Prairie plant phenology driven more by temperature than moisture in climate manipulations across a latitudinal gradient in the Pacific Northwest, USA

Abstract

Plant phenology will likely shift with climate change, but how temperature and/or moisture regimes will control phenological responses is not well understood. This is particularly true in Mediterranean climate ecosystems where the warmest temperatures and greatest moisture availability are seasonally asynchronous. We examined plant phenological responses at both the population and community levels to four climate treatments (control, warming, drought, and warming plus additional precipitation) embedded within three prairies across a 520 km latitudinal Mediterranean climate gradient within the Pacific Northwest, USA. At the population level, we monitored flowering and abundances in spring 2017 of eight range‐restricted focal species planted both within and north of their current ranges. At the community level, we used normalized difference vegetation index (NDVI) measured from fall 2016 to summer 2018 to estimate peak live biomass, senescence, seasonal patterns, and growing season length. We found that warming exerted a stronger control than our moisture manipulations on phenology at both the population and community levels. Warming advanced flowering regardless of whether a species was within or beyond its current range. Importantly, many of our focal species had low abundances, particularly in the south, suggesting that establishment, in addition to phenological shifts, may be a strong constraint on their future viability. At the community level, warming advanced the date of peak biomass regardless of site or year. The date of senescence advanced regardless of year for the southern and central sites but only in 2018 for the northern site. Growing season length contracted due to warming at the southern and central sites (~3 weeks) but was unaffected at the northern site. Our results emphasize that future temperature changes may exert strong influence on the timing of a variety of plant phenological events, especially those events that occur when temperature is most limiting, even in seasonally water‐limited Mediterranean ecosystems.

Climate manipulations differentially affect plant population dynamics within versus beyond northern range limits

“…Interestingly, since the negative effects of the warming treatments on λ within current ranges were of greater magnitude and more frequent than that of the drought treatment (which had only a single negative effect), warming itself, rather than reduced soil moisture, appears to be driving the demographic decline in this system. We have previously found that warming also had a greater influence than moisture on these and other species' phenological responses to climate change (Reed et al., 2019), and we observed a similar response for soil respiration (Reynolds, Johnson, Pfeifer‐Meister, & Bridgham, 2015). It seems that these phenomena are a function of the region's Mediterranean climate system, in which very wet soils occur throughout the winter and very dry soils throughout the summer until the onset of fall rains.…”

“…For each species and most vital rates (see Table for exceptions), we built two global models: a climate global model (using the climate treatment variable with four levels: control, drought, warming and warming + precipitation) and a warming global model (collapsing the climate treatment into two temperature categories: ambient (control and drought) and warming (warming and warming + precipitation)). We used this collapsed warming treatment (in addition to the full climate treatment) because preliminary data exploration and evidence from previous experiments at these sites suggest changes in temperature have a stronger influence than changes in moisture on plant responses in this system (Pfeifer‐Meister et al., 2013; Reed et al., 2019) and we gained degrees of freedom in doing so.…”

Experimental shifts in exotic flowering phenology produce strong indirect effects on native plant reproductive success

“…Although the timing and duration of the shifts we created do not reflect a specific prediction for future native–exotic overlaps, they represent reasonable high‐end estimates for shifts based on change observed in other systems (e.g. Wolkovich et al, 2013) and in warming experiments in Pacific Northwest prairies (Reed et al, 2019). The changes in seedset we observed could have demographic effects in plant species if population growth rate is sensitive to reproductive rate, which is not always the case (Iler et al, 2019).…”

“…In the absence of specific predictive information on phenological shifting by these exotic species, we chose a shift that would be large, but likely within the scope of possible future change. Wolkovich et al (2013) found up to ~10 days of phenological advance had occurred in exotic plants, while Lindh, MacGahan, and Bluhm (2018) detected ~2.5 days advancement per decade since 1958 and Reed et al (2019) found up to ~25 days of advancement in some forb species in a warming experiment in Pacific Northwest prairies. We therefore altered exotic phenology by adding pots at the onset of the species’ natural flowering at nearby sites for Ambient treatments, ~3 weeks before ambient flowering for Advanced treatments, or ~3 weeks later than ambient flowering for Delayed treatments.…”

Vascular Epiphytes: The Ugly Duckling of Phenological Studies

“…In temperate environments, the temperature has been considered one of the main triggers for phenological events (Prevéy et al, 2017;Flynn and Wolkovich, 2018;Reed et al, 2019). However, in the tropics, where most epiphytes are found, attention is mostly turned to rain seasonality, ignoring changes in temperature and photoperiods because of their low annual variation (van Schaik et al, 1993;Morellato et al, 2000;Sakai, 2001).…”