- Top of page
- Materials and methods
1. Mast seeding is the intermittent production of large quantities of seed across a perennial plant population. Such seeding events in many plant species are initiated by climatic cues, but whether these cues act solely as triggers or also via alterations to nutrient availability is unclear.
2. Here, we examine the effect of nitrogenous fertilization on the relationship between seed production in Nothofagus solandri var. cliffortioides and two climatic cues (rainfall and temperature) at specific stages in reproductive development from 1999 to 2008.
3. Foliar nitrogen concentrations were positively correlated with rainfall among years, suggesting rainfall was affecting nitrogen availability.
4. Seedfall mass in unfertilized stands was predominantly determined by rainfall during resource priming, while seed production in fertilized stands was more affected by temperature during floral primordia development. Similarly, seedfall mass in older stands, which contain greater internal nutrient reserves, was predominantly determined by temperature.
5. Synthesis. The results of this study demonstrate that the sensitivity of seed production to climatic cues can be altered by manipulation of resource availability and therefore establish that climatic cues involved in the synchronization of mast seeding can influence reproductive effort via an effect on resource availability. These results also indicate that alterations to resource availability have the potential to alter inter-annual patterns of seed production, but further study is required to verify this finding.
- Top of page
- Materials and methods
Several hypotheses have been proposed to explain the evolution of mast seeding (Kelly 1994). Predator satiation (e.g. Janzen 1971) and increased pollination efficiency (e.g. Nilsson & Wästljung 1987) are currently the best-supported evolutionary explanations, while climate and resource dynamics act as important proximate factors (Kelly & Sork 2002; Koenig & Knops 2005; Crone, Miller & Sala 2009). Temperature and rainfall are two climatic cues often associated with mast seeding. Cool temperatures and increased moisture availability during resource priming c. 2 years prior to seedfall can increase seed production across populations of a plant species (Piovesan & Adams 2001; Richardson et al. 2005), while relatively high temperatures during floral primordia development c. 1 year prior to seedfall cue mast seeding in various tree species (Schauber et al. 2002; Övergaard, Gemmel & Karlsson 2007). However, the process by which these climatic cues influence seed production is not clearly defined. Given the resources required to support substantial increases in seed production, nutrient availability must influence the incidence of mast seeding (Janzen 1971; Kelly & Sork 2002; Rees, Kelly & Bjørnstad 2002). Internal carbohydrate reserves appear to be an important factor in determining the incidence of mast seeding (Isagi et al. 1997; Miyazaki et al. 2002), while nitrogen (N) availability is critical to the timing of mast-seeding events due to the importance of N to photosynthesis and the generation of reproductive tissue (Yasumaru, Hikosaka & Hirose 2006; Han et al. 2008). This is supported by several studies in which the application of nitrogenous fertilizers has increased seed production in various tree species (Le Tacon & Oswald 1977; Fahey, Battles & Wilson 1998; Davis, Allen & Clinton 2004), and it appears that increasing atmospheric N deposition is a factor in both the greater frequency and magnitude of mast seeding in beech (Fagus) forests in southern Sweden (Övergaard, Gemmel & Karlsson 2007). Consequently, it has been suggested that synchronized increases in seed production are a function of the effects of the climatic cues on the availability of resources, such as the role of temperature and soil moisture in facilitating N availability (Walse, Berg & Sverdrup 1998; Paul et al. 2003). However, these cues may also be acting to synchronize reproductive effort by manipulating endogenous plant process such as internal resource allocation (Bazzaz et al. 1987; Koenig & Knops 2005; Richardson et al. 2005) or producing environmental conditions beneficial to other facets of reproduction, such as pollen dispersal (Selås et al. 2002). This position suggests that climatic cues influence the incidence of mast seeding by acting directly on internal reproductive processes to synchronize the incidence of mast seeding (Koenig & Knops 2005; Richardson et al. 2005), but this does not exclude the potential for direct and indirect pathways to operate simultaneously.
To partition the role of climate as a trigger on endogenous plant processes from effects on resource availability we examined the effect of N fertilizer addition on the relationship between seed production and selected climatic cues in a New Zealand mountain beech [Nothofagus solandri var. cliffortioides (Hook. f.) Poole: Fagaceae] forest. The relationship between seed production and the climatic cues was assessed at three specific stages of reproductive development – resource priming, floral primordia development and flowering (Richardson et al. 2005). We also assessed the effect of stand development on the relationship between seed production and the climatic cues, as internal nutrient reserves in mountain beech increase substantially with age (Clinton, Allen & Davis 2002), potentially decreasing the dependence of older trees on climatic factors to supply the nutrients required for increased seed production. Given the potential for resource availability to influence the extent of reproductive effort (Isagi et al. 1997; Satake & Iwasa 2000; Rees, Kelly & Bjørnstad 2002; Newbery, Chuyong & Zimmermann 2006), we also explored the possibility that alterations to N supply could influence the pathway by which seed production is determined (see Fig. 1).
Figure 1. Conceptual algorithm illustrating the potential for fertilization to influence the incidence of mast seeding. Temperature in the year prior to flowering is the first critical point, while rainfall 2 years prior to flowering and the presence or absence of fertilization determine if nutrient reserves available to the plant are greater than the required threshold to allow increased reproductive effort.
Download figure to PowerPoint
We investigated the following hypotheses: (i) that increased N availability in mountain beech stands would increase the mass of seed produced in all years, (ii) increased N availability would decrease the dependency of seed production on climatic cues during the resource priming phase of reproductive development and (iii) greater stand age would decrease the dependency of seed production on climatic cues during the resource priming phase of reproductive development. We also hypothesize that any fertilizer-induced variation in the relationship between climatic cues and seed production would alter inter-annual patterns of seed production relative to unfertilized plots.
Examination of the data demonstrated that the synchronization of seed production by rainfall is at least partially related to an effect of rainfall on N availability. We also showed that the relative importance of climatic cues to seed production, during different phases of reproductive development, varies significantly with resource availability and stand maturity. Increased N supply was also observed to partially alter patterns of reproductive effort in 1 year.