This integrative study is designed to evaluate the influences of microclimatic heterogeneity, associated with complex terrain, on phenology and to evaluate potential trophic responses to scenarios of change in climate, disturbance and land use. We will focus on a simplified model trophic system involving vascular plants, terrestrial and aquatic insects, and migratory neotropical and resident birds. This work will use and extend our long-term studies of plant phenology, climate, Lepidoptera, and aquatic insects in LTER5 and earlier, and allow us to expand our biotic studies to include birds. The model trophic system is ideal because the phenological behaviors across trophic levels are both independent (responding to different abiotic drivers) and dependent (due to trophic interactions), potentially leading to complex system behaviors.
Plant phenology is highly dependent on temperature. Hence, the spatially variable microclimate that occurs in complex terrain results in asynchrony (low spatial coherence) of plant phenologic stages across the landscape (Figure below).
When this spatial variability is integrated across the entire landscape, phonologic stages have the potential to become protracted (Figure 2.8).
Phenologies of terrestrial arthropods also have wide spatial and temporal variation (Miller unpublished), likely in response to temperature variation across terrestrial microclimates such as cold air drainage patterns and temperature inversions. Aquatic insect emergence is also tied to temperature (Frady et al. 2007; Anderson et al. 1984), but stream temperatures are influenced by different factors than those driving air temperatures (Johnson 2004) and may be less sensitive to complex terrain. We propose to assess how microclimatic influences on timing of phenological events affect trophic networks in the landscape. Microclimate variations between forest cover and canopy gaps affect phenology through altered heating (Frady et al. 2007) and snow dynamics. Leaf nutritional value and palatability for herbivores varies with time since leafout. Timing of insect emergence may be key to avoiding predation by neotropical migrant birds. Bird fecundity depends on food availability at nesting periods (Both et al. 2006). Timing of phenological events and trophic interactions (Figure 2.9) also affect ecosystem processes such as nutrient availability and site productivity. Seasonal behaviors of migratory neotropical birds, and habitat and nest selection by resident species may be regulated by endogenous mechanisms, as well as by local climate (Hagar 1992, Gwinner 1977). Phenologies of predators and prey, or producers and consumers, can become desynchronized if mobile predator species are sensitive to different phenologic cues than local prey species, affecting predation rates (Holtby 1988, Bradshaw and Holzapfel 2006).
In our model trophic system (Figure 2.9), producers (plants) and first-order consumers (caterpillars, aquatic insects) have limited ranges, and microclimatic factors control their phenology. In contrast, birds, which combine an array of well-developed behaviors (personal learning, environmental cues, social information etc.) with great mobility, are adapted for finding good feeding stations in a spatially heterogeneous environment, integrating phenology of many sites. The range of microclimates with differing insect activity and availability typical of complex terrain may "buffer" birds at the Andrews Forest, in comparison to other regions where species (Durance and Ormerod 2007) and trophic interactions (Hitch and Leberg 2007; Bradshaw and Holzapfel 2006) are responding to climate change.
To explore phenology and trophic interactions, we will address the following specific questions:
Measurements. Studies of phenology and trophic dynamics will occur at five pairs of sites selected to represent a broad range of environmental conditions and elevations in the Andrews Forest; they build on existing long-term study plots wherever possible (vegetation studies, small watersheds, stream gages, climate stations). Each pair of sites will contain a young deciduous stand and closed, mature conifer site, to allow comparisons of land use and microclimates across elevation gradients. We will concentrate our studies in spring, to capture the arrival of migrant songbirds and the increasing activity of insects. Our ability to support new measurements with the LTER6 budget is limited, so we will condense our studies into a concise period of optimal phenological activity and information.
We will focus on several bird species including a neotropical migrant (Orange-crowned Warbler, Vermivora celata) and a resident bird species (Black-capped Chickadee, Poecile atricapilla). Bird activity and behavior will be observed within 10m plots at each site and songs will be used to document arrival times for the migrant species. We will also experiment with collecting digital bioacoustic data (song rates, dialects) with an automated method that uses signal detection (Dietterich unpublished). Caterpillars are a rich food for birds, so we will document their abundance and size twice each season. Researchers and volunteers will quantify caterpillars on top and underside of 800 understory leaves at each site, estimating size by category and recording taxa by family (Rodenhouse et al. 2003). Emergence of aquatic and riparian insects will be captured using four emergence traps per site (Frady et al. 2007). Malaise traps will be used to collect arthropods. Organisms in traps will be collected twice per week; numbers and biomass will be determined in the lab. Timing of bud break for Douglas-fir (overstory dominant species), and flowering for rhododendron, ocean spray, and colts foot (understory species present at all sites), will be recorded daily using photos. Herbivory on maple leaves will also be monitored from leaves collected at the end of the study period (Shaw et al. 2006; Braun et al. 2002). Diversity and density of vascular plants and trees within study sites will be documented in conjunction with long-term vegetation plot studies. Physical and climatic data collected at all sites will include air, stream and soil temperatures, precipitation, incoming radiation and cover. We will be seeking additional funds to expand the scope of measurements to add additional trophic levels including bats, amphibians, and adult arthropods.
The date of first observation and date of the peak abundance or activity will be determined for birds, dominant aquatic and terrestrial insect taxa within a given year, then compared among years. These data will be assessed for correlation with the phenology, diversity and productivity of overstory and understory plants and abiotic factors, including air and stream temperature, timing and form of precipitation, and snow melt. After four years, we will begin to develop mathematical and computer simulation models to test the hypothesis that habitat selection strategies, patchiness of available prey and climate change interact to influence bird population viability. These models will allow us to test for demographic thresholds associated with rates of climate change and degrees of landscape patchiness. (For more information on this interdisciplinary study see Johnson, Betts, Shaw, Li, Miller, Bond)
This study will be closely integrated with our Schoolyard LTER6 program; the trophic interactions and phenological measurements offer an ideal opportunity to involve K-12 teachers and other volunteers. Teachers and citizen participants will be valuable field assistants for the caterpillar search and malaise trapping; in addition, they will be trained each year to observe the presence of birds, and record vegetation phenology so that they are involved in the full suite of phenological measurements and understand the theory of our trophic interaction study.