Characterizing Stand Density for Stand Tending Strategies

Transcription

1 Characterizing Stand Density for Stand Tending Strategies ESRM 323 Chpt. 4 Smith, et al.

2 or, What tools are available to help us quantify how much growing space the trees are using on this site

3 compared to the trees on this site:

4 Some Stand Attribute Definitions Trees per unit area (TPA) Average number of live trees existing on a characteristic unit of area. The most basic measure of stand density. (In U.S.A., units are Trees Per Acre.) Diameter Breast Height (DBH) average diameter of a tree stem (trunk, or bole) measured 4.5 feet above general ground level on flat or gently sloping terrain, otherwise measured from the uphill side on sloping terrain. (In U.S.A., units are inches.) 4

5 Some Stand Attribute Definitions Mean DBH (AveDBH) - The simple arithmetic mean DBH of all trees in the stand (inches). Basal area (ba, sometimes g) crosssectional area of a tree stem at breast height, assuming circular cross-sectional shape. (In U.S.A., units are square feet.) 5

6 Some Stand Attribute Definitions Quadratic Mean DBH (DBHq, QMD, or sometimes Dg) the DBH of the tree with mean basal area Stand Basal Area (SBA, TBA, BA, or G) sum of individual tree basal areas in a stand BA = x QMD 2 x TPA 6

7 Some Stand Attribute Definitions DBH distribution The frequency (histogram) of tree diameters in a stand. Characterized as the number of trees per acre in one or two inch diameter classes Information from the stand s diameter distribution is often used to characterize the stand s structure, to characterize within stand competition, and to target particular tree sizes in application of different thinning methods 7

8 Some Stand Attribute Definitions DBH distribution 8

9 Some Stand Attribute Definitions Height (H, AveHt) Arithmetic mean height of all trees in a stand (in U.S.A., ft.) Dominant Height Average height of dominant trees (may be average of dominant and codominant, there are other ways to define, also ) Top Height Average height of 100 largest diameter trees on a hectare (lgst. 40 / acre in U.S., sometimes denoted H40, units are feet) 9

10 Stocking vs. Stand Density STOCKING A loose term for the amount of anything on a given area, particularly in relation to what is considered to be optimum. In a forest, a more or less subjective indication of the number of trees as compared to the desirable number for "best" results. 10

11 Stocking vs. Stand Density STOCKING More precisely, a measure of the proportion of an area actually occupied by trees, expressed e.g., in terms of stocked quadrats or percent crown closure, as distinct from their stand density. Adequacy of a given amount of material to meet some management objective. Accordingly, stands can be referred to as "understocked," "fully stocked," or "overstocked." A particular stand that is overstocked for one management objective could be "understocked" for another objective. (Ford-Robertson 1977) 11

12 Stocking vs. Stand Density STAND DENSITY A quantitative measure of tree stocking expressed either relatively as a coefficient, taking normal numbers, basal area or volume as unity, or absolutely, in terms of number of trees per acre, total basal area, or volume, per unit area. More precisely, a measure of the degree of crowding of trees within stocked areas, -- of crown length to tree height; crown diameter to DBH, or crown diameter to tree height; or of stem spacing to tree height. (Ford Robertson 1977) 12

13 Some Stand Density Metrics Simple indicators of stand density (absolute measures): number per unit area (equivalent to "density" in ecological usage) basal area per unit area crown closure usually expressed as % crown cover (can be obtained easily from aerial photos) 13

14 Some Stand Density Metrics Density indexes (relative measures comparing a given stand to a standard ) Usually involve two (2) stand attributes Percent Normality, N% (McArdle, et al. 1930) Stand Density Index, SDI (Reineke 1933) Relative Density Index, RDI (Drew & Flewelling 1979) Relative Density, Curtis RD (Curtis 1982) 14

15 Some Stand Density Metrics Percent Normality Based on Normal Yield, i.e., the yield that results when the trees are fully occupying the site Knowing the age and site index for a particular stand, its basal area can be expressed as a percentage of normal BA for same age and site: N%= BA O 100 BA N 15

16 Some Stand Density Metrics Percent Normality 16

17 Stand Density Index (SDI) (Reineke 1933) Combines Trees Per Acre with average tree size, QMD Based on a pre-determined limiting relationship between log(qmd) and log(tpa) ln(tpa)=ln(a) ln(qmd) Expresses density of a stand in terms of an equivalent number of 10-inch trees The higher the SDI, the more crowded the stand 17

18 Stand Density Index (SDI) 18

19 Log TPA obs Stand Density Index (SDI) SDI Maximum log 10 log QMD 19

20 Stand Density Index (SDI) Shortcut formula: SDI =TPA QMD SDI increases with either an increase in number of stems per acre or an increase in QMD, or both The higher the SDI, the more crowded the stand Relative SDI, SDI REL = SDI SDI MAX 20

21 Stand Density Index (SDI) 21

22 Relative Density Index (RDI) (Drew & Flewelling 1979) Combines number per acre with average tree size, volume (cu.ft) Based on the 3/2 power law of selfthinning v=atpa 3/2 or ln(v)=ln(a) 3/2 ln(tpa) - For Douglas-fir in PNW, ln(a) =

23 Relative Density Index (RDI) (Drew & Flewelling 1979) 23

24 Relative Density Index (RDI) ln v o obs ln v Maximum Normal ln TPA o ln TPA max ln TPA 24

25 Relative Density Index (RDI) Relative Density Index, ρ r = TPA obs TPA max Relative Density Index is usually interpreted as the proportion of the total growing space being utilized by trees TPA max =e ln(v)

26 A B C The Density Management Diagram (DMD) D - Lower limit of acceptable total yield (per ac.) 26

27 Basic Tenets for using the DMD Stands growing below the crown closure line (RDI = 0.15; RD ~ 15), are not fully utilizing the site, and density could be increased without decreasing mean tree growth. Maximum tree size can be obtained most quickly by managing stands near the crown closure line. 27

28 Basic Tenets for using the DMD Stands managed near the lower bound of the zone of imminent competition-mortality down to a Relative Density Index of about 0.35 to 0.40 (RD ~ 35 to 40) will have somewhat greater total stand growth than stands managed at lesser densities, but will have smaller individual tree sizes Stands should not be allowed to enter the zone of imminent competition-mortality (RDI = 0.55; RD ~ 55) until several years before the final harvest in order to avoid a severe reduction in vigor and potential damage to the crop trees. 28

29 Relative Density (RD) (Curtis 1982) Combines stand basal area and average tree size, ie., QMD (or, DBHq) RD = BA / (QMD) 0.5 RD will increase with an increase in BA (with constant QMD) RD will increase with a decrease in QMD for constant BA Thus, higher values of RD imply a greater degree of competition 29

30 Approximate relationship between selected stand density measures Standard %Normality RDI RD (BA) (Flewelling) (Curtis) Maximum Normal Upper thinning limit Lower thinning limit Crown closure

31 Uses of Density Indexes They are useful descriptors of stand conditions (though not a complete description) They are useful predictors of growth (in combination with other variables) They serve as guides to thinning and stand treatment, defining the following Maximum size-density limit ( A line) Upper thinning limit, above which one expects substantial mortality and/or unacceptable diameter growth ( B line) Point of crown closure in young stands ( C line) Lower thinning limit, below which one expects unacceptable total volume growth ( D line) 31

32 Uses of Density Indexes They can also be used to estimate desirable planting numbers and desirable number of residual trees in pre-commercial thinning (e.g., RD). Can be used to translate multiple objectives into actionable metrics, ie., metrics that can be used to take action, eg., habitat requirements 32

33 Stand Development Stages 33

34 Impacts Wildlife Populations Stand Initiation Stem Exclusion Understory Re-iniit. Old Growth 34

35 Using the DMD for Wildlife Habitat UH - Upper limit for appreciable Wildlife Habitat (CurtisRD = 45; RDI ~ 0.45) LH - Lower limit for appreciable Wildlife Habitat (CurtisRD = 25; RDI ~ 0.25) 35

36 Summary Ideas As stand dynamics progress, the growing space is reallocated to different trees mostly as a result of competition Manipulating growing space is the major tool available to the silviculturist to accelerate (decelerate) the approach to desired (undesired) stand outcomes 36

37 Summary Ideas In order to manipulate growing space effectively, the silviculturist needs to know how it is being utilized currently, and how a growing space manipulation (thinning) will change that Ability to quantify stand growing space utilization leads to sound silvicultural decisions and sound mgt. 37