly wider floral disc diameter in C. scorpioides (19.7 ± 2.9 mm compared to 14.9 ± 3.1 mm).

Warming advanced the timing of each phenophase; however, the advancement was not constant across phenophases resulting in differing durations. The extended flowering and seed development duration in the warmer environment resulted in heavier seed in C. suaveolens whereas larger floral discs developed in C. scorpioides during a shorter development phase.

This demonstrates that reproductive phenophases may have differing advancements in response to increased mean temperature depending on species. Also, advances in first flowering date may be partly compensated through increased flowering and seed maturation durations that can reduce the advancement of seed dispersal date relative to advances in first flowering date. These phenological changes have resulted in changes to the vegetative and reproductive output which could ultimately lead to changes in species abundance, composition and distribution.