We suggest that ecological processes and physical characteristics possess an inherent scale at which the processes or characteristics occur over the landscape. We propose a conceptual spatial response model that describes the nature of this ecological scale. Based on the proposed spatial model, we suggest methods for estimating the size of study plots or transects and the distance between replicate plots needed to approach statistical independence. Using data on percent cover for Agropyron spicatum, a common arid-land bunchgrass, we demonstrated four relationships that should hold if the spatial response model is appropriate. These relationships are sample variance increases as functions of (1) transect segment length and (2) intersegment length (transect segment dispersal), and correlation decreases as functions of (3) intersegment length and (4) transect segment length. Based on evaluation of these four relationships, cover for A. spicatum is correlated over the landscape on a scale of 400 to 700 m, and a segment length of 64 to 128 m is most appropriate for measuring cover for this species.

No abstract

Project Order No. 84PP4819

Litter fall is commonly collected for a variety of ecological studies. This study was designed to test the effect of collector size on the precision of forest litter-fall estimates and on the time involved in laboratory sample sorting. Collectors varied in size from 0.010 to 0.933 m2 and were physically nested, the smaller units within larger units. Ten of these collector combinations were randomly placed on a 1-ha plot in a Douglas-fir/western hemlock (Pseudotsugamenziesii (Mirb.) Franco/Tsugaheterophylla (Raf.) Sarg.) stand in H. J. Andrews Experimental Forest. Collections were made monthly and records were kept of the time required to sort the litter into needles, epiphytes, and miscellaneous categories. Based on a definition of precision as ±10% of the mean, 90% of the time, results indicate (i) that the cost of obtaining precise estimates of needle fall decreases with decreasing collector size to 0.010 m2, (ii) that collectors of any size can be used to obtain estimates of total litter fall if the number of collectors required to obtain precise results is determined, and (iii) that precise estimates of epiphyte biomass require large numbers of samplers and are not cost effective.